Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
"PEROXIDE BLEACHING OF PULP"
FIELD OF THE INVENTION
THIS INVENTION relates to a process for peroxide
bleaching of pulp. Pulps which may be bleached in the process of the
invention include lignocellulose pulp which may be produced
mechanically and chemi-mechanically with yields in the region of
greater than 75% which are otherwise known as high yield pulps.
BACKGROUND OF THE INVENTION
In a conventional peroxide bleaching process, sodium
hydroxide is used as an alkali source. To achieve a desired brightness
with maximum efficiency, auxiliary substances are also used. Such
auxiliary substances include sodium silicate, magnesium sulphate and
chelating agents inclusive of DTPA (sodium salt of diethylene triamino
pentaacetic acid).
Reference may be made to a prior art article by Soteland
et al., 1988, TAPPI Proceedings 231-236, which describes a peroxide
bleaching process which utilises magnesium oxide as a sole alkaline
source. The pulp was pretreated with DTPA and magnesium oxide
particles were utilised in a size range of 1.00 mm - 0.25 mm or
smaller. The magnesium oxide were also used in a concentration of 2-
3% based on the dry weight of the pulp. The Mg0 used in the
process was light-burnt Mg0 and finely crushed. It was found that
brightness levels obtained were very close to that which was achieved
by conventional bleaching using NaOH. In the bleaching process, the
pulp was diluted to form a pulp suspension and the amount of Mg0
was added to the suspension under vigorous stirring. Hydrogen
peroxide was subsequently added to the suspension at a
concentration of 3% based on the weight of the pulp. This reference
also made the observation that coarse particles are less effective as
an alkaline source during peroxide bleaching.
Having regard to the abovementioned reference, an
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observation was also made in the corresponding patent specification
DE3617942 that use of Mg0 as sole alkaline source considerably
simplified the bleaching process since sodium hydroxide as alkaline
source and auxiliary chemicals such as sodium silicate could be
omitted.
Another advantage of using Mg0 as sole alkaline source
was that only a small amount of waste is produced in the bleaching
plant. Thus, for example, in integrated mills which produce
magnesium sulphite pulp and peroxide bleached high-yield pulps, the
used bleaching liquor is combusted and the Mg0 may be recovered for
re-use.
However, the use of Mg0 as sole alkaline source in a
peroxide bleaching process has not as yet achieved widespread
commercial acceptance because although the principle of utilising
7 5 Mg0 as sole alkaline source was described in the Soteland et al.
references, the means of reducing the principle to practice on a
commercial scale has not yet been fully elucidated.
SUMMARY OF THE INVENTION
Surprisingly, it has now been discovered that commercial
usage of magnesium oxide as a sole alkaline source in peroxide
bleaching of wood pulp may be achieved by employing Mg0 particles
with a particle size of less than 500 micron and more preferably less
than 75 micron and having particle surface area (PSA) of between 20
60 m2/g and more preferably between 30-50 m2/g. By using such
parameters, an efficient peroxide bleaching process may be carried
out most efficiently on a commercial scale which may be achieved
within a maximum bleaching time of 180 minutes and achievement of
a maximum target of ISO brightness of 65 in regard to freshly
prepared pulp.
Utilizing Mg0 with parameters outside those stated
above will result in a less efficient bleaching process leading to higher
usage of chemicals and therefore higher operating costs.
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'CT,~Ai) ~ 6 / 0 0 3 5
RECEIVED ~ ~ ~~R iJ~l
- w The dosages of Mg0 that may be utilised in the process
of the invention is 0.3-2% based on the weight of the pulp.
The amount of hydrogen peroxide that may be utilised in
the process of the invention is from 1-5% based on the weight of the
pulp.
To achieve maximum efficiency, the Mg0 particles are
preferably added to the pulp in the form of a powder or slurry prepared
in situ.
Preferably the Mg0 is added to the pulp simultaneously
1o with the peroxide or prior to the addition of the peroxide.
