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
  • D21C11/00—Regeneration of pulp liquors or effluent waste waters
  • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
  • D21C11/0028—Effluents derived from the washing or bleaching plants
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the present invention relates to a method for de- lignification/bleaching of pulp with peroxide, comprising a treatment step with chelating agents in order to free disturbing metal ions from the pulp. More specifically the invention relates to a method wherein the disturbing metal ions are inactivated by separating a liquid phase from the pulp after the treatment with chelating agents and where the separated liquid phase containing chelating agents is treated for oxidation of the metal ions.
• Chlorine-free bleaching with combinations of chelating steps and peroxide steps has become especially interesting.
• the first commercial bleaching of this kind was the so called Lignox method which was developed by Eka Nobel AB, Sweden, during the years 1989-1990.
• the method is described in the European Patent No. 0402335 and comprises a first step, before the peroxide step, with the object of freeing disturbing metal ions by treatment with chelating agents and thereafter eliminate disturbing metal ions such as manganese and copper by washing, as these ions can lead to deterioration of the charged peroxide.
• a pretreatment step with a chelating agent (a Q-step) before a peroxide bleaching step (P-step) has nowadays become a standard procedure for removing disturbing metal ions, especially manganese, and thereby increasing the effectiveness of the peroxide step and reducing the peroxide consumption.
• a bleaching method comprising a peroxide bleaching step it is possible, instead of removing the disturbing metal ions, to a substantial extent inactivate these ions so that they not necessarily have to be washed out from the pulp.
• An advantage with this concept is that the discharge volume from the bleaching plant can be reduced, since water outlet from the chelating step thereby can be reduced or even be eliminated completely.
• the present invention thus relates to a method of pulp bleaching, which before a peroxide bleaching step comprises a step in which the pulp is treated with a chelating agent, followed by separation of a liquid phase and a treatment of the separated liquid phase, as is defined more in detail in the following claims.
• the method according to the invention is applicable in any bleaching sequence or in any bleaching system comprising a peroxide bleaching step and can be applied on any pulp to be bleached, e.g. chemical pulp or pulp based on recycled fibres.
• the method is used for bleaching chemical pulp which as such may have been delignified according to any process and consists of softwood or hardwood pulp or pulp from annual plants such as straw or bagasse.
• the method is used in bleaching of sulphate pulp. Description of Preferred embodiments of the Invention
• the pulp is treated before the peroxide step, either directly before it or at an earlier stage, e.g. before an oxygen step, if present, before a peroxide step in the bleaching sequence in question, with a chelating agent.
• a chelating agent can be used which binds the disturbing trace metals in question, most importantly manganese, copper and iron.
• Suitable chelating agents are aminocarboxylic acids such as e.g. ethylenediaminetetra-acetic acid (EDTA) , diethylene- triaminepenta-acetic acid (DTPA) , and triethylenetetra- aminehexa-acetic acid (TTHA) , aminophosponic acids, such as e.g.
• ethylenediaminetetramethylene-phosphonic acid phosphonic acids, such as e.g. hydroxyethyldiphosphonic acids, and polyphosphates .
• EDTA and DTPA are normally used as chelating agents.
• the chelating step is usually performed at elevated temperature, from about 50°C up to about 95°C and at a pH which can vary from 4 to 11.
• the amount of chelating agent used, measured as a pure product, should be at least 0.5 kg/ton pulp. Preferably it lies within the interval from 1 to 2 kg/ton.
• the pulp concentration at the chelating step is not critical but can vary e.g. from 2 to 20 % by weight cellulose-containing material.
• the concentration lies within the interval from 10 to 14 % in order to suit conventional process equipment.
• the time of treatment in the Q-step depends on, among other things, the temperature and should be at least 5 minutes, and most preferably at least 60 minutes and can if necessary be several hours.
• no conventional wash to remove the chelated metals from the pulp with fresh water is performed.
• a separation of liquid phase from the pulp treated with a chelating agent is performed.
• a substantial amount of water is separated.
• at least 1/3 of the original amount of water is separated.
• the intent is to separate as much water as possible and depending on the type of equipment and the concentration of the treated pulp the resulting pulp consistency will be about 12 to 50 %.
• the separated liquid phase is fed to a separate reactor wherein it is treated for oxidation of the metal ions.
• This treatment is made with an oxidizing agent under those conditions that are required for the oxidizing agent to give a satisfactory oxidizing effect.
• the treatment of the liquid phase is performed with ozone or peroxide.
• the treatment is preferably performed with peroxide and in the following description the method will be described more in detail for peroxide treatment of the separated liquid phase.
• the peroxide treatment of the liquid phase, as well as the peroxide bleaching step is normally performed with hydrogen peroxide, even if it is possible to use other peroxides.
