Note: Descriptions are shown in the official language in which they were submitted.
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Method for treating ligno-cellulosic materials
Field of the invention
The present invention relates to a method for treating pectin-containing ligno-
cellulosic raw materials in a high-yield pulping process utilizing one or more
treatment stages at alkaline conditions by controlling the conditions. More
particularly the present invention relates to a method for increasing the
bleachability of the final pulp and also lowering the cationic demand of the
process
waters. The present invention also relates to pulp, paper, board or tissue
obtained
with the method
Background of the invention
Mechanical pulping aims at transforming the raw material into fibers of
sufficient
quality without severe yield losses. Enhancement of the properties of
mechanical
pulps by chemical treatment can be achieved prior to refining, during
refining, on
the coarse fibers between refining stages, or in a post-treatment after
refining. In
order to ,produce high-yield pulp of high-quality it is generally important to
increase
the brightness by removing colored or chromophoric structures without
sacrificing
too much of the yield in the process. An extensive overview of the prior art
can be
found in Sundholm, J.: Mechanical pulping, Book 5, Fapet Oy 1999.
Generally alkaline treatments are utilized in the pre-treatment of wood chips,
such
as in chemithermomechanical pulping, or in peroxide bleaching of mechanical or
chemimechanical pulps, such as groundwood, thermomechanical and
chemithermomechanical pulps.
Chromophores in mechanical pulps originate to a major part from the lignin.
These
structures are partly removed or converted to non-chromophoric structures
during
the bleaching processes. However, during the course of mechanical pulping
process stages at alkaline conditions, i.e. mainly chip pre-treatment or pulp
bleaching, new chromophoric structures are also formed due to the prevailing
process conditions, i.e. high temperatures and high alkalinities. Alkaline
darkening
is a known phenomenon involving formation of ortho-quinones and
coniferaldehydes.
Substitute Sheet (Rule 26)
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Pectin in the wood is one of the major sources of so called "anionic trash"
and
alkali-induced "unbleachable" chromophores in the final product. Dissolution
of
pectic acid, i.e. "anionic trash", and new chromophoric structures are formed
as a
result of alkaline-induced cleavage of natively occurring pectins.
When pectin is cleaved, large pectin molecules split into smaller fragments
which
are able to diffuse out of wood ("anionic trash") and new chromophoric
(terminal)
end groups are formed.
It was verified in relation to the present invention that the temperature is a
key
parameter controlling the degree of pectin cleavage at alkaline conditions.
Higher
temperature results in more intensive cleavage and smaller molar-mass
fragments, producing more chromophoric end groups.
Analogously, lower temperature results in less intensive cleavage and larger
molar-mass fragments producing less chromophoric end groups.
Alkaline-induced cleavage of pectin is a very fast reaction: once it has
reacted
(been cleaved) extended alkaline treatment of pectin at higher (or lower)
temperature than the initial temperature does not results in additional
cleavage.
Summary of the invention
The present invention relates to a method for improving bleachability in
industrial
scale by preventing the formation of stable chromophores and reducing the
release of anionic trash in alkaline treatment of pectin-containing materials,
such
as in chip pre-treatment or in peroxide bleaching. The invention is based on
the
discovery that when adding alkali to the process the temperature should be
low,
e.g. 70 C or below, because the demethylation of pectin occurs surprisingly
fast
and after that the alkaline-induced cleavage of pectin is minimized even if
the
temperature is raised or more alkali is applied. Compared to high temperature
(-100 C) generally used in the processes it results in two times less "anionic
trash"
in process water and preserves about 2% ISO brightness.
The release of pectins from spruce TMP was doubled at high temperature
treatments (80-100 C) compared to low temperature treatment (20-70 C)
according to the present invention. This was also seen as lower cationic
demand
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(-25%) of the process waters. At the same time the brightness of the peroxide
bleached spruce pulps were up to 1-2% ISO higher for the pulps impregnated
under low temperature conditions compared to high temperature conditions
before
raising the temperature to desired reaction temperature.
