Note: Descriptions are shown in the official language in which they were submitted.
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Method for reducing the extractives content of high-yield pulps and method
for producing bleached high-yield pulps
Field of the invention
The present invention relates to a method for reducing the extractives content
of
high-yield mechanical pulps. The invention also relates to a method for
producing
bleached high-yield pulps having a reduced content of extractives.
Description of the prior art
Extractives are known constituents of wood and pulp species that are
extractable
with organic solvents. The most predominant chemical groups found in
extractives
are fatty and resin acids, terpenes, phytosterols and phenolic compounds.
Extractives are found esterified, glycosylated and nonderivatized. The most
problematic extractives in wood are mainly the oleophilic compounds. They are
fatty acids, resin acids, terpenes and sterols and their function is to
protect the tree
by preventing attacks from germs, funguses etc. Scandinavian softwoods contain
approximately 1.5% extractives by weight while hardwoods of the temperate zone
usually contain 2-2.5% by weight. Generally, the problematic extractives are
commonly referred to as pitch. Problems are usually found in high shear force
areas
and when the temperature, pH or pitch concentration changes.
Common detrimental effects from pitch in different areas are as follows. In
bleach
plants, deposits are formed and may also increase consumption of chemicals. In
the
final product spots and high level of extractives are found.
A modern bleaching process for mechanical pulps includes often high-
consistency
peroxide bleaching (PHC). After peroxide bleaching the pulp is often diluted
with
white water coming from the paper machine and after that the pulp slurry is
dewatered. This washing step reduces carry-over of anionic trash to paper
machine
and also allows recirculation of residual peroxide. Alkaline peroxide oxidizes
and
removes quite efficiently extractives from the pulp. However, if the pulp in
the
bottom of PHC bleach tower or after PCH bleaching is diluted with white water
the
extractives can re-precipitate in the surface of pulp or process equipment due
to
lower pH of the water, high hardness or electrolyte (salt) concentration.
Precipitated
extractives will be transferred in the fiber surface to the paper machine
leading to
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problems in wet end and paper quality. When the PHC wash filtrate is
recirculated
back to earlier stages of the process, the extractives, possibly found in the
wash
filtrate, can be precipiteted in the process equipment or fibers in acidic or
neutral
environment.
US 4 363 699 describes a process for stabilizing alkaline solutions of
peroxidic
compounds used for bleaching wherein an alkaline salt of a poly-alfa-hydroxy-
acrylic acid is added to the solution as a stabilizer. The stabilized
solutions can be
used for the bleaching of paper pulps and textiles. When bleaching textiles,
the
bleaching solution may contain a wetting agent which is a surface active
agent.
US 4 963 157 describes a method of bleaching a cellulosic fiber material,
especially
cotton with hydrogen peroxide. The method comprises impregnating the fiber
material with a bleaching solution containing hydrogen peroxide and a
stabilizer.
The stabilizer comprises poly-alfa-hydroxyacrylic acid or a salt thereof or
the
polylactone corresponding thereto, and an organic phosphonic acid or a salt
thereof.
DE 3423452 discloses a solution for avoiding the use of water glass comprising
a
stabilizing mixture of a poly-alfa-hydroxyacrylic acid (PHAA) and a water
soluble
homopolymer of acrylic or methacrylic acid or a copolymer of acrylic acid
and/or
malefic acid. The salts, especially the sodium salt of PHAA and the sodium
salt of
the polycarboxylate polymer are mixed together. Chelating agents can be added
to
the mixture and thus an improved stabilizer mixture for bleaching processes is
obtained. According to DE 3423452 the bleaching process is carried out by
using an
alkaline peroxide bleaching liquor containing said stabilizing mixture.
Summary of the invention
The invention is based on the fact that the extractives are more soluble in
alkaline
conditions and thus, when the pulp is diluted with dilution water, for example
white
water after bleaching, the extractives may redeposit on the fiber surfaces
especially
when the pH of dilution water is lower that that of pulp coming out from
bleaching
tower. According to the present invention it has been found that certain
surface-
active agents can inhibit precipitation of extractives and thus keep them in
water
phase. Then the extractives can be washed off the pulp at next dewatering
press
which in turn leads to reduced extractives content in the pulp. Since the
extractives
are dispersed and the particles are stabilized, there is no re-precipitation
of
extractives when the filtrate is re-circulated to the earlier process stages.
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The invention is especially advantageous when the pulp is bleached without
silicate
as stabilizer, since it has been noticed that silica has some dispersing
efficiency
against extractives. However, the invention works also well in the presence of
silicate.
