Note: Descriptions are shown in the official language in which they were submitted.
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METHODS FOR BLEACHING AQUEOUS PULPS AND COMPOSITIONS FOR USE
IN THESE METHODS
FIELD OF THE INVENTION
[01] The present invention relates to compositions for use in the bleaching
of
pulps. In particular, the compositions are suitable for use as additives for
adjustment of
pH and/or control of unwanted substances within a pulp slurry during a pulp
bleaching
step. Also part of the present invention is the use of said compositions, for
example the
use of said compositions in a pulp preparation or a papermaking process, as
well as
bleaching methods for pulps using said compositions. Finally, kits of
ingredients for
forming the compositions according to the invention, or for use in a method
according to
the invention, are also part of the present invention.
BACKGROUND OF THE INVENTION
[02] Traditional bleaching processes of pulps commonly employ alkaline
hydrogen peroxide as a bleaching agent. To that effect, hydrogen peroxide and
a water
soluble alkaline compound, such as sodium hydroxide and/or water glass are
introduced
into the pulp in order to improve whiteness and/or brightness of a wood pulp,
a recycled
paper pulp or any other chemical or mechanical pulp, such as a pulp for
papermaking
processes. The use of chelating agents like EDTA to remove some of the metal
ions
from the pulp prior to adding peroxide allows the peroxide to be used more
efficiently.
Magnesium salts and sodium silicate may also be added to improve bleaching
with
alkaline peroxide. Magnesium salts typically are also used to protect
cellulose fibres
from too much degradation and therefore strength loss during delignification
and
bleaching process steps.
[03] More recently, alkaline earth metal hydroxides and/or alkaline earth
metal
oxides have been employed to replace sodium hydroxide and water glass, and to
reduce
the need for chelating agents and sodium silicate (see for example as
disclosed in Li et
al., Pulp & Paper Canada 106:6 (2005), pages T125 to T129 "Using magnesium
hydroxide (Mg(OH)2) as the alkali source in peroxide bleaching at Irving
paper").
WO 2014/195478 Al discloses the use of pitch control additives in combination
with
Mg(OH)2 and/or MgO in paper bleaching pulps, in order to avoid the formation
of
magnesium based fatty acid salts and resins in paper pulps.
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[04] Alkaline earth metal hydroxides and alkaline earth metal oxides are
only
sparingly soluble in aqueous solutions, which may lead to a reduction in
bleaching
efficiency and reduced efficiency in fibre protection. In addition, it is
known to use MgSat
in paper bleaching pulps as a radical scavenger and a slurry stabilising
agent. MgSat
is expensive and difficult to store, due to its hygroscopic properties.
[05] Finally, maintaining high pulp viscosity is a consideration in the
processing of pulps, including in pulp bleaching. Pulp viscosity is a
measurement that
indicates the strength of the fiber. The set-up of pulp processing plants,
such as for
example pulp bleaching plants, is notably designed to maintain the pulp
viscosity while
achieving desired bleaching result. Variation of pulp viscosity can therefore
be
problematic.
[06] The state of the art therefore presents several problems.
SHORT DESCRIPTION OF THE INVENTION
[07] The present invention is defined in the appended claims.
[08] In particular, the present invention is embodied by a composition for
use
in an aqueous pulp bleaching process, the composition comprising (i) one or
more
alkaline earth metal oxides and/or one or more alkaline earth metal
hydroxides; (ii) one
or more alkaline earth metal salts; and (iii) optionally, one or more pitch
control additives.
It was found that in the presence of an alkaline earth metal salt, such as an
alkaline earth
metal sulphate, the alkalinity of the described chemical preparation was
reduced, leading
to improved solubility of the alkaline earth metal oxide and/or alkaline earth
metal
hydroxide in this preparation that is then dosed into the pulp, while at the
same time the
beneficial radical scavenging and slurry stabilising effects of the alkaline
earth metal salt,
such as an alkaline earth metal sulphate, were obtained. For example, the
alkaline earth
metal may be selected from magnesium, calcium, or a mixture of magnesium and
calcium.
[09] According to one embodiment, the alkaline earth metal salt is an
alkaline
earth metal sulphate, such as for example calcium sulphate or magnesium
sulphate.
[10] According to one embodiment, the pitch control additive is selected
from
talc, bentonite, zeolite, diatomaceous earth, cationic mica, hydrophobic
carbonates,
resin-decomposing enzymes, cationic polymers designed to capture and
immobilise
resins or resinous components on fibre surfaces, aluminium sulphate,
polyaluminium
chloride, and dispersing agents specifically designed for the dispersion of
resins and
resinous components, and mixtures thereof. It was found that these pitch
control agents
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lead to a reduction in fatty acid salts and resins in paper pulps. According
to one
embodiment, the pitch reduction agent is talc, such as for example micronized
talc,
bimodal talc, cationic talc, or mixtures thereof. According to one embodiment,
the pitch
control additive is bentonite, such as an activated bentonite. In certain
embodiments,
the bentonite is activated by the alkaline earth metal oxide and/or alkaline
earth metal
hydroxide and/or the alkaline earth metal salt. Bentonite may bind heavy
metals in the
paper bleaching composition and reduce their interference with the bleaching
process,
for example by degrading the peroxide responsible for the bleaching process.
[11] According to the present invention, the composition may be a dry
particulate composition, or the composition may be an aqueous slurry, or the
composition
may be present as a constituent of an aqueous slurry comprising other
components,
such as for example, the composition may be present as a constituent of an
aqueous
paper bleaching pulp. According to the present invention, a particulate
composition
according to the invention may be mixed with water in order to prepare an
aqueous slurry,
which is also a part of the present invention.
