EP3066257B1 - Method for delignifying and bleaching pulp - Google Patents

Description

The invention relates to a process for the delignification and bleaching of pulp for which no other oxidizing agents are required in addition to oxygen and hydrogen peroxide.

For the production of paper pulp must be delignified and bleached after pulp cooking in several stages. While primary elemental chlorine was previously used for delignification and bleaching, bleaching sequences that use chlorine dioxide instead of elemental chlorine are now preferred in ECF bleaching (elemental chlorine-free bleaching). Most commonly used is the ODE _OP DP bleaching sequence, where O is an oxygen delignification under alkaline conditions, D is chlorine dioxide delignifying and bleaching agent, E _{OP is} an alkaline extraction with addition of oxygen and hydrogen peroxide, P denotes a bleach with hydrogen peroxide in the alkaline and takes place between the individual stages in each case a washing of the pulp. For the abbreviation of bleaching stages and bleaching sequences by means of letters, here and below the nomenclature rules of " Glossary of Bleaching Terms by the Bleaching Committee, Technical Section, Canadian Pulp and Paper Association (ISBN 1-895288-90-8 ) applied.

Chlorine dioxide has the disadvantage that it can not be transported or stored for a long time, so it has to be prepared for the bleaching of pulp in a separate plant in the pulp mill. Thus, in addition to the costs for the starting material sodium chlorate, there are also investment and operating costs for such a plant. In addition, chlorine dioxide also forms chlorinated compounds when delignified, which lead to an undesirable content of organochlorine substances in the pulp and in the wastewater.

In order to avoid these disadvantages of chlorine dioxide, the oxidizing agents ozone, percarboxylic acids such as peracetic acid and monoperoxy sulfuric acid have been used as alternative delignifying agents. These oxidizers allow bleaching sequences for a completely chlorine-free bleach (TCF bleach), but the bleached pulp bleached to a whiteness, as is common in ECF bleaching, has inferior mechanical properties, indicated by the significantly lower viscosity of the bleach bleached pulp. In addition, the cost of these delignifying agents is higher than for chlorine dioxide.

As a further alternative to chlorine dioxide, delignification with hydrogen peroxide in acid in the presence of molybdate or tungstate as catalyst has been proposed. US 4,427,490 describes delignification with hydrogen peroxide under acidic conditions in the presence of tungstate as a catalyst. Kubelka describes in Journal of Pulp and Paper Science Vol. 18 (1992), pages J108-J114 a delignification with hydrogen peroxide, which is carried out at a pH of 5 with sodium molybdate as catalyst. Out US 6,165,318 Heteropolytungstate and Heteropolymolybdate are known as catalysts for delignification with hydrogen peroxide in the acid.

It has now been found that by bleaching sequences comprising two bleaching stages with hydrogen peroxide in acid in the presence of a molybdate or a tungstate and an intermediate bleaching stage with hydrogen peroxide in the alkaline, a bleached pulp can be produced, which in comparison with a pulp bleached by ECF bleaching no disadvantages at whiteness and viscosity and less prone to yellowing. The invention therefore relates to a process for the delignification and bleaching of pulp comprising a first bleaching stage with hydrogen peroxide in the presence of a molybdate or a tungstate in an acidic aqueous mixture, a second bleaching stage subsequent to the first bleaching stage with hydrogen peroxide in an alkaline aqueous mixture and one of second bleaching step subsequent third bleaching step with hydrogen peroxide in the presence of a molybdate or a tungstate in an acidic aqueous mixture.

In the first bleaching stage of the process according to the invention, the pulp is reacted with hydrogen peroxide in the presence of a molybdate or a tungstate. Hydrogen peroxide is preferably used in an amount of from 0.1 to 5% by weight, based on the weight of dry pulp used. From 0.2 to 2% by weight and most preferably from 0.5 to 1% by weight of hydrogen peroxide are particularly preferably used. Hydrogen peroxide is preferably used in the form of an aqueous solution containing from 35 to 70% by weight of hydrogen peroxide.

The reaction with hydrogen peroxide takes place in the first bleaching stage 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 molybdate is used as the catalyst, the molybdate is preferably used in an amount of 10 to 2000 ppm, more preferably 100 to 1500 ppm, and most preferably 200 to 600 ppm of molybdenum, based on the weight of dry pulp. When using tungstate as a catalyst For example, tungstate is preferably used in an amount of 200 to 10,000 ppm, preferably 500 to 5000 ppm, and most preferably 1500 to 3000 ppm tungsten, based on the weight of dry pulp. The first bleaching stage is designated Pmo according to the above nomenclature rules when molybdate is used as the catalyst and Pw when tungstate is used as the catalyst.

The molybdate or tungstate used as a catalyst can be added before or after the hydrogen peroxide or simultaneously with the hydrogen peroxide. In a preferred embodiment, however, the molybdate or tungstate and the hydrogen peroxide are simultaneously but separately added in the form of two aqueous solutions.

By selecting the amounts of hydrogen peroxide and molybdate in the preferred ranges, a particularly efficient delignification and bleaching of the pulp is achieved and a pulp with reduced yellowing tendency is obtained.

In the first bleaching stage of the process according to the invention, the reaction of the pulp with hydrogen peroxide is preferably carried out at a temperature of from 50 to 150 ° C, more preferably from 60 to 120 ° C, and most preferably from 70 to 90 ° C. The reaction of the pulp with hydrogen peroxide is preferably carried out for a period of 60 to 180 minutes, more preferably 90 to 120 minutes.

