U.S. patent application number 13/321256 was filed with the patent office on 2012-03-15 for process for the bleaching of mechanical paper pulp.
This patent application is currently assigned to SOLVAY SA. Invention is credited to Pierre Dournel.
Application Number | 20120061043 13/321256 |
Document ID | / |
Family ID | 40822931 |
Filed Date | 2012-03-15 |
United States Patent
Application |
20120061043 |
Kind Code |
A1 |
Dournel; Pierre |
March 15, 2012 |
Process for the bleaching of mechanical paper pulp
Abstract
The invention relates to a process for the bleaching of paper
pulp with one or more peroxide oxidizing agents, in which the pulp
is first treated in a pretreatment step with at least one inorganic
magnesium derivative combined with at least one carboxylic acid or
its salt, except its magnesium salt, or with at least one
carboxylic acid at least partially saponified with Mg.sup.2+, the
carboxylic acid being substituted by at least one electrodonating
group or comprising at least one carbon-carbon double bond.
Inventors: |
Dournel; Pierre; (Brussels,
BE) |
Assignee: |
SOLVAY SA
Brussels
BE
|
Family ID: |
40822931 |
Appl. No.: |
13/321256 |
Filed: |
May 28, 2010 |
PCT Filed: |
May 28, 2010 |
PCT NO: |
PCT/EP2010/057372 |
371 Date: |
November 18, 2011 |
Current U.S.
Class: |
162/76 |
Current CPC
Class: |
D21C 9/00 20130101; D21C
9/1042 20130101; D21C 9/10 20130101 |
Class at
Publication: |
162/76 |
International
Class: |
D21C 9/16 20060101
D21C009/16; D21C 3/04 20060101 D21C003/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 29, 2009 |
EP |
09161461.0 |
Claims
1. A process for the bleaching of paper pulp with one or more
peroxide oxidizing agents, comprising first treating the pulp in a
pretreatment step with one or more chelating agents chosen selected
from the group consisting of (i) carboxylic acids or their salts,
except their magnesium salt, substituted by at least one
electrodonating group or comprising at least one carbon-carbon
double bond, in the presence of at least one inorganic magnesium
derivative, and (ii) carboxylic acids at least partially saponified
with Mg.sup.2+ and substituted by at least one electrodonating
group or comprising at least one carbon-carbon double bond.
2. The process according to claim 1, wherein the electrodonating
group is selected from the group consisting of amines, alcohols,
thiols, O-alkyl, and S-alkyl.
3. The process according to claim 1, wherein the carboxylic acid
has the following general formula
HOOC--C.sub.nH.sub.m(OR.sup.1).sub.p(SR.sup.2).sub.q(NR.sup.3R.sup.4).sub-
.r(COOH).sub.s--R' wherein C.sub.nH.sub.m is linear or branched and
saturated or unsaturated; n is an integer from 1 to 8; m is an
integer from 0 to 2n; p is an integer from 0 to n; q is an integer
from 0 to n; r is an integer from 0 to n; s is an integer from 0 to
n; p+q+r+s is an integer from 0 to 2n, and if p+q+r+s is 0, m is
.ltoreq.2n-2; R' is selected from the group consisting of --H,
--CH.sub.2OH, and --COOH; and R.sup.1, R.sup.2, R.sup.3, R.sup.4
are the same or different and are selected from the group
consisting of --H and linear or branched groups of formula
--C.sub.n'H.sub.m'--R'', wherein n' is an integer from 1 to 8, m'
is an integer from 0 to 2n, and R'' is selected from the group
consisting of --H, --CH.sub.2OH, and --COOH.
4. The process according to claim 1, wherein the carboxylic acid is
selected from the group consisting of lactic acid, citric acid,
tartaric acid, glucaric acid, maleic acid, nitrilotriacetic acid,
methylglycine diacetic acid, their salts, and mixtures thereof.
5. The process according to claim 1, wherein the inorganic
magnesium derivative is selected from the group consisting of
magnesium oxide, magnesium hydroxide, and magnesium sulfate.
6. The process according to claim 1, wherein the at least one
chelating agent is the carboxylic acid at least partially
saponified with Mg.sup.2+ (ii).
7. The process according to claim 1, wherein the pretreatment step
is performed at the natural pH of the paper pulp.
8. The process according to claim 1, wherein the concentration of
the inorganic magnesium derivative, expressed as Mg.sup.2+ ions, is
less than or equal to 1% by weight of the dry paper pulp and is
more than or equal to 0.01% by weight of the dry paper pulp.
