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
• • 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
  • D21C9/1005—Pretreatment of the pulp, e.g. degassing the pulp

Definitions

• This invention relates to the bleaching of chemical pulp with chlorine free bleaching chemicals.
• the invention more precisely, refers to a method of treating the pulp with chelating agent in connection with the bleaching.
• Chlorine free bleaching of chemical paperma ing pulp is carried out to a large extent with hydrogen peroxide in sequences which normally also include the bleaching chemicals oxygen gas and at times ozone. In future probably also so-called peracids, such as peracetic acid and persulphuric acid, will be used.
• peracids such as peracetic acid and persulphuric acid.
• the lignin which must be removed from the paper ⁇ making pulp in order to achieve high ISO-brightness of the pulp, is firmly bound, and for performing the bleaching in a lenient way and with economic use of chemicals it is favourable to use a combination of several bleaching chemicals. The individual chemicals are added in steps, and the pulp is washed between the steps.
• Hydrogen peroxide is a highly effective bleaching chemical, if the bleaching conditions are right ones, and by using only oxygen gas and hydrogen peroxide brightness levels of almost full brightness, i.e. al onst 90% ISO, have been achieved on mill scale.
• the presence of such metals causes parts of the hydrogen peroxide to be degraded without any bleaching effect, which tendency increases with increased metal content and increased charging of peroxide.
• the peroxide steps of today are highly sensitive even to low metal contents.
• the manganese content in pulp entering the peroxide step for example, must not exceed 1 g/ton pulp when highest brightness is to be achieved. This level, however, is difficult to attain on mill scale.
• a high brightness of the pulp is desired, because such pulp can be sold at a higher price.
• a low metal content in the pulp prior to the peroxide step is achieved by treating the pulp with a chelating agent, for example EDTA or DTPA and thereafter washing the pulp to eliminate the released metal ions prior to the peroxide step.
• the chelating agent treatment shall be carried out between 10°C and 100°C, preferably 40-90°C, for a period of up to 6 hours and at a pH between 3.1 and 9.0.
• the present invention offers a solution for the aforesaid problems.
• the invention results in a more effective chelating agent treatment which is carried out prior to the peroxide step, in that the chelating agent is activated by treating the pulp with an oxidation agent.
• activation is meant that the efficiency of the chelating agent is increased by oxidation or that the metals, which should be removed from the pulp, are oxidated. In both cases the metal binding is facilitated and the metal content in the pulp is reduced.
• the pulp from a preceding process step for example oxygen delignificaiton is wahsed in a pulp washer room. Thereafter the pH value is adjusted by the addition of acid or liquor.
• the acid/liquor shall be admixed in a homogenous way, for example in connection with a dilution of the pulp or by means of a pump or mixer. When a mixer is used in this position, a very intense and uniform admixing is ensured, which in certain cases can be of decisive importance.
• the pH value shall there be adjusted to between 2 and 7, preferably 4-6.
• the chelating agent for example EDTA or DTPA
• the chelating agent is added in a separate mixer which causes an effective admixing of the chelating agent to the pulp.
• the amount of chelating agent suitably is 1-5 kg per ton pulp, preferably 1.5-2.5 kg/t.
• An oxidizing agent is also added to the pulp.
• oxygen-containing gases such as air or oxygen gas can be used.
• hydrogen peroxide or ozone can be used.
• the charging of oxidizing agent, which contains oxygen gas shall be 1-10 kg per ton pulp, preferably 3-7 kg/t, counted as pure oxygen gas.
• hydrogen peroxide is added, the amount shall be 0.5-3 kg, preferably 1-2 kg/t.
• ozone an ozone/air mixture with 0.5-3 kg ozone per ton pulp, preferably 1-2 kg/t, is added.
• the pulp is thereafter pumped to a reactor vessel, in which a temperature of 40-130°C, preferably 100-130°C, and a pressure of 1-10 bar, suitable 1.5-10 bar, preferably 3-8 bar, are maintained.
• the stay-time of the pulp in the vessel shall be between 5 minutes and 2 hours, preferably 10-60 minutes.
• the pulp treated with chelating agent is directed from the reactor vessel to a pulp washer room where the temperature is held above 80°C whereby the metal ions, which are bound by chelating agent and thereby can be washed out, are washed out effectively.
• This pulp washer can be a pressurized filter or preferably a roll press, which is very effective and has low water consumption (4-5 m 3 /ton pulp) .
• the oxidizing agent has the object to activate the added chelating agent so that it binds undesired metal ions, such as manganese, copper and iron, more actively.
• the oxidizing besides, facilitates the separation of metals by releasing them more easily from the pulp.
• the metals thereby can be bound better by the chelating agent and be washed out from the pulp in the subsequent washing step.
• the pulp will thereby be purer from a metal aspect.
• the pulp is directed after the washing to a subsequent bleaching step where chlorine free bleaching is carried out with peroxide-containing bleaching agent.
• the low metal content in the pulp renders it possible to bleach more effectively to maximum brightness.
• the Q-step was performed at 10% pulp consistency in 1 hour and final pH 5.0-5.2.
• the Q-step was performed with and, respectively, without addition of oxygen (Q 2 ) .
• the pulp was washed and bleached in a pressurized P-step to which oxygen was added, a so called (PO)-step.
• the conditions in the (PO) - step was 12% pulp consistency, 2 hours retention time, 100-110°C and final pH 10.0-10.4. The results is shown in the table below.

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Detergent Compositions (AREA)

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• 1994
  • 1994-10-07 SE SE9403396A patent/SE504826C2/sv not_active IP Right Cessation
• 1995
  • 1995-10-05 WO PCT/SE1995/001135 patent/WO1996011298A1/en not_active Application Discontinuation
  • 1995-10-05 CA CA 2199257 patent/CA2199257A1/en not_active Abandoned
  • 1995-10-05 AU AU37124/95A patent/AU686024B2/en not_active Ceased
  • 1995-10-05 EP EP95934912A patent/EP0784720A1/en not_active Withdrawn
  • 1995-10-05 CN CN 95195457 patent/CN1159842A/zh active Pending
  • 1995-10-05 JP JP8512520A patent/JPH10506965A/ja active Pending
  • 1995-10-05 BR BR9509253A patent/BR9509253A/pt not_active Application Discontinuation
• 1997
  • 1997-04-04 FI FI971401A patent/FI971401A/fi unknown
  • 1997-04-04 NO NO971536A patent/NO971536D0/no unknown

Non-Patent Citations (1)

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