Note: Descriptions are shown in the official language in which they were submitted.
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METHODS OF TREATING CHEMICAL CELLULOSE PULP
DOMESTIC PRIORITY CLAIM
The present invention is based on and claims domestic priority benefit
under 35 USC ~119(e) from U.S. Provisional Patent Application Serial No.
60/659,077 filed on March 8, 2005, the entire content of which is expressly
incorporated hereinto by reference.
FIELD OF THE INVENTION
The present invention relates to a method of treating chemical cellulose
pulp from any chemical pulping process. In preferred embodiments, the
present invention relates to treating chemical cellulose pulp in a stage with
an
acid treatment, a subsequent neutralization treatment and a chlorine dioxide
treatment following the neutralization treatment.
BACKGROUND AND SUMMARY OF THE INVENTION
Pulp bleaching sequences using chlorine dioxide as the bleaching
chemical are popular and environmentally feasible. Many practical and
economic reasons exist for pulp bleaching sequences utilizing chlorine
dioxide. For example, the price of chlorine dioxide is very competitive
compared with that of other bleaching and delignifying chemicals. Also, the
strength and brightness of the pulp obtained by chlorine dioxide bleaching are
good, in fact at least approximately of the same order as those obtained by
using peroxide at the same chemical consumption level (kg/admt).
Conventional chlorine dioxide bleaching is usually preformed at a
temperature of 50-80°C, and with a chlorine dioxide dosage of about 10-
30 kg
per ton of pulp. Typical sequences used are, for example, DoED~ED2 and
ODoED~ED2 and modifications thereof. The treatment time in a Do stage is
usually shorter as compared to the treatment time in the other D stages, being
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for example, between about 30-90 minutes. The pH of the pulp decreases to
the range of 1-3 towards the end of the Do stage. In the D, and D2 stages the
treatment time is 2-3 hours and the pH is slightly higher than in the Do
stage.
The chlorine dioxide stage (all types) is usually performed at a temperature
of
. about 70°C, while the treatment time in the Do stage is 0.5-2 hours
and in the
D~ and D2 stages 2-3 hours. Higher temperatures than about 70°C
have
typically been avoided as the low final pH in the range of 1-3 in the D stage
combined with a high temperature and long treatment time deteriorates the
strength properties of the pulp fibers.
Although conventional chlorine dioxide sequences have achieved
widespread acceptance in the industry, some improvements are still desired.
For example, improvements in the final brightness achieved by chlorine
dioxide bleaching sequences are continually sought. It is towards fulfilling
such improvements that the present is directed.
The present invention is especially embodied in a single bleaching
stage which comprises an acid treatment, A, a neutralization treatment, n, and
a chlorine dioxide treatment, D°. The AnDo bleaching stage of the
present
invention may optionally include washing between treatments, either between
each treatment or only some treatments, represented by A-n-Do which
indicates there is washing between the Acid (A) and the neutralization (n)
treatments and the neutralization (n) and the chlorine dioxide (D) treatments,
or An-D° which indicates there is washing only between the
neutralization (n)
and the chlorine dioxide (D) treatments. The AnDo stage can be followed by
washing and further bleaching stages. Preferably, the (AnDo) stage is
followed by an extraction stage Eop with caustic in the presence of oxygen and
a peroxide.
When compared to a stage that is an acid treatment followed by a
chlorine dioxide treatment without neutralization between the treatments
(AD°), using this AnDo bleaching stage of the present invention gives
the
surprising results of higher brightness after the chlorine dioxide treatment
and
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higher brightness and lower Kappa number after a subsequent Eop stage.
This AnDo bleaching stage also results in a higher final brightness at
substantially identical bleaching conditions when bleaching does not include
such an ADo or Do stage only.
Additionally, it has been found that reduced brightness reversion (that
is, the measure of the loss of brightness of bleached pulp or paper over a
period of time) occurs for pulp bleached in accordance with the present
invention. In this regard, it is highly desirable for the bleached pulp to
retain
its white ("bleached") color over prolonged time periods and thus reduction of
brightness reversion of bleached pulp is highly desirable. Thus, brightness
reversion of bleached pulp from the AnDo bleaching stage when compared to
a conventional ADo stage is lower. It has also been found that the AnDo
bleaching stage at a given brightness level requires less bleaching chemicals,
especially chlorine dioxide. All of the improvements are a result of the
introduction of a neutralization treatment, n, between the A and Do
treatments.
