U.S. patent application number 12/160805 was filed with the patent office on 2010-07-01 for process for the bleaching of mechanical paper pulp.
This patent application is currently assigned to SOLVAY (SOCIETE ANONYME). Invention is credited to Pierre Dournel, Rodney Seccombe.
Application Number | 20100163193 12/160805 |
Document ID | / |
Family ID | 36568695 |
Filed Date | 2010-07-01 |
United States Patent
Application |
20100163193 |
Kind Code |
A1 |
Seccombe; Rodney ; et
al. |
July 1, 2010 |
Process for the bleaching of mechanical paper pulp
Abstract
A process for the bleaching of mechanical paper pulp with one or
more peroxide oxidizing agents, in which the pulp is pretreated in
a pretreatment step with one or more chelating agents chosen from
aspartic acid and its derivatives, substituted on the nitrogen
atom, characterized in that the initial pH of the pretreatment
step, just before adding the one or more chelating agents, is above
8.
Inventors: |
Seccombe; Rodney;
(Hoeilaart, BE) ; Dournel; Pierre; (Brussels,
BE) |
Correspondence
Address: |
Solvay;c/o B. Ortego - IAM-NAFTA
3333 Richmond Avenue
Houston
TX
77098-3099
US
|
Assignee: |
SOLVAY (SOCIETE ANONYME)
Brussels
BE
|
Family ID: |
36568695 |
Appl. No.: |
12/160805 |
Filed: |
January 22, 2007 |
PCT Filed: |
January 22, 2007 |
PCT NO: |
PCT/EP2007/050578 |
371 Date: |
July 14, 2008 |
Current U.S.
Class: |
162/76 |
Current CPC
Class: |
D21C 9/1042 20130101;
D21C 9/16 20130101; D21C 9/1005 20130101 |
Class at
Publication: |
162/76 |
International
Class: |
D21C 3/00 20060101
D21C003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 24, 2006 |
EP |
06075155.9 |
Claims
1- A process for the bleaching of mechanical paper pulp with one or
more peroxide oxidizing agents, in which the pulp is treated in a
pretreatment step with one or more chelating agents selected from
the group consisting of aspartic acid and its derivatives,
substituted on the nitrogen atom, wherein the initial pH of the
pretreatment step, just before adding the one or more chelating
agents, is above 8.
2- The process according to claim 1, characterized in that wherein
the pH of the pretreatment step is not measured between the
addition of one or more chelating agents, and the end of the
pretreatment step.
3- The process according to claim 1, wherein the pH is not
controlled by the addition of pH modifying compounds during the
pretreatment step.
4- The process according to claim 1, wherein the chelating agent
used in the pretreatment step is selected from the group consisting
of N-carboxymethylaspartic acid, N-(1,2-dicarboxyethyl)-aspartic
acid, N-(1,2-dicarboxy-2-hydroxyethyl aspartic acid, compounds of
formula: ##STR00002## where n is 1-3, m is 0-3, p is 1-3, R.sub.1,
R.sub.2, R.sub.3 and R.sub.4 are H, Na, K, Ca or Mg, R.sub.5 and
R.sub.6 are H, CH.sub.2OH, CH.sub.2CH.sub.2OH or
CH.sub.2O(CH.sub.2CH.sub.2O).sub.1-10CH.sub.2CH.sub.2OH, and
mixtures thereof.
5- The process according to claim 3, wherein the chelating agent is
selected from the group consisting of
ethylenediamine-N,N'-disuccinic acid, its isomers, its salts,
2,2'-iminodisuccinic acid, its isomers, its salts, and mixtures
thereof.
6- The process according to claim 1, wherein the initial pH of the
pretreatment step is from 8.1 to 11.
7- The process according to claim 1, wherein the pretreatment step
is carried out at a temperature from 40 to 95.degree. C.
8- The process according to claim 1, wherein the pretreatment step
is carried out in the presence of water in order to reach a
consistency from 0.1 to 45% by weight of dry pulp.
9- The process according to claim 1, wherein the chelating agent
used in the pretreatment step is used at a concentration from 0.1
to 1.5% by weight of dry pulp.
10- The process according to claim 1, wherein the oxidizing agent
used in the bleaching step is selected from the group consisting of
hydrogen peroxide, organic peracids and combinations thereof.
11- The process according to claim 10, wherein the organic peracid
is peracetic acid.