The conversion of the pulp takes place in the first bleaching stage in an acidic aqueous mixture. Preferably, the reaction is carried out at a pH of the aqueous mixture in the range of 1 to 7, more preferably 2 to 5 and most preferably 2 to 4. The range for the pH refers to at the end of the bleaching stage at the temperature the reaction measured pH values. The pH of the aqueous mixture is preferably by addition of an inorganic acid, more preferably by addition of sulfuric acid or hydrochloric acid.

The conversion of the pulp is preferably carried out in the first bleaching stage at a consistency in the range of 3 to 30%, i. in an aqueous mixture containing from 3 to 30% by weight of pulp, calculated as dry pulp based on the total weight of the aqueous mixture. More preferably, the consistency is in the range of 5 to 20% and most preferably in the range of 8 to 15%.

In the second bleaching stage of the process according to the invention, the pulp is reacted with hydrogen peroxide in an alkaline aqueous mixture. Preferably, the reaction takes place at a pH of the aqueous mixture in the range between 7 and 12, more preferably 8 to 11 and most preferably 9 to 11. The range for the pH refers to at the end of the bleaching stage at the temperature the reaction measured pH values. The pH of the aqueous mixture is preferably adjusted by adding an inorganic base, more preferably by adding sodium hydroxide. Hydrogen peroxide is preferably used in an amount of from 0.1 to 5% by weight, based on the weight of dry pulp used. From 0.2 to 2% by weight and most preferably from 0.5 to 1% by weight of hydrogen peroxide are particularly preferably used. The reaction of the pulp with hydrogen peroxide is preferably carried out at a temperature of 50 to 100 ° C, more preferably from 60 to 100 ° C, and most preferably from 70 to 90 ° C. The second bleaching stage is designated Ep according to the abovementioned nomenclature rules if it predominantly causes extraction of lignin in the alkaline bleaching stage formed in the first bleaching stage and P if it causes predominantly bleaching of the pulp.

The second bleaching stage can be carried out with the addition of oxygen. Oxygen is preferably used in the form of substantially pure oxygen or in the form of oxygen-enriched air. When oxygen is added, the second bleaching step is preferably carried out at a pressure of from 0.1 to 1.5 MPa, more preferably from 0.3 to 1.0 MPa, and most preferably from 0.3 to 0.5 MPa. The second bleaching stage is designated by the above nomenclature rules with the addition of oxygen according to the above nomenclature rules when it mainly causes extraction of formed in the first bleaching stage, in the alkaline soluble degradation products of lignin, with Po, if it causes predominantly a bleaching of the pulp and with Op, if it mainly causes a delignification of the pulp.

The second bleaching stage can be carried out with the addition of a bleach catalyst, preferably with the addition of one of WO 97/44520 known manganese complexes. Particularly preferred is as a bleach catalyst from WO 97/44520 known dinuclear manganese complex having the formula (Me ₂ TACN) ₂ Mn ^III Mn ^IV (μ-0) ₂ (μ-OAc)] ²⁺ 2 X ^- used, where Me ₂ TACN for 1,2-bis (4,7 -dimethyl-1,4,7-triazacyclononan-1-yl) ethane, OAc for acetate and X ^{- represents} a monovalent anion. X ^- is preferably acetate, chloride or hexafluorophosphate.

In the third bleaching stage of the process according to the invention, the pulp is reacted once more with hydrogen peroxide in the presence of a molybdate or a tungstate in an acidic aqueous mixture. The preferred conditions for the reaction in the third bleaching stage correspond to the preferred conditions for the first bleaching stage. The third bleaching step can be carried out under the same conditions as the first bleaching step or under different conditions, for example with a smaller amount of hydrogen peroxide.

In a preferred embodiment of the process according to the invention, a complexing agent is added in the first bleaching stage, in the third bleaching stage or in the first and the third bleaching stage. 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. The complexing agents used are preferably aminocarboxylic acids or aminophosphonic acids, in particular 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, dipropylenetriaminepentamethylenephosphonic acid and 1-hydroxyethane. 1,1-diphosphonic acid, and their alkali metal salts. Further suitable complexing agents are ion exchangers based on bentonite, polyoxycarboxylate-polyacrylic acid copolymers, sodium imino succinate, aspartic acid diethoxysuccinate, iminodisuccinate, ethylenediamine disuccinate, methylglycinediacetic acid, nitrilotriacetic acid, modified anionic polyamine and polyhydroxyacrylic acid. Particularly preferred complexing agents are EDTA and DTPA and their sodium salts. Complexing agents are preferably used in an amount of from 0.05 to 1% by weight, based on the weight of dry pulp used. By adding a chelating agent, equal delignification and bleaching can be achieved for the same amount of hydrogen peroxide, or the amount of hydrogen peroxide required to achieve desired delignification and bleaching can be reduced.

In the three bleaching stages of the process according to the invention, it is possible, in addition to the substances already mentioned, to use further stabilizers known from the prior art for bleaching with hydrogen peroxide, for example waterglass and magnesium sulphate.