Further, it has been found when compared to a stage that is an AnDo,
using an An-Do bleaching stage (acid treatment followed by neutralization
followed by washing followed by chlorine dioxide) of the present invention
gives even more surprising results of an even higher brightness after the
chlorine dioxide treatment and higher brightness and lower Kappa number
after a subsequent Eop stage. This An-Do bleaching stage also results in an
even higher final brightness at substantially identical bleaching conditions
when bleaching does not include such a stage.
As with the AnDo, it has been found that the An-Do results in a reduced
brightness reversion (that is, the measure of the loss of brightness of
bleached pulp or paper over a period of time) for pulps bleached in
accordance with the present invention. In this regard, it is highly desirable
for
the bleached pulp to retain its white ("bleached") color over prolonged time
periods and thus reduction of brightness reversion of bleached pulp is highly
desirable. Thus, brightness reversion of bleached pulp from the An-Do
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bleaching stage when compared to a conventional ADo stage or even an
AnDo is lower. It has also been found that the An-Do bleaching stage at a
given brightness level requires less bleaching chemicals, especially chlorine
dioxide. All of the improvements are a result of the introduction of a
neutralization treatment, n, between the A and Do treatments with washing
following the neutralization.
These and other aspects and advantages will become more apparent
after careful consideration is given to the following detailed description of
the
preferred exemplary embodiments thereof.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Reference will hereinafter be made to the accompanying drawings,
wherein like reference numerals throughout the various FIGURES denote like
structural elements, and wherein;
FIGURE 1 is a schematic view of the AnDo process;
FIGURE 2 is a schematic view of the An-Do process;
FIGURE 3 is a bar graph showing pulp brightness for chlorine dioxide
bleaching sequences with and without a neutralization, n, stage and with and
without a caustic extraction stage in the presence of oxygen and a peroxide
(E°p).
FIGURE 4 is a bar graph showing the impact of a neutralization stage
on E°p Kappa Number for chlorine dioxide bleaching stages;
FIGURE 5 is a graph of final brightness (% ISO) versus C102 charge in
the D~ stage; and
FIGURE 6 is a graph of final brightness (% ISO) versus Total C102
charge.
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DETAILED DESCRIPTION OF THE INVENTION
A schematic view of a system 10 according to the present invention is
depicted in accompanying FIGURE 1. In this regard, the pulp slurry of the
target consistency enters the treatment vessel 12 via supply line 14 so that
the pulp can be subjected to an acid treatment, A. The acid treated pulp is
discharged from the vessel 12 via line 16 and pumped by slurry pump 18 to a
mixer 20. An appropriate amount of a neutralizing chemical (e.g., NaOH) is
added to the pulp slurry at the mixer 20. The neutralized pulp slurry may then
be introduced into treatment vessel 22 via supply line 24. The vessel 22 is
sized to accommodate the desired residence time for the pulp slurry to be
neutralized therein. The neutralized pulp slurry is then withdrawn from the
vessel 22 via line 26 and directed to a mixer 28 wherein the bleaching charge
of chlorine dioxide C102 is added. The pulp slurry is thus introduced into the
bleaching vessel 30 via line 32. The bleaching vessel 30 is sized so as to
allow the pulp slurry to reside for a time sufficient to bleach the pulp
whereby
the bleached pulp may then be discharged therefrom via line 34.
The present invention is especially embodied in a single bleaching
stage which comprises an acid treatment, A, a neutralization treatment, n, and
a chlorine dioxide treatment, Do. The (AnDo) bleaching stage of the present
invention may optionally include washing between treatments represented by
(A-n-Do), or between only the acid and neutralization treatments represented
by (A-nDo) or between only the neutralization and chlorine dioxide treatments
represented by (An-Do).
FIGURE 2 shows the schematic of the system 10 where washing
occurs at a wash stage 40 between neutralization n (which occurs in vessel
22) and the addition of chlorine dioxide at mixer 28. This wash stage may
involve a press (not shown). The wash stage 40 (which can occur with or
without a press) includes the addition of wash liquid (preferably fresh water
at
about 20°C to about 100°C) via line 41 to the pulp after the
neutralization
stage such that the material leaving the wash stage 40 via line 26-1 is at a
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consistency of between about 4 to about 35%, preferably at a consistency of
between about 10 to about 14%. The wash liquid may be extracted from the
wash stage 40 via line 42.
The preferred acid treatment will be conducted at a pH of between
about 1 to about 5, and more preferably between about 2 to about 3.5. The
acid used in such acid treatment may be any of those acids conventionally
employed in the art, such as for example, sulfuric, hydrochloric acid and the
like. Hydrofluoric acid may also be employed. The pulp retention time for the
acid treatment will typically range between about 30 seconds to about 5
hours, and more preferably between about 2 to about 4 hours. The acid
treatment will most preferably be conducted at an elevated temperature of
between about 40 to about 100°C, and more preferably between about 80
to
about 90°C (e.g., about 85°C). The consistency of the pulp
slurry during acid
treatment is preferably between about 0.1 to about 50%, more preferably
about 4 to about 15%, and most preferably between about 10 to about 12%.
