EP2345760B1 - Process for delignifying and bleaching chemical pulp - Google Patents

Description

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.

For the production of paper pulp must be delignified and bleached after pulp cooking in several stages. Whereas in the past elementary chlorine was used mainly for delignification and bleaching, today bleaching sequences without elemental chlorine are preferably used in ECF bleaching. For this purpose, 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.

For the further development of ECF bleaching, a reduction in the amount of chlorine dioxide used is aimed at further reducing the formation of organochlorine compounds during pulp bleaching. In addition, a simplification of the bleaching sequence with a reduction in the number of stages to only four stages and three intervening washes is sought in order to carry out the ECF bleaching with the same number of stages as a pulp bleaching with elemental chlorine. However, the same whiteness of at least 89.5% ISO (according to PAPTAC Standard E.1) and the same stability of the whiteness must be achieved as with the bleaching sequence OD ₀ -E _OP -D ₁ -P and there must be no stronger oxidative degradation of the pulp.

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. Although the process makes it possible to reduce the required amount of chlorine dioxide compared to the stage D _{0 of} the conventional ECF bleach, it is used with the in US 6,048,437 used amounts of chlorine dioxide and hydrogen peroxide does not reach the desired whiteness of at least 89.5% ISO. Increasing the amounts of chlorine dioxide and hydrogen peroxide, then it comes to an undesirable oxidative degradation of the pulp, which is noticeable by a decrease in the pulp viscosity, as from MS Manning et al., J. Pulp Paper Sci. 32 (2006) 58-62 is known. Such unwanted degradation of the pulp can be avoided only by the introduction of an additional stage Q of extraction of the pulp with a metal ion chelating agent, corresponding to a bleaching sequence OQ-DP _cat -E _OP -D ₁ -P or QO-DP _cat -E _OP -D ₁ -P. US 6,048,437 also describes with Comparative Examples 13 and 14 that with two stages DP _cat or P _cat -D, in which P _{cat stands} for bleaching with hydrogen peroxide in the presence of a molybdate or tungstate as catalyst, a significantly lower bleaching effect compared to the stage DP _cat , is achieved.

It has now surprisingly been found that with a bleaching stage D / P _cat , in which the pulp is first reacted with chlorine dioxide and after reaction of the chlorine dioxide without an intermediate wash with hydrogen peroxide in the presence of a molybdate or tungstate, in comparison with the bleaching stage DP _cat known from 6,048,437, a further improved bleaching effect and a further saving in chlorine dioxide are achieved without causing the unwanted degradation of the pulp, so that an additional stage Q of extraction of the pulp with a metal ion chelating agent can be dispensed with ,

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. In the first step, 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. In the most preferred embodiment, the chlorine dioxide is completely reacted in the first step. Following 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.

In the first step of the bleaching stage of the process according to the invention, 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. Preferably, 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 .-%. With a kappa number of the pulp of 10 used, a concentration of chlorine dioxide of 0.5% by weight corresponds to a kappa factor of 0.5 * 2.63 / 10 = 0.1315. 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.

In the first step of the bleaching stage of the process according to the invention, 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.

In the second step of the bleaching stage of the process according to the invention, 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. Preferably, 0.2 to 2 wt .-% and particularly preferably 0.5 to 1 wt .-% hydrogen peroxide used. For this purpose, 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. The reaction with hydrogen peroxide is carried out in the presence of a molybdate or a tungstate, which acts as a catalyst for bleaching with hydrogen peroxide. According to the invention, the terms 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. ₄₀ ^3- . When using molybdate as a catalyst, 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. When using tungstate as a catalyst, 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. Alternatively, 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. In a preferred embodiment, the molybdate or tungstate and the hydrogen peroxide are simultaneously but separately added in the second step in the form of two aqueous solutions.

In the second step of the bleaching stage of the process according to the invention, 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. Preferably, 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.

In the second step of the bleaching stage of the process according to the invention, 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.

Preferably, 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. In this embodiment, 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.

In a preferred embodiment, in addition to the bleaching stage, 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.

