Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PULP BLEACHING PROCESS INCLUDING
FINAL STAGE BRIGHTENING STEP WITH
SALT OF PEROXYMONOSULFURIC ACID
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of
bleaching and brightening of pulp obtained from
lignocellulosic fiber, and in particular to a final
treatment stage in which the pulp is treated with one or
more salts of peroxymonosulfur:ic acid.
2. Description of the Related Art
Cellulosic fiber used in lhe production of pulp and
paper products is derived from wood materials and non-
wood materials, such as kenaf, bagasse, and reed. These
wood and non-wood materials are typically subjected to
mechanical treatment, chemical treatment or a combination
of mechanical and chemical treatments to provide
cellulosic fiber. The cellulosic fiber thus obtained is
typically dark in color and contains mostly cellulose and
lignin (and thus the term lignocellulosic fiber) and some
extraneous materials. The lignocellulosic fiber obtained
mainly from mechanical treatment is treated with hydrogen
peroxide or hydrosulfite in one or two stages to produce
semi-bleached pulp (80% ISO brightness) suitable for
various grades of paper and paper products. Brightness
improvement of these lignocelllllosic fibers is achieved
primarily by modifying the light absorbing chromophoric
groups of lignin and cellulose without much degradation
and dissolution. Thus the bleached pulp retains most of
the lignin and is known as lignin retaining bleaching.
Cellulosic fibers having very high brightness of 88-
90% ISO and good brightness stability are obtained frombleaching pulp obtained from chemical pulping. Chemical
pulping conducted under severe conditions reduces the
lignin content to 2-3% on wood (based on total lignin
content in the feedstock). Bleaching of chemical pulp is
conducted in multiple stages u'sing one or more bleaching
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chemicals, including for example, chlorine (C), chlorine
dioxide (D), sodium or calcium hypochlorite (H), oxygen
(O), ozone (Z), hydrogen peroxide (P), and one or more
chemicals for caustic extraction (E), such as sodium
hydroxide. Typical multi-stage bleaching sequences
include CEHD, CEHH, (C/D)EDED, (C/D)EopDED, DEopDED,
DEopD, DEopDP, and other sequences that are known in the
art. In recent times it has also been known to include
an oxygen delignification stage between the pulping
process and the multi-stage bleaching treatment to reduce
the lignin content by another 90-50% in the pulp. In all
these multi-stage bleaching sequences the first two
stages, for example, CE or DEop, or (C/D)Eop and the like
are generally considered as pre-delignification or pre-
bleaching stages. In these two-stages degradation and
dissolution of lignin occurs. The subsequent stages of
bleaching are referred to as brightening stages where
brightness improvement is the primary function of the
chemicals.
After completion of the bleaching in the multi-stage
sequences, the bleached pulp i'3 stored in a suitable
storage vessel or chest typica:Lly found in pulping
plants, such as a high density storage tower or chest.
These storage chests can store pulp for varying periods
of time, typically from 5 minutes to several hours (24-72
hours). The pulp typically moves through the vessel or
chest in a continuous or discontinuous flow before being
discharged for final processing on a papermachine into
articles such as printing and writing paper or other
paper products.
Oftentimes, the desired final brightness of the pulp
is not achieved from the multi-stage bleaching treatment
due to one or more factors suc].L as, process variations,
increased production rate, customer demand for higher
brightness, and/or brightness stability. A simple
solution for increasing the final brightness of
completely processed pulp by up to 4% ISO involves a
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post-treatment, in which the fully bleached or bleached
pulp having low brightness (semi-bleached pulp) is
further treated with small amounts of hydrogen peroxide
and caustic at an initial pH of 11.0 - 11.3 in the high
density storage chest. This hydrogen peroxide treatment,
which is typically conducted at about 45~C to 65~C
requires long retention times of about 6-8 hours to
utilize fully the peroxide and enhance the brightness by,
for example, 2% ISO to in some cases up to 4~ ISO.
