Note: Descriptions are shown in the official language in which they were submitted.
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Description
Composition and process for bleaching of mechanical wood pulp
BACKGROUND OF THE INVENTION
Chemical pulps have good strength properties and a high brightness value.
These
attributes, however, are obtained at the cost of low yields and the highly
negative effect
produced on the environment by the effluent from the bleaching process.
This had led in recent years to intensive development work aimed at producing
mechanical pulps in high yields to about 90%, and high brightness values, and
with
strength properties approaching those of the chemical pulps, while at the same
time
retaining the opacity and bulk properties unique to the mechanical pulps. The
resulting
pulps, while quite strong, are highly coloured probably due to the coloured
chromophores
in lignin.
Bleaching of mechanical wood pulps; such as ground pulp (GP), refiner ground
pulp
(RGP), thermo-mechanical pulp (TMP), bleached chemical thermo-mechanical pulp
(BCTMP), chemical ground pulp (CGP) and deinked recycled newspaper pulp has
been
in wide use, as have peroxides, such as hydrogen peroxide, sodium percarbonate
and so
on.
Hydrogen peroxide has environmental benefits over chlorine-based bleaches and
it is
effective for changing chromophores to non-coloured products in bleaching
mechanical
pulp. Hydrogen peroxide bleaching liquors conventionally employed have, in
general,
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additives inclusive of sequestering agents, for example, a metal chelating
agent, such as
sodium diethylenetriaminepentaacetate (DTPA), tetra- and disodium-
ethylenediaminetetraacetate (EDTA) and so on, and magnesium sulfate, sodium
silicate,
for preventing useless decomposition of the peroxide under the bleaching
condition due
to the presence of contaminant trace metals of natural occurrence or delivered
from waste
paper.
Caustic soda is also employed, in general, due to its low price as a pH
regulator since
peroxides exhibit their bleaching action in an alkaline condition. It has been
known that
sodium silicate also buffers and stabilizes the pH of a bleaching solution.
In terms of the typical parameters of conventional tower bleaching of pulp
with a
bleaching solution in which hydrogen peroxide is used as the peroxide, the
conditions for
bleaching include, in general, a bleaching duration of 1-4 hours at 40-80
degrees C. and a
solution pH of 10-11.
Although mechanical pulps have environmental benefits and are high-efficiency
pulps,
they suffer from some performance disadvantages, which become particularly
important
when an existing chemical pulp/paper manufacturing plant is being converted to
operate
with mechanical pulp. One problem is that the mechanical pulp brightness may
be lower
compared with the chemical pulp brightness and this may significantly reduce
the value
of the mechanical pulp. A second problem is brightness reversion of mechanical
pulps.
Recently alternative processes to improve brightness of mechanical pulp during
subsequent tower bleaching, by treating with oxidizing and reducing agents,
have been
suggested to reduce energy cost and improve optical properties of pulp. For a
number of
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reasons, well known to those in the art, hydrogen peroxide and peracetic acid
have
proven to be of particular interest, which are intended to brighten pulp
efficiently in the
presence of an alkali.
JP05186989 proposes an alkaline process using a bleach activator such as TAED
to give
a brighter pulp when it is used with oxygen and optionally also with hydrogen
peroxide.
Very few details of the process steps are given and hydrogen peroxide is not
used in the
examples. It appears that the TAED is mixed as a solid with dry pulp at the
start of the
bleaching step.
In W09521290 there is described a process in which per-acid is produced in
situ by
reaction of a bleach activator such as tetraacetylethylenediamine and hydrogen
peroxide
at a pH less than the pKa of peracetic acid formed from the reaction of these
chemicals. It
is stated that in a preferred process, the TAED is first dissolved in hot
water and then
added to the hydrogen peroxide before the reacting mixture is dosed to the
pulp. A
sequestrant may be added to the pulp before the dosing takes place. It is also
stated that
conditions must be optimized to ensure that all of the TAED is consumed. The
chemistry
must be carefully controlled to achieve consistent results when using such
powerful
bleach as peracetic acid.
In EP456032 there is described a similar pulp bleaching process using alkaline
TAED
and hydrogen peroxide. Bleaching of the pulp is done in plastic bags and no
detail is
given of how a scaled up process should be operated.
CA2041468 proposes use of TAED activated hydrogen peroxide to bleach
mechanical
wood pulps at lower temperatures within a short period of time.