Preferably, the pulp is washed after the first bleaching stage and after the second bleaching stage. For this purpose, the mixture resulting from the bleaching stage is preferably dewatered by filtration with a drum filter, a filter press or a screw press and then water is added to adjust the desired consistency for the next bleaching stage. Alternatively or additionally, a displacement wash with water can be carried out on the filter. By washing the pulp, the consumption of bleaching agent and adjuvants for adjusting the pH in the second and third bleaching stages can be kept low.

The process according to the invention preferably comprises no further bleaching stages between the first and the second bleaching stage and between the second and the third bleaching stage. Corresponding preferred embodiments comprise the bleaching sequences PmoPPmo, PmoEpPmo, PmoPoPmo, PmoEopPmo, PmoOpPmo, PwPPw, PwEpPw, PwPoPw, PwEopPw and PwOpPw.

Preferably, the process according to the invention does not comprise any further bleaching stages in which an oxidizing agent other than hydrogen peroxide and oxygen is used. The limitation to hydrogen peroxide and oxygen as the oxidizing agent has the advantage that no toxic bleaching agents are required for the process and that only storable bleaching agents are used.

In a preferred embodiment, the process according to the invention comprises an additional stage of alkaline delignification of the first bleaching stage Pulp with oxygen, which is preferably carried out under pressure. Alkaline delignification with oxygen is preferably the first stage for delignification in the process. Corresponding preferred embodiments comprise the bleaching sequences OPmoPPmo, OPmoEpPmo, OPmoPoPmo, OPmoEopPmo, OPmoOpPmo, OPwPPw, OPwEpPw, OPwPoPw, OPwEopPw and OPwOpPw. By an upstream alkaline delignification with oxygen, the need for oxygen for the inventive method can be reduced. Suitable conditions for alkaline delignification with oxygen are known to those skilled in the art.

In a further preferred embodiment, the process according to the invention after the third bleaching stage comprises an additional bleaching stage with hydrogen peroxide in an alkaline aqueous mixture. The preferred conditions for the reaction in this additional bleaching stage correspond to the preferred conditions for the second bleaching stage. The additional bleaching stage can be carried out under the same conditions as the second bleaching stage or under different conditions, for example with additional addition of oxygen. Corresponding preferred embodiments include the bleaching sequences PmoPPmoP, PmoEpPmoP, PmoPoPmoP, PmoEopPmoP, PmoOpPmoP, PwPPwP, PwEpPwP, PwPoPwP, PwEopPwP, PwOpPwP, PmoPPmoPo, PmoEpPmoPo, PmoPoPmoPo, PmoEopPmoPo, PmoOpPmoPo, PwPPwPo, PwEpPwPo, PwPoPwPo, PwEopPwPo and PwOpPwPo. The additional bleaching step with hydrogen peroxide after the third bleaching stage is preferably combined with an alkaline delignification with oxygen carried out before the first bleaching stage. Corresponding preferred embodiments comprise the bleaching sequences OPmoPPmoP, OPmoEpPmoP, OPmoPoPmoP, OPmoEopPmoP, OPmoOpPmoP, OPwPPwP, OPwEpPwP, OPwPoPwP, OPwEopPwP, OPwOpPwP, OPmoPPmoPo, OPmoEpPmoPo, OPmoPoPmoPo, OPmoEopPmoPo, OPmoOpPmoPo, OPwPPwPo, OPwEpPwPo, OPwPoPwPo, OPwEopPwPo and OPwOpPwPo. Bleaching sequences of this embodiment are particularly useful for delignification and bleaching of softwood pulp.

