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
• • 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
• • 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
• • 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
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
• • 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
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H11/00—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only
  • D21H11/02—Chemical or chemomechanical or chemothermomechanical pulp
• • 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
  • D21C9/12—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds
  • D21C9/14—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites
  • D21C9/144—Bleaching ; Apparatus therefor with halogens or halogen-containing compounds with ClO2 or chlorites with ClO2/Cl2 and other bleaching agents in a multistage process
• • 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 invention relates to a process for the delignification and bleaching of pulp with chlorine dioxide and hydrogen peroxide in the presence of a molybdate or tungstate as a catalyst.
• the bleaching sequence OD 0 -E OP -D 1 -P is most frequently used, where O stands for a delignification with oxygen under alkaline conditions, D 0 and D 1 denote a first and second stage with chlorine dioxide as a bleaching and delignifying agent, E OP for an alkaline extraction with the addition of oxygen and hydrogen peroxide, P denotes a bleach with hydrogen peroxide and each hyphen stands for a washing of the pulp, for example by adding water and filtering the resulting suspension.
• US 6,048,437 describes a process for the delignification and bleaching of pulp with a bleaching sequence O-DP cat- E OP -D 1 -P, in which instead of a first stage D 0 for delignification and bleaching with chlorine dioxide, a stage DP cat is used in which chlorine dioxide and hydrogen peroxide are used simultaneously as a catalyst in the presence of a molybdate or tungstate.
• a stage DP cat is used in which chlorine dioxide and hydrogen peroxide are used simultaneously as a catalyst in the presence of a molybdate or tungstate.
• the invention therefore relates to a process for the delignification and bleaching of pulp, comprising a bleaching stage, in which in a first step pulp in an aqueous mixture containing 3 to 30 wt .-% pulp with chlorine dioxide in an amount corresponding to a kappa factor in the range from 0.02 to 0.25 at a temperature of 50 to 150 ° C and a pH in the range of 2 to 7 is reacted until more than 90% of the chlorine dioxide and the mixture obtained in the first step, then without separation of components of Mixture in a second step with 0.1 to 5 wt .-% hydrogen peroxide in the presence of a molybdate in an amount of 10 to 2000 ppm of molybdenum or a tungstate in an amount of 200 to 10,000 ppm tungsten at a temperature of 50 to 150 ° C. is further reacted, wherein the amounts are each based on the mass of dry pulp.
• the method according to the invention comprises a bleaching stage with two steps.
• pulp is reacted with chlorine dioxide until more than 90%, preferably more than 95%, and more preferably more than 99% of the chlorine dioxide have reacted.
• the chlorine dioxide is completely reacted in the first step.
• the mixture obtained in the first step is then reacted with hydrogen peroxide in a second step in the presence of a molybdate or a tungstate without separating components of the mixture.
• the reaction of the pulp with chlorine dioxide is carried out at a consistency in the range of 3 to 30%, i. the reaction is carried out in an aqueous mixture containing from 3 to 30% by weight of pulp, calculated as dry pulp, based on the total mass of the aqueous mixture.
• the consistency is in the range of 5 to 20% and more preferably in the range of 8 to 15%.
• Chlorine dioxide is used in an amount corresponding to a kappa factor in the range from 0.02 to 0.25 and preferably in the range from 0.05 to 0.15.
• the kappa factor is a parameter familiar to the skilled worker for the amount of bleaching agent used for pulp bleaching and denotes the quotient of the amount of bleaching agent, calculated as the active chlorine concentration in% by weight based on the mass of dry pulp, and the kappa number of the bleach used pulp.
• the active chlorine content is calculated from the concentration of chlorine dioxide in wt .-%, based on the mass of dry pulp, by multiplication with the factor 2.63, i. a concentration of chlorine dioxide of 1 wt .-% corresponds to a Aktivchlorkonzentration of 2.63 wt .-%.
• the kappa number is a parameter known to those skilled in the art for the lignin content of the pulp, which is determined by the consumption of permanganate for the oxidation of residual lignin according to TAPPI Standard T 236 om 99.
• the reaction of the pulp with chlorine dioxide is carried out at a temperature of from 50 to 150.degree. C., preferably from 60 to 120.degree. C. and more preferably from 70 to 90.degree.
• the reaction takes place at a pH of the aqueous mixture in the range from 2 to 7, preferably 2 to 5 and particularly preferably 2 to 4.