[12] According to the present invention, the composition may be an aqueous
slurry having a solids content of up to 75 wt.-%, such as for example 10 to 75
wt.-%, on
the basis of the total weight of the slurry.
[13] According to the present invention, the weight ratio of (i) the
alkaline earth
metal oxide and/or the alkaline earth metal hydroxide on the one hand and (ii)
the alkaline
earth metal salt on the other hand is from 1:10 to 100:1. It was found that
the
advantageous effects according to the present invention were best obtained if
the
components are present in such proportions. For example, the said weight ratio
may be
in the range of 1:5 to 50:1, such as for example in the range of 1:2 to 25:1,
or from 1:1
to 10:1, such as for example about 2:1, or about 70:30 or about 80:20, or
about 90:10.
[14] According to one embodiment, the composition may further comprise one
or more of the following additives: dispersants; fillers; surfactants;
bleaching agents;
chelating agents; and pH-buffering agents.
[15] According to a further aspect of the present invention, there is
provided a
kit of ingredients, comprising a first container comprising an alkaline earth
metal oxide
and/or an alkaline earth metal hydroxide, and either one or both of a second
and a third
container, the second container comprising an aqueous mineral acid solution
and the
third container comprising sulphur dioxide or carbon dioxide. It was found
that the
compositions according to the present invention could be easily obtained using
the said
kit. In one embodiment, the aqueous mineral acid is aqueous sulphuric acid.
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[16] Also part of the present invention are methods for producing the
compositions of the present invention, including a step of mixing an alkaline
earth metal
oxide and/or an alkaline earth metal hydroxide with an acid such as a mineral
acid, such
as sulphuric acid, or sulphur dioxide, or carbon dioxide, or an alkaline earth
metal salt,
such as for example an alkaline earth metal sulfate or an alkaline earth metal
carbonate,
optionally in the presence of one or more pitch control agents. The alkaline
earth metal
may be selected, for example, from magnesium, calcium and mixtures thereof.
[17] According to one aspect of the present invention, the said method may
be carried out in situ within an aqueous pulp bleaching composition. It was
found that
the alkaline earth metal oxide and/or alkaline earth metal hydroxide
efficiently reacted in
situ with an acid such as a mineral acid, such as sulphuric acid, and/or
sulphur dioxide
or carbon dioxide to form a mixture of alkaline earth metal oxide and/or
alkaline earth
metal hydroxide with alkaline earth metal salt, such as an alkaline earth
metal sulphate,
in situ. For example, the method according to the present invention may be
performed
by addition of a mineral acid, such as for example sulphuric acid, or carbon
dioxide, or
sulphur dioxide to a paper bleaching pulp comprising the alkaline earth metal
oxide
and/or the alkaline earth metal hydroxide. For example, the method according
to the
present invention may be performed by addition of the alkaline earth metal
oxide and/or
alkaline earth metal hydroxide with an acidic pulp composition (e.g. wherein
the only acid
present in the acidic pulp composition is acid that is naturally present in
the pulp
composition (i.e. derived from the wood) or no external acid is added).
[18] According to one aspect of the invention, the method may include a
step
of mixing an alkaline earth metal hydroxide with a mineral acid, wherein the
base-acid
normality, i.e. the molar ratio of hydroxide ions in the alkaline earth metal
hydroxide and
the hydrogen cations in the mineral acid is between 10:1 and 1:5, such as for
example
about 10:1, or about 5:1, or about 4:1, or about 3:1, or about 2:1, or about
1.5:1, or about
1:1, or about 1:1.5, or about 1:2, or about 1:5. Corresponding acid/base
ratios shall be
employed in the case of alkaline earth metal oxide, and carbon dioxide or
sulphur
dioxide, for example.
[19] According to one aspect of the present invention, the method may be
performed by admixing the ingredients of the kit according to one aspect of
the present
invention into a paper bleaching pulp. It was found that the said kit was
easily
transported, stored and applied for use in the method of the invention.
[20] Also part of the present invention is the use of the composition, the
method or the kit as described according to the present invention in a pulp
bleaching
process. It was found that the pulp bleaching process could be rendered more
simple,
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efficient and less expensive by the use of the present invention, while at the
same time
reducing the need for expensive raw materials or complex procedures.
[21] According to some embodiments, the composition, the method or the kit
according to the present invention may be used in a process to reduce pitch in
a pulp
and/or in a process to protect the pulp from depolymerisation and/or
degradation of pulp
(e.g., resulting in the formation of stickies). In this context, methods for
reduction of
depolymerisation and/or degradation of a pulp and/or for protecting pulp from
the
formation of degradation products (e.g., stickies) are also part of the
present invention.
The depolymerisation or degradation occurs due to attack of the bleaching
chemicals on
the cellulose molecules causing reduction of polymer chain length and
consequently
lower strength properties of the pulp. Very often this degradation is measured
as fibre
viscosity. Lower chain length by depolymerisation or degradation leads to
lower viscosity
values and hence lower strength properties which reduced the value of the
produced
pulp.
[22] According to some further embodiments of the present invention, the
composition or the product of the method or one or more of the components of
the kit
according to embodiments of the present invention are added to the pulp to be
bleached,
prior to an addition of any other alkaline bleaching agents, such as water
glass, NaOH,
sodium silicate, and/or hydrogen peroxide.
[23] It is understood that the following detailed description and
references to
the figures concern exemplary embodiments of the present invention and shall
not be
limiting the scope of the claims.
