CA2247532A1 - Process for bleaching a paper pulp with ozone and chlorine dioxide - Google Patents
Process for bleaching a paper pulp with ozone and chlorine dioxide Download PDFInfo
- Publication number
- CA2247532A1 CA2247532A1 CA002247532A CA2247532A CA2247532A1 CA 2247532 A1 CA2247532 A1 CA 2247532A1 CA 002247532 A CA002247532 A CA 002247532A CA 2247532 A CA2247532 A CA 2247532A CA 2247532 A1 CA2247532 A1 CA 2247532A1
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- Canada
- Prior art keywords
- pulp
- ozone
- bleaching
- paper pulp
- paper
- 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
- D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
-
- 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
- D21C9/1015—Bleaching ; Apparatus therefor with use of means other than pressure, temperature
-
- 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
- D21C9/1026—Other features in bleaching processes
-
- 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
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
- D21C9/153—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications with ozone
-
- 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
- D21C9/16—Bleaching ; Apparatus therefor with per compounds
- D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides
Abstract
Process for bleaching a paper pulp in which the said pulp is subjected successively to the action of ozone and then of chlorine dioxide, or vice versa. The process according to the invention is characterized in that the paper pulp treated is a so-called "low concentration" pulp with a consistency of less than or equal to approximately 5% by weight, the mixing of the ozone with the pulp being carried out in a rotary stirrer in which the pulp is stirred at a low speed while ozone is injected into the pulp, so as to obtain a fluidized pulp, the tangential peripheral speed of the pulp at the walls of the rotary stirrer being less than or equal to approximately 12 m/s but greater than or equal to approximately 2 m/s, the contact time between the ozone and the pulp being between 1 min and 20 min and the amount of ozone injected into the rotary stirrer being between 1 kg and 20 kg of ozone per t of paper pulp on a dry basis, the pulp being maintained in the fluidized state for substantially the entire duration of the reaction with the ozone.
Description
ROCE88 FOR RT-F ~CHING A PA~ER ~ULP
W~H OZONE AND CHLOR~NE DIOX~DE
The present invention relates to a process for bleaching paper pulp in which the said pulp is subjected successively to the action of ozone and of chlorine dioxide, or vice versa.
In the past, chlo~ine was one of the bleachi~g agents which was most widely used for the bleaching of paper pulp, in particular for so-called chemical pulp.
Chlorine dioxide o~ hypochlorlte are also well known bleaching agents which make lt possible both to remove the lignin ln the cellulose and to improve t~e whiteness of the ~ellulose. As a general rule, the cholce of the bleaching agent depends essèntially on the type of paper which it is desired to produce from the treated pulp. When, ~or example, so-called kraft pulp is used to make very white writlng paper, i~ is necessary to use a large amount of bleaching agent and to choose this agent frcm those having a ~aster and more selective action. Wha~ever the type of chlorinated bleaching agent used, organochlorine products are produced during the bleaching which generally have little solubility in the aqueous liquo~ for washing the pulp and large amounts of which are entrained or extracted from the pulp with the e~fluents at each stage of bleaching the pulp. A relatively low amount of these organochlorine products remains in the pulp itself. As a general rule, when chl~ri~e is used and when it is desired to obtain very white products, i~ is necessary to use a much greater amount of chlorine compared wlth the processes in which the whiteness of the pulp obtained is less important.
As chlorinated effluent discharges have a particularly harmful effect on the environment, legislation has been passed in many countries so as to limit, if not completely ban, the use of chlorine and its derivatives in the bleaching of paper pulp, so as to limit the amo~nt of organochlorine compounds in the aqueous wash llquors which it is subsequently necessary to treat. Lower limit values for the emission of organochlorine substances have thus been defined, amounts whlch are quantlf~ed by t of adsorbable organohalogen substances or AOX per tonne of pulp. ~or the moment, the maximum amounts per tonne of pulp are, dependlng on the country, li~ited to 1 or 2 t but, in a few years' time, it will- be nece~sary to discharge effluents containing less than 0.5 t of AOX per tonne of pulp produ~ed.
Some solutions have already been provided for reducing or removing organochlorine products in the proeess for bleaching paper pulp.
The simplest method consists in substituting non-chlorinated chemicals, such as oxygen, peroxides, such as hydrogen peroxide, ozone, pe~acetic acid, and the like, fo~ the chlorinated chemicals used.
Unfortunately howe~er, none of these bleaching ~rocesses using non-~hlorinated products has made it possible to obtain pulps for which the properties, such as the whiteness or the vi~cos t ty, have an acceptable value at a reasonable cost Another possibility for reducing the amount of organochlorine compounds is to reduce the amount of z5 chlorine u~ed, generally in the first bleaching stage.
Varlous solutions have already been provided, such as increased delignification in the stage of coo~ing the pulp, as well as delignification with oxygen. These treatments, followed by an appropriate extraction stage, make it possible to reduce the content of lign-n in the pulp, which is conveyed to the bleaching plant.
However, these treatments do not make it possible to sufficiently reduce the concentration of organochlorine compounds in the bleached pulp and to bring the a~ount of residues to a sufficiently low value.
