Note: Descriptions are shown in the official language in which they were submitted.
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WO 2004/022842 PCT/SE2003/001296
A METHOD OF PRODUCING MECHANICAL PULP AND THE MECHANICAL
PULP THUS PRODUCED
TECHNICAL FIELD
The present invention relates to a method of mechanical pulping based on
cellulose
fiber material after pretreatment of the fiber material with pectinase prior
to mechanical
defibration. The invention is also related to the mechanical pulp thus
produced.
PRIOR ART
In mechanical pulping wood fibers are separated by mechanical defibration,
typically in
a refiner or a grinder, with or without pretreatment. When pretreatment is
applied, the
process can be either TMP, where thermal softening is applied; or CTMP, where
for
instance sulfite is used for the sulfonation of lignin; or APMP, where
allcaline peroxide
is applied resulting in a combined effect of chemical softening due to alkali
and
bleaching or brightening due to peroxide. In almost all the mechanical pulping
processes, thermal treatment is employed prior to refining to soften the wood
material
thus leading to reduced energy consumption as well as improved pulp strength.
While
alkaline treatment is an effective treatment for reducing energy consumption,
it also
induces darkening reactions that result in poorer optical properties that are
essential for
paper products based on mechanical pulps. Besides, the use of alkali results
in extensive
dissolution of wood material, which results in yield loss and necessitates
effluent
treatment.
In addition to conventional processes, there have been efforts to apply
biotechnology to
mechanical pulping. In WO 97/40194, a mechanical pulping process using various
fungi
or enzymes is described. The proposed treatment includes compression of wood
chips
followed by impregnation with a biological agent prior to refiner treatment.
Although a
great number of fungi and enzymes are referred to in the patent application,
the
biological agents used in the given examples are limited either to fungus with
ligninase
activity, or to enzymes based on xylanase activity aiming for the
hemicellulose, and
lipase activity specific for the wood resin. The examples show that the
enzymatic
treatment can reduce the refiner energy by 4 to 10%, in addition to the energy
reduction
of about 4% due to the compression treatment as compared to the reference. At
the same
time the strength properties of the pulp are improved when compared at the
same pulp
freeness.
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2
Another biological pulping process involving pretreatment of wood chips is
based on
lignin oxidizing fungi treatment (US 5,055,159). The refining energy
consumption can
be reduced by about 30% at the same freeness. The treatment however requires
long
incubation time and the pulp produced has a dark color.
Different to the above-mentioned applications, pectinase has been specifically
used in
post-treatment of mechanical pulps. In US 5,487,812 pectinase is used for
alkaline
bleached mechanical wood pulp. The purpose of such treatment is however to
remove
pectin remaining after the pulping process in order to reduce the source of so-
called
"anionic trash" which is harmful in paper machine operation.
SUMMARY OF THE INVENTION
The present invention aims at providing a method of mechanical pulping
requiring
significantly reduced refining energy consumption to reach a certain pulp
freeness, or
pulp strength, as compared to prior art. The invention also aims at improved
optical
properties in the pulp, such as an improved brightness.
This is accomplished by the method according to the invention, as defined in
the claims.
According to the invention, the method comprises pretreatment of the
comminuted
cellulose fiber material with pectinase, alone or in combination with a
chelating agent,
for instance DTPA and/or sulfite, followed by mechanical defibration and
refining to
produce a mechanical pulp.
Pectin is a group of amorphous polysaccharide substances in wood. Although the
amount of pectin in softwood and hardwood is normally less than 1 %, it is
predominantly deposited in the compound middle lamella, and the tori of
bordered pit-
membranes of coniferous species (Hafren J. and Westermark U.: Nordic Pulp and
Paper
16 (4), 284-290, 2001). The main component of pectin is polygalacturonan
consisting of
galacturonic acid units that to various extend are esterified. When in free
acid form, the
carboxylic groups of the galacturonan having a negative charge can induce
local
swelling of the fibers in contact with aqueous solutions. The esterified
groups, on the
other hand, can be de-esterified through for instance alkaline treatment and
thus
contributing to swelling.
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3
Given the technical background, the present invention is aimed to utilize the
specific
features of pectin with respect to its specific location and the potential to
induce
selective weakening in the fiber wall. Enzyme with specific activity towards
pectin is
used for the pretreatment of cellulose fiber material. Maceration of the fiber
material
prior to the enzyme treatment proves an efficient way to get the enzyme in
place. To
render the fiber separation in the region where chemical weakening occurs, the
refining
condition is optimized in respect to refining intensity and preheating
temperature. The
results according to the present invention are a significantly reduced
refining energy to
reach a given pulp freeness and improved strength and optical properties of
the pulp.