Detailed description of the invention
According to the present invention there is provided a method for reducing the
content of extractives of a high-yield pulp in a peroxide bleaching stage,
said stage
including peroxide bleaching and a subsequent dewatering or washing, said
method
comprising contacting the pulp in the peroxide bleaching with an organic
stabilizer
and in or after the peroxide bleaching with a surfactant, and thereafter
subjecting~the
bleached pulp to said dewatering or washing for removing extractives along
with
the aqueous phase.
In this context the peroxide bleaching stage includes peroxide bleaching of
the pulp,
optionally dilution of the pulp, and dewatering or washing of the pulp.
According to the present invention there is also provided a method for
producing
bleached high-yield pulp having a reduced content of extractives comprising
bleaching high-yield pulp with peroxide, the pulp being contacted with an
organic
stabilizer during the peroxide bleaching and with a surfactant during the
peroxide
bleaching or after the peroxide bleaching, and dewatering or washing the
bleached
pulp for removing extractives along with the aqueous phase and for producing
bleached high-yield pulp having a reduced content of extractives.
The extractives to be removed from the pulp according to the present invention
originate from the wood pitch. The extractives include fatty acids, resin
acids,
terpenes, phytosterols and phenolic compounds in esterified, glycosylated
and/or
nonderivatized form. The most problematic extractives include fatty acids,
resin
acids, triglycerides, steryl esters, sterols and lignans.
The organic stabilizer and the surfactant can be added separately or these
chemicals
can be mixed beforehand with the bleaching solution. Furthermore the organic
stabilizer and the surfactant can be added simultaneously or separately as one
chemical mixture, i.e. product.
The organic stabilizer and the surfactant can be added to the pulp in the
peroxide
bleaching or before the peroxide bleaching.
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It is also possible to add the surfactant to the pulp after the peroxide
bleaching. In
that case the surfactant can be added to dilution water which is added to the
pulp
between the peroxide bleaching and the dewatering or washing. The dilution
water
can be white water circulated from the paper machine.
The organic peroxide stabilizer used in the methods of the invention is
preferably a
polymeric stabilizer, such as a poly-alfa-hydroxyacrylic acid or a salt
thereof or the
corresponding polylactone, a homopolyrner of acrylic acid, methacrylic acid or
malefic acid or a copolymer of acrylic acid and/or methacrylic acid with an
unsaturated dicarboxylic acid. The polymeric stabilizer can also be a mixture
of the
poly-alfa-hydroxyacrylic acid and the homopolymer and/or the copolymer. The
unsaturated dicarboxylic acid is preferably malefic acid. The salt of the poly-
alfa-
hydroxyacrylic acid is preferably a sodium, potassium or ammonium salt. The
poly-
alfa-hydroxyacrylic acid can have a molecular weight of at least 5000,
preferably at
least 10000, and more preferably at least 15000 (calculated as the sodium salt
of the
PHAA). The homopolymer and the copolymer can have a molecular weight of at
least 4000, preferably at least 10000, and more preferably at least 20000.
The organic stabilizer can also be a phosphonic acid or a salt thereof. The
salt is
preferably a sodium, potassium or ammonium salt. Suitable phosphonic acids are
for example the (poly)aminophosphonic acids, such as aminotri(methylenephospho-
nic acid), ethylenediaminetetra(methylenephosphonic acid), i.e. EDTMPA, or
diethylenetriaminepenta(methylenephosphonic acid), i.e. DTPMPA.
In addition to said organic stabilizer it is possible to introduce another
stabilizer.
This other stabilizer can be an alkaline earth metal compound such as a
magnesium
and/or calcium compound. The alkaline earth metal compound can be in the form
of
a salt, such as a sulphate, chloride or any other water soluble salt or in the
form of a
complex with a polymer or a chelating agent. A preferred alkaline earth metal
salt is
magnesium sulphate.
The amount of the organic stabilizer is preferably from 0.1 kg to 5 kg per ton
dry
pulp, more preferably from 0.25 kg to 3 kg per ton dry pulp, and even more
preferably from 0.5 kg to 3 kg per ton dry pulp.
The surfactant used in the methods of the invention can be an anionic
surfactant,
such as naphthalene sulphonate or lignosulphonate, or a non-ionic surfactant,
such
as an O/W emulsifier, f. ex. a fatty alcohol ethoxylate or alkyl phenol
ethoxylate.
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Also a mixture of the anionic and non-ionic surfactants can be used. Defoamers
can
be used in combination with the surfactants) to prevent foaming.
The amount of the surfactant is preferably from 0.005 kg to 2 kg per ton dry
pulp,
more preferably from 0.05 kg to 1 kg per ton dry pulp.
5 The methods of the invention are used in bleaching high-yield mechanical
pulps,
like groundwood pulps, stone groundwood pulps, pressure groundwood pulps (GW,
SGW, PGW), refiner mechanical pulps (RMP), thermomechanical pulps (TMP) and
chemithermomechanical pulps (CTMP, APMP).