Another possibility consists in substituting chlorine dioxide for chlorine, which chlorine dioxide is a strong oxidizing agent compared with chlorine, maklng it possible to carry out a delignification comparable with that of chlorine, with a lower amount (less than half). This type of process is known in the literature under the abbreviation DE, ~referably DEDED, the stage D being the stage of treatment wlth chlorine dioxide, the stage ~ being the conventional extraction stage.
For further details on these various treatments, reference ~ay be made to the work by J.B. Casey, "Pulp and Paper, Chemistry and Chemical Technology", 3rd edition, Vol. l, 1980, John Wiley Son, New York, page~ 694 to 696.
An experimental process for bleaching a paper pulp of "sulphite" type is also known from the journal "Pulp and Paper Science", March 1984, F. Granum, entitled "Influence of Bleaching Chemicals and Sequences on some Properties of Sulphite Pulp", pages J.25 or J.29, in which process numerous se~uences are described, in particul~r sequences using oxygen or ozone, followed by sequenc~s using chlorine dioxide.
A process for bleach~ng a kraft pulp intended for the manufacture of a paper is also known from EP
464,157, which process comprises a sequence of bleaching with pure or mixed chlorine dioxide, ~ollowed by a stage of bleaching with ozone, without an intermediate extraction stage, followed by an alkaline extraction as the third stage and finally ending with a fourth stage of bleaching with chlorine dioxide, this stage being the final stage of this bleaching process carried out in this order.
A process for bleaching a paper pulp according to a so-called DZ sequence, that is to say a first treatment with chlorine dioxide followed by a second treatment with ozone without an intermediate extraction stage between the two, i~ also known from Canadian Patent 2,031,850, in which process the two chemicals are injected at points separated from one another in the bleaching line. Such a process is supposed to decrease the production o~ AOX. However, all the examples gi~en were carried out on pulp delignified beforehand with oxygen.
Canadian Patent 2,031,848 also discloses a process similar to that disclosed above in Canadia~
Patent 2,031,850 but in any order, that is to say in fact describing ZD or DZ sequences. Here again, a preliminary stage of dellgnification with oxygen is necessary, as is disclosed in all the examples.
In these two Canadian patents and in the abovementioned European patent, all the implementational exa~ples are exclusively based on a paper pulp of medium consistency.
It is generally expected by a person skilled in ~he art that the higher the consistency of the pulp, the better the efficiency should be of the treatment with chemicals of the D and Z type, pro~ided that the mixing is carried out ln a mixer with a ve~y high speed. This is why, until now, none of these processes has been developed with so-called low consistency pulp, that is to say comprisin~ a much greater amount of water than a medium or hig~ consistency pulp.
However, paper pulp plants oper~ting with low consistency pulp currently face a number of problems. A
number o~ plants currently operate with C~ sequences, that is to say a first stage of bleaching with chlorine, followed by a second stage of bleaching with chlorine dioxide (intermediate solution already mentioned abo~e), which process, as was indicated above, is not satisfactory with respect to standards which will soon come into force relating to discharges of AOX. In these plants, which currently operate with low consistency pulps, all the equipment used is specific to low consistency pulp, in particular the mixer~, the dimensions of the pipework and of the intermediate storage devices, and, generally, all the manufacturing equipment. With the processes currently known for preventing an excessively high concentration of AOX products in the discharges, it is thus necessary, for the conversion of these plants, to equip them with medium or high consistency equipment, so as to meet the standards. This means that it is necessary to replace all the existing eguipment by new equlpment in these plants, which ~eans an investment of millions of dollars.
There currently therefore does not exist a process which generates a low amount of AOX, that is to say less than 0.5 t of AOX per tonne of paper pulp, and which uses a treatment o~ the pulp at low consistency, that is to say a pulp with a consistency ~ S~.
It is an object of the present inventlon to provide a solution to the problem thus posed.
The process according to the invention is characterized in that the paper pulp treated is a so-called "low concentration" pulp with a co~sistency ofless than or equal to approxlmately 5~ by weight, the mixing of the ozone with the pulp being carried out in a rotary stirrer in which the PUlp i~ stirred at a low speed while ozone is injected into the pulp, so as to obtain a fluidized pulp, the tangential peripheral speed of the pulp at the walls of the rotary stirrer being le~s than or equal to approximately 12 m/s bu~
greater than or equal to approximately 2 m~s, the contact time between the ozone and the pulp being between 1 min and 20 min and the amount of ozone injected into the rotary stirrer ~eing betwee~ 1 kg and 20 kg of ozone per t of paper pulp on a dry basis, the pulp being maintained in the fluidized state for substantially the entire duration of the reaction with the ozone.
The ozone is preferably in~ected as a mixture with oxygen, preferably in the proportion of 3 to 20~
by volume of ozone in the oxygen, the oxygen also being in contact with the pulp for substantially the same duration as the ozone and producing a complementary and simultaneous dellgnlfication and/or bleaching action on the pulp.
At the end of the stirring stage, the oxygen is recovered, filtered to retain the undesirable impuritie~ and optionally recycled to the inlet of the ozonizer.