According to one aspect of the invention, the pretreatment is preferably
assisted by a
mechanical compression to facilitate liquor uptake and to give a more even
treatment of
the material. Compression screw device or twin roll presses can be used at a
compression ratio of 1:1 to 8:1. The pretreatment can be ftirther improved if
presteaming is conducted prior to the compression.
Impregnation according to the invention follows immediately after the
compression
and/or thermal pretreatment. The pectinase is charged in an amount required
for
sufficient treatment, preferably 2,000,000 to 200,000,000 polygalacturonase
units/ton
material and even more preferred 10,000,000 to 50,000,000 polygalacturonase
units/ton.
The retention time of treatment is 3 min to 24 hours, preferably 3 to 300 min,
even more
preferred 15 to 240 min and most preferred 30 to 120 min at a temperature of
20 to
100°C, preferably 35 to 70°C, and more preferably about
50°C. The treatment condition
can be adjusted depending on the charge of the pectinase to allow for a
sufficient
hydrolysis of the pectins. Other pectinase preparations can be used provided
that the
pectolytic activity is satisfied.
Defibration and refining of the pectinase treated material can be carried out
either at
conventional TMP conditions and high intensity conditions. A surprising energy
reduction effect is obtained. To reach a given pulp freeness of 100 ml CSF,
the energy
reduction is 400 kWh/t with TMP condition, from 2500 kWhlt without pectinase
treatment to 2100 kWh/t with pectinase treatment, or by about 16%. The energy
reduction is 150 kWh/t with high intensity condition, from 2150 kWh/t without
pectinase treatment to 2000 kWh/t with pectinase treatment, or by about 7%.
The
combination of high intensity and pectinase treatment gives a total reduction
of 500
kWhlt, from 2500 kWh/t to 2000 kWh/t at a pulp freeness of 100 ml CSF, which
is 20%
in reduction. The skives content decreases more rapidly after the pectinase
treatment.
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The strength properties remain the same as compared to conventional TMP, and
improved by about 10% as compared to high intensity TMP. The brightness of the
pulp
is also increased by the enzymatic treatment.
While the addition of DTPA or sulfite doesn't contribute to further energy
reduction, it
increases the brightness by 2 to 5 brightness units. The improved brightness
is
maintained after peroxide bleaching, together with the benefit of an increased
amount of
residual peroxide for recycling.
The surprising effects resulting from the pectinase treatment could be
explained by a
selective weakening of the pectin-enriched region in the fiber wall due to
hydrolysis of
the pectins, with the consequence of a more efficient fiber separation in
defibration and
refining of the material. It is evident to those skilled in the art that this
process can bring
about a significant economical benefit, in terms of improved pulp properties
and
reduced energy cost. It is also evident to those skilled in the art that any
enzyme
preparation that contains sufficient pectolytic hydrolysis activity, alone or
in
combination with other chemicals, can be used for the treatment according to
the
present invention.
The present invention describes a mechanical pulping process comprising:
DETAILED DESCRIPTION OF THE INVENTION
A) Mechanical pretreatment of the comminuted fiber material with suitable
mechanical means, for instance compression with screw device or roll press
device, to
enable an efficient uptake of aqueous solution into the fiber material during
an
impregnation immediately following the pretreatment. Replacement of resin-
containing
liquor in natural cellulose fiber material with added aqueous solution is also
positive in
a process point of view. The compression ratio is preferably in the range of
1:1 to 8:1,
preferably 2:1 to 5:1, where 1:1 means a screw conveyer transporting the fiber
material
into the impregnation bin. In combination with compression treatment, an
initial thermal
heating of the fiber material may further facilitate an efficient uptake of
the aqueous
solution, preferably by use of fresh or recycled steam at atmospheric
pressure, for 1 to
30 min, preferably 10 to 20 min. As an alternative, thermal heating followed
immediately by impregnation may be used instead of compression treatment when
no
such facility is available. The pretreatment procedure can also be repeated in
one to
several stages with the freedom of applying various impregnation chemicals in
different
stages.
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B) Impregnation of the fiber material with pectinase-containing aqueous
liquid.