In a likewise preferred embodiment, the process according to the invention before the first bleaching stage comprises an additional stage of acid hydrolysis with addition of at least one complexing agent. As complexing agents, the compounds mentioned above for the addition in the first or third bleaching stage can be used for this purpose. Complexing agents are preferably used in an amount of 0.01 to 1 wt .-%, particularly preferably 0.1 to 0.5 wt .-%, based on the weight of dry pulp used. The acidic hydrolysis is preferably carried out at a pH of the aqueous mixture in the range of 2 to 7, more preferably 3 to 6. The range for the pH value refers to pH values measured at the end of the hydrolysis stage at the temperature of the reaction. The pH is preferably adjusted by adding an inorganic acid, particularly preferably by adding sulfuric acid or hydrochloric acid. The acidic hydrolysis is preferably carried out at a temperature of 50 to 100 ° C, particularly preferably 60 to 90 ° C, preferably for a period of 60 to 480 minutes, more preferably 120 to 300 minutes, and preferably at a consistency in the range of 2 to 30%, more preferably 5 to 15%. Acid hydrolysis with the addition of a complexing agent is designated Aq according to the nomenclature rules mentioned above. Corresponding preferred embodiments include the bleaching sequences AqPmoPPmo, AqPmoEpPmo, AqPmoPoPmo, AqPmoEopPmo, AqPmoOpPmo, AqPwPPw, AqPwEpPw, AqPwPoPw, AqPwEopPw and AqPwOpPw. This embodiment can also be combined with the embodiment of a preceding alkaline delignification of the pulp with oxygen to the bleaching sequences OAqPmoPPmo, OAqPmoEpPmo, OAqPmoPoPmo, OAqPmoEopPmo, OAqPmoOpPmo, OAqPwPPw, OAqPwEpPw, OAqPwPoPw, OAqPwEopPw and OAqPwOpPw. Likewise, this embodiment can also be combined with the embodiment of a downstream additional alkaline bleaching stage with hydrogen peroxide to the bleaching sequences AqPmoPPmoP, AqPmoEpPmoP, AqPmoPoPmoP, AqPmoEopPmoP, AqPmoOpPmoP, AqPwPPwP, AqPwEpPwP, AqPwPoPwP, AqPwEopPwP, AqPwOpPwP, AqPmoPPmoPo, AqPmoEpPmoPo, AqPmoPoPmoPo, AqPmoEopPmoPo, AqPmoOpPmoPo, AqPwPPwPo, AqPwEpPwPo, AqPwPoPwPo, AqPwEopPwPo, AqPwOpPwPo, OAqPmoPPmoP, OAqPmoEpPmoP, OAqPmoPoPmoP, OAqPmoEopPmoP, OAqPmoOpPmoP, OAqPwPPwP, OAqPwEpPwP, OAqPwPoPwP, OAqPwEopPwP, OAqPwOpPwP, OAqPmoPPmoPo, OAqPmoEpPmoPo, OAqPmoPoPmoPo, OAqPmoEopPmoPo, OAqPmoOpPmoPo, OAqPwPPwPo, OAqPwEpPwPo, OAqPwPoPwPo, OAqPwEopPwPo and OAqPwOpPwPo. In addition, the acid hydrolysis step may be combined with the addition of a complexing agent with a subsequent alkaline bleaching stage with hydrogen peroxide to the bleaching sequences AqPPmoPPmo, AqPPmoEpPmo, AqPPmoPoPmo, AqPPmoEopPmo, AqPPmoOpPmo, AqPPwPPw, AqPPwEpPw, AqPPwPoPw, AqPPwEopPw, AqPPwOpPw, OAqPPmoPPmo, OAqPPmoEpPmo, OAqPPmoPoPmo , OAqPPmoEopPmo, OAqPPmoOpPmo, OAqPPwPPw, OAqPPwEpPw, OAqPPwPoPw, OAqPPwEopPw, OAqPPwOpPw, AqPPmoPPmoP, AqPPmoEpPmoP, AqPPmoPoPmoP, AqPPmoEopPmoP, AqPPmoOpPmoP, AqPPwPPwP, AqPPwEpPwP, AqPPwPoPwP, AqPPwEopPwP, AqPPwOpPwP, AqPPmoPPmoPo, AqPPmoEpPmoPo, AqPPmoPoPmoPo, AqPPmoEopPmoPo, AqPPmoOpPmoPo, AqPPwPPwPo, AqPPwEpPwPo, AqPPwPoPwPo , AqPPwEopPwPo, AqPPwOpPwPo, OAqPPmoPPmoP, OAqPPmoEpPmoP, OAqPPmoPoPmoP, OAqPPmoEopPmoP, OAqPPmoOpPmoP, OAqPPwPPwP, OAqPPwEpPwP, OAqPPwPoPwP, OAqPPwEopPwP, OAqPPwOpPwP, OAqPPmoPPmoPo, OAqPPmoEpPmoPo, OAqPPmoPoPmoPo, OAqPPmoEopPmoPo, OAqPPmoOpPmoPo, OAqPPwPPwPo, OAqPPwEpPwPo, OAqPPwPoPwPo, OAqPPwEopPwPo and OAqPPwOpPwPo. The use of an additional stage of acid hydrolysis with the addition of at least one complexing agent before the first bleaching stage has particular advantages in the bleaching of hardwood pulp and reduces the consumption of oxidizing agent in the subsequent bleaching stages.

1. a) Abtrennen des Zellstoffs von der wässrigen Mischung unter Erhalt einer Molybdat oder Wolframat enthaltenden wässrigen Lösung im Anschluss an die erste Bleichstufe, die dritte Bleichstufe oder die erste und die dritte Bleichstufe,
2. b) in Kontakt bringen der in Schritt a) erhaltenen, Molybdat oder Wolframat enthaltenden wässrigen Lösung mit einem wasserunlöslichen, kationisierten anorganischen Trägermaterial bei einem pH-Wert im Bereich zwischen 2 und 7 unter Erhalt eines mit Molybdat oder Wolframat beladenen Trägermaterials und einer an Molybdat oder Wolframat abgereicherten wässrigen Lösung,
3. c) Abtrennen des mit Molybdat oder Wolframat beladenen Trägermaterials von der an Molybdat oder Wolframat abgereicherten wässrigen Lösung,
4. d) in Kontakt bringen des mit Molybdat oder Wolframat beladenen Trägermaterials mit einer wässrigen Lösung bei einem pH-Wert im Bereich zwischen 7 und 14 unter Erhalt eines an Molybdat oder Wolframat abgereicherten Trägermaterials und einer mit Molybdat oder Wolframat beladenen wässrigen Lösung,
5. e) Abtrennen des an Molybdat oder Wolframat abgereicherten Trägermaterials von der mit Molybdat oder Wolframat beladenen wässrigen Lösung, und
6. f) Zurückführen der in Schritt d) erhaltenen, mit Molybdat oder Wolframat beladenen wässrigen Lösung in die erste Bleichstufe, die dritte Bleichstufe oder die erste und die dritte Bleichstufe.

