chapter 5 bleaching and characterization of...
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CHAPTER
5
Bleaching and Characterization of Treated and Untreated
Bamboo Pulps
5.1 Introduction
Conventional kraft pulping removes approximately 90-95% of the lignin from
wood. Further delignification results in an increased degree of carbohydrate degradation
and dissolution, causing significant loss in yield and pulp strength. Kraft pulps,
containing residual lignin, are delignified further using alternative delignification
procedures that do not cause significant yield loss (Sixta et al., 2006; Gullichsen and
Fogelholm, 2000). Chlorine is well-recognized as an effective delignification agent for
kraft pulps; it oxidizes and degrades residual lignin in such a way that a substantial
portion of the lignin is easily removed from the pulp by a subsequent alkali extraction.
The effluents from chlorine bleaching contain chlorinated organic compounds, typically
measured as absorbable organic halides (AOX) (Froass, 1996).
The bleaching of pulp with elemental chlorine and chlorine based chemicals has
become a major global environmental concern (Brunner and Pulliam, 1993). The
discharge of chlorinated phenolics (formed during bleaching with chlorine) in mill
effluents became an issue in early 1970’s when measurement techniques become
available and high concentration of chlorinated phenolics was detected in fish stock
receiving bleach plant effluent. Conventional pulp bleaching of softwoods produces
about 5 kg of organically bound chlorine per tonne of pulp, with almost all of this
discharged as effluent. Consequently environmental regulatory authority became active
and finalized norms and guidelines to reduce the discharge of chloro-organics in mill
effluents. Typical goals were to produce 1.5 to 2.5 kg AOX per tonne of pulp after
recognizing the toxic effect of chlorinated phenolics, generated during bleaching of pulp
with chlorine based chemicals. Efforts have been made by researchers and technology
suppliers to develop technologies which can reduce the kappa number of pulp and
improve pulp washing to minimize the carry over of organic matters along with pulp
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going to bleach plant as both the factors govern the consumption of bleach chemicals.
Bleaching chemicals are quite expensive and they result in increased cost of bleaching
operation. Therefore minimization of chemical usage during pulp bleaching is beneficial
for both environmental improvement and mill economics. In addition, minimal water
usage is the additional benefit while the volume of effluents is reduced by reducing kappa
number of pulp prior to bleaching (Wang et al., 1995; Malinen and Fuhrman, 1995;
Pryke and Reeve, 1997).
5.1.1 Alternatives for use in Bleaching Stages to Reduce Pollution
Several other measures have already been taken to reduce the amount of AOX
released into the environment. For example, the incorporation of chlorine dioxide (ClO2),
ozone (O3) and hydrogen peroxide (H2O2) into pulp bleaching sequences has significantly
reduced the problem (Johnston et al., 1997). For the same amount of Cl2, ClO2 produces
only one-fifth the amount of AOX and more efficient pulping methods prior to bleaching
helps to reduce the amount of lignin which reaches the bleaching stages.
A great part of the effort put into these environmental actions in the last few years
has been directed towards the reduction of chemical reagents in the pulp bleaching. The
application of biotechnology to the pulp and paper industry has been an object of many
research studies (Eriksson, 1998; Tortter, 1990). Years ago, microorganisms began to
be used in the treatment of effluents, the fermentation of sulphate liquors, the preparation
of starch for paper sizing and the prevention/ control of slime buildup on paper machines.
Now a day, research is more focused on improving tree species, pulping, modifying
fibers and bleaching. An interesting approach is the use of lignin-degrading fungi, not in
a pre-bleaching stage but as an alternative bleaching process. This alternative has already
been proven to reduce by 72% the required bleaching agents for Kraft pulp (Fujita et al.,
1991). The investigation of how these microorganisms degrade a polymer of the
structural complexity of lignin has been the object of many research studies. The lignin-
degrading capacity of these fungi is now known to be due to extracellular oxidative
enzymes that function together with low molecular weight cofactors (Barr and Aust,
1994; Kuhad et al., 1997). Secreted by the fungi in response to low levels of key
nutrients such as C, N or S, these enzymes are mainly lignin peroxidase (LiP) (Tien and
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Kirk, 1983), manganese peroxidase (MnP) (Glenn and Gold, 1985), manganese-
independent peroxidase (MIP) (Jong et al., 1992), laccase (Eggert et al., 1995), H2O2-
generating oxidases (Kuwahara et al., 1984) and hemicellulolytic enzymes such as
xylanases (Casimir-Schenkel et al., 1995). The role of each of these enzymes is still
unclear since the lignin-degrading species differ in the range of ligninolytic enzymes they
produce. MnP activity is detected in active biobleaching cultures of different strains
(Moreira et al., 1997), and has been correlated with the biobleaching ability of different
white-rot fungi. Kondo et al. (1994) proved the bleaching ability of purified MnP in in-
vitro systems, provided that Mn2+, Tween 80, malonate and H2O2 were supplied in
adequate concentrations. Moreover, the use of MnP avoids operational problems related
to the need of a mediator, as in the case of a laccase based bleaching with the larger
associated benefits related to economical and environmental points of view. However, an
application of MnP on a pilot or an industrial scale is still lacking, which would be the
first step to involve industrial partners in the uses of this enzyme.
