U.S. patent application number 14/454883 was filed with the patent office on 2015-04-23 for dissolving pulp and a method for production thereof.
The applicant listed for this patent is University of New Brunswick. Invention is credited to Zhibin HE, Yonghao NI, Jaroslav STAVIK.
Application Number | 20150107789 14/454883 |
Document ID | / |
Family ID | 52825142 |
Filed Date | 2015-04-23 |
United States Patent
Application |
20150107789 |
Kind Code |
A1 |
NI; Yonghao ; et
al. |
April 23, 2015 |
DISSOLVING PULP AND A METHOD FOR PRODUCTION THEREOF
Abstract
A method of preparing dissolving pulp. The method includes
physically separating a kraft pulp or a kraft hydrolysis pulp into
first and second fractions, the first fraction having a relatively
low lignin content and the second fraction having a relatively high
lignin content.
Inventors: |
NI; Yonghao; (Fredericton,
CA) ; STAVIK; Jaroslav; (Halifax, CA) ; HE;
Zhibin; (Fredericton, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of New Brunswick |
Fredericton |
|
CA |
|
|
Family ID: |
52825142 |
Appl. No.: |
14/454883 |
Filed: |
August 8, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61892585 |
Oct 18, 2013 |
|
|
|
Current U.S.
Class: |
162/55 |
Current CPC
Class: |
D21C 3/02 20130101; D21H
17/005 20130101; D21C 5/005 20130101; D21C 1/02 20130101; D21H
11/04 20130101 |
Class at
Publication: |
162/55 |
International
Class: |
D21D 5/24 20060101
D21D005/24; D21H 17/00 20060101 D21H017/00; D21C 5/00 20060101
D21C005/00 |
Claims
1. A method of preparing dissolving pulp, the method comprising the
steps of: (i) physically separating a kraft pulp into first and
second fractions, the first fraction having a relatively low lignin
content and the second fraction having a relatively high lignin
content; and (ii) processing the first fraction to produce a
dissolving pulp.
2. The method of claim 1, wherein step (i) includes separating pulp
particles on the basis of size, wherein particles of the first
fraction are relatively large and particles of the second fraction
are relatively small.
3. The method of claim 2, wherein the second fraction has a lignin
content of at least about 15% by weight.
4. The method of claim 3, wherein the first fraction has a lignin
content of no more than about 6% by weight.
5. The method of claim 4, wherein the unseparated kraft pulp has a
kappa number of between 10 and 50.
6. The method of claim 5, wherein the kappa number of the first
fraction is no more than about 80% the kappa number of the
unseparated pulp.
7. The method of claim 6, further comprising the step of, prior to
step (i), kraft pulping wood chips to produce the pulp.
8. The method of claim 7, wherein said wood chips are subjected to
steam and/or water prehydrolysis prior to said kraft pulping.
9. The method of claim 7, wherein the kraft pulp is obtained from a
conventional kraft process without a prehydrolysis step, and the
pulp subject of separating step (i) has a kappa number of between
10 and 40.
10. The method of claim 9, wherein the lignin content (wt %) of the
second fraction is at least 1.1 the lignin content of the first
fraction.
11. The method of claim 10, wherein between 85 and 95 wt % of the
unfractionated kraft pulp is contained in the first fraction
obtained in step (i).
12. The method of claim 11, further comprising the step of (iii)
exposing the first pulp fraction to a cellulase, xylanase, and/or
mannase enzyme to decrease the intrinsic viscosity of the pulp
fraction by between 50 and 700 ml/g (0.5% cellulose in a
cupriethylenediamine (CED) solution), optionally exposing the pulp
fraction to one or more cationic polymers wherein said polymer can
be selected from the group consisting of cationic polyacrylamide
(CPAM), polydiallyldimethylammonium chloride (DADMAC),
polyethylenimine (PEI), polyaluminum chloride (PAC), cationic
starch, a dual polymer system, wherein the cationic polymer can
further comprise an anionic polymer, microparticle-containing
system, and/or a silica-based cationic polymers.
13. The method of claim 12, wherein the enzyme comprises FiberCare
D.TM., present in a dosage 0.1 to 10 u/g of dry pulp; further
comprising the step of (iv) bleaching the first pulp fraction
before, after or as part of step (iii); further comprising the step
of (v) exposing the bleached pulp obtained in step (iv) to a
xylanase; further comprising the step of (vi) subjecting the pulp
obtained in step (v) to alkali; and wherein step (vi) comprises a
hot alkali treatment, or step (vi) comprises a cold alkali
treatment and the caustic soda concentration is in the range of 1
to 12 wt %.
14. The method of claim 13, further comprising recovering active
enzyme of step (iii) and cycling the recovered enzyme back into
step (iii), wherein amount of enzyme recycled is in the range of 30
to 90%.
15. The method of claim 6, wherein step (i) comprises centrifugal
separation.
16. The method of claim 6, wherein step (i) comprises screening of
the kraft pulp into the first and second fractions.
