Note: Descriptions are shown in the official language in which they were submitted.
CA 02620978 2008-02-07
COMBINED PROCESS OF PEROXIDE BLEACHING OF WOOD PULPS AND
ADDITION OF OPTICAL BRIGHTENING AGENTS
FIELD OF THE INVENTION
The present invention relates to a combined hydrogen peroxide bleaching
process in which optical brightening agents are incorporated into the pulp to
give
an improved hydrogen peroxide bleaching process for mechanical or high-yield
pulps for producing a very high brightness pulp economically.
BACKGROUND OF THE INVENTION
There is a need to provide various paper grades, including printing and
writing quality papers, to be produced with greater brightness. This requires
that
the pulp furnish, which is needed to manufacture the paper products, have very
high brightness.
The bleached chemi-thermo-mechanical pulp (BCTMP), also known as
high-yield pulp (HYP) has many unique properties such as high bulk and light
scattering coefficient that are desirable for printing and writing paper
products [1-
5]. However, based on currently available bleaching technologies, the
economical brightness levels of HYP are still significantly lower and its
yellowness (b*) higher than those of fully bleached kraft pulps, BKP [1-2].
For example, Aspen HYP can be bleached to about 85% ISO brightness from the
alkaline peroxide bleaching process, however, the production cost increases
exponentially when a very high brightness (e.g. higher than 80% ISO) is
required.
Also, in some commercial applications extremely high brightness (higher than
1
CA 02620978 2008-02-07
88% ISO) is required, and the conventional peroxide bleaching in these cases
would become prohibitively expensive.
Optical brightening agents (OBAs) or fluorescent whitening agents (FWAs)
are commonly used to improve the optical properties of printing and writing
paper
during the papermaking process [6]. Both the paper brightness and whiteness
can be increased with the addition of OBA. Also, OBA can decrease the photo-
yellowing (photo- reversion) of HYP by acting as an UV screen [7, 8, 9]. It
has
also been reported that the overall environmental impact of using OBA might be
better than peroxide for a certain level of brightness gain of mechanical
pulps [9].
Conventionally, OBA is applied to pulp at the wet-end of the papermaking
process. OBA molecules adsorb on pulp fibers by forming hydrogen bonding with
cellulose in fibers. When OBA is added to a mixed fumish, HYP will compete
with BKP for OBA. Ideally, OBA adsorbed on HYP fibers would be more
effective in whitening HYP-containing paper sheets. To have more OBA on HYP
fibers rather than on BKP fibers, OBA may be added to HYP at the pulp mill
during the HYP production process. Another factor affecting OBA efficiency is
its
retention on fibers. It has been reported that OBA retention decreased with
the
increase of HYP substitution rate due to the lower affinity of OBA molecules
to
HYP fibers [10, 11]. As a result, the OBA efficiency decreased. Although the
white water circulation system can improve the overall retention of OBA, the
un-
absorbed OBA molecules in the white water can undergoes a transformation
from trans- to cis- configuration and loses its fluorescent properties [6].
U.S. Patent No. 5,902,454 and European patent application Serial No. EP
0899 373 Al disclose the use of OBA to increase the brightness of HYP.
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CA 02620978 2008-02-07
Typical state of the art processes for peroxide bleaching of HYP are
described in the following references. First Presley, J.R. and Hill, R.T.,
Pulp
Bleaching: Principles and Practice, Edited by C.W. Dence and D.W. Reeve,
Page 480 discloses the so-called cascade system for preparation of bleach
liquor, in which magnesium sulfate and sodium silicate are added to water and
intimately mixed, followed by the addition of caustic soda and finally by the
addition of hydrogen peroxide. The resulting liquor is subsequently mixed with
pulp.
Presley, J.R. and Hill, R.T., Pulp Bleaching: Principles and Practice,
Edited by C.W. Dence and D.W. Reeve, Page 481 discloses the so-called in-line
system, where similar mixing and addition occurs but without the cascade
arrangement.
Ni, Y. et al., Pulp and Paper Canada, 104(12):78 (2003) discloses a
sequential addition of chemicals beginning with the addition of silicate or
DTPA,
or other peroxide stabilizers to a pulp slurry and the subsequent addition of
hydrogen peroxide. Alkali sources, such as sodium hydroxide can be added
simultaneously with the stabilizers. The following patents deal with bleaching
processes for mechanical or high-yield pulp, but do not address the process of
the present invention: Canadian Patent Nos. 686,115; 820,190; 1,294,655;
1,310,797; 2,041,588; 2,070556.
Therefore, there is a need to increase the brightness in a more cost-
effective manner. Also, there is a need to produce HYP pulps with extremely
high brightness in a more economic way.
3
CA 02620978 2008-02-07
SUMMARY OF THE INVENTION
The present invention provides embodiments of processes of peroxide
bleaching of wood pulps during which optical brightening agents are
incorporated
into the wood pulp. It has now been determined that improvement in the
bleaching process can be obtained if OBA is added to the pulp during the
peroxide bleaching process.