Description
[0001] The present application claims the benefit of the European
patent application n.sup.o 06075155.9 filed on Jan. 24, 2006,
herein incorporated by reference.
[0002] The present invention is related to a process for the
bleaching of mechanical paper pulp with one or more peroxide
oxidizing agents, in which the paper pulp is pretreated with a
chelating agent.
[0003] It is known to bleach paper pulp with peroxide oxidizing
agents such as peracids, or hydrogen peroxide.
[0004] It is also known to proceed to a pretreatment step of the
paper pulp with a chelating agent, in order to selectively
eliminate metallic ions detrimental to bleaching. Indeed, some
metallic ions catalyze decomposition reactions of the peroxide
compounds. The most detrimental ions are manganese, iron, and
copper.
[0005] Powerful chelating agents can be of the aminocarboxylic
type, such as ethylene diamine tetra-acetic acid and its salts
(EDTA), or diethylene triamine penta-acetic acid and its salts
(DTPA). Nevertheless, these chelating agents are poorly
biodegradable which can generate environmental problems.
[0006] The use of biodegradable chelating agents such as aspartic
acid and its derivatives, substituted on the nitrogen, is described
in the international application WO 97/30209. Such products include
ethylenediamine-N,N'-disuccinic acid (EDDS) and
2,2'-imino-disuccinic acid (IDS). In that previous application, the
pretreatment step is conducted at a pH from 4 to 8. A first
disadvantage of this process is that it does not conduct to an
optimal brightness of the paper pulp. A second disadvantage is that
the pH of the pretreatment step must be measured and maintained in
a range from 4 to 8 during the whole step.
[0007] The international application WO 99/46441 describes a
process for the bleaching of mechanical paper pulp with one or more
peroxide oxidizing agents, in which the pulp is pretreated with one
or more chelating agents chosen from aspartic acid and its
derivatives, substituted on the nitrogen, and at a pH above 8. As
in the previous invention, the pH of the pretreatment step must be
measured during the whole step and must be maintained above 8.
[0008] The purpose of the present invention is to provide a
simplified bleaching process, avoiding measurement and control of
the pH during the pretreatment step, conducting to an optimal
brightness of the paper pulp, while still using biodegradable
chelating agents during the pretreatment step.
[0009] The present invention therefore relates to a process for the
bleaching of mechanical paper pulp with one or more peroxide
oxidizing agents, in which the pulp is pretreated in a pretreatment
step with one or more chelating agents chosen from aspartic acid
and its derivatives, substituted on the nitrogen atom,
characterized in that the initial pH of the pretreatment step, just
before adding the one or more chelating agents, is above 8.
[0010] One of the essential features of the present invention
resides in solely controlling the initial pH of the pretreatment
step, which must be above 8 just before adding the one or more
chelating agents, and not measuring it after the addition of the
chelating agent, and so until the end of the pretreatment step. It
has indeed surprisingly been found that, as the critical pH of the
pretreatment step is its initial pH, it is therefore not necessary
to control it anymore during the rest of the pretreatment step.
This is important as it is known that, due to alkaline demand of
the paper pulp, the pH will naturally decrease with time. According
to this invention, it is not necessary to measure it during the
pretreatment step, nor to keep it above 8 by the addition of pH
modifying compounds. It follows therefrom that the pretreatment
step of the process is advantageously simplified.
[0011] 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), chemithemomechanical pulp
(CTMP) and alkaline peroxide mechanical pulp (APMP or APP).
[0012] By initial pH of the pretreatment step is meant the pH of
the paper pulp just before the addition of one or more chelating
agents. The pH is measured using equipment normally found in pulp
mills for such a purpose.
[0013] By control of pH is meant the adjustment of the pH to a
certain value. This can be done by means of pH modifying compounds.
Examples of such compounds are caustic soda (NaOH) or suitable
alternative bases. Usually, these compounds are alkalis because the
natural pH of mechanical pulps is generally lower than the one
required in practice for efficient control of metal ions.