Added in connection with or immediately after the pretreatment stage, the
enzyme can
more easily reach the sites of reaction. In addition to the pectinase enzyme,
other
chemicals for instance chelating agents such as sulfite, preferably at a
charge of 5 to 50
kg/ton, and/or DTPA, preferably at a charge of 1 to 10 kg/ton, may be added in
the
impregnation to further improve the process. The pectinase-containing liquid
may
comprise an enzymatic preparation with pectolytic activity for both un-
esterified pectins
and esterified pectins. Moreover, the aqueous liquid may comprise two or more
enzymatic preparations wherein at least one of the preparations has pectinase
activity; or
the pectinase may be added as a biological agent comprising one or more fungi
or
bacteria, at least one of which having pectolytic activity. The charge of
enzyme is
subject to process conditions and cost effectiveness, and lies in the range of
2,000,000
to 200,000,000 polygalacturonase units/ton fiber material, preferably
10,000,000 to
50,000,000 polygalacturonase units/ton. The pH of the impregnation liquor is
adjusted
by alkali or acid, preferably by caustic soda or mineral acid, to a value
optimal for
enzymatic reactions, suitably in the pH range of 3 to 10, preferably 4 to 7
and more
preferably about 5. The retention time and temperature are also adjusted
depending on
the process setup and reaction requirements, preferably being 3 to 300 min at
a
temperature of 20 to 100°C. It is evident to those skilled in the art
that any enzyme that
contains sufficient pectolytic activity, alone or in combination with other
chemicals, can
be used in the treatment as described.
C) Defibration and refining of the impregnated fiber material is carried out
under
optimized conditions. The fiber material is preferably preheated prior to
feeding into the
pressurized primary refiner, to allow for thermal softening of the fiber wall.
The pulping
conditions can comprise a refiner rotation speed of 1000 to 3000 rpm,
preferably 1500
to 2600 rpm, using either conventional TMP or high refining intensity.
Conventional
TMP conditions may comprise preheating at 0 and up to 4 or 5 bar with a
retention time
of 2-10 min in the preheater and a refiner rotation speed of 1200 to 1800 rpm.
The
pressure 0 bar means atmospheric refining. High intensity conditions may
comprise
preheating at above 4 or 5 bar and up to 8 bar with a retention time of 3 to
30 sec and a
refiner rotation speed above 2000 rpm but usually not above 3000 rpm. The
retention
time should be matched against the preheating temperature (steam pressure) as
high
preheating temperature requires shorter retention time. After the primary
refining, a
secondary refining stage can be used to reach the required pulp freeness. The
second-
dary refining stage may have the same conditions as the primary stage.
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Although disc refiners are the defibration and refining equipment used in this
study,
other mechanical devices can be used for the purpose of defibration and
refining. Some
of the examples are conical refiners and stone ground wood grinders. The
refiners can
be of the single disc or double disc type. The refining may be made in a
single stage or
in multistage. The defibration and refining can also be done at atmospheric
pressure, in
which case refiner mechanical pulping, or RMP is a more appropriate
description of the
process instead of TMP.
Although only softwood fiber material is used in this study, any fiber
material
containing pectins could be treated with this method. The fiber material could
be
hardwood fiber material, non-wood fibers such as bagasse, bamboo, reed and
straw.
The embodiment as described here can be modified according to the source of
fiber
material and the process set-up.
The pulp obtained can depending on the application of the pulp be subject to
further
treatment such as washing, screening, post-refining and bleaching according to
conventional processes.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, there will be given a few examples of preferred embodiments
according to the invention. The invention is however not limited to these
examples.
Reference is made to the enclosed drawing diagrams, of which:
Fig. is showing freeness vs. specific refining
1 energy for Example 1,
Fig. is showing freeness vs. specific refining
2 energy for Example 2,
Fig. is showing freeness vs. specific refining
3 energy for Example 3.
EXAMPLES
Mechanical pulps are produced by different chemical treatments, in a pilot
refiner plant.
The process conditions are as follows.
Material
Wood chips consisting of approximately one third white spruce, one third red
and black
spruce and one third balsam fir, all of Canadian origin in the Nova Scotia
region. The
chips are prepared with mill-size chipper and well mixed prior to the trials.
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Pretreatment
The chips are pre-steamed at atmospheric pressure for 15 min. After pre-
steaming the
chips are fed into a plug-screw device with a compression ratio of 4:1. At the
outlet of
the device, the chips are released into a solution containing various
chemicals, including
pectinase in the trials according to the invention. The pH of the solution is
kept at about
5 adjusted by use of sodium hydroxide or sulfuric acid.