The molybdate or tungstate used as the catalyst in the first and third bleaching stages of the process according to the invention is preferably recovered and recycled to the bleaching stages. These are especially suitable for WO 2009/133053 and WO 2013/110419 known methods. In a preferred embodiment, the method according to the invention therefore comprises the additional steps

1. a) separating the pulp from the aqueous mixture to obtain a molybdate or tungstate-containing aqueous solution following the first bleaching stage, the third bleaching stage or the first and third bleaching stages,
2. b) contacting the aqueous solution containing molybdate or tungstate obtained in step a) with a water-insoluble, cationized inorganic carrier material at a pH in the range between 2 and 7 to obtain a molybdate- or tungstate-loaded carrier material and a molybdate or molybdate Tungstate depleted aqueous solution,
3. c) separating the molybdate or tungstate loaded support material from the molybdate or tungstate depleted aqueous solution,
4. d) contacting the molybdate or tungstate loaded support material with an aqueous solution at a pH in the range of 7 to 14 to yield a molybdate or tungstate depleted support material and an aqueous solution laden with molybdate or tungstate;
5. e) separating the molybdate- or tungstate-depleted carrier material from the aqueous solution laden with molybdate or tungstate, and
6. f) recycling the molybdate or tungstate-laden aqueous solution obtained in step d) to the first bleaching stage, the third bleaching stage or the first and third bleaching stages.

In step a), the delignified pulp is separated from the mixture obtained in the first bleaching stage, the third bleaching stage or the first and the third bleaching stage to obtain a molybdate- or tungstate-containing aqueous solution. Preferably, the separation is carried out by filtration, in particular by filtration with a drum filter, a filter press or a screw press. Suitable filtration processes are known to those skilled in the art of pulp bleaching.

In step b), the aqueous solution containing molybdate or tungstate obtained in step a) is brought into contact with a water-insoluble, cationized inorganic carrier material at a pH in the range between 2 and 7. The pH is preferably adjusted to a value in the range of 3 to 5, more preferably in the range of 3.5 to 4. Adjusting a pH in these ranges allows almost complete recovery of molybdate or tungstate from the aqueous solution with little consumption of pH-adjusting agents. In contacting, the water-insoluble, cationized inorganic support material is preferably distributed with a stirrer or a dispersing agent in the aqueous solution containing molybdate or tungstate. The contacting can take place at any temperature, suitable temperatures in the range of 0 to 100 ° C. The cationized inorganic carrier material is in step b) in contact with the molybdate or Wolframat containing aqueous solution is preferably used in an amount of 10 to 1000 parts by weight of carrier material per part by weight of molybdenum or in an amount of 200 to 10,000 parts by weight of carrier material per part by weight of tungsten. For the recovery of molybdate, more preferably 50 to 500 and in particular 100 to 300 parts by weight of carrier material per part by weight of molybdenum are used. For the recovery of tungstate, particular preference is given to using from 1000 to 5000 and in particular from 2000 to 3000 parts by weight of carrier material per part by weight of tungsten.

Suitable cationized inorganic carrier materials are inorganic carrier materials whose surface has been modified with positively charged functional groups. The modification can be carried out, for example, by reacting the surface with reagents which covalently anchor a positively charged functional group to the surface. Suitable water-insoluble, cationized inorganic carrier materials having covalently anchored positively charged functional groups are, for example, aminosilane-modified precipitated or fumed silicas which are preferably additionally quaternized at the amino group. Alternatively, the modification can also be carried out by ion exchange of an inorganic support material negatively charged on the surface with a quaternary ammonium salt. The quaternary ammonium salt used for this purpose preferably has at least one nonpolar alkyl radical having 6 to 24, particularly preferably 12 to 22 carbon atoms, in order to prevent detachment of the quaternary ammonium ions from the carrier in the acid.

Preferably, a cationized phyllosilicate is used as the water-insoluble, cationized inorganic carrier material, particularly preferably an ion-exchanged with a quaternary ammonium salt Phyllosilicate. Kaolin, smectites, illites, bentonites (montmorillonites), hectorites, pyrophillites, attapulgites, sepiolites and Laponites, preferably bentonites ion exchanged with a quaternized ammonium salt, hectorites and attapulgites, more preferably with a quaternary ammonium salt of ion-exchanged bentonite, are suitable as layer silicates.

Bentonites, hectorites and attapulgites ion-exchanged with quaternized ammonium salts are commercially available: Quaternium-18 bentonite as Bentone 34 from Rheox Corp. and as Claytone 34, Claytone 40 and Claytone XL from Southern Clay; Stearalkonium bentonite as Tixogel LG from United Catalysts, as Bentone SD-2 from Elementis Specialties and as Claytone AF and Claytone APA from Southern Clay; Quaternium-18 / benzalkonium bentonite as Claytone GR, Claytone HT and Claytone PS from Southern Clay; Quaternium-18 hectorite as Bentone 38 from Rheox Corp .; Dihydrogenated Tallow Benzylmonium Hectorite as Bentone SD-3 from Rheox Corp .; Stearalkonium hectorite as Bentone 27 from Rheox Corp .; and cationized attapulgite as Vistrol 1265 from Cimbar. These ion-exchanged phyllosilicates can be used both as powders and in the form of the commercially available dispersions in an oil or an organic solvent.