Thus in an embodiment of the present invention there is provided a
process for bleaching wood pulp, comprising the steps of:
a) mixing a wood pulp slurry containing a chelating agent to form a first
mixture, and washing and pressing said first mixture to produce a transition
metal-depleted pulp slurry and a first discharge effluent; and
b) mixing the transition metal-depleted pulp slurry with an alkaline
bleaching liquor including at least H202 and an optical brightening agent
(OBA)
to form a second mixture and bleaching said second mixtureto form a bleached
pulp.
A further understanding of the functional and advantageous aspects of the
invention can be realized by reference to the following detailed descriptions
and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood from the following detailed
descriptions thereof taken in connection with the accompanying drawings, which
form a part of this application, and in which:
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CA 02620978 2008-02-07
Figure 1 is a block diagram illustrating the high-consistency process in
accordance with the present invention;
Figure 2 is a block diagram showing a medium-consistency process in
accordance with the present invention;
Figure 3 shows the effect of peroxide concentration and dissolved and
colloidal substances (DCS) on the brightening performance of OBA (Tetra-
sulfonic OBA);
Figure 4 shows a comparison of the adsorption of OBA on HYP and BKP
(Tetra-sulfonic OBA, pH 6.5, 2-hour contact time, room temperature);
Figure 5 shows a comparison of the brightening efficiency of OBA
between two addition methods with the presence of PEI (30% SW BKP, 20-60%
HW, 10-50% HYP (Aspen 325/85), 0.05-0.25% Di-sulfonic OBA, 0.2% PEI);
Figure 6 shows the effect of OBA on the accelerated photo-reversion of
HYP under intensive UV radiation (Photo reactor: 350 nm nominal wavelength,
2.7 mW/cm2 intensity);
Figure 7 shows a comparison of the UV reversion of HYP-containing
paper sheets between the two OBA addition methods (50% HYP + 50% HW
BKP, 0.25% Tinopal UP; Photo reactor: 350 nm nominal wavelength, 2.7
mW/cm2 intensity); and
Figure 8 shows a comparison of the fluorescent light reversion of HYP-
containing sheets between two OBA addition methods (50% HYP + 50% HW
BKP, 0.25% Tinopal UP; Photoreactor: 9-10 mW/cm2 intensity, about 53 times
greater than the intensity of normal office light).
CA 02620978 2008-02-07
DETAILED DESCRIPTION OF THE INVENTION
Generally speaking, the systems described herein are directed to
hydrogen peroxide bleaching processes for mechanical or high-yield pulps for
producing a very high brightness pulp economically. As required, embodiments
of the present invention are disclosed herein. However, the disclosed
embodiments are merely exemplary, and it should be understood that the
invention may be embodied in many various and alternative forms. The Figures
are not to scale and some features may be exaggerated or minimized to show
details of particular elements while related elements may have been eliminated
to prevent obscuring novel aspects. Therefore, specific structural and
functional
details disclosed herein are not to be interpreted as limiting but merely as a
basis
for the claims and as a representative basis for teaching one skilled in the
art to
variously employ the present invention. For purposes of teaching and not
limitation, the illustrated embodiments are directed to hydrogen peroxide
bleaching processes for mechanical or high-yield pulps for producing a very
high
brightness pulp economically.
As used herein, the term "about", when used in conjunction with ranges of
dimensions of particles or other physical properties or characteristics, is
meant to
cover slight variations that may exist in the upper and lower limits of the
ranges
of dimensions so as to not exclude embodiments where on average most of the
dimensions are satisfied but where statistically dimensions may exist outside
this
region. It is not the intention to exclude embodiments such as these from the
present invention.
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CA 02620978 2008-02-07
Figure 1 illustrates a basic procedure for a one-stage high-consistency
bleaching process of the present invention. A pulp slurry (6) is added with
chelating agent diethylene tri-amine penta-acetic acid and its salts (DTPA) or
ethylene di-amine tetra-acetic acid and its salt (EDTA) (7) and fed into a
press/washer (1). The press has discharge effluent (9) and transition metal-
depleted pulp slurry (8). A bleaching liquor (10), containing water, the
optical
brightening agents (OBA), NaOH, Na2SiO3, H202 (other H202 stabilizers, such as
DTPA, EDTA, MgSOa may be here too), is added to the pulp slurry via mixer (2),
which then discharges the mixture (11) to a bleach tower (3). After the
required
reaction time, the bleached pulp is discharged from the tower and water (12)
is
added subsequently via pump (4) and the bleached pulp slurry (13) is pumped to
a press/washer (5). The effluent discharge (15) may be added to up streams to
reuse the residual peroxide, and the bleached pulp (14) will be sent to the
subsequent unit operation.
