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
  • D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes

Definitions

• the present invention relates to a process for the delignification and bleaching of an aqueous lignocellulosic pulp under acidic conditions in the bleaching plant of a pulp mill.
• the invention also includes a process for the production of paper or pulp from said bleached pulp suspension.
• Lignin is an amorphous polymer which, like a glue, keeps the cellulose fibers together in wood. Some lignin also exists in the fiber wall. In its native state in wood lignin is of light color.
• Delignification and bleaching are chemical purifications of the fibers in a cooked lignocellulosic pulp.
• the main aim of acidic bleaching sequences is to remove lignin and other components capable of absorbing visible light and thus influence the brightness of the pulp.
• the fibers, i.e. the carbohydrate components of the lignocellulose should preferably not be materially affected by the bleaching process.
• the selectivity in the bleaching can be defined as the relative reactivity of the particular reactive species toward lignin and carbohydrate components of the pulp in a competitive situation.
• the competing initial reactions are believed to consist of the addition of an electrophile to the aromatic ring or the olefin structures in the lignin and the abstraction of a hydrogen atom from the carbohydrates.
• the bleaching stages contemplated within the scope of the present invention comprise acidic bleaching stages such as one or more chlorine dioxide (D) stages, peracetic acid (Paa) stages, ozone (Z) stages, as well as multichemical stages utilizing a combination of bleaching agents.
• acidic bleaching stages such as one or more chlorine dioxide (D) stages, peracetic acid (Paa) stages, ozone (Z) stages, as well as multichemical stages utilizing a combination of bleaching agents.
• the bleaching plant operates with bleaching sequences made up of several bleaching stages, which usually are separated by washing stages, although in some cases the washing may be omitted.
• D 0 (EOP)DED An example of a modern ECF sequence for production of bleached kraft pulp is D 0 (EOP)DED, where D stand for chlorine dioxide bleaching, E stands for extraction and EOP for oxygen and peroxide reinforced extraction.
• D stand for chlorine dioxide bleaching
• E stands for extraction
• EOP oxygen and peroxide reinforced extraction.
• the unbleached pulp enters the bleach plant with an alkaline pH.
• the chlorine dioxide bleaching is then carried out under acidic conditions and the subsequent extraction is performed under alkaline conditions.
• ozone bleaching stage Z This stage may, for instance be included in a sequence of ZQ(PO).
• the ozone may also be combined with chlorine dioxide bleaching in a (ZD) or a (DZ) stage.
• the pH of the pulp suspension should be decreased to about pH 3 for the ozone in the Z stage to provide an effective bleaching.
• the pulp may be bleached in a so called multi-chemical stage where several bleaching processes occur without inter-mediate washing.
• stages are, in addition to the above mentioned (ZD) or (DZ) combination, a (ZPaa) or an (AD) stage, where A stands for acid hydrolysis.
• the acids normally used for acidifying the pulps for the acidic bleaching stage comprises mineral acids such as sulfuric acid.
• mineral acids such as sulfuric acid.
• carbon dioxide which is a gas which in reaction with water provides carbonic acid.
• carbon dioxide is a gas which in reaction with water provides carbonic acid.
• Carbon dioxide has also been suggested in GB Patent 815,247 for use in combination with sodium hydroxide to provide a pH buffer in situ for a chlorine dioxide bleach which was to be performed at a pH of 9 to 5.
• Teder, A., et al., TAPPI 61(1978)12, pp. 59-62 have reported that in a two-step pH adjusted chlorine dioxide bleach the pH may be kept at a high pH for a longer period with the use of NaHCO 3 buffer or NaOH.
• the present invention is based on the realization that while carbon dioxide has little or no impact on the final pH of the very acidic bleaching stages such as a D stage, carbon dioxide does affect the bleaching result and can be used for controlling the chemical reactions taking place between the bleaching agent and the various lignin and carbohydrate compounds in the pulp suspension.
• Carbon dioxide is a gas, which dissolves in aqueous media under alkaline conditions, e.g. in water or a pulp suspension.
• the dissolved gas forms carbonic acid, H 2 CO 3 , which readily dissociates as shown below:
• the carbonic acid produced by the dissolving carbon dioxide is a weak acid which is capable of lowering the pH of an alkaline pulp to neutral and slightly below, down to a pH of about 6.
• carbon dioxide is capable of lowering the initial pH of an alkaline pulp entering the bleaching stage, its pH lowering effect is lost at the lower pH ranges and, contrary to previous beliefs, carbon dioxide cannot be used to lower the pH of the actual (final) bleaching stage.
• carbon dioxide can be used to control the degradation rate and the decomposition of various components of the aqueous lignocellulosic pulp.
