METHOD OF BLEACHING PULP WITH CHLORINE-FREE CHEMICALS

Description

[0001] The present invention relates to a method of bleaching kraft pulp containing heavy metals like manganese by using chlorine-free chemicals. The invention is especially related to ozone bleaching of pulp without first actually removing heavy metals.

[0002] Different regulations and marketing requirements have set higher and higher demands on producers of kraft pulp to decrease or completely eliminate organic chlorine compounds in pulp products and effluents from bleaching. In order to be able to fulfill these demands the use of chlorine gas or any other compound containing chlorine (e.g. chlorine dioxide) should be avoided. When avoiding the use of chlorine-based bleaching chemicals, it is extremely difficult to obtain the desired brightness especially if the manufactured pulp has been given acceptable strength requirements. Consequently, lignin must be removed, for example, with oxygen. By using multi-stage oxygen delignification advantages have been achieved in delignification and in selectivity, especially when chelating is added to limit the amount of harmful metallic ions, and especially when between stage washing is included in the process (see US-A-4,946,556). Practical hindrances, however, restrict both the delignification and the quality of the pulp produced merely by oxygen delignification, especially if followed by an ozone bleaching stage.

[0003] So far a common and typical purpose of chlorine-free bleaching methods has been to remove heavy metals from the pulp as completely as possible prior to ozone treatment, since heavy metals are known to destroy ozone as taught by EP-A-0 512 590. Typical bleaching sequences by which pulp has earlier been bleached, are, for example, OOAZEZPZ, OAZEZPZ, OOAZEZP and OAZEZP. These sequences thus include one or more oxygen bleaching stages (O), an A-stage (acid washing), an ozone stage (Z), an extraction stage (E), a second ozone stage (Z) and a peroxide stage (P), and possibly a third ozone stage (Z). In the acid stage (A) prior to the first ozone stage heavy metals are removed, which are flushed away, when a portion of the wash filtrate is removed. The extraction stage (E) may be an oxidizing peroxide extraction stage or a conventional oxidizing extraction stage. The ozone bleaching stages are preferably carried out with pulp having the consistency of about 5-18%.

[0004] It is characteristic of the above mentioned sequences that they include at least five washing stages, in other words bleaching stages alternate with washing stages, i.e. washers, by which the chemicals separated from the fibers as reaction products or otherwise in each bleaching stage are removed from the suspension. Since the washers form a considerable part of the investment costs in a bleaching plant, the number of the washers should, of course, be limited as much as possible, if it is only possible without risking the quality of the final product.

[0005] In the same connection chemicals necessary for some bleaching reactions, such as magnesium (Mg), can also be removed from the pulp, which requires addition of magnesium subsequent to the ozone treatment.

[0006] Other multi-stage bleaching sequences have been described for instance in EP patent applications EP-A-0 426 652, EP-A-0 512 978 and EP-A-0 402 335.

[0007] EP-A-0 426 652 teaches in Example 7 a first bleaching stage with ozone, followed by a bleaching with peroxide and oxygen, then a further bleaching with ozone. In the final stage a peroxide treatment is performed. The treated pulp is a kraft pulp. The first bleaching stage encompasses the removal of metals.

[0008] EP-A-0 402 335 describes the treatment of a pulp with a complexing agent before the peroxide step to remove metals.

[0009] According to EP-A-0 512 978 the pulp is first treated with enzymes, followed by a treatment with oxygen. In a further stage the treatment with peroxide is performed.

[0010] EP-A-0 426 652 discloses several bleaching sequences where ozone has been used in combination with peroxide and oxygen. For instance, sequences like EOP-Z-PE-Z-PE, EOP-Z-PE, O-Z-EO-Z-P, O-Z-EOP-Z-P and O-Z-EO-Z-D were discussed. However, the publication does not take into account the presence of heavy metals in the pulp in spite of the fact that at least one of the examples discusses bleaching of kraft pulp by means of a peroxide containing compound.

[0011] EP-A-0 512 978 discloses a multi-stage bleaching process using both ozone and peroxide for bleaching pulp. The document does not pay any attention to the removal of heavy metals from pulp before bleaching with peroxide.