Figure 2 illustrates a basic procedure for the medium-consistency
bleaching process of the present invention. A pulp slurry (7) is added with
chelating agent (DTPA or EDTA) (8), and fed into a press/washer (1). The press
has discharge effluent (9) and transition metal-depleted pulp slurry (10), to
which
process water (11) is added, subsequently, chemicals (12), which include OBA,
NaOH, Na2SiO3, (other H202 stabilizers, such as DTPA, EDTA, MgSOa may be
added) are added to the pulp slurry via pump (2) to produce a mixture (13),
and
H202 (14) is added to the mixture via mixer (3).
The discharge from the mixer (3) will be fed to a bleach tower (4). After
the required reaction time, the bleached pulp is discharged from the tower (4)
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and SO2 (16) may be used if the mill is practicing acidification) and is
diluted via
pump (5), pulp slurry (17) is fed into press/washer (6). The effluent
discharge
(19) may be added to up streams to reuse the residual H202 if the mill
practices
reuse of the residual H202; the bleached pulp (18) will be sent to the
subsequent
unit operation. More than one peroxide bleaching stage may be practiced. The
following non-limiting examples illustrate the invention.
The optical brightening agents include, but not restricted to, di-sulfonic
types and tetra-sulfonic types of OBA.
Thus, the invention provides a process for peroxide bleaching of
mechanical or high-yield pulp, which is comprised of adding to a pulp slurry,
an
OBA and the required bleach liquor (NaOH, Na2SiO3, H202, other H202
stabilizers, such as DTPA, EDTA, MgSO4, may be added) subjecting the pulp
slurry to pre-selected conditions to complete the bleaching process.
In a further embodiment, the process comprises adding to a pulp slurry at
least one stabilizer for stabilizing transition metal ions, subsequently
adding
hydrogen peroxide to the pulp slurry at a pre-selected point, adding NaOH to
the
pulp slurry simultaneously with or subsequent to the addition of the
stabilizer,
OBA can be added to the slurry with the stabilizer, and subjecting the pulp
slurry
to pre-selected conditions to complete the bleaching process. The stabilizer
may
be, but is not limited to silicate, DTPA, EDTA, or other sequestering agents.
OBA can be added to a peroxide bleaching process which is comprised of
more than peroxide bleaching stage. In such embodiments, OBA can be added
to all peroxide stages, or it can be added only on the last peroxide bleaching
stage.
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In a further embodiment, the process may comprise a pre-treatment step
of adding a chelating agent to the pulp slurry and subsequently removing
chelated transition metal ions from the pulp slurry.
In a further embodiment, the process may comprise a pre-treatment step
of adding a chelating agent and a reducing agent to the pulp slurry and
subsequently removing chelated transition metal ions from the pulp slurry.
Example 1
Equivalent to 25 grams of o.d. mill chelated TMP pulp from a mill in
Eastern Canada (66 ppm Mn, 56% ISO initial brightness) was mixed in a
polyethylene bag with 2.8% NaOH, 3.5% Na2SiO3, 6.0% H202, 0.4% OBA
(Tinopal UP from CIBA Chemicals). The bag, along with its contents was placed
in a temperature bath at 700. Other bleaching conditions were 3 hrs, 30% pulp
consistency.
After the completion of the required reaction time, a portion of the pulp
slurry was transferred from the bag to a beaker, diluted, and neutralized with
sulfurous acid to pH 5. A handsheet was then made following TAPPI test
method T272 [12], air-dried and determined for brightness according the TAPPI
method T525 [13]. The brightness of the resulting pulp was 82.8% ISO.
Under otherwise the same procedure and conditions as above, however
no OBA was added, the brightness was 80.7% ISO. Therefore, it can be
concluded that the addition of 0.4% OBA in the bleaching process significantly
improves the pulp brightness.
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Example 2
The same pulp sample was used as in Example 1, and the same
procedure followed, however, various amounts of OBA charges were used. The
results are shown in Table 1.
Table I
OBA charge Brightness with OBA Brightness without OBA
M added (% ISO)
(% ISO)
0.2 81.7 80.7
0.8 84.2 80.7
These results support the conclusion that the OBA addition to the peroxide
bleaching process can effectively increase the pulp brightness.
Example 3
Equivalent to 10 grams o.d. mill chelated aspen CTMP (HYP) from a mill
in Western Canada (ppm Mn, initial brightness) was mixed in a polyethylene bag
with 4.0% NaOH, 2.6% silicate, 0.05% Epsom salt (MgSO4=7 H20), 6.2% H202,
various amounts of OBA (Tinopal UP from CIBA Chemicals). The bag, along
with its contents, was placed in a temperature bath at 80 . The other
bleaching
conditions were 2 hrs, 17% pulp consistency.
After the completion of the required reaction time, a portion of the pulp
slurry was transferred from the bag to a beaker, diluted, and neutralized with
sulfurous acid to pH 5. A handsheet was then made following TAPPI test
method T272 [12], air-dried and determined for brightness. The same pulp
CA 02620978 2008-02-07
sample was subjected to the peroxide bleaching without the addition of OBA
under otherwise the same conditions. The results are given in Table 2.