In the peroxide bleaching hydrogen peroxide is preferably used as the
bleaching
agent. Other chemicals such as bleaching stabilizers, chelating agents and/or
magnesium sulphate can be used in the bleaching. Especially advantageous is to
add
the chemicals before bleaching or at the same time with bleaching chemicals.
If
there are several bleaching stages, the chemicals can be added in any of them
or in
several stages.
The residence time in the bleaching can vary within a wide range, from 30 to
240
minutes, preferably from 45 to 180 minutes and most preferably from 60 to 120
minutes. The residence time will also depend on the temperature used in the
bleaching.
The bleaching of high-yield pulps can be carried out at a temperature of 30-
90°C,
preferably at a temperature of 50-90°C. The bleaching can be carned out
at a
consistency of choice, but it is most preferably to carry out the bleaching at
a high
consistency, i.e. about 30% or higher. Bleaching can also be carried in two
stages
with a dewatering stage between the stages.
The pH in the alkaline bleaching of the invention can be from 7 to 14,
preferably
from 8 to 11. The ratio between the alkali, typically sodium hydroxide, and
hydrogen peroxide can vary in a wide range, depending on raw materials and
degree
of bleaching. Also alternative alkali sources, like sodium carbonate, can be
utilized.
The present invention will be illustrated by following examples. The
percentages
are % by weight unless otherwise specified. "kg/t" means kilograms per ton dry
pulp. "Cs" means consistency. "P" stands for peroxide stage.
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Examples
Comparative example
The purpose of this comparative example is to clarify the problem to be solved
by
the invention. TMP pulp (spruce) was bleached in high consistency with
peroxide
using waterglass or poly-a-hydroxy acrylate (PHAA) as stabilizer. Subsequently
the
pulp was diluted with ion exchanged water and then dewatered to consistency of
30%. The detailed experimental data is shown in table below.
P P
T, C 70 70
t, min 120 120
Cs, % 27 30
Initial pH 10.4 10.4
Final pH 9.5 9.5
H20z, kg/t 30 30
NaOH, kg/t 30 30
Waterglass, 24 0
kg/t
Stabilizer 0 PHAA
Dosage, kg/t 0 2
Dilution to Cs 5%
T, C 50 50
t, min 15 15
Cs, % 5 5
pH 9.3 9.2
TOC, kglt 21.0 19.3
COD, kg/t 58.9 51.1
Turbidity, NTU 301 233
Residual H202, 6.2 5.9
kg/t
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1)ewaterinto Cs
3U%
Brightness, %ISO 77.6 77.5
Water extract (SCAN-CM
44:97)
Turbidity, NTU 24.8 32.1 Unbleached
COD, mg/1 310 360 pulp
Extractives is pulp
Fatty acids, mg/kg 150 220 390
Resin acids, mg/kg 110 120 550
Lignans, mg/kg 10 13 130
Sitosterols, mg/kg 37 54 110
Steryl esters, mg/kg 230 430 720
Triglycerides, mg/kg 210 710 1300
Sum 747 1547 3200
The high turbidity of the filtrates after dilution can be attributed to
extractives. The
analysis of extractives show, that a maj or part of extractives was removed in
bleaching. The results show also that silicate (waterglass) has some
dispersing
ability against extractives since the extractives content is much higher with
acrylate-
based stabilizer. This means that especially in the case on silicate-free
bleaching,
pitch dispersants are advantageous.
Example 1
A sample of CTMP-pulp (aspen) was taken from a mill and bleached with peroxide
to brightness of 83-84% ISO. The bleaching conditions were as follows: t = 120
min, 70°C, consistency 30%, H2O2 38 kg/t, NaOH 21 kg/t, Na2C03 8 kg/t.
The pulp
was already chelated in the mill and the manganese content was 1.5 ppm, Fe 5
ppm.
The additives were dosed in the peroxide bleaching stage and after bleaching
the
pulp was diluted to 10% consistency at different pH levels, and subsequently
dewatered. The table below shows the turbidity of the filtrate after
dewatering. The
experiment according to this invention was done using per ton dry pulp 2 kg
polymer (poly-a-hydroxy acrylate) as stabilizer and 0.1 kg naphthalene
sulphonate
+ 0.1 kg non-ionic surfactant (fatty alcohol ethoxylate 7 mole EO, C 12-C 14,
HLB
12.3). In the comparative experiments waterglass and poly-a-hydroxy acrylate
were
used as stabilizers. The bleaching result and chemicals consumption was about
the
same in each case.