9. The process according to claim 1, wherein the concentration of
the carboxylic acid or its salt (i) or of the carboxylic acid at
least partially saponified with Mg.sup.2+ (ii) is less than or
equal to 1% by weight of the dry pulp and is more than or equal to
0.2% by weight of the dry pulp.
10. The process according to claim 1, being applied to a mechanical
paper pulp.
11. The process according to claim 1, wherein the pretreatment step
is carried out in the presence of water in order to reach a
consistency of 1 to 20% by weight of dry pulp.
12. The process according to claim 1, comprising the additional
steps of: (a) pressing the mixture resulting from the pretreatment
step until a consistency of from 25 to 50% by weight of dry pulp is
reached, and (b) bleaching the pressed mixture resulting from step
(a) with a peroxide oxidizing agent.
13. The process according to claim 1, wherein the peroxide
oxidizing agent is selected from the group consisting of hydrogen
peroxide, organic peracids, and a combination thereof.
14. A method of use as chelating agents of (i) a mixture of at
least one carboxylic acid or salt thereof, except its magnesium
salt, combined with at least one inorganic magnesium derivative, or
(ii) at least one carboxylic acid at least partially saponified
with Mg.sup.2+, the carboxylic acids being substituted by at least
one electrodonating group or comprising at least one carbon-carbon
double bond.
15. The method according to claim 14, being carried out in a
pretreatment step of a process for the bleaching of paper pulp with
one or more peroxide oxidizing agents.
Description
[0001] The present application claims the benefit of the European
application no. 09161461.0 filed on May 29, 2009, herein
incorporated by reference.
[0002] The present invention is related to a process for the
bleaching of 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, prior to the bleaching step, 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] Widely used chelating agents are those of the
aminocarboxylic type, such as ethylene diamine tetra-acetic acid
and its salts (EDTA) or diethylene triamine pentaacetic acid and
its salts (DTPA). Nevertheless, these chelating agents are poorly
biodegradable and will thus be released in the environment where
they will accumulate. Although their toxicity is not really an
issue, their accumulation in lakes and rivers will lead to the
mobilization of heavy metals that are present in the soil and that
will become soluble in water. These heavy metals will then be
assimilated by living organisms like fishes, therefore entering in
the food chain.
[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). According to that previous
application, the pretreatment step must be conducted at a pH from 4
to 8 while EDDS and IDS perform best at a high pH, especially pH 8
for EDDS and 10 for IDS. Thus, this process does not lead to an
optimal brightness of the paper pulp.
[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, at a pH above 8. The
natural pH of the paper pulp being usually between 5 and 6, such
process implies the measurement and the control of the pH during
the whole pretreatment step, by the addition of a base such as
caustic soda. This has the drawback of lowering the brightness of
the paper pulp fed to the subsequent bleaching step, due to a
yellowing effect of the paper pulp during the pretreatment step.
Furthermore, the addition of caustic soda during the pretreatment
step increases the amount of extractives leading to a reduced yield
in pulp and a higher chemical oxygen demand (COD) charge in the
effluent treatment plant.
[0008] A further drawback of the use of EDDS and IDS as chelating
agents is the higher operating costs resulting from the higher
price or the higher concentration required and of the additional
use of a base to maintain the pH above 8 during the pretreatment
step.
[0009] The purpose of the present invention is to provide a simple
and environmentally friendly bleaching process, avoiding the
measurement and control of the pH during the pretreatment step and
still conducting to an optimal brightness of the paper pulp.
[0010] The present invention therefore relates to a process for the
bleaching of paper pulp with one or more peroxide oxidizing agents,
in which the pulp is first treated in a pretreatment step with one
or more chelating agents chosen from the group consisting of:
[0011] (i) carboxylic acids or their salts, except their magnesium
salt, substituted by at least one electrodonating group or
comprising at least one carbon-carbon double bond, in the presence
of at least one inorganic magnesium derivative, and [0012] (ii)
carboxylic acids at least partially saponified with Mg.sup.2+ and
substituted by at least one electrodonating group or comprising at
least one carbon-carbon double bond.
[0013] Indeed, it has surprisingly been found that it is possible
to reach a high degree of brightness or to decrease the hydrogen
peroxide consumption for a given brightness or both, when the
pretreatment step is conducted in the presence of at least one
inorganic magnesium derivative in combination with at least one
carboxylic acid or its salt (i) or in the presence of a magnesium
salt of a carboxylic acid (ii), the carboxylic acid being
substituted by at least one electrodonating group or comprising at
least one carbon-carbon double bond.
[0014] One of the essential features of the present invention
resides in the use, during the pretreatment step, of an inorganic
magnesium derivative combined with a carboxylic acid or its salt
(i), or of a magnesium salt of a carboxylic acid (ii), the
carboxylic acid being substituted by at least one electrodonating
group or comprising at least one carbon-carbon double bond.