Table 2
OBA Brightness with Brightness Brightness with OBA
charge OBA without added after peroxide
(%) added to peroxide OBA bleaching
stage (% ISO) (% ISO)
(% ISO)
0.2 90.1 86.4 89.0
0.4 91.7 86.4 90.3
0.8 92.4 86.4 91.5
Another set of experiments was performed. Under a constant mixing,
various amounts of OBA were added to a pulp slurry at 1% consistency (the pulp
was bleached under the conditions of 4% NaOH, 2.6% silicate, 0.05% Epsom
salt, 6.2% H202, 17% pulp consistency, 80 C, 2 hrs, and its brightness was
86.4%). After 5 min, a handsheet was then made following TAPPI test method
T272 [12], air-dried and determined for brightness. The above procedure is
similar to that outlined in European Patent application, EP 0899 373 Al. The
results are shown in Table 2 as brightness with OBA added after peroxide
bleaching. One can find that our present process is much more efficient in
improving pulp brightness, i.e. at the same OBA dosage, one can obtain a
higher
brightness.
Example 4
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A softwood bleached kraft pulp (SW BKP), a hardwood (eucalyptus)
bleached kraft pulp (HW BKP) and commercial aspen high yield pulps (HYPs)
were obtained from Tembec Inc. The kraft pulps were refined in a PFI to about
450 and 500 ml CSF freeness, respectively, and the high yield pulps were used
as received. Optical brightening agents were obtained from Ciba. The charge of
OBA in this study was based on the liquid products.
The photo-reversion of handsheets was performed in a photoreactor, with
a total intensity of either 9-10 mW/cm2 (approximately 53 times greater than
the
intensity of normal office light) fluorescent light or 2.7 mW/cm2 UV light
(with a
nominal wavelength of 350 nm). A fan is installed in the back of the
photoreactor
for temperature control.
Incorporating OBA Brightening into the Alkaline Peroxide Bleaching
Process in HYP Production
Conventionally, OBA is used at the wet-end of the papermaking process.
When HYP is substituted for HW BKP in printing and writing paper to take
advantages of its high bulk and light scattering, the brightness and whiteness
are
affected negatively due to the inferior brightness and whiteness of HYP.
However, the negative effect of HYP on brightness and whiteness can be
compensated for by adding more OBA at the wet-end [14].
The present inventors have discovered that by adding the optical
brightening agents to the added to HYP at the pulp mill, the higher
temperature
and longer contact time can improve OBA retention and thus its brightening
efficiency. If OBA experiences no interference with the alkaline peroxide
bleaching, and the alkaline peroxide has no effect on the fluorescent
properties
of the OBA, the application of OBA can be conveniently incorporated into the
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bleaching process. Based on experimental disclosed herein, the inventors have
confirmed that there is no reaction between OBA and the peroxide-containing
bleach liquor under the normal bleaching conditions. Figure 3 also shows that
different levels of peroxide concentration and DCS have negligible effect on
the
brightening efficiency of OBA. In the mill operation, OBA can be added to the
pulp along with the bleach liquor, as shown in Figure 1 and Figure 2.
To bleach HYP to high brightness (e.g. 85% ISO), stronger bleaching
conditions (higher peroxide charge, alkalinity and temperature) are usually
needed, which can decrease the bulk and light scattering coefficient of HYP.
If
the brightness target of HYP is lowered from 85% ISO to 83% ISO in peroxide
bleaching, the production cost of HYP will decrease significantly, while the
bulk
and light scattering of HYP can be preserved. The concept can decrease the
production cost. For example, for peroxide bleaching to increase 83% ISO to
85% ISO, an additional 2% peroxide is required; for using OBA to achieve the
same brightness gain, about 0.2% Tinopal UP is needed. The cost for the OBA
and hydrogen peroxide is about $2000/t and $800/t respectively, thus the
economic benefit of using OBA is rather evident.
Adding OBA to Pulp Furnish Containing OBA-treated HYP
For high brightness printing and writing paper grades (>88% ISO), OBA is
normally added at the wet-end of the papermaking process. When the OBA-
treated HYP is partly substituted for hardwood BKP, a question arises as to
whether the brightening efficiency of OBA added at the wet-end will be
affected
negatively.
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We designed a set of experimental trials, whereby, the hardwood BKP
was partially substituted with the OBA-treated HYP (Grade 325/90); OBA was
then added to the mixed furnish at 1% pulp consistency, 100 ppm calcium
concentration, pH 6.5. Theses conditions were similar to the wet-end addition
of
OBA in the papermaking process. Handsheets were then made to determine the
brightness. Results in Table 2 shows that for a brightness level up to 92%
ISO,
the presence of the OBA-treated HYP has negligible effect on the final
brightness
for the HYP substitution of 15% or lower. However, at a brightness higher than
92% ISO (achieved with more OBA), or a HYP substitution rate of higher than
15%, slightly more OBA than the control (0% OBA-treated HYP) is needed to
reach the same brightness and whiteness.