The present invention provides a method for treating pectin-containing ligno-
cellulosic raw materials in a high-yield pulping process utilizing one or more
treatment stages at alkaline conditions wherein the alkaline chemicals are
applied
at a low temperature treatment stage (Ti), meaning the point when alkali for
the
first time is in contact with the pectin-containing material, before one or
more
consecutive treatment stages at the same or higher temperature (T2). This
provides e.g. improved brightness and lower amount of ionic substances release
into process waters.
The present invention further provides pulp or bleached pulp obtained with the
method of the invention and paper, board or tissue obtained from said pulp.
One aspect of the present invention relates to increasing the bleachability of
pectin-containing material and therefore e.g. to increasing the brightness of
bleached pulps.
Another aspect of the present invention relates to decreasing the release of
anionic pectic substances.
Still another aspect of the present invention relates to lowering the cationic
demand of the process water.
Still another aspect of the present invention relates to enhancing certain
papermaking properties, such as the quality of the pulp.
Brief description of the drawings
Figure 1 shows that pectin degradation in alkaline conditions at higher
temperature results in more chromophores formation (1 mg/ml pectin, 94% methyl-
esterified, 0.3 mi 1 M NaOH, 20-100 C, 30 min).
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Figure 2 shows the principle of the present invention. Figure 2A is a block
diagram of the stages of the invention. Figure 2B shows the desired properties
of
paper, the brightness, the water (the amount of ionic substances) and the
properties of the fibers, which depend on each other.
Figure 3 shows the kinetic of the chain splitting of model pectin (94% methyl-
esterified) treated at pH 11 and 40 C.
Figure 4 shows the treatment scheme of the preliminary series
Figure 5 shows how Xyl and GaIA are released into water after alkaline
treatment of "clean" spruce TMP at different starting temperatures.
Figure 6 shows the treatment scheme of the second series.
Figure 7 shows the percentage of ISO brightness of sheets from bleached TMP
at different starting temperatures.
Figure 8 shows the treatment scheme of the final series.
Figure 9 shows the cationic demand of water after alkaline treatment of TMP
with and without peroxide at different starting temperatures. Peroxide acts as
a
buffer lowering the effect of alkali.
Figure 10 shows the percentage of ISO brightness of sheets from bleached and
alkaline treated TMP at different starting temperatures. Lower temperature in
the
initial bleaching stage resulted in +2% ISO.
Detailed description of the invention
Chemical reactions can generally be influenced by controlling the reaction pH
and
temperature. By controlling the chemical reactions in alkaline treatment of
chips or
pulps, some important papermaking properties may be enhanced.
Pectins are polydisperse polymers and have a backbone of partly methyl-
esterified
galacturonic acid (GaIA) units interspersed with rhamnose (Rha) units. Spruce
wood contains 1.5-2% pectins and aspen wood 2-2.5% (Pranovich, A. V.
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Sundberg, K. and Holmbom, B. (2003) Chemical changes in thermomechanical
pulp at alkaline conditions. J.Wood Chem. Technol. 23(1):89-112; Sundberg, A.,
Sundberg, K., Lillandt, C. and Holmbom, B. (1996) Determination of
hemicelluloses
and pectin in wood and pulp fibres by acid methanolysis and gas
chromatography.
5 Nord. Pulp Pap. Res. J. 11(4):216-219, 226).
Release of pectic substances into the process waters will increase the amount
of
the so called "anionic trash" in the water circulation. This will increase the
overall
consumption of chemicals, such as paper chemicals in the paper machine (PM)
wet-end and decrease the PM runnability.
H O O-CH3 H 0 O-CH3 H
O OH O O OH O 0 OH O O O H O O OH O O-
H
O O-CH3 OH 0 OH O s O OH 0 O-CH3
OH
a-D-GaIA->a-D-GaIA-->a-D-GaIA-->a-L-Rha-->a-D-GaIA -xa-D-GaIA
1-->4 1-->4 1-->2 1-->4 1-->4
Lignin is usually assumed to be the predominant source of chromophores
responsible for the brightness ceiling observed for mechanical pulps, for
softwoods
-80% ISO and for hardwoods -85% ISO. Acidic hemicelluloses like pectins,
xylans and other uronans may, however, also play a key role in limiting the
maximum brightness obtained in peroxide bleaching of mechanical pulps.