• the pH of the aqueous mixture is preferably adjusted by addition of an inorganic acid, more preferably by addition of sulfuric acid or hydrochloric acid.
• the reaction time required for the conversion of the chlorine dioxide depends on the reaction temperature and the concentration of chlorine dioxide and is preferably 5 to 30 minutes and more preferably 10 to 20 minutes. At a reaction temperature of 90 ° C, with a kappa factor in the range of 0.05 to 0.15, a complete conversion of the chlorine dioxide is generally achieved within 15 minutes.
• the reaction of the pulp with hydrogen peroxide is carried out in an amount of from 0.1 to 5% by weight of hydrogen peroxide, based on the weight of dry pulp used.
• the mixture obtained in the first step of the bleaching stage is added to a corresponding amount of hydrogen peroxide, preferably in the form of an aqueous solution with a content of 35 to 70 wt .-% hydrogen peroxide.
• molybdate and tungstate include both mononuclear molybdates and tungstates, such as MoO 4 2- or WO 4 2- , and polynuclear molybdates and tungstates, such as Mo 7 O 24 6- , Mo 8 O 26 4- , HW 6 O 21 5- , W 12 O 41 10- or W 12 O 39 6- , and heteroatom-containing polynuclear molybdates and tungstates, such as PMo 12 O 40 3- , SiMo 12 O 40 3- , PW 12 O 40 3- or SiW 12 O.
• the molybdate in an amount of 10 to 2000 ppm, preferably 100 to 1500 ppm and more preferably 200 to 600 ppm of molybdenum, based on the mass of dry pulp, is used.
• the tungstate is used in an amount of 200 to 10,000 ppm, preferably 500 to 5,000 ppm, and especially preferably 1500 to 3000 ppm tungsten, based on the mass of dry pulp, used.
• the molybdate or tungstate used as the catalyst may be added to the mixture obtained in the first step before or after the hydrogen peroxide or in parallel to the hydrogen peroxide.
• the molybdate or tungstate may also be added in the first step of the bleaching stage before or after the chlorine dioxide or in parallel to the chlorine dioxide.
• the molybdate or tungstate and the hydrogen peroxide are simultaneously but separately added in the second step in the form of two aqueous solutions.
• the reaction of the pulp with hydrogen peroxide takes place at a temperature of from 50 to 150.degree. C., preferably from 60 to 120.degree. C. and more preferably from 70 to 90.degree.
• the first and second steps of the bleaching stage are carried out at the same temperature.
• the reaction of the pulp with hydrogen peroxide is carried out in the second step, preferably for a period of 60 to 180 minutes, more preferably 90 to 120 minutes.
• Reaction temperature, reaction time and the amount of molybdate or tungstate used as a catalyst are preferably selected so that more than 90%, preferably more than 95% and particularly preferably more than 99% of the hydrogen peroxide used are reacted in the second step.
• the reaction of the pulp with hydrogen peroxide is carried out in the same pH range as the reaction with chlorine dioxide in the first step. If necessary, acid is added again to adjust the pH. In general it is however after the first step no further adjustment of the pH value is necessary.
• the bleaching stage of the process according to the invention can be carried out in the devices known from the prior art for the delignification and bleaching of pulp with chlorine dioxide or hydrogen peroxide.
• the bleaching stage is carried out continuously in a riser and bleaching tower apparatus in which the top of the riser is connected to the top of the bleaching tower.
• the aqueous mixture containing 3 to 30 wt .-% pulp is thereby fed to the riser at the bottom.
• Chlorine dioxide is added to the mixture in the lower part of the riser, and the mixture, after adding the chlorine dioxide, flows through the riser in ascending flow within 5 to 30 minutes, so that the first step of the bleaching stage takes place in the riser.
• the resulting mixture is taken up the riser and fed to the top of the bleaching tower.
• Hydrogen peroxide is added to an upper portion of the riser or top of the bleaching tower of the mixture, and after addition of the hydrogen peroxide, the mixture flows through the bleaching tower in descending flow for 60 to 180 minutes so that the second step of the bleaching stage occurs in the bleaching tower.
• the bleaching stage of the process according to the invention can be carried out in a typical bleaching tower for bleaching pulp with a minimum of conversions of the apparatus so that an existing pulp bleaching plant can be converted with little effort to carry out the process of the invention.