Use will preferably be made o~ an ozonizer in which oxygen ~ s intr~duced at a press~re greater tha~
atmospheric pressure, so as, on the one hand, to optimize the yield from the ozonizer and, on the other hand, on account of the pressure drops in the latter, to obtain a mixture of oxygen and of ozone at a pressure greater than atmospheric pressure ~preferably between 1.3 bar and 2.5 bar, absolute pressure) but less than lS bar absolute, generally bet~een 1 and 3 bar absolute. The gaseous mixture can thu~ be introduced directly into the pulp, without having to be compres~ed before being iniected.
lS The tangential peripheral speed of the pulp will preferably be les~ than 9 m~s and more preferably less than 6 m/s, which, ~-~ the case of P cylindrical stirrer with a diameter ~f approxlmately 1 metre, represents a rotation~ll speed of les~ than approximately 120 revoluti~ns per minute.
The ~mount of 02. ~ne per t of dry pulp is preferably between 2 and 10 kg, whlle the duration of treatment with ozone in the stirred reactor is preferably between 2 and lO min. A~ regards the Zs chlorine dioxide, the amounts vary between 1 and 20 kg per t of dry pulp, preferably from 2 to 10 kg per t o~
dry pulp.
By uslng the process according to the invention, effluents having an amount of AOX of 0.1 to 0.2 kg per tonne of dry pulp produced are thus easily obtained. The plant designed or used for a CD or DC
sequence is not changed, the plant for injection of chlorine simply being replaced by a plant for ln~ection of ozone, in the a~ounts mentioned above. Another considerable advantage of the in~ention is that the ~echanical properties of the pulp are retained or even improved.
Rotary stirrer or stirred reactor is understood to mean a stirrer, preferably cylindrical, comprlsing one or more paddles which are capable of causing the entire liquid ~low consistency pulp) to rotate with a tangential speed, measured at the periphery of a section of the cylinder, defined above as being the tangential peripheral speed of the pulp.
The general characteri~tics o~ the stirred reactors are described in the article entitled "The mixing-rate number for agitator stirred-tank", Chemical Engineerl~g, October 11, 1976, pages 141 to 143.
Another advantage of the in~ention is that it is not necessary to use an intermediate recept~cle or ~lash tank (for example, see Canadian Patent 2,031,~98), into which receptacle, after mixing the pulp and the reactants in the mixer, the pulp is poured in order to continue the reaction between t~e pulp and the ozone, so as to improve the bleaching of the pulp.
In the brochure from the co~pany Kvaerner Pulping Technologies AB entitled "Kvaerner MC~ Mixer", second page, it i~ explained that correct fluidization o~ a pulp with a mixer especially designed for paper pulp can only be achie~ed with pulp~ having a consistency of the order of or greater than 10~ (so as to achie~e a rheological behaviour of the pulp substantially identical to that of water). For this, the rotatlonal speed must be greater than approximately 1800 revolutions per minute, i.e. a tangential peripheral speed of approximately 30 m/s for a dlameter o~
approximately 30 cm for an industrial mixer. The contact between the pulp and the ozone in the mixer lasts at most 10 seconds. Furthermore, in the process according to the invention, it is not necessary to provide a stage of delignification with oxygen prior to the DZ sequence according to the invention.
The stage of treatment with 020ne will take place before or after treatment with chlorine dioxide.
Of course, the process of the invention applies to the ca~e where ozone alone is injected, without injection of chlorine dioxide, or, if the stage of injection of ozone is preceded or followed by any type of bleaching stage or sequence with any well known bleaching agent mentioned abo~e, including another stage with ozone similar to or di~ferent from that according to the present invention.
Furthermore, throughout this description, it has been assumed that the rotational speed of the paddles of the stirrer is comparable to the rotational speed of the pulp, in particular at the walls (tangential peripheral speed). In practice, differences lo due to local turbulences in the pulp may be found.
rx~rl ~
In this example , a sample of non oxygen delignified softwood kraft pulp - Kappa 27 - is treated according the invention at low consistency in ~ ~r~ y ~ y~ mixer. Pulp is first delignified with chlorine dioxide by adding 2 ~ C102. Then the sa~ple ls divided in two portions: the first one -~ample A- , after wash~ng , is treate~ in a Ep stage accordin~ '_he follo~ing condltions :
NaOH charge : 1.5 %
H202 charge : 0.25 %
Te~perature : 75~ C
Time : 120 minutes Consistency : 10 %
The second portion -sample B- ,before any washing, is directly tre~ted with 03 under the following operating conditions :
Temperature : 50~C
Pressure : atmospheric Pulp consistency : 2.5 ~
03 concentration in 02 : 10 % ~y weight Mixing time : 6 minutes Peripherical mixer rotating speed : 4.1 m/s 03 injected : 5 kg/t or 0.5 % on pulp 03. consumption efficiency : 95 ~
_ g _ After reaction sample B is ~ubmitted to the same washing and then Ep stage as with sample A.
Sample B exhiblts after D Ep treatment a ~appa of 3.4 compared to ~.6 for sa~ple A after DZ Ep reflecting the efficiency of the 03 treatment carried out ln such conditions.
