PROCEDE AMELIORE POUR LE BLANCHIMENT DE LA PATE KRAFT

IMPROVED PROCESS FOR BLEACHING KRAFT PULP

Abrégé anglais

An improved process for bleaching Kraft pulp is disclosed. The improved
process
involves the use of hydrogen peroxide to treat brownstock before treatment
with
chlorine or chlorine containing compounds. The process may also be used to
treat
recycle pulp.

Revendications

What is claimed is:
1. A process for bleaching Kraft pulp comprising treating unbleached
brownstock pulp with hydrogen peroxide prior to treatment with chlorine or
chlorine containing compounds.
2. A process as in claim 1 wherein the amount of hydrogen peroxide is equal to
from 0.05 to 2.0% based on the weight of the pulp.
3. A process for bleaching recycle pulp comprising treating the pulp
with hydrogen peroxide prior to treatment with chlorine or chlorine
containing compounds.
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Description

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IMPROVED PROCESS FOR BLEACHING KRAFT PULP
This application claims the benefit of U.S. Provisional Application No.
60/530,180,
filed December 17, 2003.
FIELD OF THE INVENTION
The present invention relates to an improved process for bleaching Kraft pulp
and
involves the use of hydrogen peroxide in the early stages of such a process -
i.e.,
before the pulp is treated with chlorine or chlorine containing compounds.
BACKGROUND OF THE INVENTION
The production of chemical, or Kraft, pulp involves the cooking of wood chips
in a
digester at elevated temperature and pressure in the presence of suitable
chemicals,
such as a mixture of sodium hydroxide and sodium sulfide. The unbleached pulp
(brownstock) is removed from the digester, washed and bleached. The purpose of
bleaching and washing is to remove lignin from the brownstock and to brighten
the
pulp for subsequent use in paper making operations. The bleaching takes place
in a
number of successive steps involving the use of chemicals such as elemental
chlorine, chlorine dioxide, sodium hypochlorite, ozone and hydrogen peroxide.
Generally, chlorine and chlorine containing compounds have been used in the
earlier
steps followed by ozone, oxygen or hydrogen peroxide used mainly to brighten
the
pulp.
SUMMARY OF THE INVENTION
The present invention relates to an improved process in which hydrogen
peroxide is
used to treat brownstock obtained in a conventional kraft pulping process
prior to
treatment with chlorine or chlorine containing compounds.
The method of the present invention uses the hydrogen peroxide in the initial
treatment of the brownstock following its removal from the digester.
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BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
Figure 1 shows data from Example 1.
Figure 2 shows data from Example 3
Figures 3 and 4 show data from Example 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, brownstock from a conventional Kraft
pulping operation is treated with hydrogen peroxide prior to treatment with
other
bleaching agents. When this process is employed, it has been found that the
amount
of chlorine, chlorine dioxide or other bleaching agents, required in
subsequent steps
can be reduced without adversely affecting the properties, particularly
brightness, of
the resulting pulp.
The treatment of the present invention is preferably conducted by including
hydrogen peroxide in the water used in washing the brownstock. The peroxide is
used alone as the sole bleaching agent and not in combination with other
chemicals.
The amount of hydrogen peroxide can vary from 0.05 to2.0% on pulp weight. In
general, preferred results are achieved with peroxide in an amount equal to
from 0.1
to 0.8% on pulp weight.
In addition to treatment of brownstock, the present invention can be used to
treat
recycle pulp such as that obtained in the recycling of newspapers. Here again
the
recycle pulp is treated with hydrogen peroxide followed by treatment with
chlorine
or chlorine containing compounds.
The process of the present invention results in pulp having a brightness equal
to that
obtained in the conventional process while using less chlorine or chlorine
containing
compounds. It can be conducted in conventional equipment so no capital
investment
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is required and does not require the addition of alkaline chemicals since the
brownstock is already alkaline.
In order to describe the invention in more detail, the following examples are
set
forth:
As is known to those skilled in the art "kappa" relates to the lignin content
of the