DETAILED DESCRIPTION OF THE INVENTION
[24] The present invention according to the appended claims provides
improvements for paper pulping processes, including bleaching, such as oxygen
bleaching, peroxide-oxygen bleaching, oxygen-peroxide bleaching or oxygen
delignification, for example. The improvements can be obtained by the products
and kits
as defined in the appended claims, as well as by their application according
to the
methods and uses as defined in the appended claims.
[25] It has been found that the combination of alkaline earth metals
hydroxides
and/or alkaline earth metal oxides, together with certain alkaline earth metal
salts and
optionally pitch control agents provides an array of advantages over the state
of the art.
[26] One advantage is the reduction of the formation of harmful deposits in
the
pulp. This is achieved by the use of, on the one hand, the combination of
alkaline earth
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metal oxide and/or alkaline earth metal hydroxide with the alkaline earth
metal salt, and
on the other hand by the presence of certain pitch control agents.
[27] Furthermore, it was also found that the presence of certain alkaline
earth
metal salts leads to a reduction of radicals formed in the bleaching process.
The
presence of radicals, while on the one hand desirable for the bleaching, may
lead to the
formation of unwanted side products, e.g., from depolymerisation and/or
degradation of
pulp such as pitch, or "stickies", which may lead to clotting of the slurry,
and damage to
the downstream paper treatment installations.
[28] Furthermore, the presence of certain alkaline earth metal salts, such
as
alkaline earth metal sulphate, leads to a stabilising of the pulp viscosity
and improvement
of the pulp strength properties. This is important for the usability of the
pulp in paper and
board production and other pulp end uses. According to some aspects of the
present
invention, the viscosity stability is obtained by the use of the compositions
or kit of the
invention, and leads to a maintenance of viscosity or to a reduction of the
decrease in
viscosity. For example, according to the present invention, the use of the
compositions
or ingredients of the kit leads to a change in viscosity which is no more than
10%, such
as for example no more than 5%, or no more than 4%, or no more than 3%, or no
more
than 2%, or no more than 1%, or no more than 0.5%, or which essentially leads
to no
measurable change in viscosity.
[29] It is thought that this "protection" of the viscosity is caused by the
presence of free alkaline earth metal ions, such as magnesium ions, in a
slurry. The
formation of alkaline earth metal salts from the alkaline earth metal
hydroxide or alkaline
earth metal oxide and an acid in a paper making pulp leads to the formation of
free
alkaline earth metals ions, such as magnesium ions and the stabilisation of
the paper
pulp viscosity.
[30] In addition, the use of alkaline earth metal salts in the present
invention
allows to adapt the pH-value of the pulp bleaching preparation. Due to the
presence of
alkaline earth metal oxide and/or alkaline earth metal hydroxide, an alkaline
slurry is
normally obtained, with an elevated pH of 10 or more, or 11 or more. According
to the
present invention, the use of alkaline earth metal salt leads to a control of
the alkalinity
in the pulp bleaching preparation, and it is possible to reduce the pH to a
value of
between 7 and 10.5, such as for example about 8.5. It is advantageous to work
in pH
region below 10 or the near neutral region of pH value, in order to protect
the equipment
from alkali degeneration and evidently for safety reasons. In addition, the
lower pH leads
to better bleaching performance and/or reduction in pulp depolymerisation
and/or
degradation. Accordingly, the loading of alkaline earth metal hydroxide and/or
alkaline
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earth metal oxide in the papermaking pulp may be reduced according to the
present
invention.
[31] Finally, according to some embodiments of the present invention, such
as
the kit or the method according to the present invention, the said alkaline
earth metal salt
is obtained by the combination of an alkaline earth metal oxide and/or an
alkaline earth
metal hydroxide with a mineral acid, or with sulphur dioxide or carbon dioxide
in situ
before or during a bleaching process. Accordingly, for example, a composition
comprising magnesium hydroxide and magnesium sulphate may be obtained within a
pulp bleaching preparation by introducing magnesium hydroxide and an acid such
as
sulphuric acid, according to the following reaction:
Mg(OH)2 + H2SO4(aq) ¨). MgSO4 + 2 H20
[32] Acids such as aqueous sulphuric acid are readily available and easy to
store in large quantities over prolonged time at a papermaking site. On the
other hand,
alkaline earth metals salts, such as for example magnesium sulphate, are often
expensive to procure and store and difficult to handle, due to their low
durability or
hygroscopic behaviour. The methods and kits according to the present invention
therefore lead to the formation of the composition according to the present
invention, and
they also form part of the present invention, and present additional
advantages over the
prior art.
[33] The alkaline earth metal hydroxide or alkaline earth metal oxide may
be
selected from magnesium hydroxide, calcium hydroxide, magnesium oxide and
calcium
oxide, or mixtures thereof. If an alkaline earth metal oxide is used, this
acts in the same
way as an alkalinity agent by formation of the corresponding alkaline earth
metal
hydroxide in aqueous medium. These compounds have a low solubility in water
and are
therefore released into the pulp only at the rate they are being consumed
during the
bleaching process. According to the present invention, control of pH is
improved as it
can be regulated by the addition of alkaline earth metal salt, or acid, in
accordance with
the embodiments of the invention.
[34] In certain embodiments, the alkaline earth metal hydroxide may be
magnesium hydroxide. In some embodiments, the magnesium hydroxide may be
crystalline (e.g., brucite). In other embodiments, the magnesium hydroxide may
be an
amorphous form.
[35] In certain embodiments, magnesium hydroxide, calcium hydroxide, and
mixtures thereof may be provided by dolomitic limestone being converted to
dolomitic
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lime, which is then converted to magnesium hydroxide, calcium hydroxide, and
mixtures
thereof. Such embodiments including magnesium hydroxide and calcium hydroxide
may
provide the pulp bleaching benefits described herein as well as a reduction in
deposits
(e.g., calcium oxalate deposits) and/or a higher availability of soluble
hydroxide ions.