In general, OBA-treated HYP can be used to replace part of BKP in
printing and writing paper with negligible effect on the brightness, whiteness
and
yellowness of paper products even when OBA is used as well at the wet-end of
the papermaking process. Under normal conditions, the efficiency of OBA added
at the paper machine remains similar whether there is OBA already on HYP
fibers or not.
Advantages of Adding OBA to HYP during the HYP Manufacturing Process
The advantages of adding OBA to HYP during the HYP manufacturing
process over the conventional wet-end OBA addition are discussed below.
Improving the OBA Efficiency
OBA molecules adsorb on pulp fibers by forming hydrogen bonding with
cellulose fibers. When OBA is added to a furnish that contains both HYP and
BKP, OBA will adsorb preferably on BKP fibers as they are essentially free of
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lignin. As shown in Figure 4, at the same OBA concentration in the liquid
phase,
the BKP fibers adsorbed much more OBA than the HYP fibers. It would be
beneficial to have more OBA adsorbed on HYP fibers. OBA may cover up
and/neutralize the yellowish color of HYP fibers more effectively when it is
on
HYP fibers. As shown in Table 3, at the same OBA dosage, the whitening
efficiency of OBA was slightly better when it was added to HYP at the pulp
mill.
Minimizing the Interference from Cationic Polymers such as PEI
It is well known that cationic polymers and metal ions have negative
effects on the performance of OBA [6, 15]. PEI is a cationic polymer that is
commonly used in the retention systems to neutralize the negative effect of
anionic trash. It can react with OBA to form complexes and reduce the
brightening efficiency of OBA. A pre-adsorption of OBA onto pulp fibers before
PEI addition may reduce their interaction, leading to an improved OBA
efficiency.
Table 4 compares the efficiency of two OBA addition methods (A and B)
at various HYP substitution rates and OBA dosages, in the presence of PEI
(0.2%). Method A is a lab simulation of adding OBA to HYP at the pulp mill,
and
Method B is a lab simulation of the conventional wet-end addition of OBA. The
results show that at the same HYP substitution rate and OBA dosage, Method A
always gave higher brightness and whiteness, and lower yellowness (b*). The
higher OBA efficiency of Method A is due to less quenching effect from PEI.
Figure 4 compares the brightness gain from OBA (fluorescent
composition) of the two systems at various conditions, which demonstrates the
superior OBA performance when added during the HYP manufacturing process.
CA 02620978 2008-02-07
For Method B, OBA is usually added to the pulp fibers first and as far as
possible from the addition point of PEI to decrease their interactions.
However,
the conclusion does not change even when the time interval between OBA and
PEI additions was up to 60 minutes for Method B (Table 5). To minimize the
interference of PEI more effectively, OBA would have to be absorbed and fixed
on fibers. The drying process after OBA addition in the HYP manufacturing
process may help fix OBA molecules on fibers by forming more and stronger
hydrogen bonding.
Minimizing the Interference of Metal Ions in the White Water
Metal ions such as A13+ and Fe3+ are present in the white water system, in
particular for the acidic papermaking system. These metal ions can react with
OBA molecules to form deposits and thus decrease the OBA efficiency [6].
Fixing OBA molecules onto fibers by adding OBA to HYP at the pulp mill can
reduce their reactivity towards the harmful metal ions. In Table 6, various
amount of A13+ and Fe3+ ions were added to pulp suspension, and the tolerance
of OBA to the metal ions was compared between the two addition methods, with
all other conditions unchanged. The results show that the interference of
metal
ions is significantly less when OBA is added to HYP at the pulp mill (Method
A),
evidenced by the constantly higher brightness, fluorescence composition and
whiteness, and lower yellowness.
Decreasing the Color Reversion
Another advantage of adding OBA to HYP at the pulp mill is that OBA on
HYP fibers may also act as a UV screen and thus decrease the photo-yellowing
(color reversion) of HYP during shipping, storage and application [16, 17]. As
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CA 02620978 2008-02-07
shown in Figure 5, the photo-reversion of OBA-treated HYP is much less than
the regular HYP when exposed to UV radiation under the same conditions.
The same benefit can also be seen when using the OBA-treated HYP in
the production of HYP-containing paper. If OBA is added to HYP at the pulp
mill,
more OBA will stay on HYP fibers to protect them from light radiation and thus
decrease the photo-reversion of the paper. As shown in Figures 6 and 7, a
noticeable reduction in reversion was observed for the paper sheet with OBA-
treated HYP in both the UV and fluorescent light treatments.
Other Practical Considerations
In some paper mills, HYP is co-refined with hardwood BKP. A question
arises as to whether the strong mechanical force and interaction between HYP
and BKP fibers during co-refining will diminish the benefits of OBA-treated
HYP.
To answer this question, the inventors designed another set of experiments as
shown in Table 7. For Method A, 50% OBA-treated HYP was co-refined with
50% HW BKP in a PFI mill to 450 ml CSF, and was then made into handsheets
under different wet-end conditions; for Method B, 50% regular HYP was co-
refined with 50% HW BKP, and then OBA was added. The results in Table 7
show that Method A is still much better than Method B with respect to the
brightening efficiency of OBA. Therefore, one can conclude that the benefits
of
the OBA-treated HYP can be maintained after the co-refining process.