~rle 2 In this example , an 02 delignified kraft eucalyptu~
lo pulp - Kappa 9.0 , Brightness 45.8 ~ ISO- is used for comparing 03 treatments in a DZ sequence with conventi~nal high shear mixlng at medium consistency (sample A) and low shear mixing at low consistency (sample B). For reference , a first pulp ~s treated according the D Eo D sequence. Operating conditions for the reference are :
D stage Temperature : S5~C
Residence time : 45 minute~
C102 charge : 1.3 % ( as pure C102 ) Consistency : 11 ~o sta~e Te~perature : 70~C
Residence time : 125 minutes NaOH charge : 1.3 %
Consistency : 12 %
02 pressure : 2 bars Final D stage Temperature : 70~C
Residence time : 90 minutes Cl02 charge : 0.6 ~ ~ as pure C102 ) Consistency : 11 %
Results obtained on this reference sample are :
Kappa after D Eo : 0.9 ~rlghtness after D Eo : 73.~ % IS0 Brightness after D Eo D : 88.1 ~ IS0 s DZ Eo D sequence~
In both ca~es,C102 and 03 are added ~equentially, 03 being added after complete consumption of ClC2 in ~he first step, before any washing.
For sample A, D stage conditions are identical to the reference except the ClOZ charge which is reduced to 0.48~. Then 0.4 ~ (or 4 kg~t on pulp) 03 is added under high shear ~ixing condition~ which are :
03 pressure : 9 bars 03 concen~ration in 02 : 9.5 ~ by weight Pulp con~i~tency : 11 %
Temperature : 55~C
Perlpherical mixer rotating speed : 78 m/s Next stages are car~iec out according conditions ldentical ~o the referen~e except the C102 charge in the final D stage which is redueed to 0.4 %. Results obtalned are :
Kappa after DZ Eo : 0.9 Brightness after DZ Eo : 79.2 % IS0 Brightness after DZ EoD : 88.5 % IS0 Sample B is treated at l~w consistency both with C102 and 03 ; other D stage conditions are identical to the reference except the C102 charge which is reduced to 0.45 ~ and pulp consistency which is reduced to 2.5 ~.
Then 0.4 ~ ~or 4 kg/t on pulp) 03 is added under low shear mixing conditions which are :
03 pressure : 2 bars 03 con~entration in 02 : 9.5 ~ by weight Pulp conSistency : 2.5 Temperature : 55~C
Peripherical mixer rotating speed : 11 ~/s .
Next stages are carried out according condit$ons identical to the reference except the C102 charge in the final D stage which is reduced to O.g ~ . ~esults obtained are :
Kappa after DZ Eo : 0.9 Brightness after DZ Eo : 79.0 ~ IS0 Brightness after DZ ~oD : 88.3 % IS0 lo These exa~ples show clearly that the 03 treatment can be combined with a C102 stage in a DZ sequence while carrled out at low consistency under appropriate ~luidised conditions with at least the same effl~iency as when performed at medlum consistency. This is re~lected among other things by the sa~ings ~n C102 when comparing the various charges in C102 used to reach 88 % IS0 brightness. For the reference 1.9 % C102 is needed while for the D~ Eo D only 0.88 ~ and 0.85 ~
are required ~ for respectively medium and low ~on~istency treatments
W~H OZONE AND CHLOR~NE DIOX~DE
The present invention relates to a process for bleaching paper pulp in which the said pulp is subjected successively to the action of ozone and of chlorine dioxide, or vice versa.
In the past, chlo~ine was one of the bleachi~g agents which was most widely used for the bleaching of paper pulp, in particular for so-called chemical pulp.
Chlorine dioxide o~ hypochlorlte are also well known bleaching agents which make lt possible both to remove the lignin ln the cellulose and to improve t~e whiteness of the ~ellulose. As a general rule, the cholce of the bleaching agent depends essèntially on the type of paper which it is desired to produce from the treated pulp. When, ~or example, so-called kraft pulp is used to make very white writlng paper, i~ is necessary to use a large amount of bleaching agent and to choose this agent frcm those having a ~aster and more selective action. Wha~ever the type of chlorinated bleaching agent used, organochlorine products are produced during the bleaching which generally have little solubility in the aqueous liquo~ for washing the pulp and large amounts of which are entrained or extracted from the pulp with the e~fluents at each stage of bleaching the pulp. A relatively low amount of these organochlorine products remains in the pulp itself. As a general rule, when chl~ri~e is used and when it is desired to obtain very white products, i~ is necessary to use a much greater amount of chlorine compared wlth the processes in which the whiteness of the pulp obtained is less important.
As chlorinated effluent discharges have a particularly harmful effect on the environment, legislation has been passed in many countries so as to limit, if not completely ban, the use of chlorine and its derivatives in the bleaching of paper pulp, so as to limit the amo~nt of organochlorine compounds in the aqueous wash llquors which it is subsequently necessary to treat. Lower limit values for the emission of organochlorine substances have thus been defined, amounts whlch are quantlf~ed by t of adsorbable organohalogen substances or AOX per tonne of pulp. ~or the moment, the maximum amounts per tonne of pulp are, dependlng on the country, li~ited to 1 or 2 t but, in a few years' time, it will- be nece~sary to discharge effluents containing less than 0.5 t of AOX per tonne of pulp produ~ed.
Some solutions have already been provided for reducing or removing organochlorine products in the proeess for bleaching paper pulp.
The simplest method consists in substituting non-chlorinated chemicals, such as oxygen, peroxides, such as hydrogen peroxide, ozone, pe~acetic acid, and the like, fo~ the chlorinated chemicals used.
Unfortunately howe~er, none of these bleaching ~rocesses using non-~hlorinated products has made it possible to obtain pulps for which the properties, such as the whiteness or the vi~cos t ty, have an acceptable value at a reasonable cost Another possibility for reducing the amount of organochlorine compounds is to reduce the amount of z5 chlorine u~ed, generally in the first bleaching stage.