Impregnation and retention
After passing through the impregnation bin, the chips are stored in well-
isolated barrels.
The retention time is 120 min at a temperature around 50°C. The
pectinase used in this
study is a highly pectolytic enzyme preparation. This enzyme also hydrolyzes
the
methyl-esterified galacturonic acid in the pectins. The pectinase used is
generated by
submerged fermentation of a group of microorganisms containingAspergillus
aculeatus
andAspe~gillus o~yzae. The chaxge of pectinase in this study is 30,800,000
polygalacturonase units/ton. The chaxges of DTPA and sodium sulfite are 4 kg/t
and 12
kg/t, respectively, were applicable.
Defibration and refminr
The primary refiner is of the Sprout-Bauer 36-1 CP single disc refiner type.
Two
pulping conditions, conventional TMP and high intensity HI-TMP, are applied.
The
preheating steam pressure is 2.8 bar for TMP and 5.9 bar for high intensity
(HI-TMP).
The retention time at preheating is 3.4 min for TMP and 12 sec for HI-TMP. The
refiner
rotation speed is 1800 rpm for TMP and 2600 rpm for HI-TMP. The secondary
refining
is carried out with a double disc atmospheric refiner to reach the final
freeness levels.
The obtained pulps are evaluated both for strength properties and optical
properties
including a bleachability study with hydrogen peroxide bleaching.
EXAMPLE 1
In this trial TMP conditions are applied for both samples with the difference
that
reference, Sample 1, is impregnated only with water adjusted to pH 5.
Sample 1: TMP condition with only water impregnation with pH adjusted to 5.
Sample 2: TMP condition with 30,800,000 polygalacturonase units/t pectinase,
pH 5.
It is shown in Table 1 that the specific energy consumption required to reach
a given
freeness is significantly reduced by the enzymatic treatment. The degree of
reduction is
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8
approximately 400 kWh/t at a given freeness, which also can be seen in Figure
1. The
pulp properties for those samples are comparable to each other over the
freeness range
applicable for paper manufacture. The shives content is more rapidly reduced
by the
pectinase .treatment. At a freeness of 57 ml CSF, the shives content is 0.12%
as
compared to the reference, 0.34%.
Table 1.
Sam Sam
le le
1 2
Freeness, ml 665 230148 129 57 656 115 57 36
CSF
SEC (kWh/t) 902 188920282237 3078 824 1993 23863000
Density (kg/m3) 309,6344,83371,75440,53 205,76 384,62446,43462,96
Bulk (cm3lg) 3,232,9 2,69 2,27 4,86 2,6 2,242,16
Burstind.(kPa.m2lg) 1,651,982,29 3,04 0,34 2,46 3,13,35
Tear ind. (mNm2/g) 10,99,6 9,5 7,9 4,8 8,7 8 7,4
Tens ind. (N.m/g) 33,939,942,7 53,5 10,1 42,9 55,557,4
%Stretch 2,032 2,08 2,57 1,1 2,2 2,52,67
T.E.A.(J/m2) 28,4832,736,9557,6 4,6 39,7 28,664,3
%Opacity 97 97 97,4 98,4 88,2 96,1 97,597,5
Lightscatt. 50,152,152,8 61,4 35,3 53,9 59,265,1
coe.(m2/I~)
ISO% 44,5 48,848,649,8 52,3 46,6 50 52,652,2
%shives Pulmac-0.1mm 15 1,30,980,8 0,34 13,9 0,6 0,120,05
EXAMPLE 2
In this trial the high intensity condition is applied for both samples with
the difference
that the reference, Sample 3, is impregnated only with water adjusted to pH 5.
Sample 3: HI-TMP condition with only water in the impregnation adjusted to pH
5.
Sample 4: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, pH
5.
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WO 2004/022842 PCT/SE2003/001296
9
Table 2.