Another question is the affinity of OBA on HYP fibers, once added in the
HYP manufacturing process. Water extraction at 50 C is a good simulation of
the situation in a paper mill where the OBA-treated HYP will be used in the
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process. As shown in Table 8, a hot water extraction (50 C, 1 hour) changed
the
optical properties of the OBA-treated HYP only slightly.
To test the affinity of OBA on HYP fibers under a high shear force
environment, a standard disintegrator operated at 3000 rpm was used to
simulate the re-pulping, refining and pumping processes in a paper mill. As
shown in Table 9, the change of the optical properties of the OBA-treated HYP
was small, indicating that the OBA-treated HYP can survive the typical
mechanical treatments in a paper mill.
OBA can be a cost-effective solution to improve the optical properties of
HYP. A partial substitution of HW BKP with the OBA-treated HYP has negligible
influence on the final paper brightness and whiteness, which would be an
encouragement to more HYP applications in more paper grades. Neither OBA
has interference to the alkaline peroxide bleaching process, nor the bleaching
chemicals on the performance of OBA. Therefore, the OBA brightening process
can be incorporated into the peroxide bleaching process conveniently.
Adding OBA to HYP at the pulp mill can have several advantages over the
conventional wet-end addition of OBA. First, the quenching effect on OBA by
the
wet-end cationic polymers such as PEI can be decreased by fixing OBA on HYP
fibers before going into the papermaking process. Moreover, the negative
impact
of metal ions in the white water system on the OBA performance can be
minimized when OBA is pre-adsorbed and fixed on HYP fibers. Furthermore, the
photo-yellowing (color reversion) of HYP and HYP-containing paper sheets can
be decreased when more OBA is on HYP fibers to protect them from harmful UV
radiation. OBA has good affinity to HYP fibers when it is added in the HYP
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manufacturing process, and it retains well when subjected to water extraction
(50 C), disintegration and co-refining treatments.
TABLE 3. COMPARISON OF THE OBA EFFICIENCY BETWEEN TWO ADDITION METHODS
Addition Method A: OBA added to HYP B: OBA added to mixed
at the ul mill furnish at the wet-end
OBA dosa e%, on total furnish) 0.25 0.50 0.25 0.50
Brightness, % ISO 90.7 91.9 90.3 91.5
Whiteness 84.1 87.2 83.0 86.5
1.97 1.30 2.21 1.38
Note: Furnish composition: 30% SW+ 20% HW + 50% HYP; OBA: Di-sulfonic OBA; wet-
end
conditions:
1% pulp consistency; pH 6.5, 100 ppm Ca2+, 5 minutes of contact time.
TABLE 4. COMPARISON OF THE EFFECT OF PEI ON OBA EFFICIENCY BETWEEN THE
TWO OBA ADDITION SYSTEMS
PEI dosage, HYP OBA dosage Brightness b* CIE whiteness
% % % %ISO
A: OBA was added to HYP at the lp mill
0.2 10 0.05 87.54 3.38 76.44
0.2 20 0.10 88.78 2.72 79.66
0.2 30 0.15 88.72 2.46 80.31
0.2 50 0.25 89.27 2.67 80.16
0 30 0.15 90.68 2.05 83.70
B: OBA was added to mixed furnish at the wet-end
0.2 10 0.05 86.22 3.62 74.35
0.2 20 0.10 87.05 3.38 75.84
0.2 30 0.15 87.49 3.42 76.12
0.2 50 0.25 87.30 3.82 74.51
Furnish: 30% SW BKP, 20-60% HW BKP and 10-50% HYP (Aspen 325/85); OBA type: Di-
sulfonic; 0.2% PEI; OBA was added first, followed by the addition of PEI after
2 minutes of
mixing, and a handsheet was made after another 5 minutes of mixing.
TABLE 5. EFFECT OF TIME INTERVAL BETWEEN OBA AND PEI ADDITION ON OBA
EFFICIENCY
OBA addition Time interval between Brightness Bright. gain b* CIE
method OBA and PEI addition, due to OBA whiteness
min. % ISO % ISO
Method B 2 87.51 1.89 3.42 76.12
Method B 20 87.79 2.04 3.28 76.83
Method B 60 87.68 2.01 3.18 77.11
Method A - 88.72 2.75 2-.46 80.31
Other conditions: 30% SW BKP + 40% HW BKP + 30% HYP, 0.15% OBA (Di-sulfonic);
OBA was
added to the mixed furnish before the addition of PEI (0.2%).