Varlous solutions have already been provided, such as increased delignification in the stage of coo~ing the pulp, as well as delignification with oxygen. These treatments, followed by an appropriate extraction stage, make it possible to reduce the content of lign-n in the pulp, which is conveyed to the bleaching plant.
However, these treatments do not make it possible to sufficiently reduce the concentration of organochlorine compounds in the bleached pulp and to bring the a~ount of residues to a sufficiently low value.
Another possibility consists in substituting chlorine dioxide for chlorine, which chlorine dioxide is a strong oxidizing agent compared with chlorine, maklng it possible to carry out a delignification comparable with that of chlorine, with a lower amount (less than half). This type of process is known in the literature under the abbreviation DE, ~referably DEDED, the stage D being the stage of treatment wlth chlorine dioxide, the stage ~ being the conventional extraction stage.
For further details on these various treatments, reference ~ay be made to the work by J.B. Casey, "Pulp and Paper, Chemistry and Chemical Technology", 3rd edition, Vol. l, 1980, John Wiley Son, New York, page~ 694 to 696.
An experimental process for bleaching a paper pulp of "sulphite" type is also known from the journal "Pulp and Paper Science", March 1984, F. Granum, entitled "Influence of Bleaching Chemicals and Sequences on some Properties of Sulphite Pulp", pages J.25 or J.29, in which process numerous se~uences are described, in particul~r sequences using oxygen or ozone, followed by sequenc~s using chlorine dioxide.
A process for bleach~ng a kraft pulp intended for the manufacture of a paper is also known from EP
464,157, which process comprises a sequence of bleaching with pure or mixed chlorine dioxide, ~ollowed by a stage of bleaching with ozone, without an intermediate extraction stage, followed by an alkaline extraction as the third stage and finally ending with a fourth stage of bleaching with chlorine dioxide, this stage being the final stage of this bleaching process carried out in this order.
A process for bleaching a paper pulp according to a so-called DZ sequence, that is to say a first treatment with chlorine dioxide followed by a second treatment with ozone without an intermediate extraction stage between the two, i~ also known from Canadian Patent 2,031,850, in which process the two chemicals are injected at points separated from one another in the bleaching line. Such a process is supposed to decrease the production o~ AOX. However, all the examples gi~en were carried out on pulp delignified beforehand with oxygen.
Canadian Patent 2,031,848 also discloses a process similar to that disclosed above in Canadia~
Patent 2,031,850 but in any order, that is to say in fact describing ZD or DZ sequences. Here again, a preliminary stage of dellgnification with oxygen is necessary, as is disclosed in all the examples.
In these two Canadian patents and in the abovementioned European patent, all the implementational exa~ples are exclusively based on a paper pulp of medium consistency.
It is generally expected by a person skilled in ~he art that the higher the consistency of the pulp, the better the efficiency should be of the treatment with chemicals of the D and Z type, pro~ided that the mixing is carried out ln a mixer with a ve~y high speed. This is why, until now, none of these processes has been developed with so-called low consistency pulp, that is to say comprisin~ a much greater amount of water than a medium or hig~ consistency pulp.
However, paper pulp plants oper~ting with low consistency pulp currently face a number of problems. A
number o~ plants currently operate with C~ sequences, that is to say a first stage of bleaching with chlorine, followed by a second stage of bleaching with chlorine dioxide (intermediate solution already mentioned abo~e), which process, as was indicated above, is not satisfactory with respect to standards which will soon come into force relating to discharges of AOX. In these plants, which currently operate with low consistency pulps, all the equipment used is specific to low consistency pulp, in particular the mixer~, the dimensions of the pipework and of the intermediate storage devices, and, generally, all the manufacturing equipment. With the processes currently known for preventing an excessively high concentration of AOX products in the discharges, it is thus necessary, for the conversion of these plants, to equip them with medium or high consistency equipment, so as to meet the standards. This means that it is necessary to replace all the existing eguipment by new equlpment in these plants, which ~eans an investment of millions of dollars.
There currently therefore does not exist a process which generates a low amount of AOX, that is to say less than 0.5 t of AOX per tonne of paper pulp, and which uses a treatment o~ the pulp at low consistency, that is to say a pulp with a consistency ~ S~.
It is an object of the present inventlon to provide a solution to the problem thus posed.
The process according to the invention is characterized in that the paper pulp treated is a so-called "low concentration" pulp with a co~sistency ofless than or equal to approxlmately 5~ by weight, the mixing of the ozone with the pulp being carried out in a rotary stirrer in which the PUlp i~ stirred at a low speed while ozone is injected into the pulp, so as to obtain a fluidized pulp, the tangential peripheral speed of the pulp at the walls of the rotary stirrer being le~s than or equal to approximately 12 m/s bu~
greater than or equal to approximately 2 m~s, the contact time between the ozone and the pulp being between 1 min and 20 min and the amount of ozone injected into the rotary stirrer ~eing betwee~ 1 kg and 20 kg of ozone per t of paper pulp on a dry basis, the pulp being maintained in the fluidized state for substantially the entire duration of the reaction with the ozone.