[0014] In the process of the invention, the chelating agent used in
the pretreatment step is chosen from aspartic acid and its
derivatives, substituted on the nitrogen. The chelating agent can
particularly be chosen from N-carboxymethylaspartic acid,
N-(1,2-dicarboxyethyl)-aspartic acid,
N-(1,2-dicarboxy-2-hydroxyethyl)-aspartic acid, and compounds
according to the formula:
##STR00001##
where n is 1-3, m is 0-3, p is 1-3, R.sub.1, R.sub.2, R.sub.3 and
R.sub.4 are H, Na, K, Ca or Mg, R.sub.5 and Rb are H, CH.sub.2OH,
CH.sub.2CH.sub.2OH or
CH.sub.2O(CH.sub.2CH.sub.2O).sub.1-10CH.sub.2CH.sub.2OH, and
mixtures thereof. An especially suitable chelating agent is, for
example, ethylenediamine-N,N'-disuccinic acid (EDDS), its various
isomers or its alkali metal salts or its earth-alkali metal salts.
Another especially suitable chelating agent is 2,2'-iminodisuccinic
acid (IDS), its various isomers or its alkali metal salts or its
earth-alkali metal salts.
[0015] In the process of the invention, good results are obtained
using only one chelating agent. Nevertheless, more than one
chelating agent can be used. For example, several chelating agents
according to the invention can be combined. A chelating agent
according to the invention can also be combined with one or more
conventional chelating agents such as aminocarboxylated chelating
agents such as EDTA or hydroxycarboxylated chelating agents such as
glucaric acid.
[0016] In the process of the invention, the paper pulp is
pretreated with one or more chelating agents at an initial pH of at
least 8.1. The pH is in most cases maximum 11. Within this range,
the optimum must be established for each particular pulp. For some
pulps, the optimal pH is for example equal to or higher than 8.5.
For some others, the optimal pH is equal to or higher than 9, and
for others it is equal to or higher than 10.
[0017] In the process of the invention, the pretreatment step with
one or more chelating agent is usually carried out at the
temperature generated by the mechanical treatment of the wood. The
temperature can vary widely and is usually at least 40.degree. C.
It is generally less than or equal to 95.degree. C. The temperature
may vary from 40 to 95.degree. C.
[0018] In the process of the invention, the pretreatment step with
one or more chelating agent is carried out in the presence of water
in order to reach a consistency of at least 1% by weight of dry
paper pulp. The consistency of the paper pulp is usually less than
or equal to 50% by weight of dry paper pulp. The consistency of the
paper pulp can be highly variable within the range from 1 to 50% by
weight of dry paper pulp and will be adapted to the type of
equipment available for mixing the chelating agents into the
process. In the majority of the cases, a consistency of less than
or equal to 10% by weight of dry paper pulp, for instance from 3 to
4% by weight of dry paper pulp, will be suitable. In special cases,
higher values, up to 45% by weight of dry paper pulp can be
used.
[0019] In the process of the invention, the chelating agent used in
the pretreatment step is used at a concentration by weight of dry
paper pulp of at least 0.1%. The concentration of the chelating
agent by weight of dry paper pulp is usually less than or equal to
1.5%. The concentration of the chelating agent is highly variable
within this range and must be optimized, depending on the paper
mill. Typical values range from 0.2 to 1.0% by weight of dry paper
pulp.
[0020] It is recommended to have a good mixing of the paper pulp
with the chelating agent during the pretreatment step. This can be
done by introducing the chelating agent via the pump circulating
the pulp through the paper mill.
[0021] The duration of the pretreatment step with one or more
chelating agent is usually from 5 to 120 minutes, varying from mill
to mill. Good results are obtained provided the duration of the
pretreatment step is of at least 5 minutes duration before the
washing step.
[0022] In the process of the invention, the oxidizing agent used in
the bleaching step is usually chosen from hydrogen peroxide,
organic peracids such as peracetic acid, and/or a combination
thereof. The oxidizing agent is preferably hydrogen peroxide, which
is advantageously used in an alkaline medium.
[0023] A washing step is usually conducted between the pretreatment
step with one or more chelating agent and the bleaching step with a
peroxide oxidizing agent.
[0024] Other steps, such as additional bleaching steps, can be
added to the process. Nevertheless, only a pretreatment step, a
washing step and a bleaching step with one or more peroxide
oxidizing agents is usually sufficient for the bleaching of
mechanical paper pulp.
[0025] The present invention is further illustrated with some
examples below, without limiting the scope thereto.
EXAMPLES
[0026] In the examples according to the invention, the pretreatment
step is conducted at a pH above 8. In the comparative examples, the
pretreatment step is conducted at a pH between 4 and 8, according
to the international application WO 97/30209.