Polymer chain splitting according to the P-elimination mechanism requires the
presence of methyl-esterified carboxyl groups and is therefore inhibited by
demethylation (Kiss J. (1974) P-eliminative degradation of carbohydrates
containing uronic acid residues. Adv. Carbohyd. Chem. Biochem. 29:229-03;
Renard, C.M.G.C. and Thibault, J.-F. (1996) Degradation of pectin in alkaline
conditions: kinetics of demethylation. Carbohydr. Res. 286:139-150). Both
demethylation and P-elimination reactions rates increase with pH. However, an
increase in pH will increase demethylation more than P-elimination, while an
increase in temperature will increase P-elimination more than demethylation.
By
controlling the temperature in the alkaline treatment process stages it is,
therefore,
possible to decrease the release of anionic pectic substances and also
increase
the brightness of bleached mechanical pulps.
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H 0 O-CH H 0 O-CHu
O H O
O OH O O H 0 O OH O + H 0
0 O-CHv OH 0 O-CH3 OH
P-elimination
Two parallel reactions happen with pectin in alkaline conditions:
demethylation and
(3-elimination. The susceptibility of pectin to depolymerisation by (3-
elimination
depends on the presence of an ester group on the galacturonic acid. When the
methanol removal from pectin is complete, the P-elimination reaction stops. A
double bond appears between C-4 and C-5 at the non-reducing end (Kiss 1974).
The release of pectins from spruce TMP is doubled at high temperature
treatments
(80-100 C) compared to low temperature treatment (20-70 C) of the present
invention. This was also seen as lower cationic demand (-25%) of the process
waters. At the same time the brightness of the peroxide bleached spruce pulps
are
up to 1-2% ISO higher for the pulps impregnated under low temperature
conditions compared to high temperature conditions before raising the
temperature to desired reaction temperature.
The effect of low temperature pre-treatment for aspen mechanical pulps (BCTMP)
has not been evaluated. It could, however, be assumed that the effect would be
even more pronounced since aspen contains approximately 25% more pectins
than spruce.
The present invention provides a method for treating pectin-containing ligno-
cellulosic raw materials in a high-yield pulping process (before the paper
machine
head box) utilizing one or more treatment stages at alkaline conditions. The
pectin-
containing raw material may be any suitable material, such as wood, softwood
or
hardwood or combinations thereof, chopped raw ligno-cellulosic material such
as
chopped wood, straw, defiberized wood or high-yield pulp.
The alkaline treatment stage may be performed before mechanical defibration or
consecutive alkaline bleaching stage(s). The alkaline chemicals may be any
suitable alkaline chemicals which provide conditions sufficient to achieve
significant demethylation of the methyl esters in native pectins, typically
above pH
9 at room temperature. The alkali source may for example originate from
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hydroxides, carbonates or sulfites, preferably sodium, calcium, ammonium or
magnesium hydroxides, carbonates or sulfites, or combinations thereof.
In the method of the present invention the alkaline chemicals are applied at a
low
temperature treatment stage (Ti) meaning the point when alkali for the first
time is
in contact with the pectin-containing material before one or more consecutive
treatment stages at the same or higher temperature (T2). It is essential that
no
previous alkali treatments have been applied to the pectin-containing material
because the alkali-induced cleavage of pectin happens very fast. On the
contrary,
when the alkali is applied for the first time at low temperature the
demethylation
reaction occurs fast thus preventing further alkali-induced cleavage of pectin
by (3-
elimination reactions. The dosage of alkali may be in the range of 0.1% to 10%
(w/w) of the dry-based pectin-containing material.