In this preferred embodiment, 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. Preferably, 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. Between the bleaching stage according to the invention and the extraction of the pulp with an aqueous alkaline solution and between the extraction of the pulp with an aqueous alkaline solution and the additional bleaching 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.

In the additional bleaching step, in a first step, 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. Following 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.

In the first step of the additional bleaching step, 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. Preferably, 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.

In the first step of the additional bleaching stage, 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.

In the second step of the additional bleaching stage, 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. When using calcium hydroxide or magnesium hydroxide, the addition of hydrogen peroxide is preferably carried out after the addition of the base. When using caustic soda, the hydrogen peroxide is preferably added simultaneously with but separate from the caustic soda solution.

In the second step of the additional bleaching stage, 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. Preferably, 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.

Preferably, 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. In the preferred embodiment, with washes before and after the alkaline extraction, 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.

In 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. For the preferred embodiment with the additional bleaching stage according to the invention, 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.

In a further preferred embodiment, the inventive method additionally comprises a recovery of molybdate or tungstate according to the teaching of WO 2009/133053 , In this embodiment, from the mixture resulting in the second step of the bleaching stage D / P _cat 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 molybdate or tungstate and separating the molybdate or tungstate depleted carrier material from the molybdate or tungstate loaded aqueous solution. The aqueous solution separated in the last step and charged with molybdate or tungstate is then recycled to the second step of the bleaching stage.

The cationized inorganic carrier material used is preferably a cationized layered silicate, more preferably a bentonite ion-exchanged with a quaternary ammonium salt. In addition, for the recovery of molybdate or tungstate all in WO 2009/133053 for the recovery steps described preferred embodiments are used.

In the process according to the invention, it is additionally possible to use 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. For this purpose, 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 Dipropylentriaminpentamethylenphosphonsäure, 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 following examples illustrate the claimed process without, however, limiting the scope of the invention.

Examples methods:

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.

All experiments were carried out with eucalyptus kraft pulps delignified with oxygen under alkaline conditions. The kappa number and the whiteness of the pulps used are listed in Table 1.

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. For the catalyzed bleaching with hydrogen peroxide, sodium molybdate in the form of an aqueous solution was used as the catalyst.

The washes between the bleaching stages were carried out in each case by adding deionized water to a consistency of 2%, intensive stirring of the resulting suspension and separation of the pulp from this suspension by vacuum filtration and centrifugation. Table 1 Kappa number and whiteness of pulps used cellulose used in the trials Kappa number Whiteness in% ISO A 1 - 6 10.0 50.1 B 7 - 15 10.5 56.9 C 16, 17 10.3 56.6 D 18-21 10.3 52.3 e 22 - 25 10.3 55.1 F 26, 27 12.9 47.9

Comparison of bleaching stages:

In Examples 1 to 5, 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.

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.

In 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.

In Example 3 (not in accordance with the invention), 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.

In Examples 4 and 5 (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. Table 2 Two-stage bleaching of pulp A with a sequence O-bleach attempt step Chemical in% by weight Kappa factor Time in min Temperature in ° C pH Kappa number Whiteness in% ISO H ₂ SO ₄ H ₂ O ₂ Not a word ClO ₂ beginning The End 1 D / P _cat 0.3 0.76 0.20 15 90 1.0 0.05 120 3.1 1.6 73.4 2 * DP _cat 0.3 1.0 0.05 0.76 0.20 135 90 2.9 1.7 73.1 3 * P _cat / D 0.3 1.0 0.05 120 90 0.76 0.20 15 3.0 1.7 71.9 4 * P _cat 0.6 1.0 0.05 135 90 3.1 3.9 3.4 56.2 5 * D 0.76 0.20 135 90 2.9 3.1 65.9 6 * 0.8 135 90 2.8 3.0 6.4 52.6 * Not according to the invention

Three-stage sequence with bleaching stage and E _P stage:

In Examples 7 to 14, pulp B was bleached with the amounts of chemicals listed in Table 3 at the conditions shown in the table. The properties of the resulting pulp are summarized in Table 4.

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. In addition, 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 , In addition, with the bleaching stage according to the invention an improvement in the yellowing resistance of the pulp is achieved, recognizable by the lower post-color number (PC number) after heat aging.