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CA 02620978 2008-02-07
TABLE 6. EFFECT OF METAL IONS ON OBA EFFICIENCY
Wet-end conditions Bri.ghtness (% Bright. gain due b* CIE Whiteness
ISO) to OBA % ISO
A: OBA was added to HYP at the pulp mill (0.15% Di-sulfonic OBA
Blank A 90.4 3.6 2.5 82.0
AI 2.5ppm 90.3 3.6 2.5 81.7
A1+5.0 m 90.2 3.6 2.6 81.3
AI 10 ppm 89.6 3.5 2.8 80.2
AI + 20 ppm 89.4 3.4 2.9 79.6
Fe 0.1 m 90.1 3.6 2.6 81.4
Fe+0.2 m 90.0 3.5 2.5 81.3
Fe 0.4ppm 89.0 3.4 2.5 80.5
Fe 0.8ppm 88.2 3.3 2.6 79.3
B: OBA was added to the mixed furnish at the wet-end (0.15% di-sulfonic OBA)
Blank B 90.5 3.6 2.5 81.9
AI 2.5 m 88.3 2.7 3.4 77.1
AI + 5.0 ppm 88.2 2.3 3.5 76.5
AI 10 ppm 88.1 1.9 3.5 76.7
AI 20 ppm 88.0 1.7 3.8 75.7
Fe + 0.1 m 87.8 1.4 3.8 75.3
Fe 0.2ppm 87.6 1.4 3.9 74.9
Fe 0.4ppm 87.3 1.5 3.7 75.0
Fe + 0.8ppm 86.4 1.5 3.7 74.1
Other conditions: 30% SW BKP + 30% HYP + 40% HW BKP; pH 4.5; 5 minutes.
TABLE 7. COMPARISON OF THE TWO OBA ADDITION METHODS WITH CO-REFINING OF
HYP AND BKP
Wet-end conditions Brightness Bright. gain due b* CIE Whiteness
(% ISO to OBA % ISO)
A: OBA was added to HYP at the pulp mill (0.25% Di-sulfonic OBA
Blank,A, pH 6.5. 89.6 4.81 2.83 80.0
0.2% PEI, PH=6.5 89.2 4.06 2.73 79.7
0.5% PEI, PH=6.5 87.2 3.95 3.65 74.9
20 ppm AI , pH 4.5 88.2 4.21 3.41 76.6
20 ppm AI +, PH=6.5 90.1 4.86 2.45 81.6
1.0 ppm Fe , PH=4.5 84.7 4.11 2.79 74.7
1.0 ppm Fe , PH=6.5 85.0 4.26 2.53 76.0
B: OBA was added to mixed furnish at the wet-end (0.25% Di-sulfonic OBA)
Blank B, pH 6.5 89.7 3.96 3.18 79.1
0.2% PEI, PH=6.5 " 86.3 1.67 4.23 72.0
0.5% PEI, PH=6.5 * 85.2 2.12 4.87 68.8
0.5% PEI, pH=6.5 85.4 2.72 4.60 70.0
20 ppm AI , pH 4.5 85.7 2.14 4.69 70.2
20 ppm AI , PH=6.5 89.4 4.53 2.55 80.7
1.0 ppm Fe , PH=4.5 83.1 2.45 3.80 69.9
1.0 ppm Fe , PH=6.5 84.9 3.85 2.64 75.6
Furnish: 50% HYP + 50% HW BKP; OBA treated HYP was air-dried to simulate the
commercial
HYP process.
Co-refining of HYP and HW BKP: 2000 PFI revolution to about 450 ml CSF.
* PEI was added before OBA; ** OBA was added before PEI.
CA 02620978 2008-02-07
TABLE 8. EFFECT OF HOT WATER EXTRACTION ON THE OPTICAL PROPERTIES
OF OBA-TREATED HYP
OBA dosage Brightness L " a* b* CIE Fluorescent
% (%ISO) Whiteness Com . % ISO
Before hot water extraction
0 83.3 96.9 -1.37 6.93 60.6 0
0.05 84.2 96.8 -1.11 6.19 63.9 0.89
0.2 85.8 96.9 -0.68 5.04 69.2 2.57
0.6 87.7 96.9 -0.22 3.89 74.6 4.39
After hot water extraction (1.0% consistenc , 50 C for 1 hour
0 83.1 96.6 -1.25 6.65 60.2 0
0.05 84.0 96.6 -1.10 5.89 63.6 0.82
0.2 85.4 96.7 -0.66 5.03 68.8 2.15
0.6 86.9 96.6 -0.24 3.89 73.9 3.75
Note: HYP grade: Aspen 325/83; OBA type: Di-sulfonic.
TABLE 9. EFFECT OF MECHANICAL TREATMENT ON OBA-TREATED HYP
OBA dosage (%) Broghtness L" a" b* CIE whiteness Fluores cent
1oIS0 Com . /oISO
Before re- ul ping
0 83.7 97.1 -1.51 7.00 60.8 0
0.2 85.6 96.9 -0.78 5.23 68.3 2.57
0.6 87.6 97.0 -0.37 4.08 73.9 4.39
After re-pulping
disinte ration at 3,000 rpm for 5 minutes, 0.5% consistency)
0 83.2 96.8 -1.24 6.82 60.8 0
0.2 85.4 96.8 -0.70 5.22 68.2 2.23
0.6 87.1 96.8 -0.36 4.13 73.3 3.98
Note: HYP grade: Aspen 325/83; OBA type: Di-sulfonic.