Particularly preferred acid treatments may be those described more
fully in U.S. Patent No. 6,776,876 B1, the entire content of which is
expressly
incorporated hereinto by reference. Specifically, the preferred acid treatment
stage (A) will include treating chemical cellulose pulp produced by alkaline
delignification having a kappa number under 24 and a solids consistency of
between 0.1-50% by treating the pulp at a temperature over 85°C and at
a pH
between about 2-5 for sufficient time to remove at least 50% of the
hexenuronic acid and to reduce the kappa number by at least 2 units.
The acid treatment of this invention is followed by a neutralization
treatment stage (n) and optionally may include an intermediate washing
stage. As will be discussed below, especially preferred embodiments subject
the acid treated pulp to the neutralization treatment without an intermediate.
wash (i.e., so as to achieve reduced brightness reversion). The purpose of
the neutralization treatment is to neutralize the pulp slurry to a pH of
between
about 7 to about 14 (preferably about 9.5 pH) using an alkaline chemical,
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such as sodium hydroxide. The preferred neutralization treatment is
accomplished by adding the neutralizing chemical directly to the pulp slurry
directly after the acid treatment without washing at a temperature of between
about 40 to about 100°C, more preferably between about 60 to about
90°C,
most preferably 80 to about 90°C. The pulp retention time for the
neutralization treatment will typically range between about 30 seconds to
about 2 hours, and more preferably between about 5 to about 20 minutes.
The consistency of the pulp slurry during the neutralization treatment is
preferably between about 4 to about 15%, more preferably between about 10
to about 12%.
A chlorine dioxide treatment, Do, directly follows the neutralization
treatment. As discussed previously, intermediate washing of the pulp slurry
may or may not occur between the n and Do treatments. The chlorine dioxide
treatment is conducted at a pH of between about 1 to about 7, preferably
between about 2.5 to about 3.5. The target pH is achieved by adding chlorine
dioxide and/or an acid such as sulfuric, hydrochloric, or hydrofluoric acids.
The temperature of the chlorine dioxide treatment will be between about 40 to
about 100°C, more preferably between about 60 to about 90°C,
most
preferably 80 to about 90°C. Pulp retention times during the chlorine
dioxide
treatment will range between about 30 seconds to about 2 hours, preferably
between about 15 to about 30 minutes. A chlorine dioxide charge of between
about 0.05 to about 2.5% on oven dry pulp weight (odpt) or Kappa number 3
to 24 times the Kappa factor (Kappa factor of 0.05 to 1, preferably 0.15 to
0.25) will typically be employed at beginning of the Do treatment. Pulp slurry
consistency during the Do treatment will be between about 4 to about 15%,
preferably between about 10 to about 12%. The chlorine dioxide treatments
that may be employed in the practice of this invention are described more
fully
in US Patent Application Publication 2002/0056533 A1 (the entire content of
which is expressly incorporated hereinto by reference).
The AnDo or An-Do stage can be followed by washing and further
bleaching stages. Preferably, the AnD° or An-Do stage is followed by an
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alkaline extraction, preferably by an E°p stage with caustic in the
presence of
oxygen and a peroxide. Preferably the E°p stage is as disclosed more
fully in
U.S. Patent No. 4,568,420 (the entire content of which is expressly
incorporated hereinto by reference).
As noted previously, intermediate wash treatments may optionally be
practiced between the neutralization treatment and each of the acid and
chlorine dioxide bleaching treatments or between the acid and neutralization
treatment or between the neutralization and chlorine dioxide treatment. Such
a stage can be stated as A-n-Do or A-nDo or An-Do. Suitable wash liquids,
typically at a temperature of 20-100°C for such intermediate wash
treatments
may be virtually any suitable water from the pulp mill, such as fresh water or
filtrate from the last D (chlorine dioxide) stage or it can be condensate
(either
clean or dirty) from elsewhere in the mill, washer filtrate, waste water
treatment plant effluent or combinations of any of these liquids, but
preferably
is fresh water.
Alternatively, an initial quantity of chlorine dioxide may be added to the
acid treatment, A, resulting in a stage (AonDo). According to this embodiment
of the invention, the total charge of chlorine dioxide would be split between
the Ap and Do treatments so that chlorine dioxide is added in two treatments.
Specifically, chlorine dioxide would be added during or with the acid
treatment
as well as after the neutralization treatment.