The ratio of magnesium hydroxide to calcium hydroxide in such embodiments may
range
from 10:1 to 1:10, such as for example from 5:1 to 1:5, such as for example in
the range
of 1:2 to 2:1.
[36] The alkaline earth metal salt for use in the present invention may for
example be an alkaline earth metal sulphate, such as for example magnesium
sulphate
Magnesium sulphate is a hygroscopic particulate salt. The use of the alkaline
earth metal
salt helps to adjust the pH of the slurry, and may also stabilise the slurry.
Alternatively,
magnesium chloride or magnesium nitrate may be used.
[37] The pitch control agent for use in the composition according to
certain
embodiments of the present invention may be talc, a natural magnesium silicate
with a
platy structure. Alternatively, the pitch control agent for use in the
composition according
to certain embodiments of the present invention may be bentonite, an absorbent
aluminium phyllosilicate including montmorillonite. In
particular embodiments, the
bentonite may be activated by the alkaline earth metal hydroxide or alkaline
earth metal
oxide (e.g., Mg(OH)2 or Mg(OH)). In certain embodiments, the bentonite may act
as an
ion exchanger for heavy metals such as Mn, Cr, Fe, and/or Ni, which provides
the benefit
of rendering these ions less available to disturb the bleaching process. In
other
embodiments, the bentonite may stabilize the peroxide used in bleaching and/or
reduce
the silicate content.
[38] In certain embodiments, the pitch control agent may be a blend of talc
and bentonite. For instance, the ratio of talc to bentonite in such
embodiments may
range from 10:1 to 1:10, such as for example from 5:1 to 1:5, such as for
example in the
range of 2:1 to 1:2. This blend of the two different pitch control agents
shows the benefit
of treating a wider variety of detrimental substances in the process as both
substances
have different adsorption properties. The hydrophobic surface of talc
preferentially
adsorbs hydrophobic materials while bentonite has more attraction for
hydrophilic
substances.
[39] According to certain embodiments of the present invention, the
compositions for use in bleaching of pulps may be present as an aqueous
slurry.
Aqueous slurries are commonly used, for example in papermaking, for
introducing solids
into a pulp. As such, a composition according to certain embodiments of the
present
invention in the form of an aqueous slurry may be introduced into the pulp in
order to
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support hydrogen peroxide bleaching. By
putting certain embodiments of the
composition of the present invention in the form of a slurry, the overall
weight of the
composition is increased. In order to limit the weight increase, for example
to maintain
efficiency in transportation of the compositions, it is preferred that the
aqueous slurries
comprising the composition according to certain embodiments of the present
invention
have a solids content of 10 wt.-% or higher, on the basis of the total weight
of the slurry,
such as 30 wt.-% or higher, or 30 wt.-% or higher, or 40 wt.-% or higher, or
45 wt.-% or
higher, or 50 wt.-% or higher, or even 60 wt.-% or higher, such as up to 70
wt.-%, or
72 wt.-%, or 75 wt.-%. In order to achieve high solids content slurries, it
may be
necessary to include dispersants in the slurry. The slurries may also comprise
further
components such as fillers, surfactants, bleaching agents, pH-buffering
agents, or other
additives.
[40] In certain embodiments, the compositions have the form of dry
particulate
powders. In order to reduce weight of the composition and to improve
transportability,
the compositions may be transported as dry powders and slurries be formed only
at the
site of use of the compositions in a bleaching process. The said dry
particulate powders
may be obtained, for example, by use of a spray-drying process. Spray-dried
particulate
compositions according to certain aspects of the present invention may have a
low or a
very low moisture content, such as for example 5 wt.-% moisture or less, based
on the
total amount of solids in the spray-dried particulate powder, or 4 wt.-%
moisture or less,
or 3 wt.-% moisture or less, or even 2 wt.-% moisture or less, such as for
example about
1 wt.-% moisture or 0.5 wt.-% moisture. The dry compositions may also comprise
further
components such as dispersants, fillers, surfactants, bleaching agents,
chelating agents,
pH-buffering agents, or other additives.
[41] In certain embodiments, the compositions are in the form of pellets.
According to one aspect, the pellets may have a moisture content from 0.1 wt.-
% to
25 wt.-%, based on the total weight of the composition, such as for example
from 2 wt.-%
to 20 wt.-% or from 5 wt.-% to 15 wt.-%, such as for example about 15 wt.-%.
According
to a further aspect, the pellets may comprise from 2 wt.-% to 50 wt.-% pitch
control
additive and from 98 wt.-% to 50 wt.-% particulate alkaline earth metal oxide
or alkaline
earth metal hydroxide or mixtures thereof, based on the total solids content
of the pellets.
[42] According to certain embodiments of the present invention, the
compositions for use in bleaching of pulps may have a weight ratio of the
alkaline earth
metal oxides and/or alkaline earth metal hydroxides on the one hand, and the
alkaline
earth metal salt on the other hand in the range of 1:1 to 100:1, such as for
example from
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2:1 to 50:1, such as for example in the range of 3:1 to 25:1, or from 5:1 to
10:1, such as
for example about 70:30 or about 80:20, or about 90:10.
