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
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents

Definitions

• This invention relates to the bleaching of chemical pulp with chlor ⁇ ine-free bleaching chemicals.
• the invention more precisely, refers to the treatment of pulp by chelating agents in connection with the bleaching.
• Bleaching is carried out in most cases by first treating the paper- making pulp to be bleached 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 would affect the bleaching result negatively if they remain in the pulp.
• the chelating agent step ususlly is designated by the letter Q.
• the pulp is washed to eliminate the exposed metal ions from the pulp as completely as possible. Thereafter follows the bleaching, which normally is carried out with hydrogen peroxide, at times in combination with other bleaching steps and intermediate washing steps where ozone or peracids can be used. Hydrogen peroxide steps usually are designated by the letter P. According to a thumb rule, the more harmful metal ions are eliminated from the pulp in the Q-step, the more effective the subsequent bleaching will be.
• a method, at which the pulp is treated in a Q-step and thereafter in a P-step, usually is called the "Lignox"-method, as 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 implies that the obtainable maximum brightness increases and the bleaching chemical demand decreases.
• the QP-bleaching is carried out in mills most often in such a way, that the pulp first is passed through a pump, which pumps the pulp suspension through the treatment steps. Thereafter the chelating agent is admixed homogenously to the pulp in a mixer. The next step is a bleaching tower where the pulp without additional activation slowly passes through and where all remaining reaction takes place. The pulp finally is directed to a washing step where the exposed and complexed metal ions are washed out. The exposed 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 stay-time of 1-2 hours, a pulp concentration of 10-14%, a pH of 5-7 and a charge of chelating agent of 1-3 kg/ton pulp.
• the Q-step from a strict bleaching point of view, actually is an unnec- cessary treatment step, because no increase in brightness takes place here.
• the Q-treatment is necessary for a successful chlor ⁇ ine-free bleaching, as appears from above. If the Q-step could be simplified without thereby reducing the efficiency of metal eliminat ⁇ ion, great savings could be made, because a complete bleaching step with bleaching tower is expensive.
• the present invention is directed to a solution of the aforesaid problems.
• the Q-step is made more efficient by eliminating the metals effectively at the same time as the Q-step is simplified, resulting in reduced investment costs in relation to a conventional Q-step.
• the invention is based on the surprising discovery that the efficiency of the Q-step is affected considerably less by increased reaction temperature than by increased retention time. This indicates, accord ⁇ ing to classic chemical kinetics, that the Q-step is controlled by diffusion and not by reaction. The Q-step, consequently, should be carried out so that diffusion is facilitated. If this is done effect ⁇ ively, the total retention time can thereby be reduced considerably.
• the best way of reducing the diffusion resistance is by decreasing the diffusion distance for active reactants and released reaction products. This is achieved on technical scale by effective mixing, preferably in a mixer.
• the retention time in the Q-step thus, can be reduced by repeated effective mixing.
• the number of mixing steps should be at least two, preferably at least three. For economic reasons, however, the number of mixing steps, and thereby the number of mixers, must be held on a reasonable level. A suitable number can be 2-4 steps.
• a cert ⁇ ain retention time between the mixing steps is of good use, but this time need not be long, because the diffusion rate without mixing still is much slower than what is obtainable by mixing, for example in a mixer.
• the stay-time between the mixing steps shall be short, less than 15 minutes, suitably less than 10 minutes, and preferably less than 5 minutes.
• the mixing steps should be carried out one after the other, without intermediate washing.
• the washing required for re ⁇ moving the metals from the pulp suspension shall be carried out after the last mixing step.
• the chelating agent preferably is of the type EDTA (ethylene diamine tetraacetic acid) or DTPA (diethylene triamine pentaacetic acid) , but other types of chelating agent can be used, for example NTA (nitrilo- triacetic acid) or DTPM, J (diethylene triamine pentamethylene phosphonic acid).
• the total amount of added chelating agent should be 0,5-5 kg/ t pulp (counted as pure chelating agent), preferably 1-3 kg/ t pulp.
• the entire charge of chelating agent can be added in the first mixing step, whereby subsequent mixing steps bring about only repeated mix ⁇ ing.
• the chelating agent can be added in several of the mixing steps.
• the charging then should be made substantially in the first step, and in subsequent steps the added amount should be less than 2 kg/ t pulp per step.
• the temperature in the mixing steps should be above 70 C and suitably above 80°C.
• a preferable temperature interval is 80-120°C.
• the pressure in the entire Q-step preferably should be 1-10 bar.

Landscapes

• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Treatments For Attaching Organic Compounds To Fibrous Goods (AREA)
• Cosmetics (AREA)
• Chemical Or Physical Treatment Of Fibers (AREA)
• Detergent Compositions (AREA)

Priority Applications (6)

Applications Claiming Priority (2)

Publications (1)

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ID=20399294

Family Applications (1)

Country Status (11)

Cited By (1)

Families Citing this family (1)

Citations (4)

• 1995
  • 1995-08-24 SE SE9502962A patent/SE504803C2/sv not_active IP Right Cessation
• 1996
  • 1996-08-09 AU AU68399/96A patent/AU6839996A/en not_active Abandoned
  • 1996-08-09 ES ES96928765T patent/ES2153593T3/es not_active Expired - Lifetime
  • 1996-08-09 WO PCT/SE1996/000996 patent/WO1997008380A1/en active IP Right Grant
  • 1996-08-09 CA CA002229212A patent/CA2229212A1/en not_active Abandoned
  • 1996-08-09 BR BR9610199A patent/BR9610199A/pt not_active Application Discontinuation
  • 1996-08-09 EP EP96928765A patent/EP0846199B1/en not_active Expired - Lifetime
  • 1996-08-09 AT AT96928765T patent/ATE198916T1/de not_active IP Right Cessation
  • 1996-08-09 PT PT96928765T patent/PT846199E/pt unknown
  • 1996-08-09 DE DE69611680T patent/DE69611680T2/de not_active Expired - Fee Related
• 1998
  • 1998-02-23 NO NO980753A patent/NO980753L/no not_active Application Discontinuation

Patent Citations (4)

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