17. The method of claim 6, wherein the kraft pulp of step (i) is a
prehydrolysis kraft pulp.
18. The method of claim 6, further comprising the step of refining
the fiber fraction to produce the dissolving pulp.
19. The method of claim 18, wherein the dissolving pulp produced
has a lignin content of no more than about 0.3 wt %.
20. The method of claim 19, wherein the dissolving pulp produced
has an .alpha.-cellulose content of at least about 88 wt %.
21. The method of claim 20, wherein the dissolving pulp produced
has a pentosan content of no more than about 8 wt %.
22. The method of claim 21, wherein the Fock reactivity of the
dissolving pulp produced is at least 50%.
Description
FIELD
[0001] This invention relates to the treatment of kraft pulp in the
production of dissolving pulp.
BACKGROUND
[0002] Dissolving pulp is a raw material for manufacturing
cellulose derivatives and regenerated cellulose. Production of
dissolving pulp is growing. There are two commercial processes used
for production of dissolving pulp. One is the relatively well known
acid sulfite process, the other being production of dissolving pulp
from pulp produced via a prehydrolysis kraft process, which is
relatively new. It is commonly found that adaptation of current
kraft-based installations to dissolving pulp production has not
been entirely successful, at least to the extent that it seems that
dissolving pulps obtained to date have lower reactivity than those
obtained via sulfite-based processes.
[0003] Fock reactivity, defined as the % of cellulose dissolved
under the conditions specified[1], is an important parameter in
determining the suitability of a dissolving pulp for such purposes.
Reactivity is a measure of a dissolving pulp's ability to
chemically react with, for example, carbon disulfide and sodium
hydroxide in rayon production, or more specifically the ability for
hydroxyl groups on the glucose units of cellulose chains to react
with carbon disulfide.
[0004] Production of pulps described as suitable for lyocell
manufacture has been described in the patent literature, for
example, in U.S. Patent Publication No. 2009/0165969.
SUMMARY
[0005] In one aspect, the invention is a method of preparing
dissolving pulp, the method comprising the steps of:
(i) separating a kraft pulp into a fiber fraction and a fines
fraction; and (ii) processing the fiber fraction, to produce a
dissolving pulp.
[0006] The separation step is carried out so as to obtain a first
fraction having a relatively low lignin content and a second
fraction having a relatively high lignin content. In certain
embodiments, the separation step includes separating pulp particles
on the basis of size, wherein particles of the first fraction are
relatively large and particles of the second fraction are
relatively small. The second fraction can have a lignin content of
at least about 15% by weight, or 16%, of 17%, or 18%, or 19%, or
20%, or 21%, or 22%, or 23%, or at least 24%. The first fraction
can have a lignin content of no more than about 1%, or 2%, or 3%,
or 4%, or 5%, or 6%, or 7%, or 8%, or 9%, or 10%, or 11%, or 12%,
or 13%, or no more than about 14%.
[0007] In one aspect, the kraft pulp is subject of separating step
(i) is thus separated into the fiber and fines fractions according
to size to produce a fines fraction having a lignin content higher
than the lignin content of the fiber fraction.
[0008] In embodiments, the unseparated kraft pulp has a kappa
number of between 10 and 50.
[0009] The kraft pulp subject of separating step (i) can have a
kappa number of between 10 and 40.
[0010] The kappa number of the first fraction can be no more than
about 80% the kappa number of the unseparated pulp.
[0011] The method can be used with kraft pulp produced by a
conventional kraft pulping process, or the kraft pulp can be a
prehydrolysis kraft pulp. In the latter case, the kraft pulp would
generally have a kappa number of no more than about 20. The pulp
may be produced as a separate stock, and processed separately e.g.,
in a separate mill, or kraft pulping wood chips to produce the
kraft pulp may be carried out in-line along with subsequent steps
to produce the dissolving pulp. The wood chips can be subjected to
steam/water prehydrolysis prior to the kraft pulping.
[0012] The method can thus include a step of, prior to step (i),
kraft pulping wood chips to produce the pulp. Wood chips can
include softwood and the wood chips can be kraft pulped to obtain a
pulp having a kappa number of e.g., less than 40, or between 20 and
40, or between 25 and 40, or between 30 and 40. Softwood chips can
include e.g., spruce, hemlock, fir, larch, or combinations of any
of the foregoing. Hardwood chips can be kraft pulped to obtain a
pulp having a kappa number of less than 20, or between 10 and 20,
or between 10 and 15. Hardwood chips can include e.g., maple,
aspen, birch, or combinations of any of the foregoing.
[0013] According to certain embodiments, wood chips are subjected
to steam and/or water prehydrolysis prior to kraft pulping. The
kraft pulp can be obtained from a conventional kraft process
without a prehydrolysis step, and the pulp subject of separating
step (i) can have a kappa number of between 10 and 40, or between
30 and 40.