[0015] According to the present invention, the carboxylic acids or
the salts thereof, useful as chelating agents during the
pretreatment step, are usually substituted by at least one
electrodonating group and/or comprise at least one carbon-carbon
double bond within the skeleton of the molecule. Suitable
electrodonating group are usually selected from the group
consisting of amines, especially alkylamines or dialkylamines,
alcohols, thiols, O-alkyl and 5-alkyl. Preferred electrodonating
groups are the amino group (--NH.sub.2), the alkylamino group
(--NRH) or the dialkylamino group (--NRR') or the hydroxyl group
(--OH), in particular the dialkylamino group (--NRR') or the
hydroxyl group (--OH).
[0016] In the present process, suitable carboxylic acids are those
of the following general formula (I)
HOOC--C.sub.nH.sub.m(OR.sup.1).sub.p(SR.sup.2).sub.q(NR.sup.3R.sup.4).su-
b.r(COOH).sub.s--R' (I)
wherein C.sub.nH.sub.m is linear or branched and saturated or
unsaturated; n is an integer from 1 to 8, especially from 1 to 4; m
is an integer from 0 to 2n; p is an integer from 0 to n; q is an
integer from 0 to n; r is an integer from 0 to n; s is an integer
from 0 to n; p+q+r+s is an integer from 0 to 2n, preferably at most
n, and if p+q+r+s is 0, m is .ltoreq.2n-2; R' is selected from --H,
--CH.sub.2OH, --COOH; and R.sup.1, R.sup.2, R.sup.3, R.sup.4 are
the same or different and are selected from --H and linear or
branched groups of formula --C.sub.n'H.sub.m'--R'', wherein n' is
an integer from 1 to 8, m' is an integer from 0 to 2n and R'' has
the same definition as R' above; preferably R.sup.1, R.sup.2,
R.sup.3, R.sup.4 are selected from --H, --CH.sub.2--CH.sub.2OH,
--CH.sub.2--COOH and --CH(CH.sub.3)--COOH.
[0017] The carboxylic acids used in the process of the present
invention are typically linear. They can also be branched. Usually
they are staturated. They can also be unsaturated. Preferably, the
carboxylic acids are not cyclic. In particular, they are not
aromatic compounds.
[0018] In the process of the invention, the carboxylic acids or the
salts thereof comprise at least one carboxylic acid or carboxylate
group, preferably at least two carboxylic acid or carboxylate
groups, for example two, three or even four carboxylic acid or
carboxylate groups.
[0019] Especially suitable carboxylic acids are those of the
formula (II), (III), (IV) or (V) below:
HOOC--C.sub.nH.sub.m--COOH (II)
wherein n is an integer from 2 to 8, preferably from 2 to 4 and m
is an integer from 0 to 2n-2, preferably from n to 2n-2;
HOOC--C.sub.nH.sub.m(OH).sub.p--COOH (III)
wherein n is an integer from 2 to 8, preferably from 2 to 4, and m
and p=n;
HOOC--C.sub.nH.sub.m(OH).sub.p(COOH).sub.s--COOH (IV)
wherein n is an integer from 2 to 8, preferably from 2 to 4, and m
is an integer from 0 to 2n-p-s, p is an integer from 1 to n, s is
an integer from 1 to n; and
HOOC--C.sub.nH.sub.m(NR.sup.1R.sup.2)--H (V)
wherein n is an integer from 1 to 8, preferably from 1 to 4, in
particular 1 or 2, m is an integer from 1 to 2n-1, R.sup.1 and
R.sup.2 are the same or different and are selected from --H,
--CH.sub.2--CH.sub.2OH, --CH.sub.2--COOH and --CH(CH.sub.3)--COOH,
in particular from --CH.sub.2--COOH and --CH(CH.sub.3)--COOH.
[0020] Especially suitable examples of carboxylic acids, or salts
thereof, useful in the present invention are lactic acid, citric
acid, tartaric acid, glucaric acid, maleic acid, nitrilotriacetic
acid (NTA), methylglycine diacetic acid (MGDA), their salts and
mixtures thereof. Citric acid, maleic acid, nitrilotriacetic acid
(NTA), methylglycine diacetic acid (MGDA), their salts and mixtures
thereof are preferred. According to the present invention, the most
preferred carboxylic acids, or salts thereof, are citric acid,
nitrilotriacetic acid (NTA), methylglycine diacetic acid (MGDA),
their salts and mixtures thereof.
[0021] In the process of the invention, good results are obtained
using only one carboxylic acid or salt thereof, substituted by at
least one electrodonating group and/or comprise at least one
carbon-carbon double bond within the skeleton of the molecule.