[0012] EP-A-0 402 335 discloses a process for bleaching lignocellulose-containing pulps by means of peroxide containing substance. The main teaching of the document is to alter the trace metal profile of the pulp by means of a separate treatment with a complexing agent. In other words, the document teaches the importance of treating pulp prior to a peroxide stage in a separate metal removing stage. The document further teaches that the treatment comprises a washing stage between the treatment with complex formers and the peroxide stage.

[0013] On the other hand, the previously used chlorine has also prevented a screening stage subsequent to bleaching from being combined with the bleaching plant, because in chlorine bleaching the screening and/or vortex cleaning treatment of pulp would lead to serious corrosion problems. Thus it has been necessary to carry out the vortex cleaning and/or screening as a separate stage. A conventional bleaching + screening plant has thus included four or five bleaching stages and one screening and/or vortex cleaning stage, which the washing stage separates from each other.

[0014] The present invention compresses these five to six stages into three stages and thus almost halves the investment costs of a bleaching plant and a screening plant.

[0015] By utilizing the present invention it is possible to eliminate the above mentioned disadvantages occuring with the removal of heavy metals and the omission of screening in the prior art technique. At the same time a bleaching plant is provided, which includes only three washing stages.

[0016] It is characteristic of the method in accordance with the present invention that pulp is bleached with a sequence beginning with ozone and without first actually removing heavy metals preceding the ozone stage.

[0017] It is characteristic of another embodiment of the method in accordance with the present invention that kraft pulp is bleached with a three-stage sequence (ZT) (EOP) (ZP) without an actual removal of heavy metals preceding the sequence, in which

• (ZT) refers to a bleaching stage with ozone, which also includes treatment of heavy metals and which stage is followed by washing and/or thickening,
• (EOP) refers to a bleaching stage with peroxide or oxygen and peroxide in alkali conditions and the stage is followed by washing and/or thickening,
• (ZP) refers to bleaching stage with ozone and peroxide without a between stage washing and which stage is followed by washing and/or thickening.

[0018] The Z portion of the ZT stage employs ozone as a primary bleaching chemical and the T portion represents the actual metal removal by the use of chelating agents.

[0019] In each of the stages (ZT), (EOP), and (ZP) there is no interstage washing.

Fig. 1 schematically illustrates a bleaching sequence in accordance with a first exemplary embodiment of the invention;

Fig. 2 schematically illustrates a bleaching sequence in accordance with a second embodiment of the invention;

Fig. 3 schematically illustrates the later part of a bleaching sequence in accordance with a third embodiment of the invention;

Fig. 4 schematically illustrates another bleaching sequence for comparative purposes , and especially portions thereof where acid and/or alkali may be added;

Fig. 5 schematically illustrates a recirculation method for washing filtrates in accordance with another embodiment of the invention.

[0020] According to the embodiment of Fig. 1, a bleaching sequence is provided using a high consistency pulp tower 10, from which pulp is discharged and fed, for example, with an MC® (i.e. fluidizing) pump 12 into an ozone reactor 14, from which kraft pulp is preferably discharged by means of a gas separator 16 into a first reaction tower 18. Pulp is preferably discharged from tower 18 into a washer 22 by means of an MC® pump 20. The washer may be a conventional DRUM DISPLACER™ washer or a conventional pressure diffuser. Pulp is preferably pumped from washer 22 by an MC® pump 24 into can oxygen reactor 26, and from reactor 26 into a second reaction tower 28. From tower 28 pulp is fed, preferably by an MC® pump 30, to a second washer 32 (preferably a DRUM DISPLACER™ washer), from which pulp is further pumped with an MC® pump 34 into a second ozone reactor 36 and further therefrom through a gas separator 38 to a third reaction tower 40. From tower 40 pulp is pumped with an MC® pump 42 into a third washer 44 (preferably a DRUM DISPLACER™ washer).

[0021] As also seen in Fig. 1, ozone (in a carrier gas) is mixed with pulp by a mixer 60 prior to the first ozone reactor 14. Similarly, oxygen may be mixed prior to the oxygen reactor 26 with mixer 62 and the mixture of ozone and carrier gas prior to the second ozone reactor 36. The mixers 60, 62 and 64 are preferably AHLMIXER™ type fluidizing mixers, which are able to mix very large amounts of gas into fiber suspensions, including medium consistency suspensions.