It is preferred that the pectins in the treated material are methyl-
esterified.
Generally the methyl-esterification degree is 20-100%, determined as the
percentage of galacturonic units containing one methyl-ester group, preferably
50-
70%.
The method of the present invention improves the bleachability of the pectin-
containing material. This will lead to improved brightness of the pulp and
lower
bleach chemical consumptions and costs needed to achieve a given brightness.
Further, lower amount of ionic substances are released into process waters.
Lower
amount of dissolved substances in the water means less chemical costs in
papermaking or water treatment and less environmental impact. It will also
improve the paper machine performance. Also the pulp quality is better, e.g.
there
is less darkening.
Figure 2 shows the principle of the present invention. Figure 2A is a block
diagram
of the stages of the invention. Tn represents one or more (n>_2) stages T2,
T3, T4...
Figure 2B shows the desired properties of paper, the brightness, the water
(the
amount of ionic substances) and the properties of the fibers, which depend on
each other.
The high-yield pulping process may refer to thermomechanical pulping or
chemithermomechanical pulping with yield generally over 70%, preferably over
80% and more preferably over 85%.
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The low temperature in stage T, referred herein means temperature of or below
70 C, preferably below 60 C, more preferably below 50 C. Generally temperature
range from room temperature (20 C) to said 70 C may be used. In one
embodiment the temperature is 50-60 C. The length of said low temperature step
or the interval between T, and T2 may be in the range of 1 second to 24 hours,
generally less than 4 hours. In one embodiment the interval is in the range of
1
second to 2 hours. In another embodiment the interval is in the range of 2 to
10
minutes, for example about 5 minutes. It is essential that said time allows
the
demethylation reactions to occur in the lower temperature.
In one embodiment in stage T, also other chemicals are applied such as
chelating,
stabilizing and/or bleaching agents, preferably peroxygens, such as peroxide.
The
chemicals used in stage T, may also be recycled to the subsequent stage(s).
In one embodiment in stage T2 chemicals are applied such as alkali, chelating,
stabilizing and/or bleaching agents, such as EDTA, DTPA, silicate, magnesium
sulfate, peroxygens, preferably hydrogen peroxide. In another embodiment the
temperature in the T2 stage is in the range of 70-210 C.
The anionic substances released may contain for example galacturonic acid,
glucuronic acid and 4-0-methylene glucuronic acid. The method of the invention
mainly reduces the amount of galacturonic acid.
In one embodiment the alkaline treatment stage is an alkaline pre-treatment
stage
before mechanical defibration at elevated temperature (CTMP refining). In
another
embodiment the alkaline treatment stage is an alkaline pre-treatment stage
before
alkaline peroxide bleaching; before MC stage in two stage MC-HC bleaching
sequence or before a single HC stage or as a MC stage with controlled
temperature before a HC stage running at elevated temperatures.
"Chemithermomechanical pulps" (CTMP) are produced by treating a
lignocellulosic material, commonly wood chips, with one or more chemical
agents,
in combination with the operations of heating and mechanical separation of
fibers.
In the combined operation indicated above of heating, chemical treatment and
fiber separation, the chemical treatment with controlled temperature T, may be
carried out either before, during or after the fiber separation.
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CTMP pulps are generally produced to a yield, i.e. dry weight pulp relative to
the
dry weight of starting material, of 70-90%, typically 85-90%, and with TMP
pulps
85-95%, typically 90-95%.
Generally by "chemical pre-treatment" it is intended that operation, over the
course
of which the lignocellulosic material, most commonly wood chips, is treated
with
liquor containing either sulfite or mixture of sulfite and sodium hydroxide at
a
temperature equal to or greater than 100 C under saturation water vapor
pressure.
Treatment with liquors containing mixtures of sodium hydroxide and hydrogen
peroxide is typically performed at 60-80 C. The chemical treatment potentially
includes conventional impregnation with steaming of the lignocellulosic
material to
facilitate a good penetration of the solution of the selected reagents into
the
material followed by consecutive screw pressing stage(s) to remove entrained
air
and to facilitate complete chemical uptake of the wood material.
