U.S. patent application number 12/992316 was filed with the patent office on 2011-03-24 for process for the bleaching of paper pulp.
This patent application is currently assigned to SOLVAY (SOCIETE ANONYME). Invention is credited to Pierre Dournel.
Application Number | 20110067831 12/992316 |
Document ID | / |
Family ID | 39878001 |
Filed Date | 2011-03-24 |
United States Patent
Application |
20110067831 |
Kind Code |
A1 |
Dournel; Pierre |
March 24, 2011 |
Process for the bleaching of paper pulp
Abstract
Process for the bleaching of paper pulp in which an aqueous
suspension of paper pulp is subjected to a bleaching treatment with
hydrogen peroxide in the presence of magnesium hydroxide and of at
least one poly-.alpha.-hydroxyacrylic acid, its salt or their
mixture.
Inventors: |
Dournel; Pierre; (Brussels,
BE) |
Assignee: |
SOLVAY (SOCIETE ANONYME)
Brussels
BE
|
Family ID: |
39878001 |
Appl. No.: |
12/992316 |
Filed: |
May 25, 2009 |
PCT Filed: |
May 25, 2009 |
PCT NO: |
PCT/EP2009/056278 |
371 Date: |
November 12, 2010 |
Current U.S.
Class: |
162/76 ;
162/78 |
Current CPC
Class: |
D21C 9/1042 20130101;
D21C 9/163 20130101; D21C 9/1036 20130101 |
Class at
Publication: |
162/76 ;
162/78 |
International
Class: |
D21C 9/16 20060101
D21C009/16 |
Foreign Application Data
Date |
Code |
Application Number |
May 26, 2008 |
EP |
08156934.5 |
Claims
1. A process for the bleaching of paper pulp in which an aqueous
suspension of paper pulp is subjected to a bleaching treatment with
hydrogen peroxide in the presence of at least one alkaline earth
hydroxide selected from the group consisting of magnesium hydroxide
and calcium hydroxide and in the presence of at least one
poly-.alpha.-hydroxyacrylic acid, its salt or their mixture.
2. The process according to claim 1, wherein the alkaline earth
hydroxide is used in an amount of 0.1 to 10% by weight of dry
pulp.
3. The process according to claim 1, wherein the bleaching step is
carried out in the absence of sodium hydroxide.
4. The process according to claim 1, wherein the bleaching step is
carried out in the absence of sodium silicate.
5. The process according to claim 1, wherein, prior to the
bleaching treatment, the aqueous suspension of the paper pulp is
first subjected to a chelating treatment with at least one
aminopolycarboxylic acid, its salt or their mixtures.
6. The process according to claim 1, wherein the
poly-.alpha.-hydroxyacrylic acid is in the form of its sodium
salt.
7. The process according to claim 1, wherein the bleaching
treatment is carried out at a temperature equal to or less than
100.degree. C.
8. The process according to claim 1, wherein the bleaching step is
carried out at an initial pH of from 8 to 14.
9. The process according to claim 1, wherein the final pH, at the
end of the bleaching step, is of from 6 to 11.
10. The process according to claim 1, wherein the duration of the
bleaching step is of from 1 to 6 hours.
11. The process according to claim 1, wherein the hydrogen peroxide
is used in an amount of 0.1 to 8% by weight of dry pulp.
12. The process according to claim 1, wherein the
poly-.alpha.-hydroxyacrylic acid, its salt or their mixture is used
in an amount of from 0.1 to 2.0% by weight of dry pulp.
13. The process according to claim 2, wherein the
aminopolycarboxylic acid, its salt or their mixture is selected
from the group consisting of ethylene diamine tetra-acetic acid,
diethylene triamine penta-acetic acid, triethylene tetramine
hexa-acetic acid, cyclohexane diamine tetra-acetic acid,
methylglycine di-acetic acid, nitrilo tri-acetic acid, their salts,
and mixtures thereof.
14. The process according to claim 1 applied to the bleaching of
mechanical paper pulp.
15. A method for controlling the pH of an aqueous suspension of
paper pulp during the bleaching treatment of the paper pulp with
hydrogen peroxide, wherein said bleaching treatment is conducted in
the presence of at least one poly-.alpha.-hydroxyacrylic acid, its
salt or their mixture, said method comprising using at least one
alkaline earth hydroxide selected from the group consisting of
magnesium hydroxide and calcium hydroxide.
16. The process according to claim 1, wherein the bleaching
treatment is carried out at a temperature from 60 to 95.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a national stage application
under 35 U.S.C. 371 of International Application PCT/EP2009/056278,
which claims the benefit of the European application No. 08156934.5
filed on May 26, 2008, the whole content of this application being
herein incorporated by reference for all purposes.