Thus, broadly speaking, the present invention provides a process for
bleaching wood pulp, comprising the steps of mixing a wood pulp slurry
containing a chelating agent to form a mixture which is washed and pressed to
yield a transition metal-depleted pulp slurry and a discharge effluent. This
transition metal-depleted pulp slurry is mixed with an alkaline peroxide
bleaching
liquor including at least H202 and an optical brightening agent (OBA) to form
a
mixture which is bleached to give a peroxide bleached pulp containing the
optical
brightening agent.
As used herein, the terms "comprises", "comprising", "including" and
"includes" are to be construed as being inclusive and open ended, and not
21
CA 02620978 2008-02-07
exclusive. Specifically, when used in this specification including claims, the
terms
"comprises", "comprising", "including" and "includes" and variations thereof
mean
the specified features, steps or components are included. These terms are not
to
be interpreted to exclude the presence of other features, steps or components.
The foregoing description of the preferred embodiments of the invention
has been presented to illustrate the principles of the invention and not to
limit the
invention to the particular embodiment illustrated. It is intended that the
scope of
the invention be defined by all of the embodiments encompassed within the
following claims and their equivalents.
REFERENCES
1. Zhou, Y., "Overview of high yield pulps (HYP) in paper and board",
PAPTAC 90th Annual Meeting, 2004, B143-148 (Montreal, Canada)
2. Cannell, E. and Cockram, R., "The future of BCTMP", Pulp and Paper,
74(5): 61-76 (2000)
3. Reis, R., "The increased use of hardwood high yield pulps for functional
advantages in papermaking", Proceedings of the 2001 Papermakers
Conference, 2001, 87-108 (Cincinnati, OH, US)
4. Levlin, J.E., "On the use of chemi-mechanical pulps in fine papers", Paperi
ja Puu - Paper and Timber, 72(4): 301-308 (1990)
5. Ford, M. and Sharman, P., "Performance of high yield hardwood pulp is
investigated as it should be the choice of the future", Pulp & Paper
International, 38(10): 29 (1996)
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CA 02620978 2008-02-07
6. Neimo, L. (book editor), Papermaking Science and Technology, Book 4,
Papermaking Chemistry, Fapet Oy, Helsinki, Finland (1999)
7. Ragauskas, A.J., Allison, L., Lucia, L.A. and Li, C., "Brightness reversion
of mechanical puips XIV: Application of FWAs for high-brightness, high-
yield pulps", 2001 TAPPI PEER-REVIEWED PAPER/Solutions, 84(11): 55
8. Bourgoing, S. and Robert, S., "Inhibition of light induced colour reversion
by diaminostilbene derivatives fluorescent whitening agents-molecular
simulation correlation", Proceedings of PAPTAC 87th Annual Meeting,
2001, B47-B54
9. Scheringer, M., Halder, D. and Hungerbuhler, K, "Comparing the
environmental performance of fluorescent whitening agents with peroxide
bleaching of mechanical pulp", Journal of Industrial Ecology, 3(4): 77-95
(1999)
10. Zhang, H., He, Z., Ni, Y., Hu, H. and Zhou, Y., "Effectiveness of optical
brightening agent (OBA) on high yield pulps (HYP)", Proceedings of 93rd
PAPTAC Annual Meeting, Feb. 2007, B235-240 (Montreal).
11. Zhang, H., Hu, H., Z. He, Y. Ni, and Y. Zhou, Retention of Optical
Brightening Agents (OBA) and their brightening Efficiency on HYP-
containing Paper Sheets, J. Wood Chem. Tech., 27(4): 115, 2007.
12. "Forming handsheets for reflectance testing of pulp (sheet machine
procedure)", TAPPI Test Methods T272 (1992).
13. "Diffuse brightness of pulp (d/0 )", TAPPI test Methods, T525 (1992).
23
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14.Zhang, H., He, Z., Ni, Y., Hu, H. and Zhou, Y., "Using optical brightening
agents (OBA) for improving the optical properties of HYP-containing paper
sheets", Proceedings of 94~d PAPTAC Annual Meeting, Feb 5-7, 2008,
13561-B569, (Montreal).
15. Crouse B.W. and Snow G.H., "Fluorescent whitening agent in the paper
industry: Their chemistry and measurement", Tappi J., 64(7): 87-89
(1981).
16. Bourgoing, S., Leclerc, t., Martin, P. and Robert, S., "Use of fluorescent
whitening agents to inhibit light-induced colour reversion of unbleached
mechanical pulps", Journal of Pulp and Paper Science, 27(7): 240-244
(2001).
17. Bourgoing, S. and Robert, S., "Use of fluorescent whitening agents
against light-induced colour reversion of high yield pulps", Proceedings of
gth International Symposium on Wood and Pulping Chemistry (ISWPC),
Montreal, Canada, 10-1 - 10-4 (1997).
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