[0014] In embodiments, the lignin content (wt %) of the second
fraction is at least 1.1 the lignin content of the first fraction,
more preferably at least 1.2, 1.3, 1.4 or 1.5 of the first
fraction. Further, between 85 and 95 wt % of the unfractionated
kraft pulp can be contained in the first fraction obtained in step
(i).
[0015] The method can further include a step (iii), of exposing the
fiber fraction to a cellulase, xylanase, and/or mannase enzyme to
decrease the intrinsic viscosity of the pulp fraction by between 50
and 700 ml/g (0.5% cellulose in a cupriethylenediamine (CED)
solution). More preferably, the decrease is in the range of from
100 to 300 ml/g. The cellulase enzyme can be a multicomponent
enzyme or an enzyme having a single type of catalytic activity.
FiberCare D.TM. can be used in a dosage 0.1 to 10 u/g of dry pulp,
preferably, 0.3 to 2 u/g of pulp. Examples of other commercially
available enzymes are those sold under the names FiberCare U,
FiberCare R, Celluclast, Fiberzyme CS, Fiberzyme G200, Fiberzyme
LBR, Optimase Cx, Pyrolase HT cellulose, Pulpzyme HB, Pulpzyme HC,
Luminase PB-100 and Mannaway.RTM.. These can be used alone or in
combination with each other.
[0016] The method can include exposing the pulp fraction to one or
more cationic polymers during such an enzyme treatment step.
Cationic polymers can be included in enzymatic steps, particularly
cellulase-catalyzed steps to enhance enzymatic performance. The
polymer can be, but are not limited to a cationic polyacrylamide
(CPAM), polydiallyldimethylammonium chloride (DADMAC),
polyethylenimine (PEI), polyaluminum chloride (PAC), cationic
starch, or a dual polymer system, and the cationic polymer(s) can
further include an anionic polymer, microparticle-containing
system, and/or a silica-based cationic polymers. If present,
polymer is present at e.g., a dosage in the range of 0.01 to 1000
ppm.
[0017] In embodiment the method includes a step of (iv) bleaching
the first pulp fraction before, after or as part of step (iii). The
method can further include a step of (v) exposing the bleached pulp
obtained in step (iv) to a xylanase. Such method can further
include the step of (vi) subjecting the pulp obtained in step (v)
to alkali.
[0018] Step (vi) can be a hot alkali treatment, or step (vi) can be
a cold alkali treatment with the caustic soda concentration in the
range of 1 to 12 wt %, preferably, 5 to 12 wt %, even more
preferably 8 to 10 wt %.
[0019] In embodiments that include enzymatic treatment step (iii),
the method can further include recovering active enzyme of step
(iii) and cycling the recovered enzyme back into step (iii). The
amount of enzyme recycled can be in the range of 30 to 90%, more
preferably 50 to 80%. In embodiments that include step (v), the
method can include recovering active enzyme of step (v) and cycling
the recovered enzyme back into step (v).
[0020] The dissolving pulp produced typically has a relatively low
lignin content of no more than about 0.3 wt %, no more than about
0.2 wt %, or no more than about 0.1 wt %.
[0021] The dissolving pulp produced typically has an
.alpha.-cellulose content of at least about 88 wt %, or at least
about 90 wt %, or at least about 92 wt %, or at least about 94 wt
%, or at least about 96%, or higher.
[0022] The dissolving pulp produced typically has a pentosan
content of no more than about 8 wt %, or less than about 7 wt %, or
less than about 6 wt %, or less than about 5 wt %, or less than
about 4 wt %.
[0023] The pulp of step (i) can be unbleached pulp.
[0024] The separating step is carried out by e.g., centrifugal
separation, such as by use of a hydrocylone or series of screens,
or similar setups, or it can be by screening of the kraft pulp into
the fiber fraction and fines fraction.
[0025] The method can include (iv) bleaching the pulp, before,
after or as part of step (iii). The method can further include the
step of (v) exposing the bleached pulp to a xylanase. The method
can also include the step of (vi) subjecting the pulp produced in
step (v) to alkali. Step (vi) can be a hot alkali treatment or a
cold alkali treatment, as discussed further below.
[0026] The method can also include a step of refining the fiber
fraction to produce the dissolving pulp. The Fock reactivity of the
dissolving pulp produced can be at least 20% more preferably, 30%,
40%, 50%, more preferably at least 60%.
[0027] The method can also include a step of (a) exposing the
second i.e. fines fraction to a cellulase enzyme, mechanical and/or
alkali treatment.
[0028] The method can further include a step of (b) bleaching the
fines fraction i.e., step (a) can be carried out prior to,
subsequent to, or simultaneously with step (b). Such steps can be
carried out in different orders, and can be repeated.
[0029] It can be possible to process the fines fraction separately
to produce a pulp that might be used as stock to produce a paper
product, or the fines could be, or at least a portion of the fines
could be, processed into a dissolving pulp and rejoined with the
fiber fraction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 shows a flow diagram incorporating process steps of
the present invention; and
[0031] FIG. 2 shows Fock reactivity and viscosity of enzyme treated
dissolving pulps produced from the prehydrolysis kraft-based
process from a mixed of hardwood. Enzymatic treatment was performed
at pH 5.0 and temperature 55.degree. C. for 2 hours.