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W09418298 describes a bleaching process where an N-acyl bleach activator is
reacted
with a source of hydrogen peroxide under acid conditions. The product of this
reaction
may be used in many bleaching and disinfection applications, including pulp
and paper
bleaching. The activator and other components may be in the form of particles
and these
particles may be provided by techniques similar to those used in the laundry
detergent
industry. For instance by spray drying liquid slurries; by granulation
techniques using
binders, for instance synthetic or natural polymers (or derivatives); or by
melt blending
followed by extrusion or other techniques. A composite product including a
bleach
activator may also include other additives, especially heavy metal
sequestrants, and it
may include surfactants to act as wetting agents and inorganic salts to act as
a diluent or
to increase the rate of disintegration or dissolution of the product. The
composite product
should also include the source of the hydrogen peroxide as well as the bleach
activator
when it includes the wetting agent. Only two granulated activator particles
are
exemplified in this document; both contain carboxymethyl cellulose as a binder
and
neither is used for pulp bleaching.
W09725402 proposes the use of bleach activators such as TAED for various
applications
including pulp bleaching. The preferred form of the TAED is a granule, but no
details are
given of the composition of the granule.
CA2230315 describes a refiner bleaching process where the TAED as a bleach
activator
is reacted with sodium perborate or hydrogen peroxide at elevated pressure.
The pulp
after refining has improved brightness and pulp brightness after optional
tower bleaching
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has exceeded 75 ISO% points.
Fluorescence whitening agents (FWA's) have been used to reduce the brightness
reversion of high brightness mechanical pulps, as described in Brightness
Reversion of
Mechanical Pulps XIV: Application of FWA's for High Brightness, High Yield
Pulps,
IPST Technical Paper Series number 747, A.J. Ragauskas, L. Allison, and C. Li,
Tappi
Journal (1998). However, this reference describes using an FWA with a BCTMP or
BCTMP-kraft furnish to reduce the photoyellowing properties of paper
containing high
yield pulp, and does not suggest that FWA's can be used in a BCTMP bleaching
process.
There is an ongoing need for improved but inexpensive mechanical pulps having
improved brightness; and with greater stability of the optical properties,
i.e. decreased
reversion.
SUMMARY OF THE INVENTION
The present invention relates to a novel, low-energy method of producing high
yield
mechanical pulp having a final brightness value not previously achieved, and
reducing
brightness reversion of bleached mechanical pulps containing lignin.
Theoretically calculated, one mole of TAED will be required to react with two
moles of
hydrogen peroxide. Since the molecular weight of hydrogen peroxide is 34 and
that of
TAED is 228; 3.35 grams of TAED will react with 1 gram, calculated as pure
substance,
of hydrogen peroxide employed for bleaching the pulp. However, it may be
reasonable to
employ hydrogen peroxide, which has a lower price than TAED, in an excess
amount, in
order to further increase the rate of reaction of hydrogen peroxide with TAED
and to
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improve the bleaching effect. In addition, more hydrogen peroxide becomes
effectively
active in the bleaching of pulps by the addition of TAED, so that the amount
of hydrogen
peroxide actually employed can be reduced.
The major obstacle to the use of mechanical pulps in high-grade paper products
is their
tendency to photoyellow. This yellowing phenomenon, also known as brightness
reversion, occurs primarily as a result of exposure to light and is
attributable to the photo-
oxidation of lignin. It is generally accepted that the basic mechanism of
photoyellowing
involves a variety of pathways including: direct absorption of near-UV light
by
conjugated phenolic groups to form Phenoxy-radicals; abstraction of phenolic
hydroxyl
hydrogen by aromatic carbonyl triplet excited state. One of the simplest and
most
effective means of photo-stabilizing mechanical pulps is by the addition of a
fluorescent
whitening agent (FWA) that prevents light in the 300-400nm ranges from
interacting with
lignin. FWA's can absorb ultraviolet and invisible radiation at 300-400 nm and
transform
that radiation into visible blue light at 400-500 nm. The use of FWA's
provides a two-
fold benefit for mechanical pulps, as these additives enhance the initial
brightness of pulp
and act as a UV-screen. In this process, FWA's absorb harmful near-UV light
and remit
light in the visible range giving the mechanical pulp the appearance of higher
brightness.
According to the present invention there is provided a method for bleaching
mechanical
pulp. The method comprises treating (digesting) said pulp in an aqueous
hydrogen
peroxide solution containing about 1%-6% hydrogen peroxide by weight based on
the
pulp, 1%-5% caustic soda by weight based on the pulp, 1%-5% sodium silicate by
weight
based on the pulp, 0.01 %-1 % TAED by weight based on the pulp, which
liberates
nascent oxygen upon reaction with hydrogen peroxide to increase the peroxide's
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efficiency, and 0.01 %-1% FWA by weight based on the pulp wherein the
fluorescent
whitening agent is selected from the group consisting of; A) 4,4'-Bis(6-
anilino-l,4-bis(2-
hydroxyethyl)amino)-1,3,5-triazin-2-yl)amino)stilbene-2,2-disulfonic acid(CAS
No.