Nevertheless, more than one carboxylic acid or salt thereof
according to the invention can be used. For example, several
carboxylic acids and/or salts thereof according to the invention
can be combined. A carboxylic acid or a salt thereof according to
the invention can also be combined with one or more conventional
chelating agents such as ethylene diamine tetra-acetic acid and its
salts (EDTA) or diethylene triamine pentaacetic acid and its salts
(DTPA).
[0022] According to the present invention, preferably, there is no
peroxide oxidizing agent present during the pretreatment step of
the paper pulp with the chelating agents.
[0023] In the present invention, it is preferable to select
carboxylic acids which are biodegradable products. The preferred
carboxylic acids according the present invention as well as their
salts have the main advantage to be readily biodegradable
products.
[0024] In a first embodiment of the present invention, the
pretreatment step is conducted in the presence of at least one
inorganic magnesium derivative and of at least one carboxylic acid
or its salt, except its magnesium salt (i), as defined above,
namely substituted by at least one electrodonating group and/or
comprising at least one carbon-carbon double bond within the
skeleton of the molecule.
[0025] The salt of the at least one carboxylic acid may be selected
from, for example, the sodium salt, the potassium salt, the lithium
salt of the carboxylic acid.
[0026] According to this first embodiment, the inorganic magnesium
derivative is preferably selected from magnesium oxide, magnesium
hydroxide, magnesium chloride, magnesium nitrate and magnesium
sulphate. Magnesium sulphate may be in its anhydrous form or in its
hydrated form, for example in its heptahydrated form (Epsom salt).
An especially preferred inorganic magnesium derivative according to
this first embodiment is the magnesium sulphate heptahydrate or
Epsom salt.
[0027] In this first embodiment, the amount of the inorganic
magnesium derivative must be optimized depending on the paper pulp
to be pretreated and on the paper mill. The amount of the inorganic
magnesium derivative, expressed as Mg.sup.2+ ions, is usually less
than or equal to 1% by weight of the dry paper pulp, preferably
less than or equal to 0.5%, more preferably lower than or equal to
0.1%, advantageously less than or equal to 0.08%. The amount of the
inorganic magnesium derivative, expressed as Mg.sup.2+ ions, is
generally more than or equal to 0.01% by weight of the dry paper
pulp, preferably more than or equal to 0.02%, advantageously more
than or equal to 0.04%.
[0028] Still according to this first embodiment, the concentration
of the carboxylic acid or its salt (i) is typically less than or
equal to 1% by weight of the dry pulp, preferably less than or
equal to 0.8% of dry pulp. The concentration of the carboxylic acid
or its salt (i) is usually more than or equal to 0.2% by weight of
the dry pulp, preferably more than or equal to 0.3% by weight of
dry pulp, for example more than or equal to 0.4% by weight of dry
pulp.
[0029] In a second embodiment of the present invention, the
pretreatment step is conducted in the presence of at least one
carboxylic acid at least partially saponified with Mg.sup.2+ (ii)
as defined above, namely substituted by at least one
electrodonating group and/or comprising at least one carbon-carbon
double bond within the skeleton of the molecule. This second
embodiment is especially advantageous in the process of the
invention. Said magnesium salt of carboxylic acid can be purchased
as such to an industrial manufacturer or can be prepared before use
by reacting a magnesium derivative such as magnesium oxide,
magnesium hydroxide or magnesium carbonate, with the corresponding
carboxylic acid to form the at least partial magnesium salt of the
carboxylic acid, according to any method known in the art.
[0030] According to this second embodiment, the at least partial
magnesium salt of the carboxylic acid (ii) is generally less than
or equal to 1% by weight of the dry pulp, especially less than or
equal to 0.8%. The concentration of the magnesium salt of the at
least partial magnesium salt of the carboxylic acid (ii) is in
general more than or equal to 0.2% by weight of the dry pulp,
preferably more than or equal to 0.4%.
[0031] The at least partial magnesium salt of the carboxylic acid
(ii) may be in the form of the stoechiometric magnesium salt or of
a partial magnesium salt. Indeed, all the carboxylic groups of all
the carboxylic acid molecules may be saponified, or only a part of
the carboxylic groups and/or a part of the carboxylic acid
molecules may be saponified.