[0032] FIG. 3 shows the effect of the addition of 250 ppm CPAM on
the change in viscosity over time of pulp (consistency 3%, pH 4.8
and 55.degree. C.) treated with cellulase (mg/g pulp): 0.5
(.box-solid.), 1 ( ), and 1.5 (.tangle-solidup.). Cellulase charge
(mg/g pulp) with no added CPAM: 0.5 (.quadrature.), 1
(.largecircle.) and 2 (hollow inverted triangles).
[0033] FIG. 4 shows Fock reactivity of pulp (3% pulp consistency,
pH 4.8 and 55.degree. C.) after 12 hours (hatched bars) and 24
hours (unhatched bars) of cellulase treatment. The first, third and
fifth pairs of bars are 0.5, 1 and 2 mg of cellulase charge per gm
of pulp with no added CPAM. The second, fourth and sixth pairs of
bars are 0.5, 1 and 1.5 mg of cellulase charge per gm of pulp and
250 ppm CPAM.
DETAILED DESCRIPTION
[0034] This invention discloses a cost effective and
environmentally friendly method for improving the product quality
parameters and processing efficiency in the production of
dissolving pulp by the prehydrolysis kraft process or kraft
process.
[0035] Pulp quality, such as Fock reactivity, purity e.g., high
alpha-cellulose and low pentosan content, accessibility, is
important for dissolving grade pulp, these quality parameters being
important in the downstream production process regardless of the
pathways of functionalizing cellulose during viscose production and
processing.
[0036] Dissolving pulp from a prehydrolysis kraft pulping process
or from the kraft pulping process contains undesirable
carbohydrates components, including residual hemicelluloses after
cooking, such as glucomannan, glucuronoxylan. In addition, some
degraded cellulose chains might be too short or altered to suit the
properties of certain products such as cord rayon, high tenacity
staple fiber, modal fiber, and should therefore be removed.
Portions of the pulp that are smaller in size, such as ray cells
and fines, have inferior properties compared to those that are
larger in size, such as tracheid.
[0037] According to the present invention, the dissolving pulp
quality e.g, purity of the alpha-cellulose, reduced pentosan
content, increased reactivity and accessibility, can be improved
cost-effectively by additional purification steps or combining the
purification stage/stages with the existing manufacturing process,
especially in brown stock operations and bleach plant operations.
The purification of dissolving pulp can be conducted in a
controllable manner to meet the end-use specifications.
[0038] As shown in FIG. 1, a fractionation stage in which a rejects
portion containing relatively large proportions of fines and ray
cells is separated from fibers after prehydrolysis and kraft
pulping can include additional steps such as subsequent bleaching,
enzyme treatment and/or alkali purification.
Fractionation
[0039] U.S. Pat. No. 4,731,160 describes the fractionation method
for mechanical pulp into a major fraction and fines fraction. U.S.
Pat. No. 7,005,034 discloses a method for production of mechanical
pulp, wherein the pulp is fractionated after a first refinery stage
for separating the fine material from the pulp to facilitate the
refining of the fibers in the second stage. U.S. Patent Publication
No. 2007/0023329 describes a method for selective removal of ray
cells from pulp by combining screening and centrifugal
cleaning.
[0040] Unknown to the inventors is the description in literature of
the fractionation of a pulp to be used as, or in the production of,
a dissolving pulp, either from the prehydrolysis kraft pulping
process or the kraft process in order to obtain a fraction
containing a lowered lignin content.
[0041] According to the present invention, unbleached pulp from the
kraft pulping stage contains a small percentage of ray cells and
fragments of fibers and middle lamellae. This portion of pulp is
called fines. The fines portion has significant higher content of
impurities such as lignin, resin, extractives, and non-process
elements like iron, calcium, and magnesium. In addition, the fines
portion has a much slower drainage rate, and inferior
bleach-ability resulting in higher bleach chemicals consumption and
accessibility. Although the fines account for only a small amount
(usually less than 8%) of the mass of the whole pulp, their
negative impact on the performance and efficiency of the bleaching,
enzymatic treatment and alkali purification stages, as well as the
overall quality of the final dissolving pulp, is significant.
[0042] According to the invention, unbleached pulp is separated by
fractionation into two fractions: a fiber fraction and a fines
fraction, which are separately processed. This leads to overall
chemical and energy efficiencies and/or improved quality parameters
of dissolving grade pulp ultimately obtained from the fiber
fraction. FIG. 1 illustrates an embodiment of the invention.
[0043] Dissolving pulp can be produced in a more cost-effective
manner by fractionating pulp first to improve the chemical response
and drainage characteristics of the majority of the dissolving
grade pulp (the fiber fraction) in the bleaching, enzymatic
treatment and/or alkali purification step and/or mechanical
refining of fibers. The fines fraction separated by the
fractionation step, can be processed by bleaching, alkali
extraction and enzymatic treatment.
