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
  • D21C9/1015—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
  • D21C9/12—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
  • D21C9/1026—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

Definitions

• 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.
• chlorine was one of the bleaching agents which was most widely used for the bleaching of paper pulp, in particular for so-called chemical pulp.
• Chlorine dioxide or hypochlorite are also well known bleaching agents which make it possible both to remove the lignin in the cellulose and to improve the whiteness of the cellulose.
• the choice of the bleaching agent depends essentially on the type of paper which it is desired to produce from the treated pulp. When, for example, so-called kraft pulp is used to make very white writing paper, it is necessary to use a large amount of bleaching agent and to choose this agent from those having a faster and more selective action.
• organochlorine products are produced during the bleaching which generally have little solubility in the aqueous liquor for washing the pulp and large amounts of which are entrained or extracted from the pulp with the effluents 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 chlorine is used and when it is desired to obtain very white products, it is necessary to use a much greater amount of chlorine compared with the processes in which the whiteness of the pulp obtained is less important.
• the simplest method consists in substituting non-chlorinated chemicals, such as oxygen, peroxides, such as hydrogen peroxide, ozone, peracetic acid, and the like, for the chlorinated chemicals used.
• non-chlorinated chemicals such as oxygen, peroxides, such as hydrogen peroxide, ozone, peracetic acid, and the like.
• organochlorine compounds Another possibility for reducing the amount of organochlorine compounds is to reduce the amount of chlorine used, generally in the first bleaching stage.
• Various solutions have already been provided, such as increased delignification in the stage of cooking the pulp, as well as delignification with oxygen. These treatments, followed by an appropriate extraction stage, make it possible to reduce the content of lignin 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 amount 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, making 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, preferably DEDED, the stage D being the stage of treatment with chlorine dioxide, the stage E being the conventional extraction stage.
• a process for bleaching 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, followed 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, is 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 of AOX.
• all the examples given were carried out on pulp delignified beforehand with oxygen.
• Canadian Patent 2,031,848 also discloses a process similar to that disclosed above in Canadian Patent 2,031,850 but in any order, that is to say in fact describing ZD or DZ sequences.
• a preliminary stage of delignification with oxygen is necessary, as is disclosed in all the examples.
• paper pulp plants operating with low consistency pulp currently face a number of problems.
• a number of plants currently operate with CD 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 above), which process, as was indicated above, is not satisfactory with respect to standards which will soon come into force relating to discharges of AOX.
• all the equipment used is specific to low consistency pulp, in particular the mixers, the dimensions of the pipework and of the intermediate storage devices, and, generally, all the manufacturing equipment.
• 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.
• the ozone is preferably 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.
• the oxygen is recovered, filtered to retain the undesirable impurities and optionally recycled to the inlet of the ozonizer.
• Use will preferably be made of an ozonizer in which oxygen is introduced at a pressure greater than 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 15 bar absolute, generally between 1 and 3 bar absolute.
• the gaseous mixture can thus be introduced directly into the pulp, without having to be compressed before being injected.
• the tangential peripheral speed of the pulp will preferably be less than 9 m/s and more preferably less than 6 m/s, which, in the case of a cylindrical stirrer with a diameter of approximately 1 metre, represents a rotational speed of less than approximately 120 revolutions per minute.
• the amount of ozone per t of dry pulp is preferably between 2 and 10 kg, while the duration of treatment with ozone in the stirred reactor is preferably between 2 and 10 min.
• the amounts vary between 1 and 20 kg per t of dry pulp, preferably from 2 to 10 kg per t of dry pulp.
• Rotary stirrer or stirred reactor is understood to mean a stirrer, preferably cylindrical, comprising 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.
• Another advantage of the invention is that it is not necessary to use an intermediate receptacle or flash tank (for example, see Canadian Patent 2,031,848), into which receptacle, after mixing the pulp and the reactants in the mixer, the pulp is poured in order to continue the reaction between the pulp and the ozone, so as to improve the bleaching of the pulp.
• an intermediate receptacle or flash tank for example, see Canadian Patent 2,031,848
• Kvaerner Pulping Technologies AB entitled "Kvaerner MC ® Mixer” second page, it is explained that correct fluidization of a pulp with a mixer especially designed for paper pulp can only be achieved with pulps having a consistency of the order of or greater than 10% (so as to achieve a rheological behaviour of the pulp substantially identical to that of water).
• the rotational speed must be greater than approximately 1800 revolutions per minute, i.e. a tangential peripheral speed of approximately 30 m/s for a diameter of approximately 30 cm for an industrial mixer.
• the contact between the pulp and the ozone in the mixer lasts at most 10 seconds.
• the stage of treatment with ozone will take place before or after treatment with chlorine dioxide.
• the process of the invention applies to the case 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 above, including another stage with ozone similar to or different from that according to the present invention.
• a sample of non oxygen delignified softwood kraft pulp - Kappa 27 - is treated according the invention at low consistency in a mixer. Pulp is first delignified with chlorine dioxide by adding 2 % ClO2. Then the sample is divided in two portions; the first one -sample A- , after washing , is treated in a Ep stage according the following conditions :
• reaction sample B is submitted to the same washing and then Ep stage as with sample A.
• Sample B exhibits after D Ep treatment a Kappa of 3.4 compared to 5.6 for sample A after DZ Ep reflecting the efficiency of the O3 treatment carried out in such conditions.
• an O2 delignified kraft eucalyptus pulp - Kappa 9.0, Brightness 45.8 % ISO- is used for comparing O3 treatments in a DZ sequence with conventional high shear mixing at medium consistency (sample A) and low shear mixing at low consistency (sample B).
• a first pulp is treated according the D Eo D sequence. Operating conditions for the reference are :
• Sample B is treated at low consistency both with ClO2) and O3 ; other D stage conditions are identical to the reference except the ClO2 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) O3 is added under low shear mixing conditions which are :

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
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Citations (3)

• 1997
  • 1997-09-15 FR FR9711446A patent/FR2768442B1/fr not_active Expired - Fee Related
• 1998
  • 1998-09-08 NZ NZ331806A patent/NZ331806A/xx unknown
  • 1998-09-11 CA CA002247532A patent/CA2247532A1/en not_active Abandoned
  • 1998-09-14 AU AU84212/98A patent/AU732875B2/en not_active Ceased
  • 1998-09-14 EP EP98402261A patent/EP0902120A1/de not_active Withdrawn
  • 1998-09-14 BR BR9803938-5A patent/BR9803938A/pt not_active IP Right Cessation
  • 1998-09-14 JP JP10260191A patent/JPH11172589A/ja active Pending
  • 1998-09-15 CN CN98120618A patent/CN1212310A/zh active Pending
  • 1998-09-15 KR KR1019980037906A patent/KR19990029786A/ko not_active Withdrawn

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