[0022] Furthermore Fig. 1 teaches how, in order to adjust the pH value of the pulp for the first ozone stage and the removal of heavy metals subsequent thereto, acid may be supplied into the pulp, for example, in the pump 12. Similarly, prior to the removal of heavy metals in the first reaction tower 18 complex formers, such as EDTA, and/or alkali may be added to the pulp. If too much magnesium is removed from the pulp by washer 22, it may be added, for example, with the alkali either in pump 24 and/or in the discharge from the oxygen reactor 26, or in any other appropriate way. Another possibility to adjust the pH of the pulp for the second ozone treatment is to feed acid subsequent to the second washer 32 in pump 34 or in some other suitable way. Also prior to feeding the pulp into the third reaction tower 38 alkali, peroxide and/or magnesium may be added into the pulp, as illustrated in Fig. 1.

[0023] All the reaction towers 18, 28 and 40 in Fig. 1 are shown as of the down flow type. Alternatively, they may be of the up flow type, as is shown in Fig. 2. The only significant difference between Figs. 1 and 2 is the flow direction of the reaction towers. In the Fig. 2 embodiment components functionally equivalent to or the same as the components in the Fig. 1 embodiment are illustrated by the same reference numeral, only preceded by a "1". Also pumps 20, 30 and 42 of Fig. 1 are replaced by pumps 120', 130' and 142 of Fig. 2, because they have been relocated at the other side of the reaction tower, in other words instead of feeding washers 22, 32 and 44 as illustrated in Fig. 1 they feed pulp to the reaction towers 118, 128 and 140 in the embodiments of Fig. 2.

[0024] In the process in accordance with the invention pulp is cooked, for example with a continuous EMCC digester, sold by Kamyr Inc., of Glens Falls, New York, to a low kappa number, whereafter the kappa number is further decreased by oxygen bleaching to a value of about 14 or below. By utilizing efficient cooking, hot alkali extraction and oxygen bleaching a kappa range of 14 - 5 is obtained both with soft wood and birch. Usually a kappa number of 14 is sufficient in order to carry out the final bleaching with chlorine-free bleaching chemicals and yet reach a full brightness defined by ISO 86 (preferably ISO 88). Thus also the method in accordance with the present invention may be utilized succesfully to finally bleach pulp prebleached to kappa number 14.

[0025] The kraft pulp is bleached subsequent to the pulp washing and according to the invention without a separate removal of heavy metals, for example, with a sequence in accordance with Fig. 1, which is described more in detail below. Pulp may be treated, if so required, with enzymes prior to the sequence in accordance with the present invention. Pulp is brought from the high consistency pulp tower 10 to the first bleaching stage, which is a (ZT) stage. In the Z portion of the (ZT) stage pulp is bleached with ozone, the dosing being about 2-10 kg/adt, pH about 2- 7, and temperature about 40-70°C. The pH value of pulp is adjusted by adding acid to the bottom of the high consistency tower 10, pump 12 (or the discharge to pump 12 as seen in Fig. 1), or chemical mixer 60. The ozone having reacted, the residual gas is removed from the pulp preferably in a gas separator 16 and the treatment of heavy metals begins in the first reaction tower 18 in the T portion of the (ZT) stage.

[0026] A way to carry out the T portion of the (ZT) stage is to use complex formers, for example, EDTA. The T portion of the (ZT) stage is then carried out in the pH range of about 4-7 and it is advantageous also to have the pH of Z portion of the (ZT) stage preceding T portion above 4. In this way, manganese (which is harmful in the oxygen stages) may be washed off without the magnesium being discharged, so that less magnesium addition (or even no magnesium addition) is necessary in the oxygen and peroxide stages of the final bleaching.

[0027] As is described above, the actual metal treatment (T portion of the (ZT) stage) is not carried out prior to the Z portion. Previously this has been considered necessary. Tests with medium consistency ozone treatment have shown that the bleaching reactions with ozone are so rapid that heavy metals do not have time to destroy any significant amount of ozone.