[0044] It thus becomes possible for regular kraft production to be
the basis for production of dissolving grade pulp suitable for
viscose process (xanthation), or for other cellulose derivatives
production.
[0045] The process can be varied based on the desired purity of
pulp.
[0046] Unbleached pulp from the kraft pulping stage is separated
into a minor fines fraction and major fiber fraction. The
separation may be accomplished using a screen, or centrifugal
separators (cyclones). The major fiber fraction is typically
subjected to enzymatic treatment and then subsequent bleaching
sequences to produce fully bleached pulp having higher brightness
and whiteness than pulp bleached without a separation stage at the
same chemical dosage, or to produce dissolving pulp at lower
chemical dosage and lower energy consumption for given pulp
brightness and whiteness. After bleaching, the fiber fraction is
subjected to enzymatic treatment and/or cold alkali purification
and/or refining to increase the .alpha.-cellulose content to the
specific target, for example, about 98%, and increase Fock
reactivity, in a more efficient way in terms of energy and chemical
consumption when compared to a similar process that does not
include such a fractionation stage. Table 1 shows the mass
percentage of lignin content for different pulp fractions of one
kraft brown stock pulp.
TABLE-US-00001 TABLE 1 Fiber Fraction (mesh size) % of total (wt %)
Lignin (wt %) >50 89.20 5.50 50-100 4.00 7.60 100-200 1.07 11.40
200-300 3.32 13.00 <300 2.24 24.30
[0047] As shown in Table 1, the lignin content is much different in
different fractions of the brown stock pulp, the longest fraction
(>50 mesh size) accounts for about 90% of the mass, but the
lignin content is the lowest (5.5%). One can thus see that (i)
residual lignin in unbleached pulp is unevenly distributed; and
(ii) the smallest fine fraction (mostly ray cells) contains much
higher lignin content than the fiber tracheids (long fibers). If
the fines fractions would be separated from the tracheids, due to
the significantly lower lignin content, smaller amounts of
bleaching chemicals would be needed in the subsequent bleach plant.
Also, if the fines are removed, the filterability of the dissolving
pulp should be improved.
[0048] As the separation processes described herein are,
ultimately, toward the production of a dissolving pulp, the pulp
subject of the separation, fractionation or screening process has a
suitably low overall lignin content. This is reflected in the kappa
number of the pulp prior to the separation step. For conventional
softwood kraft pulp, the kappa number may be in a range of 30 to
40; for a prehydrolysis kraft-based dissolving pulp, in
particularly, hardwood pulp, the kappa number can be substantially
lower, in the range of 10 to 16, as lignin is generally considered
an undesirable component of dissolving pulps. The results shown in
Table 1 were obtained using a conventional softwood kraft pulp
having a kappa number of about 38.
[0049] The fines fraction can be treated separately, and also more
efficiently by the processes involved, since the volume of the
fraction is smaller than that of the long fiber fraction. As shown
in Table 1, the mass percentage of the fine fraction (passed 200
mesh) is about 5%, but its lignin content is several times that of
the fiber fraction (fibers retained on 100 mesh or longer). Table 2
shows lignin and metal content of pulp fibers and ray cells, the
pulp being without any fractionation, and the ray sells being
obtained by screening. As can be seen, the ray cells, a major
component of the fine fraction, have a much higher iron and other
metal content than the pulp as a whole, so fines separation by
fractionation would decrease iron content of the pulp.
[0050] The fines fraction can thus be subjected to an acid
treatment stage to remove the iron more efficiently, and then
bleached by a bleaching sequence (e.g.
D.sub.0E.sub.OPHE.sub.PD.sub.2: D.sub.0 chlorine dioxide
pre-bleaching, E.sub.OP: oxygen and peroxide re-enforced alkali
extraction, H: hypochlorite, E.sub.P: peroxide re-enforced
extraction, D.sub.2: second chlorine dioxide brightening stage).
The fines fraction may also be treated by the enzyme treatment and
alkali purification if needed, again in a more efficient way.
[0051] In tests it was found that removal of the fines leads to the
dissolving pulp having improved drainage properties. This is shown
in Table 3 by the increase of the filterability (or CSF freeness)
of the pulp that was 60% maple, 30% aspen and 10% birch, all of
which are hardwoods. In addition, as shown in Table 3, the
brightness is much higher for the long fiber fraction than the
shorter fiber fraction, while S.sub.18 extractives, and pentosan
for the long fiber fraction, are lower.