[0032] Examples of stoechiometric salts of the carboxylic acids of
the present invention are magnesium citrate, magnesium maleate and
the magnesium salt of nitrilotriacetic acid. Magnesium citrate
results from the stoechiometric reaction of two equivalents of
citric acid with 3 equivalents of Mg(OH).sub.2 and can be written
as Citrate.sub.2Mg.sub.3. Magnesium maleate results from the
stoechiometric reaction of one equivalent of maleic acid with one
equivalent of Mg(OH).sub.2. The magnesium salt of nitrilotriacetic
acid results from the stoechiometric reaction of two equivalents of
nitrilotriacetic acid (NTA) with three equivalents of Mg(OH).sub.2
and can be written NTA.sub.2Mg.sub.3. The magnesium salt of
methylglycine diacetic acid results from the stoechiometric
reaction of two equivalents of methylglycine diacetic acid (MGDA)
with three equivalents of Mg(OH).sub.2 and can be written
MGDA.sub.2Mg.sub.3.
[0033] An example of a partial magnesium salt of the carboxylic
acids of the present invention is the compound
Citrate.sub.2Mg.sub.1.5 resulting from the reaction of two
equivalents of citric acid with 1.5 equivalents of Mg(OH).sub.2,
said formula being actually an average one and the resulting
product being in reality a mixture of compounds, i.e. a mixture of
the acid and of the mono-, di- and tri-saponified molecules.
[0034] It has to be noted that, depending on the nature of the
carboxylic acids according to the present invention and on their
degree of saponification with magnesium (Mg.sup.2+), the properties
of the resulting product will be different. For example, the
stoechiometric compound Citrate.sub.2Mg.sub.3 has a solubility in
water of maximum 10% by weight at room temperature, while the
solubility of the partial magnesium salt Citrate.sub.2Mg.sub.1.5 is
of at least 30% by weight in water at room temperature. The
solubility in water at room temperature of the stoechiometric
magnesium salt of maleic acid is of maximum 20% by weight and of
the stoechiometric magnesium salt of nitrilotriacetic acid
(NTA.sub.2Mg.sub.3) is of at least 30% by weight.
[0035] The pretreatment step of the process of the present
invention, especially according to the first or the second
embodiment disclosed above, is typically performed at a neutral pH
or at a weakly acidic pH, in particular at the natural pH of the
paper pulp. In general, the pretreatment step is conducted at a pH
equal to or above 4, particularly at a pH equal to or above 5. The
pH during the pretreatment step is usually equal to or below 8, in
many cases equal to or below 7, especially equal to or below 6.5,
values equal to or below 6 being common. Suitable ranges for the pH
during the pretreatment step are for example from 4 to 8,
preferably from 5 to 7, and even more preferably from 5 to 6.
[0036] If necessary, the pH of the paper pulp may be adjusted to
preferred values by means of pH modifying compounds. Examples of
such compounds are sodium hydroxide or suitable alternative bases
and sulfuric acid or chlorhydric acid or suitable alternative
acids.
[0037] In the process of the present invention, the pretreatment
step is usually carried out in the presence of water in order to
reach a final consistency of at least 1% by weight of dry paper
pulp. By final consistency is meant the amount of paper pulp
present into the paper pulp slurry after the addition of the
chelating agents needed for the pretreatment step. The final
consistency of the paper pulp is usually less than or equal to 20%
by weight, particularly less than or equal to 15%, more
particularly less than or equal to 10%. In the majority of the
cases, a final consistency of 1 to 20% by weight of dry pulp will
be suitable, preferably from 3 to 15%, especially from 5 to
10%.
[0038] In the process of the invention, the pretreatment step is
usually carried out at the temperature generated by the mechanical
treatment of the wood. The temperature can vary widely and is
usually of at least 40.degree. C. The temperature is generally less
than or equal to 95.degree. C.
[0039] The duration of the pretreatment step is usually from 5 to
180 minutes, varying from mill to mill. In general, good results
are obtained provided that the duration of the pretreatment step is
of at least 5 minutes. Preferably, the duration of the pretreatment
step is equal to or higher than 30 minutes.
[0040] In the process of the invention, it is recommended to have a
good mixing of the paper pulp with the chelating agents during the
pretreatment step. This can be done by introducing the chelating
agent during via the pump circulating the pulp through the paper
mill.
[0041] According to the present invention, the pretreatment step
may be applied to a mechanical paper pulp, a chemical pulp or a
waste paper pulp, preferably to a mechanical paper pulp. By
mechanical paper pulp 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).
[0042] In a further embodiment, the present invention relates to a
process for the bleaching of paper pulp with one or more peroxide
oxidizing agent, comprising the steps of [0043] (a) treating the
paper pulp, in a pretreatment step, with one or more chelating
agents chosen from the group consisting of [0044] (i) carboxylic
acids or their salts, except their magnesium salt, substituted by
at least one electrodonating group or comprising at least one
carbon-carbon double bond, in the presence of at least one
inorganic magnesium derivative, and [0045] (ii) carboxylic acids at
least partially saponified with Mg.sup.2+ and substituted by at
least one electrodonating group or comprising at least one
carbon-carbon double bond. [0046] (b) pressing the mixture
resulting from the pretreatment step (a) until a consistency of
from 25 to 50% by weight of dry pulp is reached, preferably from 30
to 45%, and [0047] (c) bleaching the pressed mixture resulting from
step (b) with a peroxide oxidizing agent in the presence of a base
and optionally in the presence of a stabilizer.