[0028] If so required also enzymes may be added in the T portion of the (ZT) stage. Filtrate S₁ of the washer 22 subsequent to the (ZT) stage may be brought to pulp washing prior to the (ZT) stage, or passed to the sewer or to the recovery of cooking chemicals.

[0029] The (ZT) stage is followed in Fig. 1 by an (EOP) stage. In this stage the oxygen dose is about 2-6 kg/adt and the peroxide dose about 10-20 kg/adt. In some special cases it is possible to run the process completely without oxygen. Temperature in the (EOP) stage is about 60-95°C, pH about 9-12, and the duration is about 2-8 hours. If required, magnesium may be added as a protective chemical. The (EOP) stage is followed by washing, which gives filtrate S₂. The filtrate S₂ may be taken to pulp washing prior to or subsequent to the (ZT) stage, to sewer, or to recovery of chemicals.

[0030] The (EOP) stage is followed in Fig. 1 by a second ozone bleaching stage, i.e. an (ZP) stage. The ozone portion of the (ZP) stage is normally carried out in the processes in accordance with the prior art in cold, acid conditions in order to have the ozone react properly. Correspondingly, the P portion of the (ZP) stage is carried out according to the teachings of the prior art in hot, alkali conditions in order to have the peroxide react properly. Thus the combination thereof in an economically advantageous way according to the present concepts is conventionally considered impossible. However, this is possible, if the following conditions are utilized in the (ZP) stage:

[0031] In the Z portion of the (ZP) stage the ozone dose is small, below 3 kg/adt and the purpose of the ozone is only to activate. Although disadvantageous conditions are used and a part of the ozone reacts poorly, this is insignificant, because the dose is small. Ozone is thus mainly used for the activation of the bleaching stage. The temperature in the ozone stage may be 50-80°C, preferably, for example, 60-70°C. The pH is 4-10, preferably about 6- 10.

[0032] The dose in the P portion of the (ZP) stage is also small, usually less than 10 kg/adt. Normally about 3-7 kg/adt is sufficient. Thus the temperature in the peroxide stage may be dropped to the range of 60-80°C, preferably to 70-80°C. The pH is 9-11, preferably about 10. The duration is about 1-6 hours.

[0033] Thus the conditions of Z and P portions of the (ZP) stage are brought close to each other and washing and heating between the Z and P portions of the (ZP) stage are avoided. Moreover, small acid and alkali amounts are sufficient for the pH-control in the Z and P portions of the (ZP) stage. In some cases no between stage heating and/or acid is/are required.

[0034] After the (ZP) stage the pulp is washed and a filtrate S₃ is obtained. The filtrate S₃ may be used for the washing of pulp in connection with the earlier bleaching stages, discharged to the sewer, or led to the recovery of cooking chemicals.

[0035] According to yet another embodiment, shown in Fig. 3, the process in accordance with the present invention is significantly changed by repositioning of the equipment. For example, a vortex cleaner 66 and/or a screening plant may be added according to Fig. 3 to the last stage of the bleaching plant to precede the thickener/washer 68, which in this case does not have to be an MC® washer, as in the earlier embodiments. Pulp is diluted to the consistency range of about 0.5-1.5 % after the P-tower 140, when vortex cleaning or screening with a slotted screen is used. On the other hand, when screening with a perforated screen, a dilution to about 2-4% is usually sufficient. Subsequent to vortex cleaning or screening, the pulp is thickened and washed - usually with a suction filter 68. Previously pulp had to be diluted after washing to a screening consistency and thickened again after screening to a medium consistency.

[0036] In the embodiment illustrated in Fig. 4 which is not according to the invention, no EDTA is used, but the removal of metals is carried out with acid in a Z stage and by adding magnesium to the (EOP) and (ZP) stages. The addition may well be done in the (EOP) stage in an MC® pump 224, or in an oxygen mixer 262. This may also well be done in the (ZP) stage in an MC® pump 234, an ozone mixer 264, or in a peroxide mixer 70. The necessary total chemical amounts are given in Table 1. Thus the initial kappa number prior to bleaching is presumed to be 10.