TABLE-US-00002 TABLE 2 Lignin Mn Cu Fe Mg Ca [%] [mg/L] [mg/L]
[mg/L] [mg/L] [mg/L] Pulp 3.38 100 1.2 12 270 45 Ray Cell 8.1 178
25 146 587 123
TABLE-US-00003 TABLE 3 Long fibers Parameter Unit Feed (>100
mesh) Short fibers (<200 mesh) S.sub.10 % 6.5 5.6 N/A S.sub.18 %
4.2 3.56 4.89 Extractives % 0.15 0.10 0.41 Ash % 0.04 0.02 Pentosan
% 2.88 2.8 2.87 Brightness % 89 90.5 82.5 ISO Filterability 301 428
17
Enzymatic Treatment
[0052] In a prehydrolysis kraft process, as in the example of pulp
obtained here from the AV Nackawic mill, the majority of the
hemicelluloses in wood chips is degraded in the acidic
prehydrolysis step and dissolved in the subsequent kraft cooking
stage. Typically, unbleached pulp from the cooking process is
bleached to about 90% ISO brightness by a five-stage bleaching
sequence (for example, D.sub.0E.sub.OPHE.sub.PD.sub.2, D.sub.0
chlorine dioxide pre-bleaching, E.sub.OP: oxygen and peroxide
re-enforced alkali extraction, H: hypochlorite, E.sub.P: peroxide
re-enforced extraction, D.sub.2: second chlorine dioxide
brightening stage). Most of the lignin and wood extractives are
also removed in the cooking and bleaching processes. Upon drying,
the remaining cellulose fibrils are able to bind more closely to
each other to form a compact structure. The inter fibrillar spaces
of the pulp fibers collapse and thus the surface area and pore
volume decrease [2-5]. This irreversible process is called
hornification which causes decreased pulp accessibility/reactivity
[6,7]. The method of drying influences the size of the fibril
aggregate. Harsher drying (fast drying) produces larger lateral
fibril aggregates, resulting in lower reactivity [8]. The
hornification effect is more pronounced in the presence of
hemicellulose that has a very high bonding capability. This is one
of the reasons why the hemicellulose content of dissolving pulp has
to be reduced to a very low level.
[0053] Enzyme hydrolysis/degradation and chemical modification have
been proposed to increase the accessibility/reactivity of
dissolving pulp [5,9]. Within the past few years, a monocomponent
endoglucanase has been used to treat dissolving pulp to increase
accessibility/reactivity with promising results [10,11]. Pure
monocomponent cellulases of the endoglucanase type are commercially
available.
[0054] The activation mechanism of dissolving pulp by
endoglucanases is not completely understood, although hypotheses
have been put forward [12,13]. It is generally believed that the
attack on the less ordered cellulose regions by the endoglucanase
leads to fiber wall swelling and thus an increase in accessibility
towards solvents and reactants. Endoglucanase preferably degrades
amorphous cellulose located on the fiber surface and between the
microfibrils, which leads to increased crystalline surface exposure
and to increased swelling ability and reactivity of the pulp [14].
Endoglucanase may also increase reactivity by attacking cellulose
II [15,16]. Recent studies by Ibarra et al. reveal that the
activation effect of endoglucanase is affected by the modular
structure of the enzyme and the drying history of dissolving pulp
[17,18]. They found that endoglucanase with an inverting catalytic
domain and a cellulose binding domain is most effective in
activating cellulose, in particular when it is applied on
never-dried pulps.
[0055] Most studies, however, have focused on sulfite dissolving
pulp, and little has been reported in the literature on the effect
of enzyme treatment on the accessibility/reactivity of the
dissolving pulp from a prehydrolysis kraft process. The action of
the endoglucanase on kraft pulp differs from that on sulfite
dissolving pulp. For example, the reactivity of a sulfite
dissolving pulp increased rapidly from about 70% to almost 100%
within 10 min of incubation [9]. In contrast, post endoglucanase
treatment of a kraft pulp increased its reactivity only marginally
(from 19.1% to 22.9%) [19].
[0056] Endoglucanase may act on kraft pulp by a different
mechanism, or it may be that sulfite and kraft pulps differ in
chemical and physical properties. Modification of the bleaching
process of prehydrolysis kraft pulp production may improve its Fock
reactivity, as well as its response to enzyme treatment for
reactivity improvement. In a recent study, a spruce prehydrolyzed
kraft pulp was bleached with chlorite to remove the remaining
lignin, in order to study the effect of residual lignin on the Fock
reactivity [20]. The results showed that the Fock reactivity of the
pulp increased with decreasing kappa number (decreasing residual
lignin content), and the highest reactivity was obtained after
complete lignin removal using chlorite delignification. It was also
found that the carbohydrate composition had little influence on the
pulp reactivity, but lower intrinsic viscosity either obtained by
prolonged cooking or chlorite delignification correlated with
higher pulp reactivity.
[0057] It has been found here that treating the pulp with xylanases
and/or cellulases can significantly improve the purity and Fock
reactivity of the dissolving pulp. Xylanases can have an effect on
pore structure as well as pentosan content. The pre-bleach
application is the standard application point but xylanase can be
effectively applied at any point of the bleaching process where
conditions are acceptable. Hemicellulase (such as xylanase) is an
effective means to increase brightness and remove residual pentosan
after the bleaching.