[0048] Thus, the present invention also relates to a process for
the bleaching of paper pulp with one or more peroxide oxidizing
agent in which the pulp is first treated in a pretreatment step as
described above, and comprising the additional steps of: [0049] (a)
pressing the mixture resulting from the pretreatment step until a
consistency of from 25 to 50% by weight of dry pulp is reached,
preferably from 30 to 45%, and [0050] (b) bleaching the pressed
mixture resulting from step (a) with a peroxide oxidizing
agent.
[0051] Preferably, in the process of the present invention, the
paper pulp is submitted to only one pretreatment step before the
bleaching with one or more peroxide oxidizing agent.
[0052] A washing step may also be conducted between the
pretreatment step and the bleaching step. Other steps, such as
additional bleaching steps, can also be added to the process.
Nevertheless, only one pretreatment step, one pressing step, one
washing step and one bleaching step with one or more peroxide
oxidizing agents is usually sufficient for the bleaching of
mechanical paper pulp.
[0053] In the process of the invention, the peroxide oxidizing
agent is usually chosen from the group consisting of 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.
Hydrogen peroxide is thus preferably used in combination with an
alkaline compound, especially with at least alkaline earth
hydroxide such as sodium hydroxide, potassium hydroxide, calcium
hydroxide and magnesium hydroxide. Preferably, the bleaching step
is conducted using hydrogen peroxide, an alkaline compound and a
stabilizer, such as sodium silicate or an organic stabilizer such
as a polyacrylate and/or a polyhydroxyacrylate salt.
[0054] The present invention also relates to the use as chelating
agents of [0055] (i) a mixture of at least one carboxylic acid or
salt thereof, except its magnesium salt, combined with at least one
inorganic magnesium derivative, or [0056] (ii) at least one
carboxylic acid at least partially saponified with Mg.sup.2+, the
carboxylic acids being substituted by at least one electrodonating
group or comprising at least one carbon-carbon double bond. Is also
part of the invention, the use of said compounds as chelating
agents in the pretreatment step of a process for the bleaching of
paper pulp with one or more peroxide oxidizing agents.
[0057] The present invention is further illustrated with some
examples below, without limiting the scope thereto.
EXAMPLES
[0058] The brightness of paper pulp was measured using a Datacolor
Elrepho spectrophotometer (SF450) according to ISO standard
2470.
[0059] In the examples, the percentages of chelating agents are
expressed as the amount of pure product by weight of dry pulp.
[0060] Examples conducted in the absence of a chelating agent are
the result of a pretreatment step performed with demineralised
water only. The bleaching step was conducted in the same conditions
as the other examples of the series.
[0061] In the examples below, DTPA means diethylene triamine
pentaacetic acid. DTPA was used as a commercial solution of 40%
DTPA by weight of water.
[0062] Citrate.sub.2Mg.sub.1.5 is a partially saponified magnesium
salt of citric acid. This product results from the reaction of two
equivalents of citric acid with 1.5 equivalents of Mg(OH).sub.2,
said formula being actually an average one and the resulting
product being in reality a mixture of compounds.
[0063] Maleic Mg means the magnesium salt of maleic acid. It
results from the stoechiometric reaction of 1 equivalent of maleic
acid with one equivalent of Mg(OH).sub.2.
[0064] NTA means nitrilotriacetic acid. NTA is commercially
available, for instance as TRILON AS.RTM. from BASF. NTA Na.sub.3
means the tri sodium salt of nitrilotriacetic acid. This product is
commercially available, for example as TRILON A.RTM. sold by BASF.
NTA.sub.2Mg.sub.3 means the magnesium salt of the nitrilotriacetic
acid (NTA). NTA.sub.2Mg.sub.3 was prepared by reacting 2
equivalents of NTA (commercially available as TRILON AS.RTM. from
BASF) with 3 equivalents of Mg(OH).sub.2. NTA, NTA Na.sub.3 and
NTA.sub.2Mg.sub.3 were used as aqueous solutions (40 wt %).
[0065] MGDA means methylglycine diacetic acid. MGDA Na.sub.3 means
the trisodium salt of methylglycine diacetic acid. This product is
commercially available, for example as TRILON M.RTM. from BASF.