In the high density storage chest since a constant
level of pulp is not possible to maintain and no washing
of pulp is done following the treatment, practically,
several problems arise from the unused hydrogen peroxide
(residual) and caustic left in the pulp when it leaves
the high density storage chest. Following are the
drawbacks of this treatment.
~ Variable reaction time that could vary from 5 min to
as long as 72 hours, resulting in an uncontrolled pH
profile, and either incomplete chemical consumption
or early consumption of peroxide.
~ Unconsumed peroxide or caustic or both may remain as
residual chemical with the pulp. Depending upon the
available reaction time, the peroxide can be used up
and the pulp is left with caustic only. This
results in what is known as "Caustic yellowing" or
"Alkali darkeningn. This will revert the brightness
gain achieved from peroxide treatment. On the other
hand, if the caustic is used up and unconsumed
peroxide is left, this wi:Ll interfere with the
chemicals added in the fo:Llowing wet end operation
of papermaking. Under such circumstances the
residual peroxide has to be destroyed by reaction
with reducing agents like sodium thiosulfate, sodium
bisulfite or sulfur dioxide in water. When peroxide
and caustic remain as unconsumed then additionally
the caustic should be neutralized to reach a pH of
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around 6-8. This is the required pulp pH for
papermaking.
In order to circumvent the aforementioned problems
associated with the peroxide treatment of pulp in the
high density storage chest, application of Caro's acid or
peroxymonosulfuric acid was discovered as described
herein.
Numerous publications and patents discuss the use of
peroxymonosulfuric acid (or Caro's acid) for
delignification of unbleached chemical pulp in the pre-
bleaching stage. These discloc;ures generally involve
pre-bleaching with peroxymonosulfuric acid in acidic
solutions followed by an alkaline extraction with sodium
hydroxide solution. For examp]e, U.S. Patent No.
5,004,523 to Springer et al. discloses a process in which
lignocellulosic material is de]ignified with
peroxymonosulfuric acid in a strongly acidic solution
having a pH less than 2. U.S. Patents Nos. 5,091,054 and
5,246,543, both to Meier et al , relate to
delignification processes in which lignocellulosic
materials are treated with peroxymonosulfuric acid to
arrive at a final pH of 3-5. 'rhe Meier patents disclose
the beneficial effect of Caro's acid treatment that is
realized by employing a subsequent oxidative extraction
stage.
Ozone is also known in the field of pulp bleaching
and is primarily used in the prebleaching stage as a
delignifying agent. Treating chemical pulp with a
mixture of Caro's acid and ozone as an effective means
for removing residual lignin in pulp is discussed in U.S.
Patent No. 5,411,635 to Francis et al. However, the
Francis patent discloses that the delignification stage
is preferably conducted at the low pH range of 3-4, and
precedes an alkaline extraction or oxidative extraction
stage conducted in the presence of oxygen and/or hydrogen
peroxide. Ozone is a powerful oxidant and readily
attacks lignin and cellulose under acidic conditions.
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However, ozone is such a non-selective bleaching chemical
that it attacks cellulose much more than the lignin, thus
producing pulp having low strength propertles.
Liebergott (Pulp & Paper Canada, Vol. 97, No. 3, p.
T105) discusses the effect of peroxy chemicals such as
peracetic acid, mixed peracids and dimethyl dioxirane on
brightness improvement for semi-bleached (81.5 %ISO
brightness from DEopD sequence) and fully bleached (89.2
%ISO brightness from DEopDnD) pulps. Also, the
Liebergott publication mentions that due to anticipated
decomposition of Caro's acid occurring in a solution
having an alkaline pH, treatme:nt with Caro's acid was not
investigated for semi-bleached or fully bleached pulps at
a pH above 6Ø
The present invention is characterized by the
unexpected discovery that the ]brightness of a prebleached
lignocellulosic pulp can be improved by up to about 4
%ISO by performing a final brightening treatment in which
an oxidizer consisting essentially of at least one
peroxymonosulfuric acid salt i.3 introduced to the pulp
while avoiding a final pH of the pulp in an undesirable
range.