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. Table 3 Three-stage bleaching of pulp B with a sequence O-bleach E _P 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 7 D / P _cat 0.85 0.60 0.15 15 90 0.5 0.05 105 2.6 E _P 1.2 0.4 60 80 12.0 10.9 8th* DP _cat 0.85 0.5 0.05 0.60 0.15 120 90 2.9 E _P 1.2 0.4 60 80 12.0 10.9 9 * P _cat / D 0.85 0.5 0.05 105 90 0.60 0.15 15 2.8 E _P 1.2 0.4 60 80 11.9 10.9 10 * P _cat / D 0.85 0.5 0.05 15 90 0.60 0.15 105 2.9 E _P 1.2 0.4 60 80 12.0 10.8 11 * D 0.85 0.60 0.15 120 90 2.4 E _P 1.2 0.4 60 80 10.7 10.7 12 * P _cat 1.0 0.5 0.05 120 90 3.0 E _P 1.2 0.4 60 80 12.0 10.9 13 * D 0.75 1.0 0.25 120 90 2.5 E _P 1.2 0.4 60 80 11.9 10.9 14 D / P _cat 0.3 0.30 0,075 15 90 0.5 0.05 105 3.7 E _P 1.2 0.4 60 80 12.1 11.0 * Not according to the invention Three-stage bleaching of pulp B with a sequence O-bleach E _P attempt sequence Kappa number Whiteness in% ISO Viscosity in mPa * s Heat aging damp Heat aging dry Whiteness change in percentage points PC number Whiteness change in percentage points PC number 7 D / P _cat E _P 1.4 84.5 22.7 -1.9 0.41 -2.1 0.46 8th* DP _cat -E _P 1.6 83.6 19.9 -1.8 0.42 -2.1 0.49 9 * P _cat / DE _P 1.5 82.5 22.1 -2.1 0.53 -2.3 0.59 10 * P _cat / DE _P 1.7 81.9 20.7 -2.0 0.53 -1.9 0.50 11 * DE _P 2.7 82.7 21.7 -2.5 0.64 -2.2 0.55 12 * P _cat -E _P 3.2 71.9 21.4 -1.7 0.83 -1.9 0.94 13 * DE _P 2.2 84.9 19.3 -2.3 0.49 -2.5 0.54 14 D / P _cat E _P 2.2 82.7 21.2 -2.7 0.69 -1.7 0.42 * Not according to the invention

Five-step sequence O-bleaching step E _P -DP:

In Examples 15 and 16, pulp C and in Examples 17 to 20 Pulp E were bleached with the amounts of chemicals listed in Table 5 at the conditions shown in the table. The properties of the resulting pulp are summarized in Table 6.

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. * D 0.5 0.59 0.15 120 90 2.9 E _P 1.2 0.4 60 80 12.0 10.9 D 0.08 120 85 5.1 P 0.77 0.5 90 85 11.8 10.9 17 D / P _cat 0.4 0.57 0.15 15 90 0.5 0.05 105 3.0 E _P 1.2 0.4 60 80 11.9 10.8 D 0.15 0.23 120 80 4.1 P 0.6 0.4 60 80 11.6 10.6 18 * D 0.4 0.78 0.20 120 90 2.8 E _P 1.2 0.4 60 80 12.1 10.9 D 0.15 0.23 120 80 4.5 P 0.6 0.4 60 80 11.6 10.6 19 D / P _cat 0.4 0.57 0.15 15 80 0.5 0.10 105 3.2 E _P 1.2 0.4 60 80 12.0 11.0 D 0.15 0.23 120 80 4.1 P 0.6 0.4 60 80 11.5 10.6 20 D / P _cat 0.4 0.57 0.15 15 70 0.5 0.15 105 3.2 E _P 1.2 0.4 60 80 12.0 10.8 D 0.15 0.23 120 80 4.1 P 0.6 0.4 60 80 11.5 10.6 * Not according to the invention Five-stage bleaching of pulp C with a sequence O-bleach E- _P- DP attempt step Kappa number Whiteness in% ISO Heat aging damp Heat aging dry Whiteness change in percentage points PC number Whiteness change in percentage points PC number 15 D / P _cat E _P 2.2 D P 89.8 -2.1 0.28 -1.7 0.22 16. * D E _P 2, 9 D P 89.9 -3.7 0.54 -2.6 0.36 17 D / P _cat E _P 2.0 D P 89.9 -1.4 0.18 -1.8 0.24 18 * D E _P 2.2 D P 89.7 -1.5 0.20 -1.8 0.24 19 D / P _cat E _P 2.6 D P 89.7 -1.6 0.21 -2.2 0.30 20 D / P _cat E _P 2.9 D P 89.6 -2.1 0.29 -2.4 0.34 * Not according to the invention