The ozone is preferably in~ected as a mixture with oxygen, preferably in the proportion of 3 to 20~
by volume of ozone in the oxygen, the oxygen also being in contact with the pulp for substantially the same duration as the ozone and producing a complementary and simultaneous dellgnlfication and/or bleaching action on the pulp.
At the end of the stirring stage, the oxygen is recovered, filtered to retain the undesirable impuritie~ and optionally recycled to the inlet of the ozonizer.
Use will preferably be made o~ an ozonizer in which oxygen ~ s intr~duced at a press~re greater tha~
atmospheric pressure, so as, on the one hand, to optimize the yield from the ozonizer and, on the other hand, on account of the pressure drops in the latter, to obtain a mixture of oxygen and of ozone at a pressure greater than atmospheric pressure ~preferably between 1.3 bar and 2.5 bar, absolute pressure) but less than lS bar absolute, generally bet~een 1 and 3 bar absolute. The gaseous mixture can thu~ be introduced directly into the pulp, without having to be compres~ed before being iniected.
lS The tangential peripheral speed of the pulp will preferably be les~ than 9 m~s and more preferably less than 6 m/s, which, ~-~ the case of P cylindrical stirrer with a diameter ~f approxlmately 1 metre, represents a rotation~ll speed of les~ than approximately 120 revoluti~ns per minute.
The ~mount of 02. ~ne per t of dry pulp is preferably between 2 and 10 kg, whlle the duration of treatment with ozone in the stirred reactor is preferably between 2 and lO min. A~ regards the Zs chlorine dioxide, the amounts vary between 1 and 20 kg per t of dry pulp, preferably from 2 to 10 kg per t o~
dry pulp.
By uslng the process according to the invention, effluents having an amount of AOX of 0.1 to 0.2 kg per tonne of dry pulp produced are thus easily obtained. The plant designed or used for a CD or DC
sequence is not changed, the plant for injection of chlorine simply being replaced by a plant for ln~ection of ozone, in the a~ounts mentioned above. Another considerable advantage of the in~ention is that the ~echanical properties of the pulp are retained or even improved.
Rotary stirrer or stirred reactor is understood to mean a stirrer, preferably cylindrical, comprlsing one or more paddles which are capable of causing the entire liquid ~low consistency pulp) to rotate with a tangential speed, measured at the periphery of a section of the cylinder, defined above as being the tangential peripheral speed of the pulp.
The general characteri~tics o~ the stirred reactors are described in the article entitled "The mixing-rate number for agitator stirred-tank", Chemical Engineerl~g, October 11, 1976, pages 141 to 143.
Another advantage of the in~ention is that it is not necessary to use an intermediate recept~cle or ~lash tank (for example, see Canadian Patent 2,031,~98), into which receptacle, after mixing the pulp and the reactants in the mixer, the pulp is poured in order to continue the reaction between t~e pulp and the ozone, so as to improve the bleaching of the pulp.
In the brochure from the co~pany Kvaerner Pulping Technologies AB entitled "Kvaerner MC~ Mixer", second page, it i~ explained that correct fluidization o~ a pulp with a mixer especially designed for paper pulp can only be achie~ed with pulp~ having a consistency of the order of or greater than 10~ (so as to achie~e a rheological behaviour of the pulp substantially identical to that of water). For this, the rotatlonal speed must be greater than approximately 1800 revolutions per minute, i.e. a tangential peripheral speed of approximately 30 m/s for a dlameter o~
approximately 30 cm for an industrial mixer. The contact between the pulp and the ozone in the mixer lasts at most 10 seconds. Furthermore, in the process according to the invention, it is not necessary to provide a stage of delignification with oxygen prior to the DZ sequence according to the invention.
The stage of treatment with 020ne will take place before or after treatment with chlorine dioxide.
Of course, the process of the invention applies to the ca~e where ozone alone is injected, without injection of chlorine dioxide, or, if the stage of injection of ozone is preceded or followed by any type of bleaching stage or sequence with any well known bleaching agent mentioned abo~e, including another stage with ozone similar to or di~ferent from that according to the present invention.
Furthermore, throughout this description, it has been assumed that the rotational speed of the paddles of the stirrer is comparable to the rotational speed of the pulp, in particular at the walls (tangential peripheral speed). In practice, differences lo due to local turbulences in the pulp may be found.
rx~rl ~
In this example , a sample of non oxygen delignified softwood kraft pulp - Kappa 27 - is treated according the invention at low consistency in ~ ~r~ y ~ y~ mixer. Pulp is first delignified with chlorine dioxide by adding 2 ~ C102. Then the sa~ple ls divided in two portions: the first one -~ample A- , after wash~ng , is treate~ in a Ep stage accordin~ '_he follo~ing condltions :
NaOH charge : 1.5 %
H202 charge : 0.25 %
Te~perature : 75~ C
Time : 120 minutes Consistency : 10 %
The second portion -sample B- ,before any washing, is directly tre~ted with 03 under the following operating conditions :
Temperature : 50~C
Pressure : atmospheric Pulp consistency : 2.5 ~
03 concentration in 02 : 10 % ~y weight Mixing time : 6 minutes Peripherical mixer rotating speed : 4.1 m/s 03 injected : 5 kg/t or 0.5 % on pulp 03. consumption efficiency : 95 ~
_ g _ After reaction sample B is ~ubmitted to the same washing and then Ep stage as with sample A.