Accordingly, we have found a process for treating a
prebleached pulp in a final br:ightening stage with the
brightening agent, peroxymonosulfuric acid or its salts,
to produce a more fully bleached pulp that is more
compatible for subsequently producing paper products.
For instance, the final pH of the final brightening stage
is more compatible with the papermaking process - little
or no pH adjustment is necessary prior to papermaking.
Also, there is little or no risk of alkali darkening from
early consumption of brightening agent. Furthermore, the
process is more amenable to highly variable retention
times.
The present invention thu~s provides a significant
advancement over the above-ment;ioned final stage hydrogen
peroxide/caustic treatment in which the final pH of the
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pulp is in the range of 10 or more and has potential
deleterious effects.
S ~ ~ RY OF THE INVENTION
In accordance with the pr1nciples of the present
invention, a process is provided for performing a final
stage of brightening, in for example a high density
storage tower or chest, said b:rightening comprising
treating a prebleached lignoce:llulosic pulp having a
brightness with an oxidizing agent consisting essentially
of at least one peroxymonosulfuric acid salt to increase
the brightness of the prebleached lignocellulosic pulp
and provide a more fully bleached pulp, wherein the more
fully bleached pulp is not subjected to further
brightening or delignification operations subsequent to
said final brightening stage, and wherein said treatment
is conducted at an initial pH at commencement of said
final brightening stage in a range of from about 7 to
about 12 and a final pH at the end of said final
brightening stage in a range oi- from about 3 to about 9.
Preferably, the initial pH is in a range of from about 8
to about 11, and the final pH is in a range of from about
6 to about 8.
The prebleached pulp exitLng the preceding stage
from a conventional bleaching process may be in the range
of 4-8, in which case the initial pH at the commencement
of the final brightening stage according to the present
invention may be attained by the addition of alkali, for
example a caustic solution.
The reaction of peroxymonosulfuric acid or its salts
with prebleached lignocellulosi.c pulp in a high density
storage chest to produce a more fully bleached pulp
proceeds to completion in say ].-2 hours, and the final pH
of reaction falls in the range of preferably 6-8. Thus
the more fully bleached pulp produced according to the
process of the present invention need not be subjected to
any chemical treatment or washing before being discharged
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to the wet end of a paper machine. However, optionally
the pulp can be washed prior to such discharge.
The final brightening stage according to the present
invention can be conducted with about 0.01% on pulp to
about 3.0% on pulp as active oxygen (based on oven dried
weight of the pulp) of an oxidizing agent consisting
essentially of at least one peroxymonosulfuric acid or
its salts. Preferably, the charge of oxidizing agent is
in the range of from about 0.05% on pulp to about 1.5% on
pulp as active oxygen.
As referred to herein, an oxidizing agent consisting
essentially of at least one peroxymonosulfuric acid salt
preferably means an oxidizing agent consisting
exclusively of peroxymonosulfuric acid salt, although the
term permits for the presence of insubstantial amounts of
other oxidizing agents, including, for example, those
from previous bleaching operat:ions, so long as the
residual oxidizing agents do not materially affect the
bleaching operation of the peroxymonosulfuric acid salt
or the pH of the pulp during the final bleaching stage.
The present invention can be practiced in connection
with the preparation of more fully bleached pulp from
virgin pulp, from recycled furnish or from a combination
of virgin pulp and recycled furnish.
The principles of the precsent invention outlined
above are applicable to all types of bleached pulps, but
have particular applicability t:o the production of high
brightness chemical pulp for the production of various
writing, printing, coated and uncoated free sheets.