[43] According to certain embodiments of the present invention, the kit of
ingredients may include such amounts of ingredients that a weight ratio as
described
above may be obtained. It is understood by the skilled person in the art that
such weight
ratio as a result of the reaction of, for example, magnesium hydroxide with
sulphuric acid
to obtain magnesium sulphate, as described above, may depend on many factors,
such
as concentration, temperature, relative amounts of ingredients,
acidity/alkalinity prior to
the reaction, etc. According to certain embodiments of the present invention,
the method
for obtaining a composition according to the invention may be carried out in
such a way
that a weight ratio as described above may be obtained.
[44] If a pitch control agent is used in the present invention, the weight
ratio of
the alkaline earth metal oxides, alkaline earth metal hydroxides and the
alkaline earth
metal salt on the one hand, and the pitch control agent on the other hand may
be ranging
from 1:9 to 9:1, such as for example between 3:7 and 7:3, such as for example
ranging
from 2:3 to 3:2, for example at a weight ratio of 1:2, or 1:1, or 2:1.
[45] According to certain embodiments of the present invention, the
compositions for use in bleaching of pulps may comprise the one or more
particulate
alkaline earth metal oxides or alkaline earth metal hydroxides in an amount of
10 wt.-%
to 90 wt.-% of the total solids content of the composition, such as for
example in an
amount of 30 wt.-% to 70 wt.-% of the total solids content of the composition,
such as for
example in an amount of 40 wt.-% to 60 wt.-% of the total solids content of
the
composition, such as for example in an amount of 50 wt.-% of the total solids
content of
the composition.
[46] According to certain embodiments of the present invention, the
compositions for use in bleaching of pulps may comprise the alkaline metal
salt in an
amount of 0.1 wt.-% to 50 wt.-% of the total solids content of the
composition, such as
for example in an amount of 1 wt.-% to 30 wt.-% of the total solids content of
the
composition, such as for example in an amount of 5 wt.-% to 15 wt.-% of the
total solids
content of the composition, such as for example in an amount of 10 wt.-% of
the total
solids content of the composition.
[47] According to certain embodiments of the present invention, the
compositions for use in bleaching of pulps may comprise one or more pitch
control
additives in an amount of 1 wt.-% to 90 wt.-% of the total solids content of
the
composition, such as for example in an amount of 10 wt.-% to 70 wt.-% of the
total solids
content of the composition, such as for example in an amount of 20 wt.-% to 60
wt.-% of
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the total solids content of the composition, such as for example in an amount
of 50 wt.-%
of the total solids content of the composition.
[48] Another aspect of the present invention is the use of the compositions
or
kits of ingredients according to certain embodiments of the present invention
in the
bleaching process of pulps, such as in the bleaching process of pulps for
papermaking.
For example, the compositions according to certain embodiments of the present
invention may be used in combination with other additives such as hydrogen
peroxide,
complexing agents, water glass, or others, in a bleaching process of pulp in a
paper
making process.
[49] The compositions according to the present invention may fully or
partially
replace known alkaline compositions, such as NaOH. For example, NaOH may be
fully
omitted as the alkaline agent in a H202-bleaching process, or the amount of
NaOH may
be reduced by simultaneous addition of a composition according to the present
invention.
[50] According to certain embodiments of the present invention, it is not
required that the one or more alkaline earth metal oxides or alkaline earth
metal
hydroxides or mixtures thereof on the one hand, and the one or more alkaline
earth metal
salts on the other hand, be mixed together prior to being introduced into a
pulp to be
bleached. An additional aspect of the present invention is the use of a
composition
obtained by separately introducing one or more (e.g. all) of the components of
a
composition into a papermaking pulp, or by separately introducing one or more
(e.g. all)
of the components of a kit of ingredients into a papermaking pulp, thereby
forming the
composition within a pulp during the bleaching of said pulp.
[51] In the bleaching process, the composition or the kit of ingredients
according to certain embodiments of the present invention may be introduced
into the
pulp in such an amount that the amount of the one or more particulate alkaline
earth
metal oxides or alkaline earth metal hydroxides or mixtures thereof in the
pulp is from
0.1 wt.-% to 8 wt.-% of the total solids content of the pulp during the
bleaching, such as
from 0.2 wt.-% to 4 wt.-% of the total solids content of the pulp during the
bleaching, such
as from 0.5 wt.-% to 2.5 wt.-% of the total solids content of the pulp, such
as for example
1 wt.-% or 2 wt.-% of the total solids content of the pulp, and that the
amount of the one
or more alkaline earth metal salts in the pulp is from 0.01 wt.-% to 5 wt.-%
of the total
solids content of the pulp during the bleaching, such as from 0.02 wt.-% to 1
wt.-% of the
total solids content of the pulp during the bleaching, such as from 0.05 wt.-%
to 0.5 wt.-%
of the total solids content of the pulp, such as for example about 0.1 wt.-%
or about
0.25 wt.-% of the total solids content of the pulp.
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[52] According to certain embodiments of the present invention, the
composition or the ingredients of a kit of ingredients for use in bleaching of
a
papermaking pulp may be included in said pulp for example during the bleaching
stage,
or at the beginning of the bleaching stage, or prior to the bleaching stage.
It also may
be included during or prior to the delignification stage. For example, the
composition for
use in bleaching of a papermaking pulp may be included in said pulp as early
as the
grinding stage during which the pulp components are ground, or at any time
between the
grinding stage and the bleaching stage. The various components of the
composition
according to certain embodiments of the present invention may also be added to
the
papermaking pulp at separate stages, such as for example during the grinding
stage and
or just prior initiation of the bleaching step or the delignification step.
[53] For example, according to one embodiment of the present invention, the
composition or the ingredients of a kit of ingredients for use in bleaching of
a
papermaking pulp according to some embodiments of the present invention may be
introduced into a pulp to be bleached prior to the introduction of any
alkaline bleaching
agents, such as NaOH, waterglass, sodium silicate, and/or hydrogen peroxide.