Chelating agents also may be used in the process of the
invention and such chelating agents may comprise DTPA, EDTA or
HEDTA (hydroxy-ethylene diamine tetracetic acid). Preferably the
chelating agent is added to the pulp simultaneously with addition of
Mg0 particles, as well as prior to addition of Mg0 particles.
Bleaching times of 60-180 minutes may also be utilised
by the process of the invention to achieve a target ISO brightness of
55-65.
BRIEF DESCRIPTION OF DRAWINGS
2o In several preferred embodiments concerning the process
of the invention which are discussed hereinafter in relation to
Experiments 1 and 2:-
FIG. 1 is a graph showin the effect of particle size on
CCS (Cold Caustic Soda) pulp and more specifically showing particle
size vs brightness at different times;
FIG. 2 refers to the results of Experiment 2 whereby
various samples are plotted against final brightness;
FIG. 3 also refers to the result of Experiment 2 and
shows the effect of surface area on CCS pulp and more specifically
showing particle size vs brightness at different times; and
FIG. 4 shows the results of FIG. 3 when plotted against
time.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXPERIMENT 1
EFFECT OF MGO PARTICLE SIZE ON THE BLEACHABILITY
OF CCS PULP
Introduction
This work was performed to establish a relationship
between Mg0 particle size and alkali performance in the bleaching of
CCS (Cold Caustic Soda) wood pulp. Four grades of Mg0 were
trialled, each one identified by its particle size distribution. Each
sample of Mg0 had approximately the same surface area. Particle size
and surface area for each of the samples is given in Table 1.
Summary of bleaching work
CCS (chemi-mechanical pulp), pre-treated with DTPA to
remove metal ions, was retrieved from the washers in the bleach plant
at the Boyer mill. An equivalent mass of 20 grams OD of pulp was
weighed out and placed in a plastic breaker. DTPA was then added as
0.15 % v/w on the pulp and mixed. Mg0 as 0.4% w/w, enough water
to give a stock consistency of 12% and peroxide as 1.6% v/w on
pulp was added and mixed for 2 minutes. The pulp was wrapped in
plastic bags and placed into a constant temperature water bath at
65°C. A 3 gram OD sample was removed from the bath at intervals
of 2, 3 and 4 hours. This was then made into a brightness hand sheet
using the standard Boyer pulp mill method. These inrere dried
overnight in a constant temperature/humidity room and tested for ISO
brightness. This procedure was repeated for all Mg0 samples as well
as with control pulp containing no Mg0 (sample J)
Results
The results of this study indicate that particle size is a
key parameter for achieving efficient peroxide bleaching of chemic-
mechanical pulp. The results shown in Table 2 and FIG. 1 of this
study indicate that an Mg0 particle size of < 75 N (samples G and F,
d90 - 65 and 35 respectively) is required to achieve a target
r : ~...,. F .. r
.. . .
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brightness for a given retention time of 2, 3 or 4 hours.
To achieve an equivalent brightness with samples C (d90
- 1500) or D (d90 = 3500), the chemical dosages of Mg0 and H202
' would need to be increased.
EXPERIMENT 2
EFFECT OF MGO PARTICLE SURFACE AREA ON THE
BLEACHABILITY OF CCS PULP
Introduction
This work was performed to establish a relationship
between Mg0 particle surface area and alkali performance in the
bleaching of CCS (Cold Caustic Soda) wood pulp. Five grades of Mg0
were trialled, each one identifiable by its particle surface area. Each
sample of Mg0 had approximately the same particle size. Particle size
and surface area data for each of the samples is given in Table 3.
Summary of bleaching work
CCS (chemi-mechanical) pulp, pre-treated with DTPA to
remove metal ions, was retrieved from the washers in the bleach plant
at the Boyer mill. For each sample, a mass of 10 g O.D. pulp was
placed into a beaker and the approximate mass of chemicals added.