[0027] The pH was measured with a Knick Portamess pH-meter (911
pH), with an Ag/AgCl electrode (Mettler Toledo, U402-S7/120). The
electrolyte was a mixture KCl/Aga 3M. The temperature was
compensated with a Knick Pt1000 (ZU0156) probe. The pH-meter was
calibrated with buffer solutions from Mettler Toledo (pH 7.00
buffer with the reference 9865 and pH 4.01 buffer with the
reference 9863).
[0028] The brightness of paper pulp was measured using a Datacolor
Elrepho spectrophotometer (SF450) according to the ISO standard
2470.
Example 1
(according to the invention), C2 & C3 (comparative)
[0029] A mechanical paper pulp (TMP) with an initial brightness of
55.5.degree. ISO was pretreated with EDDS, diluted with water prior
to thickening, then submitted to a bleaching process with hydrogen
peroxide.
[0030] The pretreatment step was carried out at 60.degree. C.,
during 20 minutes, at a consistency of 10% by weight of dry pulp
and with a concentration of 0.3% in EDDS. Initial pH of the
pretreatment step of examples 1, C2 and C3 was respectively 9.8,
6.0 and 5.0. The pH was measured with the standard laboratory glass
electrode described above. The pH was not measured nor adjusted
after the addition of the chelating agent, until the end of the
pretreatment step.
[0031] The washing step was carried out by diluting the pulp to 4%
consistency with water and then thickening it with a press to a
consistency around 20%.
[0032] The bleaching step was carried out at 70.degree. C., during
120 minutes, at a consistency of 15% by weight of dry pulp, with a
bleach liquor containing 3% hydrogen peroxide, 2.3% NaOH, and 2%
sodium silicate by weight of dry pulp.
[0033] Brightness results are given in the table below.
TABLE-US-00001 Chelating agent used in the pretreatment Initial
Brightness Gain Gain Example step pH (.degree. ISO) (.degree. ISO)
(%) 1 EDDS 9.8 69.0 13.5 24.3 C2 EDDS 6.0 67.5 12.0 21.6 C3 EDDS
5.0 59.4 3.9 7.0
Example 4
(According to the Invention), C5, C6 & C7 (Comparative)
[0034] A mechanical paper pulp (TMP) with an initial brightness of
56.9.degree. ISO was pretreated with IDS, washed as per the process
in Examples 1, C2 and C3, then submitted to a bleaching process
with hydrogen peroxide.
[0035] The pretreatment step was carried out at 60.degree. C.,
during 45 minutes, at a consistency of 10% by weight of dry pulp
and with a concentration of 0.3% in IDS. Initial pH of the
pretreatment step of examples 4, C5, C6 and C7 were respectively
9.0, 7.0, 6.0 and 5.0. The pH was measured with the standard
laboratory glass electrode described above. The pH was not measured
nor adjusted after the addition of the chelating agent, until the
end of the pretreatment step.
[0036] The washing step was carried out as in Examples 1, C2 and
C3. The bleaching step was carried out at 70.degree. C., during 120
minutes, at a consistency of 15% by weight of dry pulp, with a
bleach liquor containing 3% hydrogen peroxide, 2.08% NaOH, and 2%
sodium silicate by weight of dry pulp.
[0037] Brightness results are given in the table below.
TABLE-US-00002 Chelating agent used in the Initial Brightness Gain
Gain Example pretreatment step pH (.degree. ISO) (.degree. ISO) (%)
4 IDS 9.0 67.6 10.7 18.8 C5 IDS 7.0 63.7 6.8 12.0 C6 IDS 6.0 62.3
5.4 9.5 C7 IDS 5.0 62.8 5.9 10.4
Example 8, 9
(According to the Invention), C10 & C11 (Comparative)
[0038] A mechanical paper pulp (PGW) with an initial brightness of
65.6.degree. ISO was pretreated with EDDS, washed as per the
process in Examples 1, C2 and C3, then submitted to a bleaching
process with hydrogen peroxide.
[0039] The pretreatment step was carried out at 60.degree. C.,
during 20 minutes, at a consistency of 10% by weight of dry pulp
and with a concentration of 0.5% in EDDS. Initial pH of the
pretreatment step of examples 8, 9, C10 and C11 were respectively
10.9, 9.5, 6.9 and 5.1. The pH was measured with the standard
laboratory glass electrode described above. The pH was not measured
nor adjusted after the addition of the chelating agent, until the
end of the pretreatment step.