[0058] Accessibility to the substrate can limit the effectiveness
of enzyme treatments, and especially so for dissolving grade pulps.
For this reason, it is most effective to have complementary
activities working simultaneously. Cellulases and hemicellulases
can complement each other and achieve greater results than either
alone. It is not always feasible to remove most of a single
component without disrupting other substrate structures. Multiple
applications of a particular treatment can be more effective than a
single treatment at the same net dose. Pentosan removal is more
efficient when a series of enzyme treatments are carried out at
different points in the process, especially when there is a wash
step or a different reactive step in between. This leads to the
prospect of a brown stock treatment and a post bleach treatment
with hemicellulases.
[0059] According to the present invention, the Fock reactivity of
dissolving pulp can increase from the enzyme treatment, as shown in
FIG. 2 (the reactivity and viscosity of resulting dissolving pulp
as a function of the cellulase (a multi-component cellulase sample,
FiberCare D, from Novozymes) dosage. It is evident that the
reactivity increased from 48.0% to 93.5% when increasing the
cellulase dosage from 0 to 2 ug.sup.-1 dry pulp, and the viscosity
decreased from 665.8 mlg.sup.-1 to 354.7 mlg.sup.-1; a further
increase in the cellulase dosage only resulted in a slight increase
in the Fock reactivity.
[0060] Other enzymes, such as FiberCare U, FiberCare R, Celluclast,
Fiberzyme CS, Fiberzyme G200, Fiberzyme LBR, Optimase Cx, Pyrolase
HT cellulose, may also be used for this purpose.
[0061] As shown in FIG. 2, the decrease in the pulp viscosity
corresponds roughly with the observed increase in the reactivity of
dissolving pulp.
[0062] One or more cationic polymers, such as CPAM (cationic
polyacrylamide) can be included in the cellulase treatment solution
to enhance the performance of the cellulase treatment step. FIG. 3
shows the effect on viscosity of the inclusion of CPAM at a
concentration of 250 ppm in the cellulase enzymatic treatment of
dissolving pulp for different enzyme concentrations. It can be seen
that the decrease in intrinsic viscosity over time is greater in
the presence of CPAM for a given charge of enzyme. The viscosity of
cellulase charge of 1 mg/g from the CPAM addition was lower than
that of 2 mg/g cellulase charge without CPAM, for example,
demonstrating that the cellulase efficiency was significantly
improved due to the addition of CPAM.
[0063] The use of CPAM in the cellulase treatment can also enhance
the increase in Fock reactivity, as shown in FIG. 4. As evident
from the results shown, treating samples with cellulase-CPAM
compositions resulted in pulp compositions have significantly
higher Fock reactivity.
[0064] According to present invention, the pentosan content of the
pulp can be decreased significantly with the enzyme treatment,
Ecopulp TX-800 (an industrial enzyme product) as shown in Table 4.
The yield losses for pulp treated by enzyme are from 1 to 1.3% per
1% increase in .alpha.-cellulose content.
[0065] Other enzymes may also be used, including Pulpzyme HB,
Pulpzyme HC, Luminase PB-100, and Mannaway.RTM..
TABLE-US-00004 TABLE 4 Addition Retention Enzyme (L/T) pH
Consistency Temp. Time Washed K# Pentosan Ecopulp 0 7.4 14.23%
60.degree. C. 30 min Yes*** 5.6 3.56 TX-800 A* Ecopulp 0.04** 7.4
14.23% 60.degree. C. 30 min Yes*** 5.5 3.48 TX-800 A* Ecopulp
0.08** 7.4 14.23% 60.degree. C. 30 min Yes*** 5.4 3.04 TX-800 A*
Ecopulp 0.10** 7.4 14.23% 60.degree. C. 30 min Yes*** 5.2 2.2
TX-800 A* 60.degree. C. 30 min Yes 60.degree. C. 30 min Yes
60.degree. C. 30 min Yes Samples were tested 24 days after
treatment. *Diluted 1 ml/1000 ml. **Additions - 0.4 ml/10 g, 0.8
ml/10 g, 1.0 ml/10 g of diluted enzyme. ***Washed in sheet former
with 15 L of water purified by reverse osmosis; dried in oven. BOD
Used the 4L/T sample 1 ml enzyme diluted to 1000 ml 0.4 ml of the
enzyme diluted to 2000 ml: 1/1000 .times. 0.4/20000 = 0.0000002 ml
of enzyme that sample gave 12 mg/l BOD.
Alkali Treatment
[0066] Alkaline treatment/purification of pulp can principally be
carried out in two ways, cold and hot alkaline purification: [0067]
a) Cold purification consisting of the treatment of pulp in
concentrated lye around room temperature, permitting short chain
material and microfibril fragments to dissolve. Cold purification
thus primarily involves physical changes to the pulp, and only a
small amount of alkali is consumed; and [0068] b) Hot alkali
purification, performed at a higher temperature (usually higher
than 70.degree. C., and in some cases, higher than 100.degree. C.)
with relatively a lower alkali dosage (0.2-4 wt % on pulp). The
more accessible parts of the fiber react under these conditions,
with the formation of organic acids.