TECHNICAL FIELD
[0002] The present invention relates to a process for the bleaching
of paper pulp, especially mechanical paper pulp, through a
bleaching treatment with hydrogen peroxide and an alkali source.
Such bleaching treatment usually involves the use of peroxide
stabilizers.
BACKGROUND
[0003] It is known to bleach mechanical paper pulp with oxidizing
agents such as hydrogen peroxide in an alkaline medium and in the
presence of a peroxide stabilizer.
[0004] For instance, the U.S. Pat. No. 6,221,209 of SOLVAY INTEROX
relates to a process for bleaching a chemical paper pulp by first
purifying the pulp so as to reduce its manganese content and then
by bleaching it with hydrogen peroxide in alkaline medium,
especially at a pH adjusted to at least 10 by the addition of an
alkaline compound (i.e. sodium hydroxide), and in the presence of a
stabilizing agent, namely sodium silicate.
[0005] It is also known to use other alkaline sources than sodium
hydroxide, such as magnesium hydroxide. For example, the U.S. Pat.
No. 7,052,578 discloses the use of magnesium hydroxide for the
hydrogen peroxide bleaching of mechanical paper pulp.
[0006] However, even if, according to U.S. Pat. No. 7,052,578, it
is possible to reach high brightness results using magnesium
hydroxide alone as alkaline source, it is more difficult compared
to the use of sodium hydroxide and sodium silicate as it requires
an extended reaction time, an increased peroxide charge, and the
addition of chelating agents such as DTPA, even if a chelating step
was conducted prior to the bleaching treatment.
[0007] When magnesium hydroxide is used as alkaline source in the
hydrogen peroxide bleaching of mechanical paper pulp, it is
difficult to achieve a satisfactory brightness, and the consumption
of hydrogen peroxide for a given brightness is high.
SUMMARY
[0008] The purpose of the present invention is to provide a process
which does not present the above disadvantages and which enables to
obtain a higher paper pulp brightness compared to the known use of
other alkaline sources than sodium hydroxide, such as magnesium
hydroxide or calcium hydroxide. The purpose of the present
invention is also to provide a process which allows decreasing the
consumption of hydrogen peroxide for a given brightness.
[0009] The present invention therefore relates to a process for the
bleaching of paper pulp in which an aqueous suspension of paper
pulp is subjected to a bleaching treatment with hydrogen peroxide
in the presence of at least one alkaline earth hydroxide selected
from magnesium hydroxide and calcium hydroxide and of at least one
poly-.alpha.-hydroxyacrylic acid, its salt or their mixture.
BRIEF DESCRIPTION OF THE DRAWING
[0010] For a detailed description of the preferred embodiments of
the invention, reference will now be made to the accompanying
drawing in which:
[0011] FIG. 1 illustrates the influence of the addition of sodium
poly-.alpha.-hydroxyacrylate (PHAS) during the bleaching of a
thermomechanical paper pulp with hydrogen peroxide and in the
presence of magnesium hydroxide.
DETAILED DESCRIPTION
[0012] Indeed, it has surprisingly been found that, in the presence
of at least one poly-.alpha.-hydroxyacrylic acid, its salt or their
mixture, it is possible to reach higher brightness, or to decrease
the hydrogen peroxide consumption for a given brightness, when at
least part of the alkaline compound is selected from alkaline earth
hydroxides and especially from magnesium hydroxide and calcium
hydroxide.
[0013] Furthermore, sodium silicate cannot be used when using
alkaline earth hydroxides such as magnesium hydroxide and/or
calcium hydroxide as alkaline source, because it forms precipitates
with those products, leading to scaling. Such scaling creates
problems in the whole production line including in the paper
machine, when the mill is an integrated site. Thus, when alkaline
earth hydroxides such as magnesium hydroxide and/or calcium
hydroxide are used, the common practice is to bleach without any
stabilizer, except aminopolycarboxylic acids or their salts, such
as diethylene triamine penta-acetic acid (DTPA) or its sodium salt
which will act as chelating agents and will therefore limit the
decomposition of the hydrogen peroxide which is catalyzed by
transition metals such as manganese or iron. Nevertheless, the
conditions present during the bleaching are usually not suitable
for a good efficiency of such chelating agents and therefore the
effect of such stabilizers is very limited.