[0040] The washing step was carried out as in Examples 1, C2 and
C3.
[0041] The bleaching step was carried out at 70.degree. C., during
120 minutes, at a consistency of 15% by weight of dry pulp, with a
bleaching liquor containing 3% hydrogen peroxide, 1.8% NaOH, and
1.5% sodium silicate by weight of dry pulp.
[0042] Brightness results are given in the table below.
TABLE-US-00003 Chelating agent used in the pretreatment Initial
Final Brightness Gain Gain Example step pH pH (.degree. ISO)
(.degree. ISO) (%) 8 EDDS 10.9 9.6 78.8 13.2 20.1 9 EDDS 9.5 7.8
79.2 13.6 20.7 C10 EDDS 6.9 6.5 76.0 10.4 15.8 C11 EDDS 5.1 5.7
71.6 6.0 9.1
[0043] The final pH is the pH of the reacting medium at the end of
the pretreatment step.
Example 12 to 16
According to the Invention
[0044] After the pH optimization done with examples 8, 9, C10 and
C11, the dosage of EDDS was optimized on the same paper pulp as the
one used in these examples (initial brightness of 65.6.degree.
ISO).
[0045] The following results were obtained at a pH of 9.5, with
respectively 0, 0.2, 0.4, 0.6 and 0.8% of EDDS by weight of dry
pulp. The other conditions are the same as in examples 8, 9, C10
and C11.
TABLE-US-00004 Concentration of EDDS used in the Brightness Gain
Gain Example pretreatment step (.degree. ISO) (.degree. ISO) (%) 12
0 69.7 4.1 6.2 13 0.2 77.8 12.2 18.6 14 0.4 79.1 13.4 20.5 15 0.6
79.0 13.4 20.5 16 0.8 79.2 13.6 20.7
Example 17, 18
(According to the Invention), C19 & C20 (Comparative)
[0046] A mechanical paper pulp (PGW) with an initial brightness of
67.0.degree. ISO was pretreated with IDS, washed as per the process
in Examples 1, C2 and C3, then submitted to a bleaching process
with hydrogen peroxide.
[0047] The pretreatment step was carried out at 60.degree. C.,
during 20 minutes, at a consistency of 10% by weight of dry pulp
and with a concentration of 0.5% in IDS. Initial pH of the
pretreatment step of examples 17, 18, C19 and C20 were respectively
11.0, 9.0, 7.1 and 5.1. The pH was measured with the standard
laboratory glass electrode described above. The pH was not measured
nor adjusted after the addition of the chelating agent, until the
end of the pretreatment step.
[0048] The washing step was carried out as in Examples 1, C2 and
C3.
[0049] The bleaching step was carried out at 70.degree. C., during
120 minutes, at a consistency of 15% by weight of dry pulp, with a
bleaching liquor containing 3% hydrogen peroxide, 1.8% NaOH, and
1.5% sodium silicate by weight of dry pulp.
[0050] Brightness results are given in the table below.
TABLE-US-00005 Chelating agent used in the pretreatment Initial
Final Brightness Gain Gain Example step pH pH (.degree. ISO)
(.degree. ISO) (%) 17 IDS 11.0 9.2 76.7 9.7 14.5 18 IDS 9.0 8.1
78.2 11.2 16.7 C19 IDS 7.1 6.97 77.0 10.0 15.0 C20 IDS 5.1 5.30
73.6 6.6 9.9
[0051] The final pH is the pH of the reacting medium at the end of
the pretreatment step.
Example 21 to 25
According to the Invention
[0052] After the pH optimization done with examples 17, 18, C19 and
C20, the dosage of IDS was optimized on the same paper pulp as the
one used in these examples (initial brightness of 67.0.degree.
ISO).
[0053] The following results were obtained at a pH of 9.0, with
respectively 0, 0.2, 0.4, 0.6 and 0.8% of IDS by weight of dry
pulp. The other conditions are the same as in examples 17, 18, C19
and C20.
TABLE-US-00006 Concentration of IDS used in the Brightness Gain
Gain Example pretreatment step (.degree. ISO) (.degree. ISO) (%) 21
0 70.1 3.1 4.6 22 0.2 75.6 8.6 12.8 23 0.4 77.2 10.2 15.2 24 0.6
77.7 10.7 16.0 25 0.8 78.2 16.7 16.7
* * * * *