The pulp was mixed for 2 minutes in a bench top mixer. The pulp was
then wrapped in plastic bags and placed into a constant temperature
water bath at 65°C. After two hours retention, the samples were
removed from the bath and divided into two. Half the sample was
returned to the bath for a further hour of reaction while the other half
was made into 5 gram brightness hand sheets. These were dried
overnight and then tested for ISO brightness. The work was repeated
with samples taken at 2, 3 and 4 hours.
Results
In the previous study (Experiment 1 ), we determined that
Mg0 particle size was important for peroxide bleaching efficiency. The
results of this study indicate that particle surface area is also a key
parameter for achieving maximum brightness for a given chemical
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dose. The results from these two independent studies (Tables 4 and
5, FIGS. 2 and 3) indicate that a surface area in the range 30-50 m2/g
(samples B and C) is required to achieve maximum brightness for a
given retention time and chemical dose. Surprisingly, when the
surface area is either decreased or increased, the peroxide bleaching
efficiency is reduced as indicated in FIGS. 2 and 3 by the bell shaped
curves with brightness plateaus between samples B and C. To achieve
an equivalent brightness to samples B and C with samples A, D or E,
the chemical charges of H20 and Mg0 would need to be increased.
The results in FIG. 3, when plotted against time (FIG. 4),
appear to indicate that a similar brightness will be achieved with four
of the five samples when the bleaching time is extended indefinitely.
However, indefinite bleaching time is not a commercial reality and
there is a clear benefit, based on these results, in employing Mg0
particles with a specific size and surface area. In fact, if Mg0
particles, with parameters outside those stated in this document are
used, then the target brightness may not be achieved without
increasing chemical dose rates.
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TA-
TABLE 1
Sample Particle size d90 Surface area m2/g
micron
35 38
G 65 35
1500 30
I 3500 30
TABLE 2
Sample Surface AreaBrightness
m21
g
2 hr 3 hr 4 hr
F < 40 61.39 62.17 62.88
G < 75 61.22 61.94 62.69
H < 2000 56.85 57.98 59.17
I < 5000 56.32 56.19 56.97
J 0 54.3 55.03 55.04
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TABLE 3
PCT/AU96/00354
Sample Particle size d90 Surface area mzlg
micron
A 14 1
B 10 35
C 10 43
0 15 (d90 = 70) gg
E 11 142
TABLE 4
Sample Surface AreaBrightness
mzl
9
2 hr 3 hr
A 1 58.5 59.1
B 35 60.2 60.5
C 43 60.1 60.6
g8 58.3 59.0
142 56.8 58.8
TABLE 5
Sample Surface AreaBrightness
mz/
9
2 hr 3 hr 4 hr
1 56.02 57.75 58.54
35 58.89 60.58 60.96
C 43 59.17 31.37 61.09
98 58.15 59.1 60.29
E 142 57.45 59.13 60.27
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LEG-
TABLE 2
Bleaching conditions:
Mg0 % wlw on oven dry pulp = 0.4%
H202 - 1.6%
DTPA = 0.15
Temperature = 65C
Initial brightness 47.1
TABLE 4
Bleaching conditions:
Mg0 % w/w on oven dry pulp = 0.3%
H2O2 = 1.8%
DTPA = 0.1
Temperature = 65C
Initial brightness 43.5
TABLE 5
Bleaching conditions:
Mg0 % w/w on oven dry pulp = 0.4%
HZO2 = 1.6%
DTPA = 0.15
Temperature - 65 C
Initial brightness 47.1
FIG. 1
Effect of particle size pulp - Particle size vs brightness
on CCS at
different times.
F1G. 2
Sample number vs final final
brightness.
FIG. 3
Effect of surface area on pulp - Surface area vs brightness
CCS at
different
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FIG. 4
Effect of surface area on CCS pulp - Time vs brightness for different
surface areas