4404-43-7), B) disodium 4,4'- bis(6-anilino-1,4-bis(2-hydroxyethyl)amino)-
1,3,5-
triazin-2-yl)amino)stilbene-2,2-disulphonate (CAS No. 4193-55-9), C) potassium
sodium 4,4'-bis[6-anilino-4-[bis(2-hydroxyethyl)amino]-1,3,5-triazin-2-
yl]amino]stilbene-2,2'-disulphonate (CAS No. 70942-01-7), D) 2,2'-
Stilbenedisulfonic
acid, 4,4'-bis((4-anilino-6-((2- hydroxyethyl)methyl amino)-s-triazin-2-
yl)amino)-
,disodium salt (CAS No. 13863-31-5), E) disodium 4,4'-bis[(4-anilino-6-
morpholino-
1,3,5-triazin-2-yl)amino]stilbene-2,2'-disulphonate (CAS No. 16090-02-1), F)
tetrasodium 4,4'-bis[[4-[bis(2-hydroxyethyl)amino]-6-(4-sulphonatoanilino)-
1,3,5-
triazin-2-yl)amino]stilbene-2,2'-disulphonate] (CAS No. 16470-24-9), G)
tetrasodium
4,4'-bis[[4-[bis(2-hydroxypropyl)amino]-6-(4-sulphonatophenyl)amino]-1,3,5-
triazin-2-
yl]amino]-stilbene-2,2'-disulphonate] (CAS No. 67786-25-8), and H) 2,2'-
stilbenedisulfonic acid, 4,4'-bis[[4-[(2-carbamoylethyl)(2-
hydroxylethyl)amino]-6-(p-
sulfoanilino)-s-triazin-2-yl]amino]-tetrasodium salt (CAS No.29637-52-3) at a
temperature in the range of 65 to 80 degrees C., and preferably 70-75 degrees
C. At such
temperatures, the reaction may proceed for about one to three hours. The pH
range of the
method is preferably about 10 to about 12.
We have found that residual hydrogen peroxide, after the bleaching tower,
could be
decreased by 2.3 kg (kilogram) to 9.9 kg using 0.1 kg TAED per tonne of dry
pulp in a
pre-reaction with 50 kg hydrogen peroxide (calculated at 100% hydrogen
peroxide) per
tonne of dry pulp and 37.5 kg caustic soda per tonne of dry pulp, 30 kg sodium
silicate
per tonne of dry pulp and 0.1 kg FWA per tonne of dry pulp to form a bleaching
solution
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for use in a mechanical pulp bleaching process.
By using the inventive process, an ISO brightness value of at least 86 is
preferably
obtained, still more preferably an ISO brightness value of at least 92 is
obtained.
The invention provides several advantages over conventional hydrogen peroxide
bleaching: 1. Lower cost benefits due to the amount of hydrogen peroxide,
alkali and
silicate actually employed that can be reduced while still achieving the same,
or greater,
ISO brightness values. 2. Increased brightness values for mechanical pulp over
conventional bleaching. This increases the number and variety of applications
for the
product, significantly expanding its marketability. 3. Environmental benefits
due to the
amount of hydrogen peroxide, alkali and silicate actually employed that can be
reduced.
The invention and its advantages will be illustrated in more detail by the
examples below
which however, are only intended to illustrate the invention without limiting
the same.
The percentages and parts stated in the description, claims and examples,
refer to percent
by weight and parts by weight, respectively, unless otherwise stated.
DETAILED DESCRIPTION OF THE INVENTION
The invention will be further described with reference to the following non-
limiting
examples:
Pulp brightness (ISO brightness) is measured with a brightness meter, which
determines
the brightness of a split sheet at a wavelength of 457 nm (ISO D65 Standard
Method)
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Using a bleaching solution of hydrogen peroxide, TAED is made to a 0.5%
solution and
FWA is made to a 5% solution.
EXAMPLE 1
Laboratory Studies
A mechanical softwood pulp sample (from a pulp mill in British Columbia,
Canada) has
a brightness of 58.6% ISO, and a concentration of 24.51%, treated with DTPA.
Laboratory bleached at 20% concentration pulp, 70 degrees C and 150 minutes of
retention time.