• the object of the present invention is thus to provide an acidic bleaching process wherein the degradation of lignin and carbohydrates in a pulp suspension is controlled.
• Another object of the invention is to improve the selectivity of the bleach by directing the attack of the bleaching agent primarily towards a reaction with lignin.
• a further object of the invention is to provide a bleaching process wherein the degradation chain of carbohydrates and/or lignin is influenced so as to suppress the decomposition of said compounds to smaller fragments.
• An object of the invention is to improve the selectivity of the bleach and thus to obtain a higher brightness and/or lower kappa number of the bleached pulp with a given amount of bleaching agent.
• Another object of the invention is to improve the selectivity of the bleach and thus to obtain a the same brightness and/or kappa number with a reduced amount of bleaching agent.
• a further object of the invention is to decrease the chemical oxygen demand (COD) of the bleach filtrate.
• the present invention concerns a process for bleaching an aqueous lignocellulosic pulp under acidic conditions in the bleaching plant of a pulp mill, said process comprising the following stages: providing an aqueous lignocellulosic pulp suspension in said bleaching plant; adding bleaching agent to said pulp suspension and providing an acidic pH therein for causing a reaction between lignin and said bleaching agent; prior to or simultaneously therewith adding a carbon dioxide providing medium to said lignocellulosic pulp suspension to provide carbon dioxide in said acidic pulp suspension for controlling the degradation of lignin and its derivatives in their reactions with said bleaching agent; and subsequently subjecting said pulp suspension to an alkaline extraction step to solubilize and remove reacted lignin compounds from said pulp.
• the preferred carbon dioxide providing medium comprises carbon dioxide in gaseous form.
• the carbon dioxide may, however, also be introduced in liquid or solid form.
• the carbon dioxide in the acidic aqueous suspension can also be provided by compounds which dissociate and/or decompose at the pH in question and thereby provide carbon dioxide.
• Such compounds comprise gases or liquids capable of providing carbonate and/or bicarbonate ions in the aqueous suspension under alkaline or neutral conditions.
• the compounds are exemplified by alkali metal bicarbonates and carbonates which decompose at the lower pH ranges freeing carbon dioxide.
• the carbon dioxide providing medium used according to the present invention is preferably a carbon dioxide containing gas which is fed directly into the aqueous pulp suspension. It may, however, also be a carbon dioxide providing aqueous liquid, such as dilution water, which contains carbon dioxide, bicarbonate or carbonate. For the purpose of the invention it is not critical how the carbon dioxide enters the suspension, it is only required that it will provide carbon dioxide in the aqueous pulp suspension.
• the carbon dioxide should preferably be added directly to the pulp suspension prior to or in immediate connection to the bleaching stage in question, but after any washing stage, that might precede the bleaching stage. If added to the washing stage, most of the carbon dioxide will be removed with the washing water and the effect on the bleach will be inadequate.
• the carbon dioxide may, however, be added to any dilution water used to dilute the pulp after a preceding washing stage.
• the carbon dioxide providing medium comprises gaseous carbon dioxide which is injected into a flowing stream of said pulp suspension just prior to the addition of the bleaching agent of a D, Z, Paa or multichemical stage.
• an improved brightness and kappa number of the pulp may be obtained after extraction as the carbon dioxide will channel the attack of the bleaching agent to the initial bleaching reactions with lignin
• the improved bleaching action may be used for reducing the consumption of bleaching chemicals
• the pulp suspension to be treated according to the present invention is not critical. Almost any kind of lignin containing pulp can be bleached according to the procedures of the present invention. Examples of such pulps are chemical pulps, organosolv pulps, mechanical pulps, chemi-mechanical pulps, semi chemical pulps, pulps containing recycled fibers or broke, or mixtures of any of these in a bleaching plant of a pulp mill.
• the pulp may be introduced into the bleaching plant after cooking or it may be directed to the bleaching plant via an oxygen delignification stage and possibly other treatments.
• the consistency of the pulp may be in the low, medium or high consistency range. For better handling it is generally preferable to use a low or medium consistency pulp.
• a typical consistency is 1 to 18%, preferably 3 to 15%.
• the present invention also includes the processing of the bleached pulp further in a pulp or paper mill to provide dried pulp and/or paper containing said bleached pulp.
• the production of dried pulp and paper from the bleached pulp can be performed in the conventional way which is well known to those skilled in the art.