[0014] One of the essential features of the present invention
resides in the combined use of an alkaline earth hydroxide selected
from magnesium hydroxide and calcium hydroxide as alkaline source
(referred to as "alkaline earth hydroxide according to the
invention") and of poly-.alpha.-hydroxyacrylic acid, its salt or
their mixtures (referred to as PHAA) as hydrogen peroxide
stabilizer, during the hydrogen peroxide bleaching of paper
pulp.
[0015] The PHAA used in the process of the invention generally has
a molecular weight within a specific range. As the molecular weight
is rather difficult to measure, it is inferred by measuring the
viscosity, the higher the molecular weight, the higher the
viscosity. Viscosity in the range of 10 to 100 mPas is usual but
more particularly it can be in the range of 20 to 60 mPas.
Commonly, the PHAA is used as a sodium salt of
poly-.alpha.-hydroxyacrylic acid, corresponding to sodium
poly-.alpha.-hydroxyacrylate (referred to as PHAS). In the
invention, good results are obtained using only one PHAA.
Nevertheless, more than one PHAA can be used.
[0016] The bleaching treatment according to the present invention
is generally carried out at an initial pH of at least 8, in
particular of at least 8.5, values of at least 9 being preferred.
The initial pH is usually at most 14, in special cases at most 13,
values of at most 12 being convenient. The final pH of the aqueous
paper pulp suspension, at the end of the bleaching treatment is in
general at least 6, especially at least 7, in particular at least
8. The final pH of the aqueous paper pulp suspension at the end of
the bleaching treatment is in many cases at most 11, particularly
at most 10. The pH of the aqueous suspension before, during or
after the bleaching treatment is measured using equipment normally
found in pulp mills for such a purpose. According to the invention,
the natural pH of the paper pulp suspension is adjusted to the
required value by means of an alkaline compound comprising at least
one alkaline earth hydroxide selected from magnesium hydroxide and
calcium hydroxide.
[0017] The amount of alkaline earth hydroxide according to the
invention, used in the bleaching step of the present invention, is
generally such that the pH of the suspension is maintained within
the above-mentioned range. This amount is usually at least 0.1% by
weight of dry pulp, preferably at least 1% by weight of dry pulp,
more preferably at least 2% by weight of dry pulp, especially above
2% by weight of dry pulp. The amount of alkaline earth hydroxide
according to the invention, used in the bleaching step of the
present invention, is generally at most 10% by weight of dry pulp,
with particular preference at most 5% by weight of dry pulp. The
amount of alkaline earth hydroxide according to the invention, used
in the bleaching step of the present invention, is for instance of
from 2 to 5% by weight of dry pulp, for example about 3% by weight
of dry pulp.
[0018] The amount of alkaline earth hydroxide according to the
invention depends on the amount of hydrogen peroxide used and on
the type of pulp. The ratio alkaline earth hydroxide according to
the invention/hydrogen peroxide can for example vary from 0.3 to
2.5, preferably from 0.5 to 2.0 depending on the amount of hydrogen
peroxide added and on the type of pulp. For example, the ratio may
be about 1.
[0019] The alkaline earth hydroxide according to the invention is
usually added to the paper pulp suspension in the form of an
aqueous slurry. Preferably, the particle size of the alkaline earth
hydroxide according to the invention is sufficiently small for the
slurry to be readily suspendible, advantageously having a mean
particle size of less than about 10 .mu.m, preferably less than 6
.mu.m, for example about 2 .mu.m.
[0020] The bleaching step of the invention is generally carried out
at a consistency of at least 10% of dry pulp based on the weight of
pulp suspension. The efficiency of the process increases as the
consistency is increased up to a value of around 30% by weight of
dry pulp. The consistency can be up to 40% by weight of dry
pulp.
[0021] The bleaching treatment of the invention is usually carried
out using an amount of hydrogen peroxide required to achieve the
final target brightness. The amount of hydrogen peroxide is
generally equal to or higher than 0.1% by weight of dry pulp,
preferably equal to or higher than 0.5% by weight of dry pulp, more
preferably equal to or higher than 1% by weight of dry pulp. The
amount of hydrogen peroxide is usually equal to or lower than 8% by
weight of dry pulp, especially equal to or lower than 6% by weight
of dry pulp, and more particularly equal to or lower than 5% by
weight of dry pulp. For example, the amount of hydrogen peroxide
may be about 2 to 4% by weight of dry pulp.
[0022] The bleaching treatment of the invention is usually carried
out at a temperature equal to or higher than 40.degree. C.,
especially equal to or higher than 60.degree. C. The temperature
can be up to about 100.degree. C. The temperature is preferably
less than 100.degree. C., more preferably equal to or less than
95.degree. C., for example equal to or less than 85.degree. C. The
optimum bleach temperature depends on the wood type and pulping
process but is normally in the range of 60 to 95.degree. C.