MGDA and MGDA Na.sub.3 were used as aqueous solutions (40 wt
%).
[0066] In the examples below, MGDA Na.sub.3/NTA means a mixture of
the trisodium salt of methylglycine diacetic acid (MGDA Na.sub.3)
and of nitrilotriacetic acid (NTA). This product was prepared by
adding 1.6 g of NTA stepwise (by 0.4 g) to 10 g of MGDA Na.sub.3.
The resulting mixture was diluted with water in order to obtain a
concentration of active product of 40% by weight. This solution
contained 72% by weight of MGDA Na.sub.3 and 28% by weight of NTA,
both being partially saponified and present under the form of their
partial sodium salt. The pH of this solution was about 5.0.
Examples 1 to 11
[0067] A sample of a refiner mechanical pulp (RMP) was pretreated
with the chelating agents summarized in Table I below.
[0068] The pretreatment step was carried out at 60.degree. C.
during 30 minutes on pulp samples at a final consistency in water
of 10 wt %, at the natural pH of the paper pulp. After said
pretreatment, the pulp was pressed up to a consistency of 30 wt
%.
[0069] The resulting paper pulp was then subsequently bleached at a
final consistency of 15 wt % and at a temperature of 70.degree. C.
during 120 minutes in the presence of 5.0% of hydrogen peroxide,
3.1% of caustic soda and 3.5% of sodium silicate, all these
percentages being expressed by weight of dry pulp.
[0070] The brightness of the resulting bleached paper pulp and the
hydrogen peroxide consumption are summarized in Table I below.
TABLE-US-00001 TABLE I Ex- H.sub.2O.sub.2 am- Amount Brightness
consumption ples Chelating agent (%) (.degree.ISO) (%) 1 -- -- 71.7
93.4 2 DTPA 0.16 74.5 66.0 3 Mg.sup.2+ 0.06 72.6 71.6 4 Citric acid
+ 0.2 + 73.5 72.6 Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06 5
Citric acid + 0.4 + 73.9 70.0 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.06 6 Citric acid + 0.6 + 74.4 69.2
Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06 7 Citric acid + 0.8 +
74.5 70.0 Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06 8
Citrate.sub.2Mg.sub.1.5 0.69 74.0 73.8 9 Maleic Mg 0.31 73.5 75.4
10 MGDA Na.sub.3 0.16 73.5 75.0 11 MGDA Na.sub.3 + 0.16 + 74.4 70.0
Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06
[0071] The amount of 0.69% of Citrate.sub.2Mg.sub.1.5 by weight of
dry pulp corresponds to an amount of 0.06% of Mg.sup.2+ and of
0.63% of citric acid by weight of dry pulp. The amount of 0.31% of
the magnesium salt of maleic acid (Maleic Mg) by weight of dry pulp
corresponds to an amount of 0.06% of Mg.sup.2+ and of 0.19% of
maleic acid by weight of dry pulp.
Examples 12 to 27
[0072] A sample of a thermomechanical pulp (TMP) was pretreated
with the chelating agents summarized in Table II below.
[0073] The pretreatment step was carried out in the same conditions
as in previous examples.
[0074] The resulting paper pulp was then subsequently bleached at a
final consistency of 15 wt % and at a temperature of 80.degree. C.
during 60 minutes in the presence of 3.9% of hydrogen peroxide,
1.9% of caustic soda and 1.5% of sodium silicate, all these
percentages being expressed by weight of dry pulp.
[0075] The brightness of the resulting bleached paper pulp and the
hydrogen peroxide consumption are summarized in Table II below.
TABLE-US-00002 TABLE II Bright- H.sub.2O.sub.2 Exam- Amount ness
consumption ples Chelating agent (%) (.degree.ISO) (%) 12 -- --
58.2 98.7 13 Mg.sup.2+ 0.04 66.5 98.0 14 DTPA 0.16 73.1 61.6 15
DTPA 0.24 74.5 54.0 16 DTPA + 0.16 + 74.1 58.5 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.04 17 Citric acid 0.6 64.7 97.9 18
Citric acid + 0.6 + 70.2 76.1 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.05 19 Citrate.sub.2Mg.sub.1.5 0.5
70.3 82.0 20 Citrate.sub.2Mg.sub.1.5 0.75 71.6 75.1 21 MGDA
Na.sub.3 0.32 63.9 73.4 22 MGDA Na.sub.3 + 0.32 + 74.1 55.7
Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.05 23 MGDA Na.sub.3 + 0.32
+ 74.1 74.7 Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06 24 MGDA
Na.sub.3 + 0.24 + 73.2 79.5 Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O)
0.06 25 MGDA Na.sub.3/NTA + 0.32 + 72.9 78.6 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.06 Final pH at pretreatment step =
4.5 26 MGDA Na.sub.3/NTA + 0.32 + 74.5 62.9 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.06 pH maintained between 6 and 6.7
27 MGDA Na.sub.3/NTA + 0.24 + 74.3 66.6 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.06 pH maintained between 6 and
6.7
[0076] The amount of 0.5% of Citrate.sub.2Mg.sub.1.5 by weight of
dry pulp corresponds to an amount of 0.04% of Mg.sup.2+ and of
0.46% of citric acid by weight of dry pulp. The amount of 0.75% of
Citrate.sub.2Mg.sub.1.5 by weight of dry pulp corresponds to an
amount of 0.06% of Mg.sup.2+ and of 0.69% of citric acid by weight
of dry pulp.