pulp and "brightness" relates to the whiteness of paper made from the pulp.
EXAMPLE 1
A softwood pulp was produced in a digester. After a few stages of washing, the
pulp
was stored in a high-density storage tank. The pulp kappa number before
peroxide
treatment was about 30 for several hours prior to the point where peroxide
addition
was started as shown on Figure 1.
Hydrogen peroxide was added to the high-density tank at the rate of 10
pounds/ton
(0.5% on weight of pulp). Pulp stayed in the tank for approximately two hours
at a
temperature of 140 degrees Fahrenheit and a pH of 10.5. No caustic was added
to
the pulp.
The pulp kappa as recorded by the analyzer was about 22 after the peroxide
treatment. The pulp was discharged from the tank, washed with additional water
and
fed to the bleach plant. A reduction of about 20 pounds/ton pulp as equivalent
chlorine was noted in the bleach plant. A reduction of about 8 pounds/ton of
caustic
was also realized in the bleach plant. Pulp quality was maintained and a
slight
increase in the final pulp brightness was achieved. The kappa and chemical
demand
as total equivalent chlorine (TEC) are shown in Figure 1. As can be seen,
during the
period when peroxide was added at 10 pounds/ton the TEC was reduced. TEC was
also reduced when the peroxide was added at 5 pounds/ton. Further reduction in
caustic (% NaOH) it also shown in Figure 1.
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EXAMPLE 2
Hardwood pulp was produced in a digester. After a few stages of washing, the
pulp
was stored in a high-density storage tank. The pulp kappa number before
peroxide
S treatment was about 15 ( 14.7).
Hydrogen peroxide was added to the high-density tank at the rate of 2
pounds/ton
(Example 2-1) (0.1% on weight of pulp) and 6 poundslton (Example 2-2) (0.3% on
weight of pulp). Pulp stayed in the tank for one hour at a temperature of 140
degrees Fahrenheit and a pH of 10.5. No caustic was added to the pulp.
The pulp was discharged from the tank, washed with additional washer and fed
to
the bleach plant. The pulp kappa after the peroxide treatment was about 13. A
reduction of about 4 poundslton pulp of chlorine dioxide (Example 2-1) and
about
10 poundslton pulp of chlorine dioxide (Example 2-2) was noted in the bleach
plant.
About 2 pounds/ton caustic was also reduced from the bleach plant in (Example
2-
2). Pulp quality and brightness were maintained. The data is shown in the
following
Table where the results of Example 2-1 and 2-2 are compared to a control in
which
no hydrogen peroxide was used.
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TABLE
Control 2-1 2-2
HZOZ to HD (IbsJton)0.0 2.0 6.0
Decker Kappa 14.7 13.6 13.2
Decker Brightness 33.1 32.4 35.1
Bleach Plant:
D100 C102 (IbsJton)393 35.1 28.9
EOP kappa 2.2 2.2 2.3
EOP caustic (lbsJton)26.3 26.3 25.6
EOP Brightness 76.0 76.2 75.0
Dl CIOZ (lbsJton) 13 5 13.4 13.4
Dl Brightness 89.4 88.9 89.2
EXAMPLE 3
A softwood pulp was produced in a digester. After a few stages of washing, the
pulp
was stored in a high-density storage tank. The pulp kappa number before
peroxide
treatment was about 26.
Hydrogen peroxide was added to the high-density tank at the rate of 8
pounds/ton
(0.4% on weight of pulp). Pulp stayed in the tank for two hours at a
temperature of
140 degrees Fahrenheit and a pH of 10Ø No caustic was added to the pulp.
The pulp was discharged from the tank and fed to the bleach plant. A reduction
of
about lOpounds/ton pulp chlorine dioxide was noted in the bleach plant. About
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pounds/ton caustic was also reduced in the bleach plant. Pulp quality was
maintained
and an increase in the final pulp brightness was achieved. Figure 2 shows the
reduction in chlorine dioxide from about 45 pounds/ton pulp when the hydrogen
peroxide was first added to 40 pounds/ton or less over time.
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EXAMPLE 4
Softwood pulp of kappa 28.5 was collected from a pulp mill and evaluated in
the
lab. Two levels of hydrogen peroxide, 0.2 and 0.4% based on the weight of the
pulp,
were added to the pulp. A base line experiment was also conducted without any
hydrogen peroxide. No caustic was added.
Pulp was kept in bags at 140 degrees Fahrenheit for two hours. The pulp
consistency
was 12%.
The pulp was taken out of the bags, washed and the brightness and kappa number
determined. A drop in kappa number and an increase in pulp brightness were
noted.
The effect was greater for the higher amount of hydrogen peroxide. The results
are
shown in Figures 3 and 4.
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Dessin représentatif