The temperature at which the treatment is carried out generally does not
exceed
200 C and usually ranges from about 120 to 160 C. The treatment medium is at
an initial pH usually ranging from 6 to 12.5.
The duration of the chemical pre-treatment depends on the selection of other
process parameters, but generally does not exceed 1 hour.
Expressed in terms of SO2, the amount of the sulfite in the pre-treatment
ranges,
for example, from approximately 0.1% to 10%, most typically from 0.5% to 3% on
oven dry lignocellulosic material and the sodium hydroxide amount from 0% to
7%
depending on wood species and product and process requirements The amount of
H202 of the alkaline peroxide pre-treatment may range from 0.5% to 12%, most
typically from 3% to 5% and the sodium hydroxide amount from 0.5% to 10%,
most typically from 2% to 7% depending on wood species and product and
process requirements.
Certain chemical agents may be used in the pre-treatment together with the
alkaline sulfite, or alkaline peroxide for example complexing or sequestering
agents, such as diethylenetriaminepentaacetic (DTPA) acid,
ethylenediaminetetraacetic (EDTA) acid, sodium silicate and magnesium sulfate
(Epsom salt)
State of the art bleaching of the TMP and/or CTMP pulps by means of hydrogen
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peroxide in an alkaline medium is typically carried out by introducing an
amount of
hydrogen peroxide of approximately 0.5% to 10%, in the presence of about 1% to
6% sodium silicate solution at a pH of from approximately 9 to 11 and at a
temperature of from about 40 to 100 C for about 0.5 to 2 hours, at a
consistency of
5 approximately 10% to 30%. The bleaching bath may also contain certain
additives,
principally one or more sequestering or complexing agents, such as, for
example,
DTPA.
Tower bleaching refers to bleaching which normally takes place at high
10 consistency. 2-stage bleaching with recycling of HC (high consistency)
bleach
filtrate to the initial MC (medium consistency) stage, is normally employed
when
high brightness targets (>80% ISO) are required. The temperature at the
initial
contact between pulp and bleaching liquor is normally the on the same level as
the
temperature in the bleaching stage, i.e. well above 60 C, typically between 70
C
and 90 C.
Steep bleaching refers to bleaching at high stock consistency in a pulp pile
at
lower temperatures, normally 20 C to 45 C, and for longer bleach times than
for
conventional tower bleaching.
Refiner bleaching refers to bleaching conducted during the refining stage by
adding alkaline peroxide to the feed to either the primary or secondary
refiner. This
means that the temperature at the initial contact initial contact between pulp
and
bleaching liquor is normally very high, i.e. 100 C to 160 C.
Examples
Example 1. Color formation by pectins under alkaline conditions at different
temperatures
Commercial methylesterified pectin was used (1 mg/ml pectin, 94% methyl-
esterified, 0.3 ml 1 M NaOH, 20-100 C, 30 min). In Figure 1 it can be seen how
pectin degradation in alkaline conditions at higher temperatures (over 70 C)
results in more chromophores formation and as a consequence severe darkening
of the waters occur.
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In Figure 3 it can be seen that the pectin chain splitting is a very fast
reaction even
at low temperatures, in this case 40 C. Already after 20 seconds the chain
length
has decreased by 30%.
Example 2. Release of pectins from "clean" TMP pre-treated at different
temperatures
Thermomechanical pulp from Norway spruce (Picea abies L.) was sampled in a
Finnish mill, after the second-stage refiner at approximately 35% consistency
and
the pulp was stored at -24 C until use.