[0023] The duration of the bleaching treatment of the process of
the invention is usually from 10 min to 6 hours, varying from mill
to mill. For instance, it can vary from 30 to 300 minutes.
[0024] For example, the bleaching treatment can be carried out at a
pH of from 8 to 14, at a consistency of from 10 to 40%, and at a
temperature of from 60 to 95.degree. C., during 1 to 4 hours.
[0025] The amount of PHAA, especially PHAS, used in the bleaching
step of the invention is usually equal to or higher than 0.05% by
weight of dry pulp, especially equal to or higher than 0.1% by
weight of dry pulp. The amount of PHAA is commonly equal to or
lower than 2% by weight of dry pulp, values of at most 1% being
suitable. For example, typical amounts of PHAA are about 0.1 to
0.5% by weight of dry pulp. The optimum amount is dependent on the
amount of metallic ions in the pulp and must be optimized for each
particular mill.
[0026] The bleaching step of the invention can be carried out in
the presence of any other alkaline or alkaline earth hydroxides
than the alkaline earth hydroxides according to the invention, such
as in the presence of sodium hydroxide. The bleaching step of the
invention may also be carried out in the absence sodium hydroxide.
The bleaching step of the invention can also be carried out in the
presence of any other additives, such as magnesium sulfate or other
soluble magnesium salts. Preferably, the bleaching step is carried
out in the absence of any other stabilizer, and especially in the
absence of sodium silicate.
[0027] In a preferred embodiment, prior to the bleaching treatment,
the aqueous suspension of the paper pulp may be subjected to a
chelating treatment to reduce the metallic ion content of the
suspension, and especially its manganese content. It is for example
usual to carry out a pretreatment step (or chelating treatment),
with at least one aminopolycarboxylic acid, its salt or their
mixture (referred to as APCA) prior to the bleaching treatment.
[0028] The APCA used in the pretreatment step is generally chosen
from ethylene diamine tetra-acetic acid (EDTA), diethylene triamine
penta-acetic acid (DTPA), triethylene tetramine hexa-acetic acid
(TTHA), cyclohexane diamine tetra-acetic acid (CDTA), methylglycine
di-acetic acid (MGDA), nitrilo tri-acetic acid (NTA). The APCA may
be used as the free acid, its salt or their mixtures. The APCA is
preferably used as its salt. The salt is usually the alkali metal
salt such as sodium or potassium or the ammonium salt or a mixture
thereof. Sodium salts give good results. The most preferred APCA is
the sodium salt of DTPA. Good results are generally obtained using
only one APCA. Nevertheless, more than one APCA can be used. The
chelating pretreatment step may be conducted as described in the
International patent application filed as EP2008/052056, the text
of which is incorporated herein by reference.
[0029] In a variant of the invention, the bleaching treatment may
be carried out in the presence of at least one chelating agent, in
particular of at least one APCA, such as those disclosed above.
According to such a variant, the amount of APCA used in the
bleaching treatment of the invention is usually at least 0.05% wt
based on the weight of dry pulp. The amount of APCA is commonly at
most about 1% wt based on the weight of dry pulp, values of at most
0.8% wt being suitable.
[0030] If at least one APCA is present during the bleaching
treatment of the invention, the weight ratio between PHAA,
especially PHAS, and APCA is usually at least 1:10, preferably at
least 1:5, ratios of at least 1:3 being satisfactory. The weight
ratio PHAA/APCA, especially PHAS/APCA, is generally at most 10:1,
especially at most 5:1, values of at most 3:1 giving the best
results.
[0031] In another variant of the invention, the bleaching treatment
is carried out in the absence of a chelating agent or of another
stabilizer than PHAA.
[0032] The paper pulp treated in the present invention can be
chosen from chemical paper pulps, mechanical paper pulps or
recycled paper pulps. The best results are obtained with mechanical
paper pulps. By mechanical paper pulps are meant paper pulps
obtained by mechanical treatment. Examples of such paper pulps are
pressure groundwood (PGW), stone groundwood (SGW), thermomechanical
pulp (TMP), refiner mechanical pulp (RMP), chemithermomechanical
pulp (CTMP) and alkaline peroxide mechanical pulp (APMP or APP).
The present invention therefore also relates to the bleaching
process of the invention applied to mechanical paper pulp.
[0033] The process of the invention can further comprise other
treatment steps such as one or more additional dilution step(s),
one or more additional chelating step(s), one or more additional
bleaching step(s), one or more washing step(s) and/or one or more
extraction step(s).