• the process according to the invention comprises an extraction of the pulp following the bleaching stage with an aqueous alkaline solution and an adjoining additional bleaching stage in which first step of the pulp in an aqueous mixture containing 3 to 30 wt .-% pulp with 0.04 to 0.4 wt .-% chlorine dioxide, based on the weight of dry pulp, at a temperature of 50 to 150 ° C and a pH in the range of 2 to 7 is reacted until at least 90% of the chlorine dioxide used, and the mixture obtained in the first step without separation of components of the mixture in a second step with 0.1 to 5 wt .-% hydrogen peroxide, based on the mass of dry pulp, at a temperature of 50 to 150 ° C and a pH in the range of 10 to 12.5 further reacted.
• the extraction of the pulp with an aqueous alkaline solution can be carried out in the same manner as the alkaline extraction used in the known ECF bleaching sequences following a D 0 bleaching stage.
• the extraction is carried out with the addition of oxygen as E O stage, with the addition of hydrogen peroxide as E P stage or with the addition of both oxygen and hydrogen peroxide as E OP stage.
• a washing of the pulp is preferably carried out to reduce the consumption of alkali in the extraction and the consumption of acid reduce in the additional bleaching stage and to remove in the bleaching stage and the extraction of pulp-cleaved compounds.
• the pulp is again reacted with chlorine dioxide at a pH in the range of 2 to 7 in an aqueous mixture, to more than 90%, preferably more than 95% and more preferably more than 99% of the chlorine dioxide are reacted. Most preferably, the chlorine dioxide is completely reacted in the first step.
• the mixture obtained in the first step is then reacted with hydrogen peroxide at a pH in the range of 10 to 12.5 in a second step without separating components of the mixture.
• the reaction of the pulp with chlorine dioxide is carried out at a consistency in the range of 3 to 30%, i. the reaction is carried out in an aqueous mixture containing from 3 to 30% by weight of pulp, calculated as dry pulp, based on the total mass of the aqueous mixture.
• the consistency is in the range of 5 to 20% and more preferably in the range of 8 to 15%.
• Chlorine dioxide is used in an amount of 0.04 to 0.4 wt .-% chlorine dioxide, preferably 0.08 to 0.2 wt .-% chlorine dioxide, based on the mass of dry pulp used.
• the reaction of the pulp with chlorine dioxide at a temperature of 50 to 150 ° C, preferably from 60 to 120 ° C and particularly preferably from 70 to 90 ° C.
• the reaction takes place at a pH of the aqueous mixture in the range from 2 to 7, preferably 3 to 6 and particularly preferably 4 to 6.
• the pH of the aqueous mixture is preferably by adding an inorganic acid, more preferably by adding sulfuric acid or hydrochloric acid.
• the reaction time required for the conversion of the chlorine dioxide depends on the reaction temperature and the concentration of chlorine dioxide and is preferably 5 to 30 minutes and more preferably 10 to 20 minutes.
• the reaction of the pulp with hydrogen peroxide takes place in one pH in the range of 10 to 12.5, preferably 11 to 12.
• the pH of the aqueous mixture is preferably adjusted by adding an inorganic base, particularly preferably by adding sodium hydroxide solution, calcium hydroxide or magnesium hydroxide.
• Hydrogen peroxide is used in an amount of from 0.1 to 5% by weight, based on the weight of dry pulp used. Preferably, 0.2 to 2 wt .-% and particularly preferably 0.25 to 1 wt .-% hydrogen peroxide used.
• Hydrogen peroxide is preferably added in the form of an aqueous solution containing from 35 to 70% by weight of hydrogen peroxide.
• the addition of hydrogen peroxide is preferably carried out after the addition of the base.
• caustic soda the hydrogen peroxide is preferably added simultaneously with but separate from the caustic soda solution.
• the reaction of the pulp with hydrogen peroxide at a temperature of 50 to 150 ° C, preferably from 60 to 120 ° C and particularly preferably from 70 to 90 ° C.
• the first and second steps of the bleaching stage are carried out at the same temperature.
• the reaction of the pulp with hydrogen peroxide is carried out in the second step, preferably for a period of 60 to 180 minutes, more preferably 90 to 120 minutes.
• the additional bleaching step is carried out in the same manner as described above for the bleaching stage in a riser and bleaching tower apparatus.
• the preferred embodiment with the additional bleaching stage according to the invention has over that in the usual ECF bleach as well as in US 6,048,437 used bleaching stage sequence with the final stages D 1 -P the advantage that a bleaching stage and a laundry saved become.