5.1.2 Bleaching Chemicals and Bleaching Sequences
Chemicals commonly used for pulp bleaching include oxidants (chlorine, chlorine
dioxide, oxygen, ozone and hydrogen peroxide) and alkali (NaOH). Hydrogen peroxide is
commonly used as a bleaching agent, and is simply called peroxide". These chemicals are
mixed with pulp suspensions and the mixture is retained at a prescribed pH, temperature
and concentration for a specific minimum period of time (Akida, 2001). The pulp is
normally bleached with elemental chlorine and hypochlorite, which leads to the
formation of variety of chlorinated phenolic compounds. In aqueous chlorine system,
chlorine can exist in three different forms, depending on the pH of the solution. These are
molecular chlorine (Cl2), hypochlorous acid (HOCl) and its anion (OCl-). In the region up
to pH 5, a concentration dependent equilibrium exists between molecular chlorine and
hypochlorous acid, while at higher pH values hypochlorous acid and its anion are both
present in proportions directly determined by the pH of the solution. Two pH regions are
of particular interest for bleaching chemistry (Dence and Reeve, 1996). The pre-
chlorination of pulp is carried out at pH 2, and the purpose of this operation is to make
the bulk of the residual lignin soluble in both water and alkali. There is no color
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improvement of pulp accomplished at this stage as well as in the following alkali-wash,
but the subsequent hypochlorite bleaching stage is greatly facilitated because of the
decreased residual lignin content. The optimum region for the hypochlorite bleaching is
pH 8 to 9, and the effect of the bleaching at this pH differs significantly from that of pre-
chlorination. The difference between the effects of pre-chlorination and hypochlorite
bleaching is probably due to the difference in the nature of the reacting chlorine species.
At the lower pH, molecular chlorine, a species much more reactive than hypochlorous
acid, directs the reaction, while in the hypochlorite bleaching, hypochlorous acid, or a
reactive intermediate derived from it, probably plays the role of the primary attacking
species (Sarkanen, 1962).
With increasing environmental awareness and recognition of the adverse and
toxic effects of these chlorinated phenolic compounds, most of the pulp mills in
developed countries have adopted modified pulping and bleaching processes to reduce
the discharge of chlorinated phenolic compounds. In India, due to economic
considerations molecular chlorine and its compounds are used for producing bleached
grade paper. The most common bleaching sequences adopted by the Indian pulp and
paper mills are CEH or CEHH. Use of chlorine dioxide, hydrogen peroxide and oxygen
reinforced alkali extraction which is also limited to very few mills and are producing
rayon grade pulp and papers with high brightness quality. The small and medium scale
pulp mills are normally using CEHH sequence for bleaching of the pulp to the required
brightness level and a few mills use only hypochlorite. The bleaching chemicals are
applied in multistage sequences wherein chemicals are mixed with pulp and allowed a
period of retention for bleaching reactions to complete. The spent chemicals and
dissolved impurities are removed by washing of pulp (Keski-Santti, 2007). The various
bleaching chemicals used in pulp bleaching are given in Table-5.1. Oxygen is another
effective delignifying agent which is widely used to enhance the extraction stage and it is
being used in advance of chlorine in order to reduce the carryover of organic matter to
bleach plant (Sjostrom, 1993; Johansson and Clark, 1995). The hypochlorite and ClO2
are mainly used for brightening of pulp. CEH is the traditional sequence used by the
Indian paper industries to produce bleached pulp. But with increasing environmental
pressure to reduce or eliminate organo-chlorine, the use of chlorine is decreasing rapidly
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with oxygen, peroxide and ClO2 providing more environmentally compatible bleaching
(Gullichsen and Fogelholm, 2000; Ansari et al., 2007). Non wood pulps are easier to
bleach than wood pulps. Shorter bleaching sequences and lower chemical charges are
used to bleach non woods. Globally, most non woods still are bleached using chlorine in
a typical CEH (Chlorination-Extraction-Hypochlorite) or CEHH (Chlorination -
Extraction – Hypochlorite - Hypochlorite) bleaching sequence.