Table 1

Stage	Chemical consumption adt		Duration min.	Temperature °C
Z	O3	0.5	2	40 - 50
E	O2	0.4	60	80 - 90
P	H2O2	1.5	180	80 - 90
	MgSO4	0.4
Z	O3	0.1	2	70
P	H2O2	0.4	180	70
	MgSO4	0.3

[0037] In the practice of the method set forth in Table I, about 20-30 kg of NaOH/adt, and 15-25 kg of H₂SO₄/adt is consumed, depending upon water usage.

[0038] In addition to the chemicals of Table 1 bleaching may be intensified by utilizing enzymes. Appropriate places for the enzyme treatment are:

• HD-tower 210 prior to the (ZT) stage,
• Drop leg 218 subsequent to the Z reactor,
• Drop leg 72 between washer 222 and the (EOP) stage,
• Drop leg 74 between washer 232 and the (ZP) stage.

[0039] The effluent flows from the bleaching plant may be decreased by recirculating the filtrates within the process according to Fig. 5. Figure 5 illustrates an oxygen delignification stage 80, which is followed by a two-stage washing 82. Pulp is transferred from the washing stage to the (ZT) stage 83, and from there via washing 84 to (EOP) stage 85, and from there via washing 86 to the (ZP) stage 87, which is followed by a washing stage 88. The amount of effluent, which is brought to the effluent clarification, discharge channel 90, is 0-5 m3/adt. Part of the effluent may alternatively be transferred to the manufacture of cooking chemicals, via discharge channel 92, to be used instead of fresh water. Thus the amount of effluent that must be treated is minimized.

[0040] As may be seen from the above description, a new method has been developed for bleaching pulp with chlorine-free chemicals in a short sequence without the removal of heavy metals preceding the bleaching sequence. The present invention also includes a new method of arranging the screening subsequent to the pulp bleaching in such a way that a separate washing between the last washing stage and screening/vortex cleaning is unnecessary, but only dilution to screening/cleaning consistency is needed.

Claims

1. A method of bleaching kraft pulp containing heavy metals like manganese to a brightness of at least about 86 ISO without the use of chlorine bleaching chemicals, comprising the steps of:

a) providing a kraft pulp of a cooked and delignified comminuted cellulosic fibrous material, having a kappa no. of about 14 or less;

b) optionally treating said pulp with enzymes;

c) optionally performing a washing stage with the pulp obtained in step a) or b);

d) then, without first actually removing the heavy metals, bleaching the pulp in a plurality of stages, the first stage being a (ZT) stage, wherein the (Z) portion employs ozone as a primary bleaching chemical and wherein the (T) portion represents the actual metal removal by the use of chelating agents with no interstage washing in the (ZT) stage.

2. A method as recited in claim 1, characterized in practising the bleaching in a sub-sequence (ZT) (EOP) (ZP).

3. A method as recited in claim 1, characterized in that, in the (ZT) stage about 2-10 kg of ozone is provided per air dried ton of pulp.

4. A method as recited in claim 1, characterized in that the pH of the pulp during the (ZT) stage is about 2-7, and the temperature is between about 40-70°C.

5. A method as recited in claim 1, characterized in that the pH during the T portion of the (ZT) stage is about 4-7.

6. A method as recited in claim 1 or 5, characterized by adding said heavy metals chelating agents to the pulp during or prior to said metals removal in the (ZT) stage.

7. A method as recited in claim 2, characterized in that, during the (EOP) stage, the amount of peroxide added is about 10-20 kg/adt.

8. A method as recited in claim 7, characterized in that the amount of oxygen added is about 2-6 kg/adt.

9. A method as recited in claim 2, characterized in that, during the practice of the (EOP) stage, the pH of the pulp is maintained within the range of about 9 to 12, the temperature within the range of about 60-95°C, and the treatment time is about 2-8 hours.

10. A method as recited in claim 2, characterized by the further step of adding magnesium to the (EOP) stage.

11. A method as recited in claim 2, characterized in that, during the practice of the (ZP) stage, the amount of ozone added is less than about 3 kg/adt, and the amount of peroxide added is less than about 10 kg/adt.