• the pulp suspension entering the bleaching plant is generally alkaline and in the preferred embodiment of the invention the pH is adjusted to an acidic or neutral pH by adding said carbon dioxide providing medium.
• the pH is adjusted to an acidic or neutral pH by adding said carbon dioxide providing medium.
• another acid such as sulfuric acid, sulfurous acid, alum, waste acid from the chlorine dioxide production may be used also for providing the required lower pH.
• the strong acidic conditions of the bleach will liberate carbon dioxide from the carbonates and bicarbonates present in the solution at a higher pH.
• the freed carbon dioxide effects the bleach to provide a controlled bleaching effect.
• any further reactions between lignin derivatives and bleaching agent are superfluous from the bleaching point of view. If the bleaching agent is allowed freely to attack the lignin derivatives, the lignin will react in a chain of reaction steps ultimately leading to decomposition. Among the fragments produced by such a decomposition there is specifically also carbon dioxide.
• the bleaching agent such as chlorine dioxide or ozone
• the fibers will degrade and finally decompose.
• the result of such an attack can be seen as a decrease in the viscosity of the bleached pulp.
• the carbon dioxide provided by the carbon dioxide providing medium of the present invention will suppress the secondary reactions degrading the carbohydrate chains and/or the lignin derivatives.
• the introduction of an effective amount of carbon dioxide into the pulp pushes the bleaching agent into the preferred primary reactions which cause an active bleaching action.
• the amount of carbon dioxide in the suspension should be at an effective level and that increasing the amount of carbon dioxide much above an optimum level will counter the beneficial effects of the added carbon dioxide and will lower the brightness and/or increase the kappa number of the bleached and extracted pulp.
• the present invention forms an improvement in a conventional process in a pulp mill and that the final product of the process is paper or pulp bleached according to the described invention. Consequently, the present invention also concerns a process for producing paper or pulp.
• an aqueous unbleached pulp suspension is bleached in one or more acidic bleaching stages with the assistance of a carbon dioxide providing medium, and subsequently extracted in a bleaching plant.
• the processing of the pulp after said bleaching stage(s) is then performed in a conventional way and the pulp is processed to form a web of paper or pulp.
• the mill was provided with a system which automatically altered the chemical consumption to provide a desired bleaching result.
• Carbon dioxide was fed at the same point as in the trials of Example 1.
• the bleaching was followed by an EOP extraction stage.
• the chemical consumption was altered according to the automation system.
• the chemical consumption had to be calculated through the whole fibreline.
• the sodium hydroxide and hydrogen peroxide consumptions were calculated alongside with the chlorine dioxide consumption. All of these results were compared to the three reference periods before and after these trials, representing a time of totally four weeks.
• a pulp from the brown stock filters of a mill fibreline was delignified in plastic bags with approximately 30 kg/ADt of chlorine dioxide, as active chlorine, at 60° C. for 60 minutes. Carbon dioxide was added as dry-ice, at approximately 2 and 4 kg/ADt carbon dioxide dosages.
• a pulp was taken from an MC-pump before a D 0 -stage of a fibreline of a pulp mill.
• the tests were performed in a laboratory with a CRS-reactor capable of controlling the reaction temperature and pressure, the carbon dioxide dosage and the chlorine dioxide dosage, and of measuring the pH of the filtrate.
• Example 4 Laboratory tests similar to those of Example 4 were performed for a D 1 stage but without performing an alkaline extraction afterwards. The results showed that without an extraction stage, the same advantages as those gained in Example 4 could not be obtained.
• the viscosities of the bleached pulps are measured.
• the viscosity of the pulp treated with ozone has decreased from 22 mPas to 17.5 mPas, while the carbon dioxide treated pulp has a final viscosity of 18.0 mPas.

Landscapes

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

Applications Claiming Priority (2)

Publications (1)

Family

ID=8558126

Family Applications (1)

Country Status (6)

Cited By (4)

Families Citing this family (2)

Citations (1)

Family Cites Families (3)

• 2000
  • 2000-04-05 FI FI20000801A patent/FI109209B/fi not_active IP Right Cessation
• 2001
  • 2001-04-04 EP EP01925600A patent/EP1278909A1/fr not_active Withdrawn
  • 2001-04-04 WO PCT/FI2001/000326 patent/WO2001075220A1/fr not_active Ceased
  • 2001-04-04 CA CA002405325A patent/CA2405325A1/fr not_active Abandoned
  • 2001-04-04 AU AU2001252300A patent/AU2001252300A1/en not_active Abandoned
  • 2001-04-04 US US10/240,685 patent/US20030168190A1/en not_active Abandoned

Patent Citations (1)

Also Published As

Similar Documents

Legal Events