"Clean" TMP fibers were obtained by extraction in a Soxhlet apparatus with
hexane-acetone (9:1 v/v) for 24 h to remove lipophilic material. Water-soluble
substances (hemicelluloses and low-molar-mass aromatics) were removed by
thorough washing. The pre-extracted TMP was suspended in distilled water (pH
5.5) at 60 C at 2% consistency and agitated for 3 h with a blade propeller (-
200
rpm). The TMP suspension was filtered under vacuum on a Buchner funnel with a
paper machine wire. To prevent the loss of fines, the filtrate was passed
twice
through the formed fiber mat. The TMP was re-suspended in distilled water and
the washing procedure was repeated 5 times. The TOC value of the final
filtrate
was 4 mg/1. The washed TMP was air-dried and stored in the dark at +4 C.
10 g o.d. (oven-dry) TMP in 95 g suspension, in polyethylene plastic bags, was
mixed well. All pulps were pre-heated at 70 C for 3 hours before adjusting the
target temperatures for the trials.
The pulps were preconditioned at different temperatures (4 C, 20 C and 70 C)
for
1 hour before adding NaOH (see table 1). The pulps pre-treated at 4 C and 20 C
had retention times of 2 and 1 hours respectively, before raising the
temperature
to 70 C in a water bath. The pulp pre-treated at 70 C was directly placed into
water bath at 70 C for 90 minutes (Figure 4).
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Finally the pulps were cooled down in ice bath to 20 C. After measuring the
end
pH, the pulps were acidified with 6% S02-water to pH 5. Samples of the pulp
filtrates were taken for chemical analysis.
In Figure 5 it can be seen how Xyl and GaIA are released into water after
alkaline
treatment at different starting temperatures. The difference in the release of
GaIA
between 20 C and 70 C is extensive, i.e. over 100% while the release of the
neutral xylans (determined as xylose monomers) does not increase by increasing
the treatment temperature. This clearly proves that the initial temperature
when
alkali meets the pulp is determining the rate of the pectin chain cleavage.
Table 1.
Pre-treatment Main treatment
Temp. NaOH Time Temp. time End pH pH after
( C) (% on pulp)* (min) ( C) (min) at 20 C neutralization
2 1 120 70 60 8.5 4.6
2 2 120 70 60 10.5 5
1 60 70 60 8.0 4.8
20 2 60 70 60 10.1 4.8
70 1 0 70 60 8.6 4.4
70 2 0 70 60 10.5 5.1
* Temperature of 1 M NaOH adjusted to specific treatment temperature
Example 3. Bleachability of TMP pre-treated at different temperatures
The pulp used in example 3 was the same as in example 2, except that in these
trials it had not been hexane extracted or extensively washed. Part of the
water-
soluble substances (hemicelluloses and low-molar-mass aromatics) were,
however, removed by diluting the pulp to 2%, agitating the pulp at 60 C for 1
hour
before thickening the pulp to 20% dryness on a Buchner funnel. To preserve
fines
the first portion of filtrate was recirculated through the fiber mat
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Washed TMP, DTPA (0.25% calculated on dry pulp) and MgSO4, (0.05%) were
mixed well in a polyethylene plastic bag and kept overnight at room
temperature.
The pulp was mixed well and divided into plastic bags containing 10 g o.d.
pulp
each. Three pulps were treated in parallel for each pre-treatment temperature
test
(2 C, 20 C and 70 C).
The treatment scheme for example 2 is shown in Figure 6. 10 ml cold 3% H202
was added to the pulps pre-treated at 2 C, mixed and placed in cold (0 -+2 C)
ice
bath for 1 hour. 3 ml cold Na-silicate (0.1g/ml) solution was added, mixed at
cold,
before adding 2.5 ml or 5 mi 1 M NaOH (also cold), mixed well and kept for 2
hours
at 0-+2 C. The pulps pre-treated at 20 C were treated analogously except
adding chemicals at room temperature and keeping the pulp at 20 C for 1 hour
before raising the temperature to the target bleaching temperature of 70 C.
The
pulps pre-treated at 70 C went directly into the bleaching stage after adding
the
corresponding chemicals.
All three series of pulps were placed into water bath at 70 C for 90 minutes.
The
pulps were cooled down to 20 C in an ice bath. After measuring end pH, samples
were acidified with 6% S02-water to pH 5 before sheet forming according to a
modified ISO 5269-1979 standard.