12. A method as recited in claim 11, characterized in that the amount of peroxide added in the (ZP) stage is about 3-7 kg/adt.

13. A method as recited in claim 2, characterized in that the pulp is immediately screened after the (ZP) stage, and then is washed following the screening.

14. A method as recited in claim 13, characterized in that screening is accomplished by diluting the pulp to a consistency of about 0.5-1.5%, and then treating it in a vortex cleaner (66).

15. A method as recited in claim 13, characterized in that the pulp is diluted to a consistency of about 0.5-4% prior to screening, and that screening is practiced at said consistency.

16. A method as recited in claim 1, characterized in that enzymes are added to the pulp prior to the (ZT) stage.

17. A method as recited in claim 1, characterized in that during the practice of said at least one bleaching stage at least one filtrate is produced, and that the pH of the filtrate is adjusted by adding alkali or acid thereto.

18. A method as recited in claim 1, characterized in that during the (ZT) stage, after treatment of the pulp with ozone it is treated with a filtrate having a pH which is acidic enough to prevent rethickening of heavy metals.

19. A method as recited in claim 18, characterized in that the filtrate is maintained acidic enough to prevent rethickening of heavy metals by adding acid thereto.

20. A method as recited in claim 18, characterized in that the pH of the filtrate remains below 4, and the pulp has a pH below 4 during the treatment of the pulp with acidic filtrate.

21. A method as recited in claim 1, characterized in that the pulp of step (a) has been delignified in an oxygen stage prior to said (ZT) stage.

22. A method as recited in claim 1, characterized in that the pulp of step (a) has been delignified by using hot alkali extraction or has been treated with enzymes in said step b).

Ansprüche

1. Verfahren fürs Bleichen von Schwermetalle wie Mangan enthaltendem Kraftzellstoff auf einen Reflexionsfakior im Blauen von zumindest ungefähr 86 ISO ohne Verwendung von Chlor-Bleichchemikalien, welches Verfahren folgende Schritte umfaßt:

a) Bereitstellen eines Kraftzellstoffs eines aufgeschlossenen und delignifizierten zerkleinerten cellulosehaltigen Fasermaterials mit einer Kappazahl von ungefähr 14 oder darunter;

b) wahlweise Behandlung besagten Zellstoffs mit Enzymen;

c) wahlweise Durchführung einer Waschstufe mit dem in Schritt a) oder b) erhaltenen Zellstoff;

d) anschließend, ohne vorangehende eigentliche Entfernung von Schwermetallen, Bleichen des Zellstoffs in einer Vielzahl von Stufen, wobei die erste Stufe eine (ZT)-Stufe ist, wo der (Z)-Abschnitt Ozon als hauptsächliche Bleichchemikalie verwendet und wo der (T)-Abschnitt die tatsächliche Metallbeseitigung durch die Verwendung von Chelatbildnern, ohne Zwischenwäsche in der (ZT)-Stufe, darstellt.

2. Verfahren nach Anspruch 1, gekennzeichnet durch die Durchführung der Bleiche in einer Subsequenz (ZT) (EOP) (ZP).

3. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß in der (ZT)-Stufe ungefähr 2-10 kg Ozon pro Tonne luftgetrockneten Zellstoffs vorgesehen werden.

4. Verfahren nach Anspruch, 1, dadurch gekennzeichnet, daß der pH des Zellstoffs während der (ZT)-Stufe ungefähr 2-7 und die Temperatur zwischen ungefähr 40-70 °C ist.

5. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der pH im T-Abschnitt der (ZT)-Stufe ungefähr 4-7 ist.

6. Verfahren nach Anspruch 1 oder 5, gekennzeichnet durch Zugabe besagter Schwermetall-Chelatbildner zum Zellstoff während oder vor der Metallbeseitigung in der (ZT-Stufe).

7. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß während der (EOP)-Stufe die Menge des zugesetzten Peroxids ungefähr 10-20 kg/adt ist.