Preferably, according to this embodiment, a treatment Ap is practiced
to achieve an acid treatment with the addition of between 1 and 99% of the
total chlorine dioxide to be used in the stage. For example, if 10% of the
total
chlorine dioxide to be added in this stage is added with the acid, A,
treatment,
then the remaining 90% of the chlorine dioxide is introduced in the
subsequent chlorine dioxide, Do, treatment. The operating conditions of this
Ao treatment are preferably substantially the same as those of the acid
treatment described previously, that is a pH of 1 to 4 preferably 2 to 3.5; a
retention time for the treatment of 30 seconds to 5 hours, preferably 2 to 4
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hours; a temperature of treatment 40 to 100°C, preferably 80 to
90°C; and a
pulp slurry consistency during treatment of 4 to 15%, preferably 10 to 12%.
Preferably, the acid used is sulfuric, hydrochloric or hydrofluoric.
The Ap treatment is followed directly by a neutralization treatment
without intermediate washing. That is, there is no washing of the pulp slurry
between the Ap and n treatments. The Do treatment follows directly the
neutralization treatment without intermediate washing. That is, there is no
washing of the pulp slurry between the n and the Do treatments. The
operating conditions for the n and the'Do treatments are substantially the
same as those described previously with the neutralization and Do treatments
except of course that the balance of the chlorine dioxide charge is added in
the Do treatment which depends on the amount of chlorine dioxide added in
the Ap treatment.
Although no intermediate washings of the pulp are preferred,
intermediate wash treatments may optionally be practiced between the
neutralization treatment and each of the Ap and Do treatments. Such a
sequence can be stated as (Ap-n-Do). Suitable wash liquids for such
intermediate wash treatments may be virtually any suitable water from the
pulp mill, such as washer filtrate, waste water treatment plant effluent and
the
like.
Another embodiment in accordance with the present invention involves
having successive chlorine dioxide treatments whereby bleaching is
accomplished first at a relatively high pH value and then at a relatively low
pH
value. Such a sequence is identified as (AnDH,D~o). In this regard the acid
and neutralization treatments are preferably practiced substantially the same
as described previously. Preferably, no intermediate washing of the pulp
slurry occurs either before or during the chlorine dioxide treatments DH,,
Duo.
The total chlorine dioxide charge is split between the DH, and Duo treatments.
Thus, between about 1 to about 99% of the total chlorine dioxide charge will
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be added in the DHi treatment, whereas the balance (i.e., between 99% to 1 %)
of the total chlorine charge is then added in the Duo treatment.
The DH, chlorine dioxide treatment is conducted at a relatively high pH
of between about 4 to about 7, preferably between about 5 to about 6. The
target pH is achieved by adding sodium hydroxide or other alkaline chemicals
with pH adjustment occurring after the addition of 1 to 99% of the chlorine
dioxide charge. The DH, treatment is preferably practiced at a temperature 40
to 100°C, preferably 60 to 90°C; a retention time at high pH of
30 seconds to
2 hours, preferably 5 to 30 minutes; and a pulp slurry consistency of 4 to
15%,
preferably 10-12%.
The Duo chlorine dioxide treatment is practiced at a relatively low pH,
without intermediate washing of the pulp slurry after the DH, treatment, of
between about 1 to about 4, preferably between about 2 to about 3. The pH
adjustment is achieved by adding a suitable acid, such as sulfuric acid,
hydrochloric acid and/or hydrofluoric acid. The addition of acid for pH
adjustment can occur concurrently with or subsequent to the addition of the
balance of the chlorine dioxide charge. Preferably, the Duo chlorine dioxide
treatment is practiced at a temperature 40 to 100°C, preferably 60 to
90°C; a
retention time at low pH of 30 seconds to 2 hours, preferably 5 to 30 minutes;
and at a pulp slurry consistency of 4 to 15%, preferably 10-12%.
Although no intermediate washings are preferred, it is possible to
practice intermediate washings between each of the treatments, specifically
between the neutralization treatment and each of the acid and high pH
chlorine dioxide treatments as well as between the high and low chlorine
dioxide treatments. Such a stage can be stated as (A-n-DH,-Duo). Wash
liquid could be any water from the mill, such as washer filtrate, waste water
treatment plant effluent, and the like.
The present invention will be further understood from the following non-
limiting Example.
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Example 1
Hardwood pulp was treated in accordance with Table 1 below to
compare the results of the AnDo treatment in accordance with the present
invention, with and without a subsequent Eop stage, with a conventional ADo
stage. The results appear in Table 1 below and are graphically presented as
FIGURES 3-5.