• amounts relating to peroxides are given in the following specification these relate to amounts of hydrogen peroxide or a corresponding amount of another peroxide .
• the added amount of peroxide for treatment of the separated liquid phase should be sufficient for oxidizing at least substantially all metal bound in complex form and normally at least 1 kg pro ton treated pulp is used and the amount of added peroxide can amount to 10 kg/ton.
• the reaction is performed under alkaline conditions and suitably at a pH within the interval 9 to 13, and preferably within the interval from 10 to 12.
• the reaction time should be at least 1 minute, suitably at least 5 and preferably at least 10 minutes. Most preferably the reaction time is at least 1 hour and it is suitable to perform the reaction at a temperature of at least 50°C up to 110°C.
• the temperature is preferably at least 70°C and even more preferred at least 80°C. It is especially suitable to perform the peroxide treatment of the liquid phase at a pH of 10 to 12 and a temperature of 80 to 95°C in order to secure with a conventional process equipment an as complete oxidation as possible of the metal ions in the shortest time possible.
• the liquid phase can be returned to the system without giving any noticeable problems at subsequent peroxide bleaching steps since the disturbing metal ions have been deactivated.
• the points for return are chosen in view of the bleaching sequence in question and the water balance therein.
• the separation of liquid phase can be made by dewatering the pulp after the Q-step and such a dewatering step can be made with conventional equipment such as a dewatering press, a filter, a screw, etc..
• the liquid phase can, after the oxidation, advantageously, completely or partly, be returned as a dilution liquid to the dewatered pulp resulting after the Q-stage.
• the separation of liquid from the pulp can be made after the Q-stage in a conventional washer, usually a drum filter, where the liquor added to the washer is not a new water, which is the normal procedure in conventional washing steps, but instead liquid previously withdrawn from this step and the liquid separated in this way is treated with peroxide as described above or with another oxidizing agent .
• the pulp can undergo a peroxide bleaching step which is performed in a conventional way, i.e. at alkaline pH, usually within the interval 10 - 12, a temperature of from about 50°C to about 110°C and with a peroxide charge which usually lies within the interval 5 to 50 kg/ton pulp.
• the peroxide step can if desired be complemented with an oxygen addition and then suitably at least 1 kg/ton pulp.
• the oxygen addition in this step is advantageously at the most 10 kg/ton pulp and preferably not higher than 5 kg/ton pulp.
• the pulp concentration in the P- step can vary, among other things depending on the amount of liquid that is returned, wether the Q-step and the separation of liquid phase are performed directly before the P-step or not etc., and it normally lies within the interval from 8 to 16 % and suitably within the interval from 10 to 14 %.
• the P-step can be preceded by other bleaching steps, e.g. most often it is preceded by an oxygen step, and the treatment with chelate forming agents and treatment of a separated liquid phase according to the invention, preferably with peroxide, can then be performed before the oxygen step.
• the Q- step, dewatering and treatment of liquid phase for oxidation are performed directly before the peroxide bleaching step in order to reduce the risk for the metal ions to be reduced to their lower oxidation state, and thus resulting in markedly increased risk for peroxide deterioration in the actual peroxide step.
• the pulp is washed and is optionally transferred to a further bleaching step, e.g. a chlorine dioxide step, an ozone step or a further peroxide step.
• a further bleaching step e.g. a chlorine dioxide step, an ozone step or a further peroxide step.
• Example An oxygen delignified softwood sulphate pulp was treated at a concentration of 5 % with DTPA in an amount of 2 kg/ton pulp. The treatment was made during 60 minutes at a pH of 6 and a temperature of 90°C. Thereafter the pulp was transferred to a press and a liquid phase was removed. The re- maining pulp had a concentration of 12 %. The liquid phase was treated separately in a reactor with hydrogen peroxide in an amount of 3 kg per ton in the process. The treatment of the liquid phase was performed at a pH of 11.5 and a temperature of 80 °C during 60 minutes.
• the liquid phase was then returned to the pulp which thereafter was subjected to conventional peroxide bleaching with hydrogen peroxide in an amount of 40 kg/ton.
• the bleaching step was performed during a time of 2 hours, at a temperature of 105°C and a pH of 10.2 - 11.3.
• the brightness of the incoming oxygen delignified pulp was 41.9 % ISO and after the above described treatment with chelating agents, separation and separate treatment of the liquid phase and maximum return of this phase and peroxide bleaching the brightness of the pulp became 77 % ISO. If instead non-oxidized liquid phase was returned, with otherwise identical conditions, the brightness of the pulp became only 70.5 % ISO.

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• Life Sciences & Earth Sciences (AREA)
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• Wood Science & Technology (AREA)
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Cited By (1)

Citations (3)

• 1996
  • 1996-10-04 SE SE9603630A patent/SE507505C2/sv not_active IP Right Cessation
• 1997
  • 1997-09-30 AU AU45793/97A patent/AU4579397A/en not_active Abandoned
  • 1997-09-30 WO PCT/SE1997/001642 patent/WO1998014657A1/en active Application Filing

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