These and other objects, features and advantages of
the present invention will become apparent from the
following detailed description which explains, by way of
examples, the principles of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Lignocellulosic materials such as untreated wood
chips and annual plants such ac~ wheat straw, kenaf, reed
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and the like can be used in accordance with the present
invention, although it is understood that the scope of
the invention is not limited to these exemplary
materials. Especially suitable are materials that have
been produced in a mechanical pulping process, a chemical
pulping process, or a combination of both mechanical and
chemical pulping processes, such as groundwood (GW),
thermomechanical, chemithemomechanical, NSSC, sulfite,
soda, kraft, organosol, and the like, and any
combinations thereof. It is this kind of material in an
aqueous suspension, hereafter referred to as pulp, which
is treated in accordance with the present invention with
at least one salt of peroxymonosulfuric acid in the final
stage of a bleaching process. It should be noted that
where no prebleaching occurs p:rior to a final
peroxymonosulfuric acid treatment according to the
process of the present invention, that is the
peroxymonosulfuric acid treatment is the only bleaching
step, then such treatment is considered to be within the
scope of the present invention. This may be applicable to
the mechanical type pulps or sulfite pulp. Kraft pulp
and/or recycled pulp are prefe]red materials for use in
the present invention for the production of the bleached
articles.
The peroxymonosulfuric acid salt used in the
treating step can be obtained by dissolving commercial
grades of its salts such as Caroat~ supplied by Degussa
AG or Oxone~ supplied by Dupont:~ in water and/or an
alkaline material, and thereaft:er treating the pulp with
the mixture. Alternatively or in addition thereto,
peroxymonosulfuric acid, also hnown as Caro's acid can be
produced on-site by mixing concentrated sulfuric acid and
hydrogen peroxide prior to mixing with caustic solution
and/or pulp. Also, in practicing the treating step the
alkaline material can be added to the pulp prior to or at
the same time that the peroxymonosulfuric acid is added
to the pulp. Suitable alkaline materials include, sodium
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hydroxide, potassium hydroxide, calcium hydroxide, sodium
carbonate, magnesium oxide, and the like.
The peroxymonosulfuric acid salt final treatment
stage is carried out, preferably, with about 0.01% on
pulp to about 3.0% on pulp (based on oven dry weight of
the pulp) as active oxygen of Caro's acid or one or more
of its salts, although it is understood that the present
invention is not so limited. Less than about 0.01% on
pulp may be insufficient to effect desirable brightness
improvements, whereas more than about 3.0% on pulp is
unnecessary to obtain satisfactory results and becomes
quite uneconomical. More preferably, the chemical charge
is in a range of from about 0.05% on pulp to about 1.5%
on pulp as active oxygen of Ca:ro's acid or one or more of
its salts.
Additives which serve to :Eacilitate the
peroxymonosulfuric acid salt f:inal treatment stage may
optionally be used. Among the additives that can be used
are hydrogen peroxide stabiliz:ing agents, such as sodium
silicate; chelating agents such as Na5DTPA, Na4EDTA,
DTMP, and the like; or, cellulose protecting agents, such
as magnesium sulfate and the l-ike; and any combination
thereof. These additives can be applied to the pulp prior
treating the pulp with the mixture of peroxymonosulfuric
acid or salt and alkali, or sirnultaneously therewith, or
added to the peroxymonosulfuric acid or its salt prior to
the addition to pulp.
The consistency of the pu]p in the
peroxymonosulfuric acid salt final treatment stage is not
critical and can be, for examp]e, in a range of from
about 1% to about 45%, and pref-erably in a range of from
about 5~ to about 15%.
The pH at the commencement: of the peroxymonosulfuric
acid salt final treatment stage is preferably in a range
of from about 7 to about 12, and more preferably in a
range of from about 8 to about 11. Over the course of
the reaction, the pH drops due to the liberation of
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sulfuric acid from the peroxymonosulfuric acid.
Generally, the pH drops to a final pH in a range of from
about 3 to about 9, and preferably in a range of from
about 6 to about 8.
By way of example and without limitation, the
peroxymonosulfuric acid salt final treatment stage can be
carried out at temperatures in a range of from about 10~C
to about 100~C. Preferred temperatures for the
peroxymonosulfuric acid final treatment stage are in a
range of from about 40~C to ab~ut 70~C.