Sample B exhiblts after D Ep treatment a ~appa of 3.4 compared to ~.6 for sa~ple A after DZ Ep reflecting the efficiency of the 03 treatment carried out ln such conditions.
~rle 2 In this example , an 02 delignified kraft eucalyptu~
lo pulp - Kappa 9.0 , Brightness 45.8 ~ ISO- is used for comparing 03 treatments in a DZ sequence with conventi~nal high shear mixlng at medium consistency (sample A) and low shear mixing at low consistency (sample B). For reference , a first pulp ~s treated according the D Eo D sequence. Operating conditions for the reference are :
D stage Temperature : S5~C
Residence time : 45 minute~
C102 charge : 1.3 % ( as pure C102 ) Consistency : 11 ~o sta~e Te~perature : 70~C
Residence time : 125 minutes NaOH charge : 1.3 %
Consistency : 12 %
02 pressure : 2 bars Final D stage Temperature : 70~C
Residence time : 90 minutes Cl02 charge : 0.6 ~ ~ as pure C102 ) Consistency : 11 %
Results obtained on this reference sample are :
Kappa after D Eo : 0.9 ~rlghtness after D Eo : 73.~ % IS0 Brightness after D Eo D : 88.1 ~ IS0 s DZ Eo D sequence~
In both ca~es,C102 and 03 are added ~equentially, 03 being added after complete consumption of ClC2 in ~he first step, before any washing.
For sample A, D stage conditions are identical to the reference except the ClOZ charge which is reduced to 0.48~. Then 0.4 ~ (or 4 kg~t on pulp) 03 is added under high shear ~ixing condition~ which are :
03 pressure : 9 bars 03 concen~ration in 02 : 9.5 ~ by weight Pulp con~i~tency : 11 %
Temperature : 55~C
Perlpherical mixer rotating speed : 78 m/s Next stages are car~iec out according conditions ldentical ~o the referen~e except the C102 charge in the final D stage which is redueed to 0.4 %. Results obtalned are :
Kappa after DZ Eo : 0.9 Brightness after DZ Eo : 79.2 % IS0 Brightness after DZ EoD : 88.5 % IS0 Sample B is treated at l~w consistency both with C102 and 03 ; other D stage conditions are identical to the reference except the C102 charge which is reduced to 0.45 ~ and pulp consistency which is reduced to 2.5 ~.
Then 0.4 ~ ~or 4 kg/t on pulp) 03 is added under low shear mixing conditions which are :
03 pressure : 2 bars 03 con~entration in 02 : 9.5 ~ by weight Pulp conSistency : 2.5 Temperature : 55~C
Peripherical mixer rotating speed : 11 ~/s .
Next stages are carried out according condit$ons identical to the reference except the C102 charge in the final D stage which is reduced to O.g ~ . ~esults obtained are :
Kappa after DZ Eo : 0.9 Brightness after DZ Eo : 79.0 ~ IS0 Brightness after DZ ~oD : 88.3 % IS0 lo These exa~ples show clearly that the 03 treatment can be combined with a C102 stage in a DZ sequence while carrled out at low consistency under appropriate ~luidised conditions with at least the same effl~iency as when performed at medlum consistency. This is re~lected among other things by the sa~ings ~n C102 when comparing the various charges in C102 used to reach 88 % IS0 brightness. For the reference 1.9 % C102 is needed while for the D~ Eo D only 0.88 ~ and 0.85 ~
are required ~ for respectively medium and low ~on~istency treatments
Claims (8)
1. Process for bleaching a paper pulp in which the said pulp is subjected successively to the action of ozone and then of chlorine dioxide, or vice versa, which process is characterized in that the paper pulp treated is a so-called "low concentration" pulp with a consistency of less than or equal to approximately 5%
by weight, the mixing of the ozone with the pulp being carried out in a rotary stirrer in which the pulp is stirred at a low speed while ozone is injected into the pulp so as to obtain a fluidized pulp, the tangential peripheral speed of the pulp at the walls of the rotary stirrer being less than or equal to approximately 12 m/s but greater than or equal to approximately
by weight, the mixing of the ozone with the pulp being carried out in a rotary stirrer in which the pulp is stirred at a low speed while ozone is injected into the pulp so as to obtain a fluidized pulp, the tangential peripheral speed of the pulp at the walls of the rotary stirrer being less than or equal to approximately 12 m/s but greater than or equal to approximately
2 m/s, the contact time between the ozone and the pulp being between 1 min and 20 min and the amount of ozone injected into the rotary stirrer being between 1 kg and 20 kg of ozone per t of paper pulp on a dry basis, the pulp being maintained in the fluidized state for substantially the entire duration of the reaction with the ozone.
2. Process for bleaching a paper pulp according to Claim 1, characterized in that the ozone is injected as a mixture with oxygen, preferably in the proportion of
2. Process for bleaching a paper pulp according to Claim 1, characterized in that the ozone is injected as a mixture with oxygen, preferably in the proportion of
3 to 20% by volume of ozone in the oxygen, the oxygen also being in contact with the pulp for substantially the same duration as the ozone and producing a complementary and simultaneous delignification and/or bleaching action on the pulp.
3. Process for bleaching a paper pulp according to Claim 2, characterized in that the oxygen is recovered in order to be optionally recycled.
3. Process for bleaching a paper pulp according to Claim 2, characterized in that the oxygen is recovered in order to be optionally recycled.
