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EP 0 687 321 B1 |
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EUROPEAN PATENT SPECIFICATION |
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Mention of the grant of the patent: |
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17.02.1999 Bulletin 1999/07 |
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Date of filing: 25.05.1993 |
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International Patent Classification (IPC)6: D21C 9/153 |
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International application number: |
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PCT/FI9300/222 |
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International publication number: |
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WO 9420/673 (15.09.1994 Gazette 1994/21) |
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METHOD OF BLEACHING PULP WITH CHLORINE-FREE CHEMICALS
VERFAHREN ZUM BLEICHEN VON ZELLSTOFF MIT CHLORFREIEN CHEMIKALIEN
PROCEDE DE BLANCHIMENT DE PATE AU MOYEN DE PRODUITS CHIMIQUES SANS CHLORURE
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Designated Contracting States: |
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AT DE ES FR PT SE |
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Priority: |
03.03.1993 FI 930954
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Date of publication of application: |
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20.12.1995 Bulletin 1995/51 |
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Divisional application: |
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98113684.9 / 0884415 |
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Proprietor: Ahlstrom Machinery Oy |
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00440 Helsinki (FI) |
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Inventors: |
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- HENRICSON, Kaj
FIN-00330 Helsinki (FI)
- STROMBERG, Bertil
Glens Falls, NY 12801 (US)
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Representative: Hansen, Bernd, Dr. Dipl.-Chem. et al |
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Hoffmann Eitle,
Patent- und Rechtsanwälte,
Arabellastrasse 4 81925 München 81925 München (DE) |
(56) |
References cited: :
EP-A- 0 402 335 EP-A- 0 512 590 US-A- 4 450 044
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EP-A- 0 426 652 EP-A- 0 512 978
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Note: Within nine months from the publication of the mention of the grant of the European
patent, any person may give notice to the European Patent Office of opposition to
the European patent
granted. Notice of opposition shall be filed in a written reasoned statement. It shall
not be deemed to
have been filed until the opposition fee has been paid. (Art. 99(1) European Patent
Convention).
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[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
S
1 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 S
2. The filtrate S
2 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 S
3 is obtained. The filtrate S
3 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 H
2SO
4/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.
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).
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.
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).