8. Verfahren nach Anspruch 7, dadurch gekennzeichnet, daß die Menge des zugesetzten Sauerstoffs ungefähr 2-6 kg/adt ist.

9. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß während der Durchführung der (EOP)-Stufe der pH des Zellstoffs in einem Bereich von ungefähr 9 bis 12, die Temperatur in einem Bereich von ungefähr 60-95 °C gehalten wird, und die Behandlungszeit ungefähr 2-8 Stunden beträgt.

10. Verfahren nach Anspruch 2, gekennzeichnet durch den weiteren Schritt des Zusatzes von Magnesium zur (EOP)-Stufe,

11. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß, während der Durchführung der (ZP)-Stufe, die Menge an zugesetztem Ozon kleiner als ungefähr 3 kg/adt und die Menge an zugesetztem Peroxid kleiner als ungefähr 10 kg/adt ist.

12. Verfahren nach Anspruch 11, dadurch gekennzeichnet, daß die Menge an in der (ZP)-Stufe zugesetztem Peroxid ungefähr 3-7 kg/adt ist.

13. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß der Zellstoff unmittelbar nach der (ZP)-Stufe sortiert und nach dem Sortieren gewaschen wird.

14. Verfahren nach Anspruch 13, dadurch gekennzeichnet, daß das Sortieren durchgeführt wird, indem der Zellstoff auf eine Konsistenz von ungefähr 0,5-1,5 % verdünnt und anschließend in einem Wirbelreiniger (66) behandelt wird.

15. Verfahren nach Anspruch 13, dadurch gekennzeichnet, daß der Zellstoff vor dem Sortieren auf eine Konsistenz von ungefähr 0,5-4 % verdünnt und daß das Sortieren bei besagter Konsistenz durchgeführt wird.

16. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß dem Zellstoff vor der (ZT)-Stufe Enzyme zugesetzt werden.

17. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß während der Durchführung der zumindest einen Bleichstufe zumindest ein Filtrat produziert wird und daß der pH des Filtrats durch Zugabe von Alkali oder Säure eingestellt wird.

18. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß während der (ZT)-Stufe, nach Behandlung des Zellstoffs mit Ozon er mit einem Filtrat behandelt wird, das einen pH hat, der sauer genug ist, um eine Wiedereindickung von Schwermetallen zu verhindern.

19. Verfahren nach Anspruch 18,dadurch gekennzeichnet, daß das Filtrat durch Zugabe von Säure sauer genug gehalten wird, um eine Wiedereindickung von Schwermetallen zu verhindern.

20. Verfahren nach Anspruch 18, dadurch gekennzeichnet, daß der pH des Filtrats unter 4 bleibt und der Zellstoff während der Behandlung des Zellstoffs mit saurem Filtrat einen pH unter 4 hat.

21. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Zellstoff von Schritt (a) in einer Sauerstoffstufe vor der (ZT)-Stufe delignifiziert worden ist.

22. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß der Zellstoff von Schritt (a) durch Verwendung von Heißalkaliextraktion delignifiziert oder mit Enzymen in dem Schritt (b) behandelt worden ist.

Revendications

1. Procédé de blanchiment de la pâte kraft contenant des métaux lourds ; tel que du manganèse, jusqu'à une blancheur d'au moins 86 ISO environ sans l'utilisation de substances chimiques de blanchiment au chlore, comprenant les étapes suivantes consistant à :

a) disposer d'une pâte kraft d'une matière fibreuse cellulosique désintégrée, lessivée et délignifiée, présentant un indice Kappa de l'ordre de 14 ou moins ;

b) traiter optionnellement ladite pâte avec des enzymes ;

c) exécuter optionnellement un stade de lavage avec la pâte obtenue dans l'étape a) ou b);

d) ensuite, sans d'abord effectivement enlever les métaux lourds blanchir, la pâte au cours de plusieurs stades, dont le premier est un stade (ZT), dans lequel la portion (Z) utilise de l'ozone en tant qu'agent chimique de blanchiment principal et dans lequel la portion (T) représente l'élimination effective des métaux par l'utilisation d'agents séquestrants sans aucun lavage intermédiaire lors du stade (ZT).

2. Procédé selon la revendication 1, caractérisé par la mise en oeuvre du blanchiment au cours d'une sous-séquence (ZT) (EOP) (ZP).