• the bleaching sequence D / P cat -ED / P results for the embodiment with the additional bleaching stage according to the invention, where D / P stands for the additional bleaching stage according to the invention, or for the embodiments of FIGS alkaline extraction with the addition of oxygen and / or hydrogen peroxide, the bleaching sequences D / P cat -E O -D / P, D / P cat -E P -D / P and D / P cat -E OP -D / P stand.
• the method according to the invention is preferably carried out before the bleaching stage D / P cat according to the invention, a delignification of the pulp in a stage O with oxygen under alkaline conditions, more preferably under pressure. All processes known from the prior art for delignifying pulp with oxygen can be used for this purpose.
• the bleaching sequences according to the invention then result OD / P cat -ED / P, OD / P cat -E O -D / P, OD / P cat -E P -D / P and OD / P cat -E OP -D / P.
• the inventive method additionally comprises a recovery of molybdate or tungstate according to the teaching of WO 2009/133053 ,
• an aqueous solution containing molybdate or tungstate is separated and from this solution molybdate or tungstate is separated with the steps of contacting the solution with a water-insoluble, cationized inorganic carrier material a pH in the range between 2 and 6 to obtain a molybdate or tungstate loaded support material and an aqueous solution depleted in molybdate or tungstate, separating the molybdate or tungstate loaded support material from the molybdate or tungstate depleted aqueous Solution, contacting the molybdate or tungstate loaded support material with an aqueous solution at a pH in the range of 6 to 14 to yield a molybdate or tungstate depleted support material and an aqueous solution laden with mo
• the cationized inorganic carrier material used is preferably a cationized layered silicate, more preferably a bentonite ion-exchanged with a quaternary ammonium salt.
• a cationized layered silicate more preferably a bentonite ion-exchanged with a quaternary ammonium salt.
• a complexing agent together with the hydrogen peroxide in the bleaching stage D / P cat according to the invention and / or in the additional bleaching stage D / P according to the invention.
• any known from the prior art for the reduction of decomposition of hydrogen peroxide in a bleach of pulp complexing agent can be used.
• Preferred complexing agents are aminocarboxylic acids or aminophosphonic acids, such as, for example, ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), N-hydroxyethyl-N, N ', N'-triacetic acid, cyclohexanediaminetetraacetic acid, aminotrimethylenephosphonic acid, ethylenediamine tetramethylenephosphonic acid, diethylenetriaminepentamethylenephosphonic acid, propylenediamine tetramethylenephosphonic acid or Dipropylentriaminpentamethylenphosphonklare, and their salts used.
• Particularly preferred complexing agents are EDTA and DTPA and their sodium salts.
• the complexing agents are preferably used in an amount of from 0.05 to 1% by weight, based on the weight of dry pulp used.
• the kappa number of the pulp was determined according to TAPPI Standard T 236 om 99.
• the whiteness of the pulp was determined according to PAPTAC Standard E.1.
• the determination of the whiteness loss by heat aging and the post-color number was carried out using the methods TAPPI UM 200 and TAPPI T 260.
• the viscosity of the pulp was determined according to TAPPI Standard T 236 om 99.
• the bleaching stages were each carried out at a consistency of 10% by mixing the pulp with the appropriate amount of water and the amounts of bleaching chemicals given in the examples and keeping them in a plastic bag in a thermostated water bath at the temperature given in the example. Deviating from this, the alkaline oxygen-assisted extraction became E OP in Examples 25 and 26 carried out in a rotating autoclave at an oxygen pressure of 3 bar.
• the indicated amounts of bleaching chemicals are based on the mass of the dry pulp used in the bleaching sequence.
• sodium molybdate in the form of an aqueous solution was used as the catalyst.
• pulp A was bleached with the amounts of chemicals listed in Table 2 at the conditions shown in the table.
• Table 2 also shows the kappa number and whiteness of the pulp after bleaching.
• Example 1 In Example 1, according to the invention, only chlorine dioxide and sulfuric acid were added first, and hydrogen peroxide and sodium molybdate were added only after 15 minutes. At the time of adding hydrogen peroxide, all of the chlorine dioxide was reacted. At the end of the bleaching stage could still be detected 34% of the hydrogen peroxide used.
• Example 2 (not according to the invention) according to the teaching of US 6,048,437 Chlorine dioxide, sulfuric acid, hydrogen peroxide and sodium molybdate are added simultaneously. At the end of the bleaching stage could still be detected 11% of the hydrogen peroxide used.