Depending upon the temperature, pH and chemical
charge selected, the retention time or reaction time of
the peroxymonosulfuric acid salt treatment can be, for
example and without limitation, in a range from about 10
minutes to about 60 hours, and preferably in a range from
about one hour to about 6 or 8 hours.
Pressure conditions selec~ed for the
peroxymonosulfuric acid salt t:reatment stage can vary for
this process, as is conventional in pulp operations.
Suitable pressures can range f:rom, but are not restricted
to, atmospheric to 0.5 MPa.
The above process conditions are merely illustrative
and are not considered limiting.
The peroxymonosulfuric acid salt final treatment is
the final bleaching stage prior to any other post-
bleaching operations, which mav generally include
refining of pulp, treatment with additives and chemicals
to make it suitable for pulp or paper sheets production.
It is noted that minor adjustments to the pH of the pulp
can optionally be effected prior to these post-bleaching
operations by including a washing and filtering step.
In carrying out the procecss according to the present
invention, conventional equipment well known in the pulp
industry can be used. By way of example, the final
treatment stage of the present invention can be carried
out by injecting or mixing the chemicals with pulp in a
mixer or at the suction side of- the pump discharging the
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pulp in to a suitable storage tower or chest. A high
density storage chest or tower, which is typically found
in pulping plants, is well suited for the final bleaching
treatment (prior to any post-brightening operations).
The following examples serve to illustrate the
present invention without limiting it in any way.
EXAMP].ES
Example 1
Southern pine kraft pulp bleached to a brightness of
86.7 %ISO by DEopD sequence was treated with 0.05% Caro's
acid (AO) in accordance with the process of the present
invention as follows.
A 45-60 gram sample of the Southern pine kraft pulp
received from the mill was pressed down (filtered) to a
consistency of 24% and fluffed. The sample was then
treated with 0.49 wt% (based on the total weight of the
pulp sample) of sodium hydroxide. The sample was then
treated with a mixture of 0.05 wt% Caro's acid, 0.49 wt%
sodium hydroxide, and 0.1 wt% ~a5DTPA (based on the total
weight of the pulp sample) and diluted with water to
bring the consistency of the pulp sample to 10%. The
pulp sample was bleached in a plastic bag immersed in a
water bath having a temperature of 45~C. Following the
reaction time, the pH of the pulp was measured and then
the pulp sample was diluted to 0.2% consistency, before
being pressed down to about 30~; consistency without
further washing. The brightness of the bleached pulp
was measured with an Elrepho-Data Color 2000 meter using
3 gram handsheet.
The reverted brightness was estimated by subjecting
the handsheet to 105~C + 3~C in a conventional oven for
60 minutes, and thereafter meas,uring the brightness with
the Elrepho-Data Color 2000 meter.
For comparative purposes, a substantially identical
procedure was conducted with hydrogen peroxide applied in
an equivalent basis, which is 0.1 wt%. The experimental
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conditions and results of Example 1 are reported below in
TABLE I:
TABLE I
Caro's AcidHydrogen Peroxide
(0.05 wt%) (0.1 wt%)
Temperature 65~C 65~C
Magnesium --- 0.1 wt%
Sulfate
Na5DTPA 0.1 wt% ---
Reaction 2 4 6 2 4 6
time (hr)
Brightness88.588.7 88.6 88.1 88.5 88.5
(%ISO) 2
Reverted 86.6 87.0 87.3 86.7 87.3 87.2
Brightness
(%ISO)
Brightness1.9 1.7 1.3 1.4 1.2 1.3
Loss (%ISO)
NaOH (wt%)0.980.98 0.98 0.22 0.22 0.22
Initial pH10.010.0 10.0 11.3 11.3 11.3
Final pH 6.5 6.2 6.1 10.3 10.2 10.0
Residual 0.02 0.02 0.02 0.06 0.04 0.02
(wt%)
Viscosity31.7 -- 30.5 33.3 -- 30.3
(mPa-s)
The Caro's acid treatment at 65~C with 0.05~ Caro's
acid over 6 hours resulted in a 1.9 %ISO increase in
brightness and a 0.6 %ISO increase in reverted brightness
of the pulp, which had an initial brightness of 86.7 %ISO
prior to being subjected to the inventive operation. The
brightness and reverted brightness increases realized by
the Caro's acid treatment were slightly better than that
realized with the hydrogen peroxide treatment (which were
1.8 %ISO and 0.5 %ISO, respectively).