In addition to the commercially available bentonites, hectorites and attapulgites ion-exchanged with tetraalkylammonium ions, it is also possible to use the corresponding materials exchanged with quaternized alkanolamine fatty acid esters, in particular with dimethyldiethanolammonium mono- and di-fatty acid esters, and methyltriethanolammonium mono-, di- and tri-fatty acid esters of ion-exchanged bentonite. Preferably, corresponding esters are used with saturated fatty acids, especially saturated fatty acids having 12 to 18 carbon atoms.

In step c), the carrier material loaded with molybdate or tungstate is separated from the aqueous solution depleted in molybdate or tungstate. The separation can be carried out with all known in the art solid-liquid separation method, for example by sedimentation, filtration, centrifugation or by flotation. The separated, molybdate- or tungstate-depleted carrier material may additionally be washed with an aqueous solution having a pH between 6 and 14 to complete the removal of molybdate or tungstate from the carrier material. The washing liquid resulting from washing is preferably combined with the aqueous solution laden with molybdate or tungstate.

In step d), the molybdate- or tungstate-loaded support material is contacted with an aqueous solution at a pH in the range of 7 to 14. The pH is preferably chosen in the range from 8 to 12 and particularly preferably in the range from 9 to 11. The contacting can take place at any temperature, suitable temperatures in the range of 0 to 100 ° C.

In step e), the molybdate- or tungstate-depleted carrier material is separated from the aqueous solution laden with molybdate or tungstate. The separation can be carried out with all known in the art solid-liquid separation method, for example by sedimentation, filtration, centrifugation or by flotation. The separated, molybdate- or tungstate-depleted carrier material may additionally be washed with an aqueous solution having a pH between 6 and 14 to complete the removal of molybdate or tungstate from the carrier material. The washing liquid resulting from washing is preferably mixed with the aqueous containing molybdate or tungstate Solution united. The molybdate or tungstate-depleted carrier material separated off in step e) is preferably used again in step b).

In a preferred embodiment, the water-insoluble, cationized inorganic carrier material is arranged in a fixed bed. Steps b) and c) are then carried out by passing the aqueous solution containing molybdate or tungstate through a fixed bed containing the water-insoluble, cationized inorganic support material. Already when passing the molybdate or tungstate-containing aqueous solution through the fixed bed, the molybdate or tungstate contained in the solution is bound to the water-insoluble, cationized inorganic support material and the aqueous solution leaving the fixed bed is depleted in molybdate or tungstate. After loading the water-insoluble, cationized inorganic carrier material arranged in the fixed bed, steps d) and e) are carried out by passing an aqueous solution which has a pH in the range from 6 to 14 through the process described in steps b) and c). passed with molybdate or tungstate loaded fixed bed. The aqueous solution leaving the fixed bed contains most of the molybdate or tungstate bound in step b) to the water-insoluble, cationized inorganic carrier material, and the fixed bed can again be used to recover molybdate or tungstate in steps b) and c) after carrying out these steps. be used. The passage of the molybdate or tungstate-containing aqueous solution through the fixed bed is preferably stopped before the content of molybdate or tungstate in the aqueous solution leaving the fixed bed rises above the desired residual content. In addition to the water-insoluble, cationized inorganic carrier material, the fixed bed preferably also contains a water-insoluble filling material for increasing the porosity of the fixed bed. suitable water-insoluble fillers are made WO 2009/133053 known. The fixed bed preferably contains the water-insoluble, cationized inorganic carrier material and the water-insoluble filler material in a weight ratio of 10: 1 to 1: 100. Preferably, at least two fixed beds connected in parallel are used, in which steps b) and c) and steps d) and e) are carried out alternately, ie in a first fixed bed in steps b) and c) the recovery of molybdate or tungstate from an aqueous solution, while in a parallel connected second, already loaded with molybdate or tungstate fixed bed in steps d) and e) the molybdate or tungstate is detached from the carrier again. In a particularly preferred embodiment, switching between the fixed beds arranged in parallel is then carried out in such a way that the passing through of the aqueous solution containing molybdate or tungstate is carried out continuously through a fixed bed.

In step f), the molybdate or tungstate-laden aqueous solution obtained in step d) is recycled to the first bleaching stage, the third bleaching stage or the first and third bleaching stages.

Preferably, in two parallel steps a), molybdate or tungstate is separated both from the aqueous mixture obtained in the first bleaching stage and from the aqueous mixture obtained in the third bleaching stage. The recovery of molybdate or tungstate may then be carried out in such a way that steps b) to f) are each carried out separately from one another with the aqueous solutions containing molybdate or tungstate obtained in the two steps a). In this embodiment, the aqueous solution laden with molybdate or tungstate is preferably returned to the bleaching stage in each step f), from which in the respective step a) the molybdate or tungstate was separated. Preferably, however, the aqueous solutions containing molybdate or tungstate obtained in two parallel steps a) are combined with one another, then steps b) to e) are carried out and, in step f), the aqueous solution obtained in step e) is charged with molybdate or tungstate split the desired amount of catalyst in the respective bleaching stages and recycled to the first and the third bleaching stage.

The following examples illustrate the invention without, however, limiting the scope of the invention.

Examples

All experiments were carried out with Kraft pulps delignified with oxygen under alkaline conditions. In Examples 1 to 4, an oxygen-delignified eucalyptus kraft pulp having a whiteness of 64.7% ISO was used, in Examples 5 and 6 an oxygen-delignified spruce kraft pulp having a brightness of 48.1% ISO.