Table 5.1: Chemicals Used in Pulp Bleaching.
Oxidants Form Advantages Disadvantages
Chlorine Gas Effective, economical
delignification. Good
practical removal
Can cause loss of pulp strength
if used improperly. Organo
chlorine formation.
Hypochlorite Ca(OCl2), NaOCl
solution 40 gpl as Cl2
Easy to make and use Can cause loss of pulp strength
if used improperly. Cholorform
formation
Chlorine dioxide 7-10 g/l ClO2 solution in
water
Achieves high brightness
without pulp degradation.
Good particle removal.
Must be made on site.
Expensive. Some organo
chlorine formation
Oxygen Gas used with NaOH
solution
Low chemical cost. Provides
chloride-free effluent from
recovery
Used in large amounts requires
expensive equipment. Can
cause loss of pulp strength.
Hydrogen peroxide 2-5% solution Easy to use low capital cost Expensive, poor particle
bleaching.
Ozone Gas in low concentration
in oxygen
Effective, provides chloride-
free effluent for recovery
Expensive, Degrades pulp.
Poor particle bleaching
Reductant
Hydrosulfite (for
mechanical pulps
only)
Solution of Na2S
2O
4 or
made onsite from NaBH4
solution plus SO2
Easy to use. Low capital
cost.
Decomposes readily. Limited
brightness gain
Alkali Sodium
Hydroxide
5-10% NaOH solution Effective and economical Darkens pulp
In this chapter fungal treated and untreated bamboo kraft pulps from non
destructured and destructured samples were subjected to conventional CEHH bleaching to
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study the effect of fungal treatment on bleaching chemical consumption. The strength and
optical properties of the fungal treated and untreated pulps after bleaching were evaluated
to estimate the effect of fungal treatment on the pulp properties.
5.2 Materials and Methods
5.2.1 Bleaching Conditions and Sequences
Kraft pulp of bamboo samples were bleached by the CEHH bleaching sequence.
All the bleaching experiments were carried out in the laboratory, using batch vessels
immersed in water bath at constant temperature. The conditions maintained during
bleaching are given in Table 5.2 and also detailed in text below.
Table 5.2: Conventional Bleaching Conditions for All Bleaching Sequences.
Conditions Stages
C E HI HII
Chemical charges Kappa no.× 0.22 × 0.60 3.5% Kappa no.× 0.22 × 0.24 Kappa no.× 0.22 × 0.16
Pulp consistency% 3 10 10 10
Temperature0C 30 60 45 45
Time (minutes) 30 60 120 120
pH 2.0 10 9 9
Chlorination (C): Chlorine was dissolved in water by bubbling slowly from a chlorine
cylinder. The chlorine water strength was determined by pipetting out 5 ml of the chlorine
water into a 250 ml conical flask containing 10 ml of 10% potassium iodide and 20 ml
distilled water. The top of the pipette was kept under the surface of this solution to prevent
escape of chlorine. The iodine liberated was titrated against 0.2N sodium thiosulfate using
starch indicator.
R×0.2×35.5
Chlorine, g/l =
5
Where, R = volume in ml of thiosulfate used in the titration
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Freshly prepared chlorine water was added to 3% consistency pulps in capped
plastic jar at pH 2. Reaction time maintained was 30 minutes at 30°C. At the end of
reaction, pulps were washed and subjected to alkali extraction. For CEHH sequences total
chemical charge was calculated on the basis of kappa number and 60% of the total chlorine
demand was utilized in the C stage, while 40% of total chlorine demand was divided in
each H stage. The kappa factor was 0.22 for each sample.
Extraction (E): Laboratory grade sodium hydroxide pellets were dissolved in distilled
water. After the solution was cooled down, the clear liquid was decanted and titrated as
given in 4.3.2.1.
Chlorinated pulps were extracted with NaOH (3.5%) at 60°C temperature, 10%
consistency and 60 minutes time. After the end of reaction time, the pulps were washed
using terylene cloth.
Hypo-chlorination (H): The calcium hypochlorite solution was prepared by dissolving
bleaching powder (calcium hypochlorite) in water. 1 kg of bleaching powder was added to
10 liters of water into a plastic bucket, thoroughly agitated, closed with a lid and set aside
for settling, for overnight. The supernatant liquor was filtered through a terylene cloth over
a Buchner funnel using suction. The available chlorine in hypochlorite was determined
iodometrically. 5 ml of hypochlorite solution was pipetted into a 250 ml conical flask
containing 10 ml of 10% acetic acid and titrated against 0.2N thiosulfate using starch as a
indicator.