Four-step sequence O-bleaching step E _P -D / P:

In Examples Examples 21 to 24, pulp E was bleached with the amounts of chemicals listed in Table 7 at the conditions shown in the table using the additional bleaching stage of the invention. The properties of the resulting pulp are summarized in Table 8.

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. Table 7 Four-stage bleaching of pulp E with a sequence O-bleach E _P -D / P 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 21 D / P _cat 0.5 0.59 0.15 15 90 3.0 0.5 0.05 105 90 3.0 E _P 1.1 0.5 60 85 11.7 10.6 D / P 0.1 0.08 10 85 5.5 0.7 0.5 90 85 11.3 10.7 22 * DP _cat 0.5 0.5 0.05 0.59 0.15 120 90 3.1 E _P 1.1 0.5 60 85 11.7 10.6 D / P 0.1 0.08 10 85 5.5 0.7 0.5 90 85 10.6 23 * P _cat / D 0.6 0.5 0.05 105 90 3.5 0.59 0.15 15 90 3.0 E _P 1.1 0.5 60 85 11.6 10.5 D / P 0.1 0.08 10 85 5.7 0.7 0.5 90 85 10.6 24 * D 0.5 0.78 0.25 120 90 3.0 E _P 1.1 0.5 60 85 11.6 10.4 D / P 0.1 0.08 10 85 5.5 0.7 0.5 90 85 10.6 * Not according to the invention Four-stage bleaching of pulp E with a sequence O-bleach E _P -D / P attempt Whiteness in% ISO Heat aging damp Heat aging dry Whiteness change in percentage points PC number Whiteness change in percentage points PC number 21 90.1 -1.7 0.22 -1.7 0.22 22 * 89.7 -1.8 0.24 -2.3 0.31 23 * 88, 9 -3.4 0.53 -3.0 0.46 24 * 90.2 -4.3 0, 63 -2.4 0.32 * Not according to the invention

Bleaching sequence OD / P _cat -E _OP -D / P:

In Examples Examples 25 and 26, pulp F was bleached with the amounts of chemicals listed in Table 9 at the conditions shown in the table using the additional bleaching stage of the invention. The properties of the resulting pulp are summarized in Table 10.

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. Table 9 Bleaching of pulp F with an E _OP stage 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 C10 ₂ beginning The End 25 D / P _cat 0.3 0.61 0,125 15 90 3.2 0.5 0.05 105 90 3.5 E _OP 1.0 0.35 90 85 10.4 D / P 0.2 0.19 10 85 4.5 0.9 0.25 90 85 11.1 10.7 26 * D ₀ 0.5 0.98 0.20 100 90 3.0 E _OP 1.0 0.35 90 85 10.3 D ₁ 0.15 0.19 70 80 4.5 P 0.4 0.25 70 80 11.4 10.2 * Not according to the invention Four-stage bleaching of pulp F with a sequence O-bleach-E _OP -D / P attempt step Kappa number Whiteness in% ISO Viscosity in mPa * s Heat aging damp Heat aging dry Whiteness change in percentage points PC number Whiteness change in percentage points PC number 25 D / P _cat E _OP 2.7 D / P 89.5 26.0 -1.7 0.23 -1.7 0.23 26 * D ₀ E _OP 2.7 D ₁ P 89.7 20.1 -3.2 0.46 -2.9 0.41 * Not according to the invention

Claims (10)