In this
case, the alkaline earth metal oxide or the alkaline earth metal hydroxide has
been
activated by the presence of alkaline earth metal salt (such as earth metal
sulphate),
and/or organic acids of wood origin present in the pulp mixture, and/or the
mineral acid
prior to the beginning of the bleaching process. For example, according to one
embodiment of the invention, the alkaline earth metal hydroxide and/or
alkaline earth
metal oxide is added to an acidic pulp mixture (e.g. wherein the only acids
present are
those naturally found in the pulp and/or no external acid is added) prior to
the beginning
of the bleaching process and/or prior to the addition of any alkaline
bleaching agents.
[54] As discussed above, it is thought that this "protection" of the
viscosity is
caused by the presence of free alkaline earth metal ions, such as magnesium
ions, in
pulp mixture or slurry. This effect is enhanced when the alkaline earth metal
ions are
formed prior to the start of bleaching by introduction of any alkaline
bleaching agents.
[55] According to a further embodiment of the present invention, the
composition of ingredients or the ingredients of a kit of ingredients may be
added at an
earlier step, such as during an acidic bleaching step, during the addition of
chelating
agents such as EDTA or chlorine dioxide. This embodiment includes all uses of
the
composition of ingredients or the ingredients of a kit of ingredients in a
bleaching step
which occurs at a pH of 7 or below.
[56] In order to avoid confusion, the embodiments in accordance with the
following numbered sentence all form part of the present invention:
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1. A composition for use in a pulp bleaching process, the composition
comprising one
or more alkaline earth metal oxides and/or one or more alkaline earth metal
hydroxides;
one or more alkaline earth metal salts; and optionally, one or more pitch
control additives.
2. A composition according to numbered sentence 1, wherein said pitch
control
additive is selected from talc, bentonite, zeolite, diatomaceous earth,
cationic mica,
hydrophobic carbonates, resin-decomposing enzymes, cationic polymers designed
to
capture and immobilise resins or resinous components on fibre surfaces,
aluminium
sulphate, polyaluminium chloride, and dispersing agents specifically designed
for the
dispersion of resins and resinous components, and mixtures thereof.
3. A composition according any of the previous numbered sentences, wherein
the
alkaline earth metal is selected from magnesium, calcium, and mixtures
thereof.
4. A composition according to any of the previous numbered senctences,
wherein
said pitch control additive is selected from micronized talc, bimodal talc,
and cationic talc.
5. A composition according to any one of numbered sentences 1 to 3, wherein
said
pitch control additive is a bentonite, such as for example activated
bentonite.
6. A composition according to any of the previous numbered sentences which
is an
aqueous slurry.
7. A composition according to numbered sentence 6, wherein said aqueous
slurry
has a total solids content of from 10 to 75 wt.-%, on the basis of the total
weight of the
slurry.
8. A composition according to any of the previous numbered sentences,
wherein the
weight ratio of (i) an alkaline earth metal oxide and/or an alkaline earth
metal hydroxide
and (ii) an alkaline earth metal salt is from 1:2 to 100:1.
9. A composition according to any of the previous numbered sentences,
further
comprising at least one of the following additives: (a) dispersants; (b)
fillers; (c)
surfactants; (d) bleaching agents; (e) chelating agents; and (f) pH-buffering
agents.
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10. A kit of ingredients comprising, an alkaline earth metal oxide and/or
an alkaline
earth metal hydroxide stored in a first container, and either one or both of a
second and
third container with an aqueous mineral acid solution stored in the said
second container
and sulphur dioxide and/or carbon dioxide stored in the said third container.
11. A method for forming a composition according to any one numbered
sentences 1
to 9, comprising a step of mixing an alkaline earth metal oxide and/or an
alkaline earth
metal hydroxide with a mineral acid, or sulphur dioxide, or carbon dioxide,
optionally in
the presence of one or more pitch control agents.
12. A method according to numbered sentence 11, when performed within an
aqueous
pulp bleaching composition.
13. A method according to numbered sentence 12, wherein the step of mixing is
performed by addition of sulphuric acid or sulphur dioxide or carbon dioxide
to a paper
bleaching pulp comprising the alkaline earth metal oxide and/or the alkaline
earth metal
hydroxide.
14. A method according to any one of numbered sentences 11 to 13,
comprising the
step of admixing the ingredients of the kit of numbered sentence 10 into a
paper
bleaching pulp or a delignification pulp.
15. A use of a composition or a method or a kit according to any one of the
previous
numbered sentences, in a pulp bleaching process or a pulp delignification
process.
16. A use according to numbered sentence 15 in a process to reduce pitch in
the pulp.
17. A use according to numbered sentence 15 in a process to protect the
pulp from
depolymerisation and/or degradation.
18. A use according to numbered sentence 15 in or after an acidic bleaching
step.
19. A use according to any one of numbered sentences 15 to 18, wherein the
said
composition or the product of said method or the components of the said kit
are added
to the pulp to be bleached, prior to an addition of any other alkaline
bleaching agents,
such as water glass, sodium silicate, NaOH, and/or hydrogen peroxide.
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20. A method of reducing pitch in a pulp comprising the use according to
any one of
numbered sentences 15 to 19.
EXAMPLES
[57] As used herein, pulp viscosity was measured in accordance with TAP P1 T
230 om-08. Furthermore, as used herein, the kappa value indicating the
residual amount
of lignin in the pulp was measured in accordance with ISO-302:2012.