Cold Alkali Purification
[0069] The cold alkali treatment is a selective process of
increasing the alpha cellulose content of the dissolving pulp,
decreasing the pentosan content and increasing the pulp viscosity
of dissolving pulp from the prehydrolysis kraft pulping process.
Degraded cellulose can have a good solubility at about a 10 wt %
NaOH concentration at room temperature, while the solubility of
hemicelluloses increases with increasing NaOH concentration.
[0070] Shown in Table 5 are the results obtained using a cold
alkali purification process. As can be seen, after cold alkali
purification under the conditions specified, the pulp viscosity
increased from 605 to 641.6 ml/g, the .alpha.-cellulose content
increased from 94.3 to 97.9% while the pentosan content decreased
from 4.42 to 1.41%. Reactivity decreased from 39.2% to 20.8%. The
decrease reactivity is due to the increased crystallinity of the
treated fibers.
TABLE-US-00005 TABLE 5 Reactivity Viscosity Pentosan Sample ID (%)
(ml/g) (%) .alpha.-Cellulose (%) Before alkaline 39.2 605.0 4.42
94.3 treatment After alkaline 20.8 641.6 1.41 97.9 treatment
Conditions: 9 wt % NaOH concentration, 9 wt % pulp consistency, 30
minutes, 35.degree. C. Each sample was washed with deionized water
until pH 6-7 then air dried.
[0071] Suitable ranges for the temperature and alkali concentration
for the cold alkali purification are 5 to 40.degree. C. and 5 to
18%. Under these conditions, mainly physical changes occur.
35.degree. C. is a preferred temperature for the prehydrolysis
kraft dissolving pulp. Cold alkali treated pulp is sensitive to
oxidative attack, which may lead to a decrease in the pulp
viscosity. Reaction time can be short and the swelling reaction
takes place almost instantaneously. Pulp consistency can vary from
3.5 to 20%. The pentosan content of the pulp decreases as the
.alpha.-cellulose content is increased after the cold alkali
treatment.
Hot Alkali Purification
[0072] In contrast to cold purification, the hot alkali treatment
involves chemical reactions in the purification process. Swelling
is limited, as the concentration of lye in the reaction mixture is
only 0.2-4 wt % NaOH. The degree of purification is regulated by
the alkali concentration, as well as by the temperature, time and
pulp consistency. In comparison to the cold alkali treatment, the
main drawbacks of the hot alkali method are the consumption of
steam for heating the pulp and higher yield loss of pulp. A yield
loss of about 3% is expected per 1% increase in .alpha.-cellulose
content.
Mechanical Treatment
[0073] Mechanical treatment of pulp fibers, such as grinding,
refining, can improve the dissolving pulp reactivity. Fock
reactivity was determined at 18.degree. C., by a dissolving pulp
sample ground in a coffee grinder, i.e., for refining/grinding. As
indicated by the results shown in Table 6, Fock reactivity
increases with grinding time. Fiber length was determined by
FQA.
TABLE-US-00006 TABLE 6 No. Grinding Time Fiber Length (length
weighted) Fock Reactivity 1 0 min 0.545 mm 55.90% 2 0.5 min 0.536
mm 59.10% 3 1 min 0.514 mm 61.08% 4 3 min 0.486 mm 65.56% 5 5 min
0.466 mm 68.67% 6 10 min 0.433 mm 75.13%
Enzyme Recycling
[0074] Enzyme recycling relates to the process economics of the
enzyme treatment. After use, the filtrate containing active enzymes
and can be recycled/reused. Table 7 shows effects of recycle of
filtrate from enzymatic treatment on dissolving pulp
reactivity.
TABLE-US-00007 TABLE 7 No Addition Addition Addition Fresh
additional of 20% of 35% of 50% enzyme fresh fresh fresh fresh
Sample Control treatment enzyme enzyme enzyme enzyme Reactivity/%
47.67 94.42 84.59 86.45 89.19 90.99
Enzymatic treatment conditions were: pulp weight 10 g o.d.; pulp
consistency, 4%; temperature 55.degree. C.; pH 5; and time, 2
hours.
[0075] The enzyme dosage of the fresh enzyme treatment was 1.0 u/g
o.d. pulp.
[0076] As used herein, the terms, "comprises" and "comprising" are
to be construed as being inclusive and open ended, and not
exclusive. Specifically, when used in this specification including
claims, the terms, "comprises" and "comprising" 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. As used herein,
the terms "about", and "approximately" when used in conjunction
with ranges of dimensions, concentrations, temperatures or other
physical or chemical properties or characteristics is meant to
cover slight variations that may exist in the upper and lower
limits of the ranges of properties/characteristics.
[0077] The contents of all documents mentioned herein are
incorporated herein by reference as though reproduced in their
entirety.
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