• Example 3 the order of addition of chlorine dioxide and hydrogen peroxide to the bleaching stage of the invention was reversed, i. Sulfuric acid, hydrogen peroxide and sodium molybdate were added first and chlorine dioxide was added only after 120 minutes. At the end of the bleaching stage, the hydrogen peroxide used was completely reacted.
• Example 4 (not according to the invention), bleaching was carried out without chlorine dioxide or without hydrogen peroxide and in Example 6 (not according to the invention) only sulfuric acid and no bleaching agent were added.
• Example 1 shows in comparison with Example 2, that with the inventive single-stage bleaching D / P cat equally strong delignification and bleaching is achieved as with the US 6,048,437 known bleach DP cat .
• This is surprising and well known in the art US 6,048,437 unpredictable, since the prior art teaches a synergistic effect of the simultaneous action of chlorine dioxide, hydrogen peroxide and molybdate, which is said to result in greater bleaching than separate bleaching with chlorine dioxide and hydrogen peroxide.
• Example 3 shows that the order of addition of chlorine dioxide and hydrogen peroxide arrives in the process according to the invention for the effectiveness of the bleaching, which is evident from the experimental data of US 6,048,437 was also unpredictable.
• Example 7 shows in comparison with the noninventive Examples 9 to 12, which were carried out with comparable amounts of bleach that the unexpectedly high compared to the prior art delignification and bleaching in combination with a downstream alkaline extraction stage E P is achieved.
• the examples also show that in the bleaching stage according to the invention, the oxidative degradation of cellulose is particularly low, recognizable by the high viscosity of the bleached pulp, ie the bleaching stage according to the invention is particularly gentle on the fiber and therefore provides a pulp with which tear-resistant papers can be produced ,
• an improvement in the yellowing resistance of the pulp is achieved, recognizable by the lower post-color number (PC number) after heat aging.
• PC number post-color number
• Example 14 in comparison with Example 13, not according to the invention, shows that with the bleaching stage according to the invention significantly less bleaching chemicals than with bleaching with chlorine dioxide are needed to achieve the same degree of delignification, ie the same kappa number, and that the required Reduce the amount of chlorine dioxide by 70%, which leads to a corresponding reduction in the formation of undesirable organochlorine compounds.
• Examples 15 (according to the invention) and 16 (not according to the invention) and 17 (according to the invention) and 18 (not according to the invention) show that with a complete bleaching sequence from the bleaching stage according to the invention D / P cat and subsequent stages E P -DP according to the prior art with a smaller amount of chlorine dioxide than a bleach with chlorine dioxide in a sequence D 0 -E P -D 1 -P the same whiteness of more than 89.5% can be achieved with a lower use of chlorine dioxide and at the same time a pulp with significantly improved yellowing resistance, recognizable by the lower post-color number.
• Examples 19 and 20 show that with the bleaching stage according to the invention, the same degree of whiteness can be achieved even with bleaching at lower temperatures and thus reduced energy consumption, if the amount of molybdate is increased.
• Table 5 Five-stage bleaching of pulp C with a sequence O-bleach E- P- DP attempt step Chemical in% by weight Kappa factor Time in min Temperature in ° C pH H 2 SO 4 NaOH H 2 O 2 Not a word ClO 2 beginning The End 15 D / P cat 0.2 0.29 0,075 15 90 0.05 0.5 0.05 105 2.6 E P 1.2 0.4 60 80 12.0 10.9 D 0.11 120 85 4.9 P 0.7 0.5 90 85 11.8 10.8 16.
• Example 21 in comparison with Examples 22 to 24 not according to the invention, shows that the bleaching stage advantages of the invention are also achieved if, in a complete bleaching sequence, bleaching stages D according to the invention instead of the bleaching stages DP with intermediate laundry are used / P is used without washing between bleaching with chlorine dioxide in acid and with hydrogen peroxide in alkaline.
• Example 25 shows that with the bleaching stage according to the invention and the additional bleaching stage according to the invention compared with the most frequently used ECF bleaching sequence OD 0 -E OP -D 1 -P with a 38% reduced amount of chlorine dioxide and saving a bleaching stage, ie lower equipment Effort, a pulp is obtained, which has virtually the same degree of whiteness, but in addition significantly yellowing resistant and less oxidatively damaged.

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