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
  • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents
• • 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
  • D21C9/1042—Use of chelating agents

Definitions

• This invention relates to the bleaching of chemical pulp with chlorine-free chemicals.
• the invention more precisely, refers to a method of pretreating the pulp with chelating agents in combination with the bleaching.
• Bleaching is carried out in most cases by treating the papermaking pulp to be bleached with chlorine-free chemicals first with a chelating agent of the type EDTA or DTPA.
• the chelating agent has the object to remove from the pulp certain metal ions, particularly manganese, copper, iron a.o., which affect the bleaching result negatively, if they remain in the pulp.
• the chelating agent usually is designated by the letter Q.
• the pulp is washed after the Q-step, so that the exposed metal ions are eliminated from the pulp as completely as possible. Thereafter the bleaching takes place, which normally is carried out with hydrogen peroxide, at times in combination with oxygen. Hydrogen peroxide steps are designated with the letter P, and P-steps reinforced with oxygen are designated with OP or PO where the dominating chemical is mentioned first.
• the thumb rule is that the greater the number of harmful metal ions eliminated from the pulp is, the more effective the subsequent bleaching will be.
• a method, at which the pulp is treated in a Q-stage and thereafter bleached in a P-stage usually is called the "Lignox"-method, which is disclosed, for example, in EP-A-402 335.
• the original pulp normally is oxygen delignified to a relatively low kappa number prior to the QP-bleaching.
• a low kappa number before the bleaching implies that the obtainable maximum bleaching result increases in the subsequent step, and that the bleaching chemical demand is reduced.
• the QP-bleaching most often is carried out in such a way, that the pulp first is passed through a pump, which pumps the pulp suspension through the treatment steps. Thereafter follows a mixer where the chelating agent is added to the pulp. The next step is a bleaching tower, where the pulp without additional agitation slowly passes through and where all remaining reaction takes place. The pulp finally passes to a washing step where the exposed and complexed metal ions are washed out. The freed liquid enriched on metal ions most often is discharged.
• the conditions normally applied in a Q-step are a temperature of 70-90°C, a retention time of 1-2 hours, a pulp consistency of 10-14%, a pH of 5-7 and a charge of chelating agent of 1-3 kg per ton pulp.
• a very good metal separation can be achieved, which is necessary if subsequent bleaching is carried out on pulp with low lignin content (the kappa number is below about 5) and if a large amount of hydrogen peroxide is used in the stage (more than 15 kg/t) .
• the pulp still contains a considerable amount of lignin (kappa number above about 5) , and the hydrogen peroxide charge in the subsequent step is limited to about 5-10 kg/t, the demand for a complete metal elimination is less critical.
• the Q-step per se is an unnecessary treatment step, because no proper bleaching takes place there.
• the Q-treatment is necessary for a following successful P- or PO-bleaching, as appears from the aforesaid. If the Q-step could be simplified without reducing the effectiveness in the elimination of metals, great savings could be made, because a complete bleaching stage is expensive.
• the present invention is directed to a simplification of the Q-step, especially for pulps with kappa number above about 5, and for chelating agent treatments which require less than 60% metal reduction.
• the simplification implies, that the investment cost decreases in relation to a conventional Q-step.
• the invention is based on the surprising observation, that the effectiveness of the Q-step is considerably less affected by increased reaction temperature than by increased retention time. This indicates, according to classical chemical cinetics, that the Q-step is diffusion controlled and not reaction controlled. This means, that the Q-step should be carried out so that the diffusion is facilitated, and if this is made effectively, the total retention time can thereby be reduced substantially.
• the best method to reduce the diffusion resistance is to decrease the diffusion distance for active reactants and released reaction products. On a technical scale this is made by effective mixing, for example in a mixer with long retention time. Thus, by effective mixing, the retention time in the Q- step can be shortened.
• the number of mixing steps is not critical, but the total mixing time must be above a certain critical level, alternatively at least two mixing apparatuses shall be used. For economic reasons, however, the number of mixing steps must be held on a reasonable level . For a pulp with kappa number above about 5, which shall be bleached further with at maximum about 5-10 kg hydrogen peroxide exclusive possible oxygen, a suitable number of mixing steps is 1-2. The same applies, if the metal reduction to be achieved in the chelating agent step is limited and need not be above 60%.
• the stay-time between the mixing steps shall be short, less than 10 minutes and preferably less than 5 minutes.
• the mixing steps then shall be carried out one after the other without intermediate washing.
• the washing required for removing the metals from the pulp suspension shall be carried out less than 15 minutes after the last mixing step.
• the entire charge of chelating agent shall be added in the first mixing step, and subsequent mixing steps bring about only a renewed mixing.
• the chelating agent preferably is of the type EDTA (ethylene diamine tetraacetic acid) or DTPA (diethylene triamine pentaacetic acid) , but also other types of chelating agent can be used, for example NTA (nitrilotriacetic acid) or DTPMP (diethylene triamine pentamythylene phosphoric acid) .
• the total amount of added chelating agent should be 0.5-3 kg per ton pulp (calculated as pure chelating agent) , preferably 1-2 kg per ton pulp.
• the temperature in the Q-step should be above 75°C, suitably over 80°C and more preferably over 85°C. A preferred temperature interval is 85-110°C.
• the desired pH interval is pH 5 to pH 11.
• the pulp consistency should be between 2 and 16%.
• An especially effective mixing can be achieved by using high-intensity mixers or with another type of mixer, where the mixing is effective and the retention time is sufficient, so that a homogenous mixture of fibres and chelating agent is obtained.
• the starting pulp was oxygen bleached and of coniferous sulphate type with kappa number 10.1, viscosity 975 dm 3 /t and brightness 43.0% ISO.
• the bleaching was carried out according to Q(OP) with 10 kg H 2 0 2 /t .

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Detergent Compositions (AREA)
• Chemical And Physical Treatments For Wood And The Like (AREA)
• Hybrid Cells (AREA)
• Noodles (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 1997
  • 1997-01-03 SE SE9700030A patent/SE508239C2/sv not_active IP Right Cessation
  • 1997-11-06 ES ES97912616T patent/ES2195123T3/es not_active Expired - Lifetime
  • 1997-11-06 PT PT97912616T patent/PT951599E/pt unknown
  • 1997-11-06 DE DE69722445T patent/DE69722445T2/de not_active Expired - Fee Related
  • 1997-11-06 CA CA002275257A patent/CA2275257A1/en not_active Abandoned
  • 1997-11-06 AU AU49741/97A patent/AU726599B2/en not_active Ceased
  • 1997-11-06 BR BR9714252-2A patent/BR9714252A/pt not_active Application Discontinuation
  • 1997-11-06 NZ NZ336204A patent/NZ336204A/xx unknown
  • 1997-11-06 AT AT97912616T patent/ATE241725T1/de not_active IP Right Cessation
  • 1997-11-06 CN CN97181066A patent/CN1103832C/zh not_active Expired - Fee Related
  • 1997-11-06 JP JP52990798A patent/JP2001508837A/ja not_active Withdrawn
  • 1997-11-06 WO PCT/SE1997/001853 patent/WO1998029597A1/en not_active Ceased
  • 1997-11-06 EP EP97912616A patent/EP0951599B1/de not_active Expired - Lifetime
  • 1997-11-19 ZA ZA9710418A patent/ZA9710418B/xx unknown
  • 1997-12-23 ID IDP973960A patent/ID20003A/id unknown
• 1999
  • 1999-07-02 NO NO993290A patent/NO993290L/no unknown

Non-Patent Citations (1)

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