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Waterglass, Acrylate According to
polymer
25 kg/t 2 kg/t invention
pH Turbidity,pH Turbidity, pH Turbidity,
NTU NTU NTU
6.6 1510 6.3 1353 6.5 1464
7.5 1430 7.3 1317 7.2 1440
7.8 1465 7.7 1389 7.5 1481
As can be seen from the results, silica originating from the waterglass
(sodium
silicate) increases turbidity. This may be attributed to higher extractives
content in
the filtrate. When silicate is replaced by a polymer-based stabilizer,
turbidity is
lower and thus extractives content in pulp is higher. The chemical composition
according to this invention yields higher turbidity, which proves that the
extractives
dissolved in alkaline peroxide are not re-precipitated after dilution.
Example 2
A sample of CTMP-pulp (aspen) was taken from a mill and bleached with peroxide
in high consistency. The bleaching conditions and results are shown in the
table
below. The surfactants were added in the bleaching stage, and after bleaching
the
pulp was diluted with ion-exchanged water. The composition of the surfactants
here
was naphtalene sulfonic acid condensation product (Na-salt) and fatty-alcohol
ethoxylate (same as in previous example) in weight-ratio 2:1 (dosage as 20%
water
solution). The polymer-based stabilizer was a mixture of poly-alfa-
hydroxyacrylate
and maleate-acrylate copolymer (1:4 w:w) having a active substance content of
25%.
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P P P P
According to invention No No Yes Yes
t, min 120 120 120 120
T,C 70 70 70 70
Cs, % 30 30 30 30
Initial pH 8.7 8.6 8.8 8.8
Final pH 8.4 8 8.4 8.3
Stabilizer, kg/t 2 2 2 2
NaZC03, kg/t 8 8 8 8
NaOH, kg/t 21 21 21 21
H202, kg/t 38 38 38 38
Surfactant, kg/t 0 0 1 1
Polyvinyl alcohol, kg/t 0 1 0 1
Polyethyleneglycol, kg/t 0 1 1 0
Residual HzOz, kg/t 27.4 27.3 28.3 27.7
Bri htness, %ISO 73.4 81.0 80.9 81.2 81.2
Dilution with ion-exchange water
t, min 10 10 10 10
T,C 70 70 70 70
Cs, % 5 5 5 a 5
pH 8.2 8.1 8.2 8.3
Turbidity, NTU 608 607 627 651
DCM extract *, % 0.38 0.35 0.27 0.25
* from pulp
DCM stands for dichloromethane.
The results show, that the method according to the invention reduces
extractives in
the pulp and does not affect the bleaching result.
Example 3
A sample of CTMP-pulp (aspen) was taken from a mill and bleached with peroxide
to brightness on 83-84% ISO. The surfactant additives were dosed in peroxide
bleaching stage and after bleaching the pulp was diluted to 10% consistency at
different pH levels, and subsequently dewatered. In one experiment a
surfactant
additive was also dosed after the dilution. The results are shown in the table
below.
The surfactant additives used here were naphtalenesulphonate condensation
product
(A) and fatty alcohol ethoxylate (B, 25 mole EO, C16-C18, HLB 16). The polymer
based stabilizer was a mixture of poly-alfa-hydroxyacrylate and maleate-
acrylate
copolymer (1:4 w:w) having a active substance content of 25%.
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P P P P
t, min 120 120 120 120
T, C 70 70 70 70
Cs, % 30 30 30 30
Initial 9 8.9 9 9
pH
Final pH 8.5 8.3 8.5 8.3
Stabilizer,0 2 2 2
kg/t
Waterglass,25 0 0 0
kg/t
NaZC03, 8 8 g g
kg/t
NaOH, kg/t 19 21 21 21
H20z, kg/t 38 38 38 38
Additive - B A _
Dose kg/t a 1 1 -
~1~. 1~. 1 1
According to No No No Yes Yes Yes No
invention
t, min 60 60 60 60 60 60 60
T,C 70 70 70 70 70 70 70
Cs, % 10 10 10 10 10 10 10
Initial pH 5.5 6.5 8.2 6.4 6.5 6.6 6.6
Final pH 5.6 6.6 7.9 6.4 6.5 6.8 6.6
Additive - - - _ A _ _
Dose, kg/t - - - - 0.5 - -
Residual, HZOZ,27.3 27.6 27.2 27.4 27.9 27.5 27.1
kg/t
Brightness, 82.7 83.5 83.2 83.2 83.2 82.6 82.7
%I50
Extractive,
mg/kg
Fatty acids 550 470 530 410 420 430 490
Resin acids 170 140 130 120 120 130 130
Lignans 71 56 55 41 42 67 55
Sitosterols 8 6 7 3 3 8 7
Steryl esters 420 280 250 240 230 280 310
Triglycerides 1300 1010 940 810 800 980 1020
Sum 2519 1962 1912 1624 1615 1895 2012
5 The results show that the combination of polymer-based stabilizer and
anionic
and/or nonionic surfactant gives lower extractives content than waterglass.