However, the final pH levels of the pulp sample
after 2, 4, and 6 hours of treatment with 0.05% Caro's
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acid were 6.5, 6.2, and 6.1, respectively. These pH
levels are significant less than the pH levels of the
pulp sample treated with 0.1% :hydrogen peroxide, which
ranged from 10.0 to 10.3. Thus, unlike the comparative
samples treated with hydrogen peroxide, the inventive
samples advantageously did not have to be neutralized
prior to post-bleaching operations.
Example 2
The procedures set forth above in connection with
Example 1 were repeated, with lhe exception that 0.1 wt%
Caro's acid and 0.2 wt% peroxide were employed, and the
temperature of the bath was 45''C. The experimental
conditions and results of Example 2 are reported below in
TABLE II:
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TABLE II
Caro's Acid Hydrogen Peroxide
(0.1 wt%) (0.2 wt%)
Temperature 45~C 45~C
Magnesium --- 0.1 wt%
Sulfate
Na5DTPA 0.1 wt% ---
Reaction 2 4 6 2 4 6
time (hr)
Brightness 88.5 89.589.488.3 88.7 88.9
(%ISO)
Reverted 86.6 87.487.686.0 86.4 86.6
Brightness
(%ISO)
Brightness 1.9 2.1 1.8 2.3 2.3 2.3
Loss (~ISO)
NaOH (wt%) 2.0 2.0 2.0 0.28 0.28 0.28
Initial pH 10.0 10.010.011.3 11.3 11.3
Final pH 6.1 6.0 6.0 11.0 10.8 10.8
Residual -- -- -- 0.16 0.15 0.14
(wt%)
Viscosity 32.4 -- 31.9 32.8 -- 27.2
(mPa s)
The Caro's acid treatment at 45~C with 0.1% Caro's
acid over 6 hours resulted in a 2.7 ~ISO increase in
brightness and a 0.9 %ISO increase in reverted brightness
of the pulp, which had an initial brightness of 86.7 ~ISO
prior to being subjected to the inventive operation. The
brightness and reverted brightness increases realized by
the Caro's acid treatment were better than that realized
with the hydrogen peroxide treatment (which were 2.2 %ISO
and -0.1 %ISO, respectively).
However, the final pH levels of the pulp sample
after 2, 4, and 6 hours of treatment with 0.1% Caro~s
acid were 6.1, 6.0, and 6.0, respectively. These pH
levels are significant less than the pH levels of the
14
CA 022~8443 1999-03-22
pulp sample treated with 0.2~ hydrogen peroxide, which
ranged from 10.8 to 11Ø Thus, unlike the comparative
samples treated with hydrogen peroxide, the inventive
samples advantageously did not have to be neutralized
prior to post-bleaching operations.
The foregoing detailed description of the preferred
embodiments of the invention has been provided for the
purposes of description and elaboration. It is not
intended to be exhaustive or to limit the invention to
the precise embodiments disclo,ed. Obviously, many
modifications and variations w:ill be apparent to
practitioners skilled in this art. The embodiments were
chosen and described in order to best explain the
principles of the invention and its practical
application, thereby enabling others skilled in the art
to understand the invention for various embodiments and
with various modifications as are suited to the
particular use contemplated. It is intended that the
scope of the invention be defined by the following claims
and their equivalents.