Examples 28 to 34
[0077] A sample of a chemithermomechanical pulp (CTMP) was
pretreated with the chelating agents summarized in Table III
below.
[0078] The pretreatment step was carried out in the same conditions
as in the previous examples.
[0079] The resulting paper pulp was then subsequently bleached at a
final consistency of 15 wt % and at a temperature of 65.degree. C.
during 120 minutes in the presence of 2.5% of hydrogen peroxide,
1.7% of caustic soda and 2.3% of sodium silicate, all these
percentages being expressed by weight of dry pulp.
[0080] The brightness of the resulting bleached paper pulp and the
hydrogen peroxide consumption are summarized in Table III
below.
TABLE-US-00003 TABLE III H.sub.2O.sub.2 Exam- Amount Brightness
consumption ples Chelating agent (%) (.degree.ISO) (%) 28 -- --
64.4 97.6 29 DTPA 0.12 73.4 64.1 30 Mg.sup.2+ 0.06 71.2 88.6 31
Citrate.sub.2Mg.sub.1.5 0.69 72.5 72.6 32 Maleic Mg 0.31 70.5 90.4
33 NTA Na.sub.3 0.24 73.4 76.1 34 NTA.sub.2Mg.sub.3 0.28 73.4
68.8
[0081] The amount of 0.69% of Citrate.sub.2Mg.sub.1.5 by weight of
dry pulp corresponds to an amount of 0.06% of Mg.sup.2+ and of
0.63% of citric acid by weight of dry pulp. The amount of 0.31% of
the magnesium salt of maleic acid (Maleic Mg) by weight of dry pulp
corresponds to an amount of 0.06% of Mg.sup.2+ and of 0.19% of
maleic acid by weight of dry pulp. The amount of 0.28% of
NTA.sub.2Mg.sub.3 by weight of dry pulp corresponds to an amount of
0.04% of Mg.sup.2+ and of 0.24% of NTA by weight of dry pulp.
Examples 35 to 41
[0082] A sample of a stone ground wood pulp (SGW) was pretreated
with the chelating agents summarized in Table IV below.
[0083] The pretreatment step was carried out in the same conditions
as in the previous examples.
[0084] The resulting paper pulp was then subsequently bleached at a
final consistency of 15 wt % and at a temperature of 80.degree. C.
during 180 minutes in the presence of 3.5% of hydrogen peroxide,
1.17% of Kaliol (magnesium hydroxide suspension) and 0.2% of DTPA,
all these percentages being expressed by weight of dry pulp.
[0085] The brightness of the resulting bleached paper pulp and the
hydrogen peroxide consumption are summarized in Table IV below.
TABLE-US-00004 TABLE IV Bright- H.sub.2O.sub.2 Exam- Amount ness
consumption ples Chelating agent (%) (.degree.ISO) (%) 35 -- --
71.8 86.6 36 DTPA 0.16 75.8 43.8 37 NTA Na.sub.3 0.5 75.0 54.3 38
NTA Na.sub.3 0.24 74.0 60.0 39 NTA Na.sub.3 + 0.24 + 75.3 55.7
Mg.sup.2+ (MgSO.sub.4.cndot.7H.sub.2O) 0.06 40 NTA.sub.2Mg.sub.3
0.24 75.4 52.0 41 MGDA Na.sub.3 + 0.16 + 75.0 61.8 Mg.sup.2+
(MgSO.sub.4.cndot.7H.sub.2O) 0.08
[0086] The amount of 0.24% of NTA.sub.2Mg.sub.3 by weight of dry
pulp corresponds to an amount of 0.04% of Mg.sup.2+ and of 0.28% of
NTANa.sub.3 by weight of dry pulp.
[0087] Should the disclosure of any patents, patent applications,
and publications which are incorporated herein by reference
conflict with the description of the present application to the
extent that it might render a term unclear, the present description
shall take precedence.
* * * * *