3. Procédé selon la revendication 1, caractérisé en ce que, au cours du stade (ZT) on prévoit de l'ordre de 2 à 10 kg d'ozone par tonne de pâte séchée à l'air.

4. Procédé selon la revendication 1, caractérisé en ce que le pH de la pâte au cours du stade (ZT) est de l'ordre de 2 à 7, et la température est comprise entre environ 40°C et 70°C.

5. Procédé selon la revendication 1, caractérisé en ce que le pH au cours de la première portion du stade (ZT) est de l'ordre de 4 à 7.

6. Procédé selon la revendication 1 ou 5, caractérisé par le fait que l'on ajoute à la pâte lesdits agents séquestrants de métaux lourds au cours du ou avant ledit enlèvement de métaux au cours du stade (ZT).

7. Procédé selon la revendication 2, caractérisé en ce que, au cours du stade (EOP), la quantité de péroxyde ajoutée est de l'ordre de 10 à 20 kg/adt.

8. Procédé selon la revendication 7, caractérisé en ce que la quantité d'oxygène ajoutée est de l'ordre de 2 à 6 kg/adt.

9. Procédé selon la revendication 2, caractérisé en ce que, au cours de la mise en oeuvre du stade (EOP), le pH de la pâte est maintenu dans la plage allant d'environ 9 à 12, la température allant de l'ordre de 60° C à 95°C, et la durée de traitement est de 2 à 8 heures environ.

10. Procédé selon la revendication 2, caractérisé par l'étape supplémentaire consistant à ajouter du magnésium au stade (EOP).

11. Procédé selon la revendication 2, caractérisé en ce que, au cours de la mise en oeuvre du stade (ZP), la quantité d'ozone ajoutée est inférieure à 3 kg/adt environ, et en ce que la quantité de peroxyde ajoutée est inférieure à 10 kg/adt environ.

12. Procédé selon la revendication 11, caractérisé en ce que la quantité de peroxyde ajoutée au cours du stade (ZP) est de l'ordre de 3 à 7 kg/adt.

13. Procédé selon la revendication 2, caractérisé en ce que la pâte est tamisée immédiatement après le stade (ZP) et elle est ensuite lavée à la suite du tamisage.

14. Procédé selon la revendication 13, caractérisé en ce que le tamisage est effectué en diluant la pâte jusqu'à une consistance de l'ordre de 0,5 % à 1,5%, et ensuite en la traitant dans un épurateur à tourbillons (66).

15. Procédé selon la revendication 13, caractérisé en ce que la pâte est diluée jusqu'à une consistance de l'ordre de 0,5% à 4% avant le tamisage, et en ce que le tamisage est effectué à ladite consistance.

16. Procédé selon la revendication 1, caractérisé en ce que des enzymes sont ajoutés à la pâte avant le stade (ZT).

17. Procédé selon la revendication 1, caractérisé en ce que, lors de la mise en oeuvre dudit au moins un stade de blanchiment, on produit au moins un filtrat, et en ce que le pH du filtrat est régulé en y ajoutant de l'alcali ou de l'acide.

18. Procédé selon la revendication 1, caractérisé en ce qu'au cours du stade (ZT), après un traitement de la pâte à l'ozone, elle est traitée avec un filtrat présentant un pH qui est suffisamment acide pour empêcher un réépaississement des métaux lourds.

19. Procédé selon la revendication 18, caractérisé en ce que le filtrat est maintenu suffisamment acide pour empêcher un réépaississement des métaux lourds en ajoutant de l'acide à celui-ci.

20. Procédé selon la revendication 18, caractérisé en ce que le pH du filtrat demeure en-dessous de 4, et la pâte présente un pH en-dessous de 4 au cours du traitement de la pâte avec un filtrat acide.

21. Procédé selon la revendication 1, caractérisé en ce que la pâte de l'étape a) a été délignifiée au cours d'un stade oxygène avant ledit stade (ZT).

22. Procédé selon la revendication 1, caractérisé en ce que la pâte de l'étape a) a été délignifiée en utilisant une extraction alcaline chaude ou qui a été traitée avec des enzymes au cours de ladite étape b).

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