The bleaching stages were in each case carried out at the fabric densities given in Tables 1 to 6 and under the specified test conditions by mixing the pulp with the appropriate amount of water and the amounts of bleaching chemicals indicated in the tables and in a plastic bag in a thermostated water bath on the specified Temperature was kept. In Examples 5 and 6, exceptionally, the alkaline, oxygen and peroxide-assisted extraction Eop, the peroxide-promoted oxygen stage Op, and the oxygen-supported peroxide stage Po were carried out in a high-shear mixer at the respectively indicated oxygen pressure. The specified Levels of bleaching chemicals are based on the mass of dry pulp used in the bleaching sequence. In EDTA, they refer to the amount of commercial 40 wt .-% aqueous solution used. For the catalyzed bleaching with hydrogen peroxide in the presence of molybdate, the catalyst used was sodium molybdate in the form of an aqueous solution. The pH values at the beginning of a bleaching stage were determined at room temperature, the pH values at the end of a bleaching stage at the temperature of the bleaching stage, each using a glass electrode combination electrode.

In each case, washes were carried out between the bleaching stages by addition of demineralized water to a consistency of 2% by weight, intensive stirring of the suspension obtained and separation of the pulp from this suspension by vacuum filtration and centrifugation. Table 1 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 1 with the bleaching sequence DEpDP bleaching stage Amounts used and bleaching parameters D Ep D P ClO ₂ in% by weight of active chlorine 2.6 0.5 H ₂ SO ₄ in% by weight 0.4 0.1 H ₂ O ₂ in% by weight 0.4 0.2 NaOH in% by weight 0.4 0.4 Temperature in ° C 90 85 80 80 Time in min 120 75 120 120 Consistency in% 10 10 10 10 pH onset 10.7 11.2 pH end 2.8 9.0 4.3 10.2 Bleaching of oxygen delignified eucalyptus kraft pulp in Example 2 with the bleach sequence AqPPmoPPmoP bleaching stage Amounts used and bleaching parameters Aq P pMO P pMO P H ₂ O ₂ in% by weight 2.0 0.5 2.0 0.1 2.0 H ₂ SO ₄ in% by weight 0.25 0.28 0.28 NaOH in% by weight 1.4 1.4 1.4 Mo in% by weight 0,025 0.01 EDTA in% by weight 0.2 0.1 0.1 Temperature in ° C 90 85 90 85 90 85 Time in min 300 90 120 90 60 90 Consistency in% 10 10 10 10 10 10 pH onset 4.2 11.6 3.5 11.8 3.5 11.5 pH end 4.0 10.7 4.0 10.5 4.1 10.7 Bleaching of oxygen-delignified eucalyptus kraft pulp in Example 3 with the bleaching sequence DEpDP bleaching stage Amounts used and bleaching parameters D Ep D P ClO ₂ in% by weight of active chlorine 1.86 0.2 H ₂ SO ₄ in% by weight 0.5 0.15 H ₂ O ₂ in% by weight 0.2 0.2 NaOH in% by weight 0.4 0.4 Temperature in ° C 90 85 80 80 Time in min 120 75 120 120 Consistency in% 10 10 10 10 pH onset 10.8 11.4 pH end 2.8 9.1 4.5 10.2 Bleaching of oxygen delignified eucalyptus kraft pulp in Example 4 with the bleach sequence AqPPmoPPmoP bleaching stage Amounts used and bleaching parameters Aq P pMO P pMO P H ₂ O ₂ in% by weight 0, 6 0.5 0, 6 0.5 0, 6 H ₂ SO ₄ in% by weight 0.25 0.35 0.35 NaOH in% by weight 1.3 1.3 1.3 Mo in% by weight 0.01 0.01 EDTA in% by weight 0.2 0.1 0.1 Temperature in ° C 90 85 90 85 90 85 Time in min 300 240 120 240 120 240 Consistency in% 10 10 10 10 10 10 pH onset 4.6 11.9 4.1 11.9 3.5 11.9 pH end 4.7 10.7 4.3 10.7 3.9 10.7 Bleaching of oxygen-delignified spruce kraft pulp in Example 5 with the bleaching sequence DEopD bleaching stage Amounts used and bleaching parameters D eop D ClO ₂ in% by weight of active chlorine 2.95 1.0 H ₂ SO ₄ in% by weight 0.15 0.03 H ₂ O ₂ in% by weight 0.5 NaOH in% by weight 1.0 O ₂ in MPa 0.3 MgSO ₄ in% by weight 0.1 Temperature in ° C 90 80 75 Time in min 60 90 120 Consistency in% 10 11 10 pH onset pH end 2.8 10.8 3.9 Bleaching of oxygen-delignified spruce kraft pulp in Example 6 with the bleaching sequence PmoOpPmoPo bleaching stage Amounts used and bleaching parameters pMO operating room pMO po H ₂ O ₂ in% by weight 0.5 0.7 0.5 2.9 H ₂ SO ₄ in% by weight 0.25 0.25 NaOH in% by weight 1.0 1.6 O ₂ in MPa 0.5 0.5 Mo in% by weight 0.02 0.02 EDTA in% by weight 0.1 0.1 MgSO ₄ in% by weight 0.1 0.15 Temperature in ° C 90 100 90 107 Time in min 120 75 120 160 Consistency in% 10 11 10 12 pH onset 4.7 - 4.3 - pH end 5.2 10.4 5.1 10.8

For the bleached pulp, the whiteness of the pulp was determined according to PAPTAC Standard E.1 and the viscosity of the pulp according to TAPPI Standard T 236 om 99. In addition, the whiteness loss due to heat aging and the post-color number (PC number) were determined by the methods TAPPI T 260 (wet) and TAPPI UM 200 (dry). The results are summarized in Table 7.