As can be seen, improvements in the final brightness are achieved by
chlorine dioxide bleaching stages according to the present invention.
Table 1
Furnish Mill
02
Pulp
Sample ID 2004-00 102403
Kappa Number 10.9
Vscosity, mPa.s 37.6
BS HexA, meq/kg 63.9
A Do Stage: 90C+85C, 180+15
min., 10h cons.
Sample ADo AnDo
Kappa Factor 0.16 0.16
HzSO, % 1.32 1.32
NaOH, % --- 1.1
CIO z, % 0.37 0.37
Consumed CIO z 0.37 0.37
Final pH 2.3+ 2.3+
2.2 9.9+
4.1
Brightness, ISO 64.7 70.7
Kappa number --- ---
HexA, meq/kg --- ---
Eop Stage: 60 min.,85C,
40psi.,10% cons.
H20z, % 0.3 0.3
NaOH, % 0.8 0.8
Final pH 10.6 10.8
Consumed H zOz 0.3 0.27
Kappa Number 3.0 2.5
Brightness, % ISO 78.0 83.8
D Stage: 70C, 180 min.,10%
cons.
CIO z, % 0.2 0.4 0.6 0.20.4 0.6
NaOH, % --- --- 0.08------ 0.08
H zS0 a, % 0.110.03 --- 0.110.03 ---
Final pH 4.2 3.8 3.9 4.24.1 3.4
CIO z Consumed, % 0.2 0.4 0.6 0.20.4 0.6
ISO Brightness, % 85.887.6 89.088.489.6 90.6
Reverted ISO units 2.3 2.3 2.5 2.52.1 2.3
1 O 4 Hrs @ 105C
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Example 2
Pulp was treated in accordance with Table 2 below to compare the
results of the AnDo and An-Do treatments in accordance with the present
invention, with and without a subsequent EoP stage, with a conventional ADo
stage. The results appear in Table 2 below and are graphically presented as
FIGURES 6. Hardwood pulp was used in the study summarized in Table 2.
As can be seen, improvements in the final brightness are achieved by
chlorine dioxide bleaching stages according to the present invention.
Specifically, as depicted in FIGURE 6, a comparison of three bleaching
sequences (ADo), (AnDo) and (An-Do) reveals that the addition of the
neutralization treatment between the acid and the chlorine dioxide treatments
results in improved final brightness over the sequence with acid and chlorine
dioxide treatments alone. It is also apparent the addition of washing between
the neutralization and chlorine dioxide treatments results in an even higher
final brightness than the case of neutralization between the acid and chlorine
dioxide treatments without washing.
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Table 2
Furnish Hardwood
Mill Oz
Pulp
Sample ID 2004-00 102640
Kappa Number 10.4
Viscosity, mPa.s 40.3
HexA, me 63.9
AD Staae: 90C+g5C.
180+15 min..
10% C011S.
AnDo Staae: 90Ct90C+85C.
120+5+15 min.. 10%
cons.
An-Do Staae: 90C+90C+85C.
120+5+15 min.. 10h
cons.
Sample ADo AnDo An-Do
Kappa Factor 0.18 0.18 0.18
HZSOs, % 1.42 1.96 1.25
NaOH, % - 1.1 0.9
CIOz, % 0.42 0.4 0.42
Consumed CIOz 0.42 0.4 0.42
2.5; 2.4;
9.4;
Final pH, each Stage 2.4 2.4; 10.8; 2.6
2.6
Brightness, ISO 65.9 66.0 72.8
E, Stake: 60 min..
85C. 40 nsi..
1~o C011S.
HzOz, % 0.5 0.5 0.5
NaOH, % 0.8 0.8 0.8
Final Ph 10.8 10.7 10.7
Consumed HzOz 0.5 0.5 0.5
Kappa Number 2.6 2.5 2.4
Brightness, % ISO 81.5 81.3 84.5
D Staae: 70C. 180
min.. 10% cons.
CIOz, % 0.2 0.4 0.6 0.20.4 0.6 0.2 0.4 0.6
NaOH, % --- --- 0.08 -- --- 0.08--- --- 0.08
HZSO,, % 0.110.03--- 0.110.03 --- 0.110.03 ---
Final pH 4.5 4.0 3.9 4.23.9 3.8 4.2 4.0 3.9
Consumed CIOz 0.2 0.4 0.6 0.20.4 0.6 0.2 0.4 0.6
Brightness, % ISO 87.389.089.9 87.789.6 90.488.789.9 91.1
Reverted ISO units 2.7 2.5 2.8 2.32.5 2 1.7 1.5 1.9
4 Hrs 105C