From the result shown in TABLE 1, it is evident that TAED and FWA bleaching
with
hydrogen peroxide provides maximum brightness gain and minimum residual
hydrogen
peroxide, and is superior to conventional hydrogen peroxide bleaching.
TABLE 1
Residual
Unbleached Hydrogen
ISO Peroxide Caustic Silicate TAED FWA Peroxide
Brightness (%) (%) (%) (%) (%) (%) ISO Brightness
58.6 5.0 3.75 3.0 0.00 0.00 1.22 77.5
58.6 5.0 3.75 3.0 0.01 0.10 1.15 83
58.6 5.0 3.75 3.0 0.03 0.20 0.99 86
58.6 5.0 3.75 3.0 1.00 0.01 0.51 84
58.6 5.0 3.75 3.0 0.10 0.30 0.78 89
58.6 5.0 3.75 3.0 0.03 0.50 0.99 90
58.6 5.0 3.75 3.0 0.01 1.00 1.15 92
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EXAMPLE 2
Laboratory Studies
A mechanical softwood pulp sample (from a pulp mill in British Columbia,
Canada) has
a brightness of 52% ISO, and a concentration of 31 %, treated with DTPA.
Laboratory
bleached at 24% concentration pulp, 70 degrees C and 150 minutes of retention
time
using a single-stage process.
It is evident from the TABLE 2 result that increasing the amount of FWA does
have a
positive effect on pulp brightness. Bleached pulp with ISO brightness of more
than 84
points has been obtained.
TABLE 2
Incoming
ISO Peroxide Caustic Silicate TAED FWA
Brightness (%) (%) (%) (%) (%) ISO Brightness
52 4.0 3.6 3.0 0.00 0.00 71
52 4.0 3.6 3.0 0.01 0.09 76
52 4.0 3.6 3.0 0.02 0.18 80
52 5.8 4.2 3.2 0.00 0.00 76
52 5.8 4.2 3.2 0.01 0.09 81
52 5.8 4.2 3.2 0.02 0.18 84
EXAMPLE 3
Laboratory Studies
Bleaching studies were performed on partial two-stage process bleached
mechanical
softwood pulp samples, which had been bleached in the first stage to have a
brightness of
66% ISO, and a concentration of 39.5%, treated with conventional hydrogen
peroxide
bleaching. Laboratory bleached at 24% concentration pulp, 70 degrees C and 150
minutes
of retention time.
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About 26 points of brightness were gained with TAED, FWA and hydrogen
peroxide,
compared to about 14 points with hydrogen peroxide. It is further evident that
TAED and
FWA with hydrogen peroxide bleaching is more effective in pulp brightening
during the
second-stage bleaching system compared to conventional hydrogen peroxide
bleaching
during the second-stage bleaching system.
TABLE 3
Incoming
ISO Peroxide Caustic Silicate TAED FWA
Brightness (%) (%) (%) (%) (%) ISO Brightness
66 2.0 1.0 2.0 0.00 0.00 74
66 2.0 1.0 2.0 0.01 0.09 78
66 2.0 1.0 2.0 0.02 0.18 81
66 3.5 2.0 2.5 0.00 0.00 80
66 3.5 2.0 2.5 0.02 0.18 89
66 3.5 2.0 2.5 0.03 0.27 92
EXAMPLE 4
Mill-Trial
A plant test was performed at a pulp and paper mill in British Columbia,
Canada.
Mechanical softwood pulp was bleached using a single-stage bleaching process.
The
pulp, after the bleaching tower, was diluted and neutralized at pH 5.6 with a
sulphur
dioxide solution in preparation for papermaking stock, was then run through a
pulp
refiner and finally made into paper at the paper machine.
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TABLE 4
Bleaching
Unbleached Tower
ISO Peroxide Caustic Silicate TAED FWA ISO Paper Machine
Brightness (%) (%) (%) (%) (%) Brightness ISO Brightness
50 6.0 4.5 3.5 0.000 0.000 76.4 76.2
50 6.0 4.5 3.5 0.010 0.090 81.9 81.8
50 6.0 4.5 3.5 0.013 0.117 82.7 82.5
50 6.0 4.5 3.5 0.020 0.18 86.0 85.2
Evidently, TAED, FWA and hydrogen peroxide bleaching gives the best optical
properties with about a 9-point increase in the ISO brightness points after
the paper
machine. The brightness does not change between the bleaching tower and paper
machine, and even after pulp refining, in preparation for papermaking stock.
Therefore, it
is clear that FWA's are effective in stabilizing pulp or paper and have a high
affinity for
pulp or paper essential.