Sam Sam
le le
3 4
Freeness,mICSF 714 293 197 12654 725254 236 121 72
SEC (kWh/t) 531 14821652 18972655 41213621462 17932344
Density(kg/m3) 311,53362,32389,11454,55 317,46321,54387,6423,73
Bulk (cm3/g) 3,212,76 2,572,2 3,153,11 2,582,36
Burstind.(kPa.m2/g) 1,471,78 2,072,96 1,821,85 2,6 3,2
Tearind. (mNm2/g) 10,18,3 8,57,7 11,111,1 9,4 9,2
Tens ind. (N.m/g) 30,234,9 40,851,6 34,735,4 47,353,3
%Stretch 1,972,08 2,032,63 2 2,1 2,5 2,54
T.E.A.(J/m2) 24,7329,7435,2758,53 30,132,4 49,357,3
%Opacity 94,695,4 96,797,2 95,394,8 96,697,1
Lightscatt.coe.(m2/kg) 41,249,3 61,662,6 49,448,9 59,360,8
ISO% 45,350,350,1 52 53,9 44,850,650,8 51,654,4
%shives Pulmac-0.lmm 8,420,620,5 0,380,04 8,760,520,4 0,140,06
It is shown in Table 2 and Figure 2 that the enzymatic treatment has a
surprising effect
on energy reduction also with the high intensity condition. To reach a given
freeness of
down to about 100 ml CSF, the specific energy consumption is reduced by about
150
kWh/t. The reduction is less pronounced at lower freeness. The pulp properties
at the
same time are improved by about 10% both for tensile strength and tear
strength, while
the optical properties remain at the same level. The potential of the
enzymatic treatment
makes is obvious to people skilled in the art that even better strength can be
achieved if
the same amount of energy is applied to the enzymatically treated pulps, or
even less
energy is required if the same strength properties are to be acquired.
EXAMPLE 3
In this trial the HI-TMP condition is applied for all the samples with Sample
3 and
Sample 4 from Example 2 as references.
Sample 3: HI-TMP condition with only water in the impregnation adjusted to pH
5.
Sample 4: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, pH
5.
Sample 5: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, 4
kg/t DTPA, pH 5.
Sample 6: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, 4
kg/t DTPA and 12 kg/t sodium sulfite, pH 5.
CA 02497904 2005-03-04
WO 2004/022842 PCT/SE2003/001296
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WO 2004/022842 PCT/SE2003/001296
11
The refining energy consumption, SEC, is not affected by additional charge of
DTPA or
DTPA/sulfite. The SEC is at the same level for all samples with pectinase
treatment,
about 150 kWh/t lower compared to Sample 3, i.e. high intensity condition
without
pectinase treatment, also see Figure 3. A surprising effect is however
obtained in the
brightness of unbleached pulp. With DTPA together with pectinase, the
brightness is
increased by about 2 units ISO. With a combined of pectinase, DTPA and
sulfite, the
brightness improvement is even higher, 4-5 units ISO. The brightness
improvement is
fairly constant in the entire freeness range.
EXAMPLE 4
Pulps with different treatments are bleached with alkaline peroxide with
following
bleach conditions:
- Pretreatment with 7 kg DTPA/t at 7% consistency, 90°C, pH 5 for 45
min before
dewatering to 30%.
- Bleaching at 25% consistency at 30kg/t hydrogen peroxide charge, 80°C
for 90
min. Three sodium hydroxide charges, 10, 20 and 30kg/t, are used to find the
optimal brightness. The optimal brightness and corresponding residual peroxide
are given as bleach results.
Sample 7: TMP condition with only water impregnation with pH adjusted to 5.
Sample 8: TMP condition with 30,800,000 polygalacturonase units/t pectinase,
pH 5.
Sample 9: HI-TMP condition with only water in the impregnation adjusted to pH
5.
Sample 10: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, pH
5.
Sample 11: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, 4
kg/t DTPA, pH 5.
Sample 12: HI-TMP condition with 30,800,000 polygalacturonase units/t
pectinase, 4
kg/t DTPA and 12 kg/t sodium sulfite, pH 5.
Table 4
Unbl. Bleached Res. H2O2
ISO% ISO%
Sample 7 52,3 68,1 7,5
Sample 8 54,7 70,6 16,8
Sample 9 53,9 68,3 5,6
Sample 10 54,4 71,9 19,9
Sample 11 56.0 71,4 12,9
Sam le 12 59 5 74 3 25 3
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12
It is shown in Table 4 that the brightness improvement is maintained after the
enzymatic
treatment. The improvement is about 2.5 brightness units for the TMP
condition, and
3.6 units for the HI-TMP condition. Combination of DTPA and sulfite in the
impreg-
nation gives fiu-ther improvement to a brightness of 74.3%ISO, compared to the
TMP
reference of 68.1%ISO.
Noticeably, the residual peroxide is much higher with the enzymatically
treated
samples. This is beneficial because the residual chemicals are recycled back
to the
process after bleaching. The need of make-up peroxide is reduced accordingly.
The invention is not limited by the embodiments described above, but may be
varied
within the scope of the claims.