[58] In a bleaching process of a paper pulp using sodium hydroxide as the
bleaching agent, the use of magnesium hydroxide as an additive was compared to
the
use of magnesium hydroxide in combination with mineral acid, as well as in
combination
with mineral acid and EDTA as a chelating agent.
[59] An unbleached kraft softwood pulp was used as starting material. The
pulp was taken from industrial production at a softwood kraft pulp mill. It
was washed in
the mill and then subjected to a bleaching process without further washing in
the
laboratory. Commercially available magnesium hydroxide, diluted before use to
a
concentration of 1 mol/L was used in these trials.
[60] In Comparative Examples 1 and 2, magnesium sulfate and magnesium
hydroxide solution respectively were added to the paper making pulp. In
Examples 1 and
2, magnesium hydroxide solution and laboratory grade sulfuric acid was added.
In
Example 3, laboratory grade EDTA (10 g/L) was additionally charged. Sulfuric
acid and
EDTA were first mixed with the commercial magnesium hydroxide. The additive
mixtures
are shown in Table I. Then this mixture was mixed to the pulp. In all the
Examples, the
charge of pulp was 120 g (oven dry). The bleaching was carried out in 2L a
Teflon-lined
autoclave, put in a preheated silicone oil bath. The total reaction time was
about 2 and
a half hours in each case.
Table I
Comp. Ex. 1 Comp. Ex. 2 Ex. 1 Ex. 2 Ex. 3
MgSat (wt.-%) 0.20 0 - - -
Mg(OH)2 (wt.-%) 0 0.15 - - -
1:1 Mg(OH)2 + H2504 (wt.-%) 0- 0.15
-
2:1 Mg(OH)2 + H2504 (wt.-%) 0- 0 0.15 0.15
EDTA (wt.-%) 0 0 0 0 0.02
The wt.-% indications are with respect to the total dry weight of the pulp
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[61] Pulp testing was performed to the usual standards according to German
Thunen Institute (Hamburg, Germany). The viscosity was determined according to
ISO
5351 using a copper ethylenediamine (cuen) solution. The results are shown in
Table
II.
Table II
Comp. Ex. 1 Comp. Ex. 2 Ex. 1 Ex. 2 Ex. 3
Brightness (ISO) 38.8 41.8 38.9 36.9 37.8
Pulp viscosity (mL/g) 940 860 920 930 920
Kappa 11.4 9.9 11.8 12.0 11.8
yield 87.43 87.44
87.36 87.34 87.26
[62] It is clear from the obtained data that (i) the viscosity values for
the
invention Examples are equivalent to those of Comparative Example 1, while the
use of
magnesium sulfate has been avoided. At the same time, the values for viscosity
and
kappa value are improved over those of Comparative Example 2. Finally, the
values for
brightness, kappa value and overall yield were not adversely affected over
Comparative
Example 1.
[63] Example 3, making use of EDTA in addition to the combination
Mg(OH)2 / H2SO4, does not substantially modify the expected technical
properties of the
pulps.
[64] In a separate bleaching process of a paper pulp using sodium hydroxide
and hydrogen peroxide as bleaching agents, the effect of using increasing
amounts of
magnesium hydroxide to replace sodium hydroxide on residual peroxide and
brightness
was determined.
[65] An unbleached kraft softwood pulp was used as starting material. The
pulp was taken from industrial production at a softwood kraft pulp mill. It
was washed in
the mill and then subjected to a bleaching process without further washing in
the
laboratory. The water pressed out from the pulp sample used in the laboratory
bleaching
trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially
available
magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used
in these
trials.
[66] Pulp mixtures shown in Table III were prepared by first mixing the
additive
components together and then adding this mixture to an aqueous pulp. The
bleaching
was carried out in plastic bags, put in a preheated water bath of 70 C with
the bag
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opening outside the water to avoid additional water getting into the bag
during the
bleaching. The total reaction time was about 2 hours in each case.
Table III
All values Comp.
Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8
in grams Ex. 3
Pulp (wet,
16.3% dry
100 100 100 100 100 100
pulp
content)
Water 69.2 69.2 69.2 69.2 69.2 69.1
NaOH
2.2 2.1 2.0 1.8 1.1 0
(7.4%)
Mg(OH)2
0 0.1 0.2 0.5 1.1 2.2
(5.3%)
Mg(SO4)
0.07 0 0 0 0 0
(12%)
H202
0.45 0.45 0.44 0.45 0.45 0.45
(30%)
[67] After bleaching for about 2 hours, the residual peroxide was
determined
by pressing out 1 ml of water from the pulp and using iodometric titration
well known by
experts in this field and described in literature. ISO brightness of the pulp
mixture was
also determined from a dried filter cake of the bleached pulp using a Konica
Minolta
CM3700d spectrocolorimeter following ISO 2469 (R457). The results are shown in
Table
IV. The chemical oxygen demand (COD) was determined from the water pressed out
of
the pulp after bleaching using the Hach Lange cuvette test LCK614.
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Table IV
Comp.
Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8
Ex. 3
Residual
peroxide
(% of
0 0 0 0 32 45
original
amount
added)
ISO
Brightness 83.3 84.1 82.7 83.2 81.3 80.1
(0/0)
COD
21.4 16.3 18.2 17.3 13.2 13.9
(kg/t)
[68] It was found that magnesium hydroxide can be used to replace sodium
hydroxide in the bleaching process when using an acidic pulp mixture. Even at
relatively
low substitution rates of NaOH by Mg(OH)2, a significant drop in COD values
can be
observed. This is advantageous for pulp yield since each kg produced COD is
lost pulp
yield. It also has a positive effect on the COD in the waste water, which thus
provides an
improved environmental impact.