In trial pairs 1 and 2, 3 and 4, and 5 and 6, the conditions of the bleaching sequences were chosen so that the pulp was bleached to a comparable whiteness. For eucalyptus kraft pulp with the bleaching sequence according to the invention in comparison with the technically usual bleaching sequence with chlorine dioxide less fiber damage is achieved, recognizable by a higher viscosity. The pulps bleached by the process according to the invention also showed a better whiteness stability, ie a lower yellowing tendency, than the pulps bleached with chlorine dioxide. Table 7 Properties of the pulps bleached in Examples 1 to 6 example 1* 2 3 * 4 5 * 6 bleaching sequence DE _p DP A _q PP _mo PP _mo P DE _p DP A _q PP _mo PP _mo P DE _op D P _mo O _P P _mo P _o Whiteness in% ISO 91.8 91.4 90.1 89.5 87.3 87.2 Viscosity in mPa * s 14.4 16.0 15.0 17.2 15.2 12.7 Heat aging wet: Whiteness change in% ISO -2.5 -1.4 -2.7 -0.9 -3.4 -2.3 PC number 0,275 0,147 0.364 0,117 0,621 0.403 Heat aging dry: Whiteness change in% ISO -1.9 -2.0 -1.9 -1.4 -2.8 -2.3 PC number 0.201 0.224 0.245 0.188 0.498 0.403 * not according to the invention

Claims (12)

1. Process for the delignification and bleaching of pulp, comprising

   a) a first bleaching stage with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture,

   b) subsequent to the first bleaching stage, a second bleaching stage with hydrogen peroxide in an alkaline aqueous mixture, and

   c) subsequent to the second bleaching stage, a third bleaching stage with hydrogen peroxide in the presence of a molybdate or tungstate in an acidic aqueous mixture.
2. Process according to Claim 1, characterized in that in the first and third bleaching stages the pulp is reacted with 0.1 to 5 wt% of hydrogen peroxide in the presence of 10 to 2000 ppm of molybdenum in the form of molybdate or 200 to 10 000 ppm of tungsten in the form of tungstate, based in each case on the mass of dry pulp, at a temperature of 50 to 150°C and at a pH in the range from 1 to 7, and in the second bleaching stage the pulp is reacted with 0.1 to 5 wt% of hydrogen peroxide, based on the mass of dry pulp, at a temperature of 50 to 100°C and at a pH in the range between 7 and 12.
3. Process according to Claim 1 or 2, characterized in
   that the pulp is washed after the first bleaching
   stage and after the second bleaching stage.
4. Process according to any one of Claims 1 to 3, characterized in that it comprises no further bleaching stages between the first and second bleaching stages and between the second and third bleaching stages.
5. Process according to any one of Claims 1 to 4, characterized in that it comprises no further bleaching stages in which an oxidizing agent other than hydrogen peroxide and oxygen is used.
6. Process according to any one of Claims 1 to 5, characterized in that the second bleaching stage is carried out with addition of oxygen at a pressure of 0.1 to 1.5 MPa.
7. Process according to any one of Claims 1 to 6, characterized in that in the first bleaching stage, in the third bleaching stage or in the first and third bleaching stages a complexing agent is added.
8. Process according to any one of Claims 1 to 7, characterized in that it comprises an additional stage of alkaline delignification of the pulp with oxygen before the first bleaching stage.
9. Process according to any one of Claims 1 to 8, characterized in that it comprises an additional bleaching stage with hydrogen peroxide in an alkaline aqueous mixture after the third bleaching stage.
10. Process according to any one of Claims 1 to 9, characterized in that it comprises an additional stage of acidic hydrolysis with addition of a complexing agent before the first bleaching stage.
11. Process according to any one of Claims 1 to 10, comprising the additional steps of

    a) separating the pulp from the aqueous mixture subsequent to the first bleaching stage, the third bleaching stage or the first and third bleaching stages to give a molybdate- or tungstate-containing aqueous solution,

    b) contacting the molybdate- or tungstate-containing aqueous solution obtained in step a) with a water-insoluble, cationized inorganic carrier material at a pH in the range between 2 and 7, to give a molybdate- or tungstate-laden carrier material and a molybdate- or tungstate-depleted aqueous solution,

    c) separating the molybdate- or tungstate-laden carrier material from the molybdate- or tungstate-depleted aqueous solution,

    d) contacting the molybdate- or tungstate-laden carrier material with an aqueous solution at a pH in the range between 7 and 14, to give a molybdate- or tungstate-depleted carrier material and a molybdate- or tungstate-laden aqueous solution,

    e) separating the molybdate- or tungstate-depleted carrier material from the molybdate- or tungstate-laden aqueous solution, and

    f) returning the molybdate- or tungstate-laden aqueous solution obtained in step d) to the first bleaching stage, the third bleaching stage or the first and third bleaching stages.
12. Process according to Claim 11, characterized in that the molybdate- or tungstate-depleted carrier material separated in step e) is used again in step b) .