[0034] It is recommended to have a good mixing of the paper pulp to
be treated with the hydrogen peroxide, the alkaline earth hydroxide
according to the invention, and the PHAA. The order of addition of
the alkaline earth hydroxide of the invention, the PHAA and the
hydrogen peroxide is either first the PHAA, then the alkaline earth
hydroxide of the invention, and lastly the hydrogen peroxide or
first the alkaline earth hydroxide of the invention, then the PHAA,
and lastly the hydrogen peroxide. Those three products may also be
premixed all together or two of them can be premixed. The alkaline
earth of the invention is usually added as a suspension in water.
Dilution water may also be added. Usually, the reactants are
introduced in the paper pulp suspension via a pump and a high
consistency mixer, especially when the bleaching is performed at a
consistency close to 30%. An example of configuration is: the
chemicals are added in the preheating screw of the HC mixer.
[0035] The present invention also relates to the use of at least
one alkaline earth hydroxide selected from magnesium hydroxide and
calcium hydroxide to control the pH of an aqueous suspension of
paper pulp during the bleaching treatment of the paper pulp with
hydrogen peroxide, wherein said bleaching treatment is conducted in
the presence of at least one poly-.alpha.-hydroxyacrylic acid, its
salt or their mixture.
[0036] The present invention is further illustrated below without
limiting the scope thereto.
EXAMPLES
Examples 1 (comparative) and 2 (According to the Invention)
[0037] According to Example 1, a mechanical pulp produced by a TMP
process using spruce as raw material, at a consistency of about
15%, was bleached at 80.degree. C. during 3 hours. Hydrogen
peroxide was added in an amount of 3% by weight of dry pulp.
Magnesium hydroxide was added in an amount of 3% by weight of dry
pulp. The brightness of the paper pulp obtained in these conditions
was 75.1.degree. ISO and the residual peroxide content after the
bleaching treatment was 9% of the initial charge.
[0038] In Example 2, the same conditions as in Example 1 were
applied except that 0.5% of PHAS were added to the bleaching step.
The resulting brightness of the paper pulp was 76.6.degree. ISO and
the residual peroxide content was 17% of the initial charge.
[0039] Those two examples illustrate that, according to the
invention, a higher brightness is obtained when a PHAA is added
during the hydrogen peroxide bleaching of paper pulp in the
presence of magnesium hydroxide as basic compound.
Examples 3 (Comparative) and 4 (According to the Invention)
[0040] In Example 3, a mechanical pulp produced by a TMP process
using spruce as raw material, at a consistency of about 15%, was
bleached at 70.degree. C. during 2 hours. Hydrogen peroxide was
added in an amount of 3% by weight of dry pulp. Magnesium hydroxide
was added in an amount of 3% by weight of dry pulp. The brightness
of the paper pulp obtained in these conditions was 73.8.degree. ISO
and the residual peroxide content after the bleaching treatment was
27% of the initial charge.
[0041] In Example 4, the same mechanical pulp as in example 3 was
bleached at 70.degree. C. during 2 hours in the presence of an
amount of hydrogen peroxide of 2% by weight of dry pulp, of
magnesium hydroxide of 3% by weight of dry pulp, and of an amount
of PHAS of 0.3% by weight of dry pulp. The resulting brightness of
the paper pulp was the same as in Example 3 (73.8.degree. ISO) and
the residual peroxide content was 31% of the initial charge.
[0042] Those two examples illustrate that, according to the
invention, it is possible to decrease the hydrogen peroxide
consumption for a given brightness.
Examples 5 and 6 (Referring to FIG. 1)
[0043] In Example 5, a mechanical pulp produced by a TMP process
using spruce as raw material, at a consistency of about 15%, was
bleached at 70.degree. C. during 2 hours. Hydrogen peroxide was
added in an amount of 3% by weight of dry pulp. Magnesium hydroxide
was added in an amount of 3% by weight of dry pulp. PHAS was added
in an amount of 0 to 0.5% by weight of dry pulp.
[0044] The same conditions as in Example 5 were reproduced in
Example 6, except that the amount of hydrogen peroxide was 2% by
weight of dry pulp.
[0045] These results show that an amount of PHAS of 0.35% by weight
of dry pulp can reduce the hydrogen peroxide consumption by 1% by
weight of dry pulp for the same brightness. These results also show
that for an amount of hydrogen peroxide of 2% by weight of dry
pulp, the brightness of the resulting paper pulp can be increased
from 72.4 to 73.9.degree. ISO by the addition of an amount of PHAS
of 0.35% by weight of dry pulp.
* * * * *