[69] In another bleaching process of a paper pulp using sodium hydroxide
and
hydrogen peroxide as bleaching agents, the effect of adding magnesium
hydroxide to
the pulp before sodium hydroxide and any other alkaline additives was
determined.
[70] An unbleached kraft softwood pulp was used as starting material. The
pulp was taken from industrial production at a softwood kraft pulp mill. It
was washed in
the mill and then subjected to a bleaching process without further washing in
the
laboratory. The water pressed out from the pulp sample used in the laboratory
bleaching
trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially
available
magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used
in these
trials.
[71] Pulp mixtures shown in Table V were prepared by either mixing the
additive components and then adding this mixture to aqueous pulp, or, where
the
magnesium hydroxide was added to the pulp first, adding the magnesium
hydroxide to
the aqueous pulp and mixing the pulp mixture by hand for around 1 minute, then
mixing
the remaining additive components and adding this mixture to the magnesium
hydroxide
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and pulp mixture. The bleaching was carried out in plastic bags, put in a
preheated water
bath of 70 C with the bag opening outside the water to avoid additional water
getting into
the bag during bleaching. The total reaction time was about 80 minutes in each
case.
Table V
Ex. 11 Ex. 13
All values Comp. (magnesium
(magnesium
Ex. 9 Ex. 10 Ex. 12
in grams Ex. 4 hydroxide hydroxide
added first) added
first)
Pulp (wet,
16.3%
100 100 100 100 100 100
dry pulp
content)
Water 69.2 69.2 69.2 69.2 69.2 69.2
NaOH
2.2 2.0 1.9 1.9 1.9 1.9
(7.4%)
Mg(OH)2
0 0.2 0.3 0.3 0.3 0.3
(5.3%)
Mg(SO4)
0.07 0 0 0 0 0
(12%)
Benton ite
0 0 0 0 0.04 0.04
(2%)
H202
0.44 0.44 0.44 0.44 0.44 0.44
(30%)
[72] After bleaching for about 80 minutes, the residual peroxide and
ISO
brightness of the pulp mixtures were measured as described above. The results
are
shown in Table VI below.
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Table VI
Ex. 11 Ex. 13
Comp. (magnesium
(magnesium
Ex. 9 Ex. 10 Ex. 12
Ex. 4 hydroxide hydroxide
added first) added
first)
Residual
peroxide
(% of
5 5 4 29 4 35
original
amount
added)
ISO
Brightness 83.0 82.5 81.7 82.8 80.8 82.8
(0/0)
[73] It was found that the residual peroxide and ISO brightness of the pulp
mixtures to which magnesium hydroxide was added first is better than that of
the pulp
mixture to which magnesium hydroxide was added at the same time as the other
additive
components.
[74] In another bleaching process of a paper pulp using sodium hydroxide
and
hydrogen peroxide as bleaching agents, the effect of using a reduced amount of
hydrogen peroxide together with magnesium hydroxide was determined.
[75] An unbleached kraft softwood pulp was used as starting material. The
pulp was taken from industrial production at a softwood kraft pulp mill. It
was washed in
the mill and then subjected to a bleaching process without further washing in
the
laboratory. The water pressed out from the pulp sample used in the laboratory
bleaching
trials had a pH value of 2.8 so the pulp was clearly acidic. Commercially
available
magnesium hydroxide, diluted before use to a concentration of 1 mol/L was used
in these
trials.
[76] Pulp mixtures shown in Table VII were prepared by either mixing the
additive components and then adding this mixture to aqueous pulp, or, where
the
magnesium hydroxide was added to the pulp first, adding the magnesium
hydroxide to
the aqueous pulp and mixing the pulp mixture by hand for around 1 minute, then
mixing
the remaining additive components and adding this mixture to the magnesium
hydroxide
and pulp mixture. The bleaching was carried out in plastic bags, put in a
preheated water
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bath at 70 C with the bag opening outside the water to avoid additional water
getting into
the bag during bleaching. The total reaction time was about 80 minutes in each
case.
Table VII
All values in Comp. Ex. 5 Ex. 14 Ex. 15 Ex. 16
grams (magnesium (magnesium
hydroxide hydroxide
added first) added
first)
Pulp (wet, 225 225 225 225
16.3% dry
pulp content)
Water 155.6 155.7 155.8 155.9
NaOH (7.4%) 5.0 4.2 4.2 4.2
Mg(OH)2 0 0.8 0.8 0.8
(5.3%)
Mg(SO4) 0.2 0 0 0
(12%)
H202 (30%) 0.98 0.98 0.86 0.73
[77] After bleaching for about 80 minutes, the residual peroxide and
ISO
brightness of the pulp mixtures were measured as described above. The results
are
shown in Table VIII below.
Table VIII
Ex. 15 Ex. 16
(magnesium (magnesium
Comp. Ex. 5 Ex. 14
hydroxide hydroxide
added first) added first)
Residual
peroxide (% of
3 29 29 23
original amount
added)
ISO Brightness
81.3 82.6 82.6 82.7
(%)
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[78] It was found that similar residual peroxide and ISO brightness results
were obtained even when using reduced amounts of hydrogen peroxide when the
magnesium hydroxide was added first.
[79] It should be noted that the present invention may comprise any
combination of the features and/or limitations referred to herein, except for
combinations
of such features which are mutually exclusive. The foregoing description is
directed to
particular embodiments of the present invention for the purpose of
illustrating it. It will be
apparent, however, to one skilled in the art, that many modifications and
variations to the
embodiments described herein are possible. All such modifications and
variations are
intended to be within the scope of the present invention, as defined in the
appended
claims.
