Note: Descriptions are shown in the official language in which they were submitted.
X7109
1
METHOD OF PROCESSING PULP WITH CHLORINE-FREE CHEMICALS
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a new kind of a method
of producing pulp by using chlorine-free chemicals. The
invention is especially related to ozone bleaching of
pulp without preceding removal of heavy metals.
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-patent
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.
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
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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%.
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.
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.
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.
EP-A-0 426 652 discloses several bleaching sequences
where ozone has been used in combination with peroxide
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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.
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.
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.
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. The present invention
compresses these five to six stages into three stages and
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1
21571~9
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thus almost halves the investment costs of a bleaching
plant and a screening plant.
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.
It is characteristic of the method in accordance with the
present invention that pulp is bleached with a sequence
beginning with ozone and without the removal of heavy
metals preceding the ozone stage.
It is characteristic of another embodiment of the method
in accordance with the present invention that pulp is
bleached with a three-stage sequence (ZT)(EOP)(ZP)
without the 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.
In each of the stages (ZT) , (EOP) , and (ZP) there is no
interstage washing.
BRIEF DESCRIPTION OF THE DRAWINGS
'~ 7 1
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
5 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 in accordance with the present invention, 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;
Fig. 6 schematically illustrates a recirculation method
for washing filtrates in accordance with the second
embodiment of the invention illustrated in FIGURE 2; and
Fig. 7 schematically illustrates results of a research
carried out with a DRUM DISPLACERTM washer.
DETAILED DESCRIPTION OF THE DRAWINGS
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 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 DISPLACERTM washer or a conventional
pressure diffuser. Pulp is preferably pumped from washer
22 by an MC~ pump 24 into an 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 DISPLAGERTM) washer, from
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6
which pulp is further pumped with an MC~ pump 34 into a
second ozone reactor 36 and further therefrom through a
as separator 38 to a third reaction tower 40. From tower
40 pulp is pumped with an MC~ pump 42 into a third washer
5 44 (preferably a DRUM DISPLACERTM) washer.
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
10 reactor 26 with mixer 62 and the mixture of ozone and
carrier gas prior to the second ozone reactor 36. The
mixers 60, 62 are preferably AHhMTXERTM type fluidizing
mixers, which are able to mix very large amounts of gas
into fiber suspensions, including medium consistency
suspensions.
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.
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
1570
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', 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.
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 our
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.
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
8
bleached with ozone, the dosing being about 2-l0 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.-
The T portion of the (ZT) stage may be carried out, for
example, in the following ways. The first alternative is
to allow the pH value of the pulp to decrease to the
range of 2 - 4, whereby the majority of the heavy metals
are dissolved into the filtrate phase and may be washed
off in the thickener or washer 22 following that stage.
The disadvantage here is that the majority of the
magnesium (Mg) is also discharged, so that it is possible
that magnesium must be added to the pulp, mostly in the
form of magnesium sulphate, for the oxygen and/or
peroxide stages following later on in the sequence.
Another 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.
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
S
X157109.-
9
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.
If so required also enzymes may be added in the T portion
of the (ZT) stage. Filtrate Sl 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.
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, sewer,
or recovery of chemicals.
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. In the
(ZP) stage the following conditions may be utilized:
In the Z portion of the (ZP) stage the ozone dose is
small, below 3 kg/adt and the purpose of the ozone is
t
to 1 5 ~ 1 0 -9
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.
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.
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.
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.
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
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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.
In the embodiment illustrated in Fig. 4 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
Chemical consumption Duration Temperature
Stage o
adt min. C
Z 03 0.5 2 40 - 50
E 02 0.4 60 80 - 90
p M202 1.5 180 80 - 90
MgS04 0.4
Y
217ao _
12
5
Z O~ 0.1 2 70
p H202 0.4 180 70
MgS04 0.3
In the practice of the method set forth in Table I, about
20-30 kg of NaOH/adt, and 15~25 kg of H2S04/adt is
consumed, depending upon water usage.
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.
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.
The reutilization of filtrates illustrated in Fig. 5 may
further be intensified by dividing the filtrates from the
.~a,.~
157109
13
washer into two fractions with different pH according to
Fig. 6. Fig. G utilizes the reference numbers of Fig. 5
with a preceding "1". The washers used in Fig. 6 are
manufactured and marketed by A. Ahlstrom Corporation, and
known as DRUM DISPLACERTM washers. For example, the pulp
for the last washer 188 comes from the alkali (ZP) stage.
Then the first outwashed filtrate 1881 is clearly
alkaline and the filtrate 1882 coming out later is less
alkaline or even neutral, because water 1880 flowing to
the last washer 188 is generally neutral or slightly
acid.
In this way two circulation waters 1881 and 1882 of
different pH values are obtained, which may be used to
adjust the pH of the pulp appropriate before the
bleaching sequence, or a particular stage of one
bleaching sequence. Filtrates 1881 and 1882 of the last
washer 188 in Fig. 6 are brought to the preceding washer
186 in a way that drops the pH of the pulp prior to the
(ZP) stage. Thus acid and alkali are saved in the
bleaching plant. In the embodiment of Fig. 6 two alkali
filtrates 1861 and 1862 are obtained from the washer 186
preceding the (ZP) stage, which are supplied to washer
the 184 preceding the (EOP) stage. According to Fig. 6
preferably acid (H2S04) 1840 is added to the filtrate
1862, whereby the first filtrate obtained from the
washer, of which one portion 1841 is led to effluent
clarification and the rest 1842 to the washer 182
preceding the (ZT) stage, is acid and the second filtrate
1843 alkaline.
The pH of the filtrates may also be adjusted by adding
acid or alkali to them before they are used again. In
some cases it may, for example, be necessary to add
alkali to the filtrates prior to their being brought to
the brown stock washing or it may be necessary to add
acid in order to maintain the pH low during the removal
4
° 709
14
of the metals in the (ZT) stage (point 1840). It is
possible that heavy metals are thickened again and attach
to the fibers, if the pH increases during washing. Thus
heavy metals are entrained to the P portion of the (ZP)
stage following the Z portion and disturb the peroxide
stage. Preferably the pH value is maintained less than
4, or at least maintained at 4, during the removal of
heavy metals.
Example:
In an experiment a DRUM DISPLACERTM washer was used to
wash the incoming 100°C pulp with 60°C water. The washed
pulp was discharged from the washer at a temperature of
65°C. The temperatures of the exiting filtrates were
95°C and 75°C. Consequently, two significantly different
filtrates were obtained from one washer. This may be
utilized in a bleaching plant to obtain two filtrates
having different pH values, because the pH value of the
filtrate to some extent correlates to the temperature of
the filtrates.
The experiment did not follow the pH value, only the
temperature, but based on the temperature distribution
the expected pH values are marked in Fig. 7 at respective
places.
pH - incoming pulp 10
pH - exiting pulp 7.5
pH - filtrate I 9.5
pH - filtrate II 8.0
Such differences in the pH values between the filtrates
is extremely significant when optimizing the chemical
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215710.
consumption of the bleaching plant. The pH values of the
filtrates are close to the pH values of the entering and
exiting pulp, preferably closer to these than to each
other.
5
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
10 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.
While the invention has been described in connection with
what is presently considered to be the most practical and
preferred embodiment, it is to be understood that the
invention is not to be limited to the disclosed
embodiment, but on the contrary, is intended to cover
various modifications and equivalent arrangements
included within the spirit and scope of the appended
claims.
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