4. Process for bleaching a paper pulp according to one of Claims 1 to 3, characterized in that the pressure of the ozone and of the oxygen introduced into the pulp is between 1 and 15 bar absolute.
5. Process for bleaching a paper pulp according to Claim 4, characterized in that the pressure of the ozone and of the oxygen introduced into the pulp is between 1 and 3 bar, preferably 1.3 and 2.5 bar, absolute.
6. Process for bleaching a paper pulp according to one of Claims 1 to 5, characterized in that the tangential peripheral speed is less than or equal to 9 m/s, preferably 6 m/s.
7. Process for bleaching a paper pulp according to one of Claims 1 to 6, characterized in that the amount of ozone per t of dry pulp is between 2 and 10 kg.
8. Process for bleaching a paper pulp according to one of Claims 1 to 7, characterized in that the amount of chlorine dioxide introduced into the pulp is between 1 and 20 kg per t of dry pulp, preferably from 2 to 10 kg per t of dry pulp.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR97/11446 | 1997-09-15 | ||
| FR9711446A FR2768442B1 (en) | 1997-09-15 | 1997-09-15 | PROCESS FOR BLEACHING A PULP WITH OZONE AND CHLORINE DIOXIDE |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2247532A1 true CA2247532A1 (en) | 1999-03-15 |
Family
ID=9511079
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002247532A Abandoned CA2247532A1 (en) | 1997-09-15 | 1998-09-11 | Process for bleaching a paper pulp with ozone and chlorine dioxide |
Country Status (9)
| Country | Link |
|---|---|
| EP (1) | EP0902120A1 (en) |
| JP (1) | JPH11172589A (en) |
| KR (1) | KR19990029786A (en) |
| CN (1) | CN1212310A (en) |
| AU (1) | AU732875B2 (en) |
| BR (1) | BR9803938A (en) |
| CA (1) | CA2247532A1 (en) |
| FR (1) | FR2768442B1 (en) |
| NZ (1) | NZ331806A (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI981808A (en) * | 1998-08-24 | 2000-02-25 | Crs Reactor Engineering Uk Ltd | Method for bleaching pulp |
| JP2002069879A (en) * | 2000-09-04 | 2002-03-08 | Nippon Paper Industries Co Ltd | Method for bleaching pulp of cellulosic fiber material |
| SE532470C2 (en) * | 2008-05-13 | 2010-02-02 | Metso Paper Inc | Final bleaching of oxygen delignified pulp with ozone, chlorine dioxide and peroxide |
| JP6411182B2 (en) * | 2014-11-13 | 2018-10-24 | 大王製紙株式会社 | Sludge dewatering aid and sludge dewatering method |
| CN110656527A (en) * | 2019-09-12 | 2020-01-07 | 华南理工大学 | Method and device for efficiently bleaching medium-consistency paper pulp by using ozone and chlorine dioxide in synergy mode |
| CN111040049A (en) * | 2019-12-02 | 2020-04-21 | 广西大学 | Method for separating and purifying hemicellulose in bagasse |
| CN111979818A (en) * | 2020-08-07 | 2020-11-24 | 齐鲁工业大学 | Sulfate wood pulp short-procedure ECF bleaching process X/Z/D-EOP-D or X/D/Z-EOP-D |
| SE545758C2 (en) * | 2022-06-20 | 2024-01-02 | Valmet Oy | Method for bleaching pulp from recycled textile material |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4216054A (en) * | 1977-09-26 | 1980-08-05 | Weyerhaeuser Company | Low-consistency ozone delignification |
| CA2186176C (en) * | 1995-09-28 | 2002-11-26 | Derek Hornsey | Method and apparatus for ozone bleaching of cellulosic pulp at low consistency |
| FR2743094B1 (en) * | 1995-12-27 | 1998-02-13 | Centre Tech Ind Papier | PROCESS FOR BLEACHING CHEMICAL PULP |
-
1997
- 1997-09-15 FR FR9711446A patent/FR2768442B1/en not_active Expired - Fee Related
-
1998
- 1998-09-08 NZ NZ331806A patent/NZ331806A/en unknown
- 1998-09-11 CA CA002247532A patent/CA2247532A1/en not_active Abandoned
- 1998-09-14 EP EP98402261A patent/EP0902120A1/en not_active Withdrawn
- 1998-09-14 JP JP10260191A patent/JPH11172589A/en active Pending
- 1998-09-14 BR BR9803938-5A patent/BR9803938A/en not_active IP Right Cessation
- 1998-09-14 AU AU84212/98A patent/AU732875B2/en not_active Ceased
- 1998-09-15 KR KR1019980037906A patent/KR19990029786A/en not_active Application Discontinuation
- 1998-09-15 CN CN98120618A patent/CN1212310A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| AU8421298A (en) | 1999-03-25 |
| JPH11172589A (en) | 1999-06-29 |
| FR2768442B1 (en) | 1999-10-15 |
| AU732875B2 (en) | 2001-05-03 |
| NZ331806A (en) | 2000-01-28 |
| CN1212310A (en) | 1999-03-31 |
| BR9803938A (en) | 1999-12-07 |
| EP0902120A1 (en) | 1999-03-17 |
| KR19990029786A (en) | 1999-04-26 |
| FR2768442A1 (en) | 1999-03-19 |
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