1. Process for delignifying and bleaching chemical pulp, comprising a bleaching stage in which

   a) in a first step, chemical pulp is reacted in an aqueous mixture containing 3 to 30% by weight of chemical 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 from 2 to 7 until more than 90% of the chlorine dioxide has reacted and

   b) the mixture obtained in the first step is further reacted, without separating off constituents of the mixture, in a second step with 0.1 to 5% by weight of hydrogen peroxide in the presence of a molybdate in an amount of 10 to 2000 ppm of molybdenum or of a tungstate in an amount of 200 to 10 000 ppm of tungsten at a temperature of 50 to 150°C and a pH in the range from 2 to 7, the amounts being based in each case on the mass of dry chemical pulp used.
2. Process according to Claim 1, characterized in that the first and the second step are carried out at a temperature of 60 to 120°C, preferably 70 to 90°C.
3. Process according to Claim 1 or 2, characterized in that the mixture is reacted in the first step for 5 to 30 min, preferably 10 to 20 min.
4. Process according to any one of the preceding claims, characterized in that the mixture is reacted in the first step until the chlorine dioxide has completely reacted.
5. Process according to any one of the preceding claims, characterized in that the mixture obtained in the first step is reacted with hydrogen peroxide in the second step for 60 to 180 min, preferably 90 to 120 min.
6. Process according to any one of the preceding claims, characterized in that the bleaching stage is carried out continuously in an apparatus comprising a riser tube and a bleaching tower, wherein the aqueous mixture containing 3 to 30% by weight of chemical pulp is fed to the riser tube at the lower end, the chlorine dioxide is added to the mixture in the lower region of the riser tube, in the first step the mixture, after addition of the chlorine dioxide, flows through the riser tube in an ascending flow within 5 to 30 min, the resulting mixture is removed from the riser tube at the top and is fed to the bleaching tower at the top, the hydrogen peroxide is added to the resulting mixture in an upper region of the riser tube or at the top of the bleaching tower and in the second step the mixture, after addition of the hydrogen peroxide, flows through the bleaching tower in a descending flow within 60 to 180 min.
7. Process according to any one of the preceding claims, additionally comprising an extraction of the chemical pulp with an aqueous alkaline solution following the bleaching stage and subsequent thereto an additional bleaching stage in which

   a) in a first step, the chemical pulp is reacted in an aqueous mixture containing 3 to 30% by weight of chemical pulp with 0.04 to 0.4% by weight of chlorine dioxide, based on the mass of dry chemical pulp used, at a temperature of 50 to 150°C and a pH in the range from 2 to 7 until at least 90% of the chlorine dioxide used has reacted and

   b) the mixture obtained in the first step is further reacted, without separating off constituents of the mixture, in a second step with 0.1 to 5% by weight of hydrogen peroxide, based on the mass of dry chemical pulp used, at a temperature of 50 to 150°C and a pH in the range from 10 to 12.5.
8. Process according to Claim 7, characterized in that in the additional bleaching stage the mixture obtained in the first step is reacted in the second step for 60 to 180 min, preferably 90 to 120 min.
9. Process according to any of the preceding claims, characterized in that an aqueous solution containing molybdate or tungstate is separated from the mixture resulting in the second step of the bleaching stage, molybdate or tungstate being separated from this solution by the steps

   a) bringing the solution into contact with a water-insoluble, cationized inorganic support material at a pH in the range between 2 and 6 to give a support material laden with molybdate or tungstate and an aqueous solution depleted in molybdate or tungstate,

   b) separating the support material laden with molybdate or tungstate from the aqueous solution depleted in molybdate or tungstate,

   c) bringing the support material laden with molybdate or tungstate into contact with an aqueous solution at a pH in the range between 6 and 14 to give a support material depleted in molybdate or tungstate and an aqueous solution laden with molybdate or tungstate and

   d) separating the support material depleted in molybdate or tungstate from the aqueous solution laden with molybdate or tungstate,

   and the aqueous solution separated off in the last step and laden with molybdate or tungstate is recycled into the second step of the bleaching stage.
10. Process according to Claim 9, characterized in that a cationized phyllosilicate, preferably a bentonite ion-exchanged with a quaternary ammonium salt, is used as the cationized inorganic support material.