In Figure 7 the percentage of ISO brightness of sheets from bleached TMP at
different starting temperatures can be seen. At higher temperatures the
brightness
is significantly decreased by about 1 % ISO.
Example 4. Bleachability and cationic demand of filtrates of unwashed TMP
pre-treated at different temperatures
The pulp used in example 4 was the same as in example 3. Part of the water-
soluble substances (hemicelluloses and low-molar-mass aromatics) were removed
by was agitating the pulp at 2% consistency at 60 C for 2 hours. The pulp was
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thickened to 20% on a Buchner funnel. To preserve fines the first portion of
filtrate
was recirculated through the fiber mat.
Washed and filtered TMP was mixed with DTPA (0.25%) and MgSO4 (0.05%) and
kept overnight before dividing the pulp into plastic bags containing 10 g o.d.
pulp
each.
The treatment scheme for some of the trials in example 4 is shown in Figure 8.
Pulps for treatment at "low" temperatures (30 C, 40 C, 50 C and 60 C) were pre-
heated at 80 C for 2 hours. Pulps for treatment at "high" temperatures (70 C,
80 C
and 90 C) were not pre-heated. These trials were done by adding NaOH and
peroxide or only alkali to the pulp. All other parameters were kept the same.
Each pulp was pre-heated for 1 h at desired temperature, 10 ml 3% H202 was
added, then 3 ml Na-silicate (0.1 g/ml) and 2.5 mi 1M NaOH were added fast,
mixed well (adding chemicals and mixing during 5 min) and the suspension was
kept for 5 min more at pre-heating temperature (all together - 10 min with
alkali at
pre-heating temperature). All series of bags were placed into water bath at 70
C
for 90 minutes.
All pulps were cooled down to 20 C in ice bath. After measuring of the end pH,
samples were acidified with 6% S02-water to pH 5 before sampling the filtrate
and
sheet forming.
In Figure 9 the cationic demand of water after alkaline treatment of TMP with
and
without peroxide at different starting temperatures can be seen. 80 C REF
refers
to a reference treatment at 80 C without any chemicals. At higher temperatures
(over 50 C), especially for samples treated with only alkali (no peroxide),
the
release of "anionic trash" increases remarkably by over 100%. For the samples
treated with alkaline peroxide the increase in cationic demand is not as
dramatic
as for treatment with only alkali since peroxide acts as a buffer lowering the
alkali
effect.
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Figure 10 shows the percentage of ISO brightness of sheets from bleached and
alkaline treated TMP at different starting temperatures. Lower temperature in
the
initial bleaching stage resulted in +2% ISO brightness compared to higher
temperature at the initial stages of the alkaline treatment. This difference
is very
5 significant and important since the top brightness units of high brightness
pulps
are very cost-inefficient in terms of chemical consumption, COD load and loss
of
bulk and yield.
The analyses were performed using the following equipment and
10 methodology.
The paper sheets from bleached pulp were examined by ISO brightness test
according to SCAN.
15 The sugar composition of hemicelluloses and pectins was determined using
methanolysis (2 M HCI in dry methanol), followed by gas chromatographic (GC)
analysis of TMS-derivatives of corresponding sugar monomers formed (Sundberg
et al. 1996). The samples were freeze-dried prior to methanolysis.
Cationic demand (CD) of TMP waters was determined by polyelectrolyte titration
using 0.0005 N potassium polyvinyl sulfate (KPVS) as anionic polymer with a
Mutek particle charge detector 03. TMP-water samples, containing dissolved and
colloidal substances were mixed with 0.0005 N polybrene directly in the
measuring
cell and were then titrated with KPVS.
A pH-meter Handylab pH 12 (Schott-Gerate GmbH, Mainz, Germany) with Schott
pH-electrode BlueLine 28 pH (pH 0-14/-5 C-80 C/Ge) was used to monitoring the
pH-value in water solutions/suspensions during alkaline treatment.