R×0.2×35.5
Chlorine, g/l =
5
Where, R = volume in ml of thiosulfate used in the titration
The hypochlorite, 1st and 2
nd stages were performed in polyethylene bags at
reaction time 120 minutes, pulp consistency 10%, pH 9 and temperature 45°C. For H stage
40% of total chlorine demand was utilized by dividing again in 60% and 40% ratio in two
stage of hypo-chlorination. After the end of reaction pulps were washed using terylene
cloth.
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5.2.2 Hand Sheet Preparation and Testing
Hand sheets were prepared of both treated and untreated bamboo pulp samples
with the help of laboratory sheet former. The experimental details for preparation and
testing for strength properties of bleached pulp sheets have been discussed in details in
4.3.4.
Optical Properties: The most important properties of fully bleached pulps are the optical
properties, i.e. the light scattering coefficient and the opacity. Opacity characterizes the
ability of paper to hide text or pictures on the back side of the sheet. Brightness is
reflectance of paper using blue light. Blue light is used because papermaking fibres have
a yellowish color and because the human eye perceives blue color as brightness.
“Lorentzen & Wettre – Elrepho” was used to measure optical properties.
The intrinsic reflectance factor measured at an effective wavelength of 457 nm
with a reflectometer conforming to the requirements described in AS/NZS 1301.436s-91 -
Measurement of diffuse reflectance factor and calibrated against ISO reference standards.
Units of brightness are a ratio, expressed as a percentage of the radiation reflected by a
body to that reflected by a standard reflecting diffuser under the same conditions.
5.3 Results and Discussion
Pulps obtained from kraft pulping of treated and untreated bamboo samples (non
destructured and destructured) were bleached by CEHH sequences. The kappa factor was
0.22 for each sample. Bleached pulps obtained from bleaching of NDC, DC, NDT and DT
were used to prepare hand sheets to evaluate the strength and optical properties (Fig. 5.1).
On the basis of pulp kappa number and assuming uniform kappa factor for all the pulp
samples, utilization of bleaching chemicals (as chlorine) was found to reduce 5kg/t in
destructured control (DC), 12kg/t in non destructured treated (NDT), 23kg/t in destructured
treated (DT) samples when compared from non destructured control (NDC) sample.
5.3.1 Strength and Optical Properties of Bleached Pulps
The effect of bleaching on pulp yield, strength and optical properties in each
sample was studied and evaluated for effect of fungal treatment on brightness of pulp.
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Table 5.3 shows the effect of fungal treatment and cooking time on the strength and
optical properties of bamboo bleached pulps.
The properties of bleached pulp sheets from NDC, NDT, DC and DT bamboo
samples were compared to find out the variation in tear index, tensile index, burst index,
brightness and opacity. The tear index is influenced by fiber length, while tensile and
burst index are influenced by both fiber length and extent of hydrogen bonding.
Table 5.3: Strength and Optical Properties of Bleached Treated/Untreated Bamboo
Non destructured and Destructured Pulps Obtained at Various Cooking Time.
S.
No.
Pulp
Samples
Yield% Tensile
Index,
N.m/g
Tear Index,
mNm2/g
Burst
Index,
K.Pa.m2/g
Brightness%
Opacity%
1 NDC 1
NDC 2
NDC 3
NDC 4
45.39
43.93
43.76
42.40
56.98
68.05
70.74
66.78
19.50
19.80
19.96
19.57
3.97
4.91
4.90
4.16
74.02
74.08
74.53
74.76
97.44
98.10
98.19
95.94
2 DC 1
DC 2
DC 3
DC 4
47.45
47.69
45.85
44.18
68.69
67.56
66.73
59.42
10.17
10.69
10.43
10.19
4.10
4.33
4.45
4.31
75.00
75.32
76.43
75.97
93.96
94.42
93.99
94.44
3 NDT 1
NDT 2
NDT 3
NDT 4
44.72
43.45
43.37
38.80
58.50
66.13
69.54
63.72
18.57
18.72
18.23
15.53
4.28
4.97
4.53
4.14
76.64
80.06
79.80
79.97
94.05
98.51
99.21
98.14
4 DT 1
DT 2
DT 3
DT 4
44.88
44.35
43.33
42.30
62.19
61.76
61.67
59.11
6.73
6.45
5.95
5.90
4.64
4.26
4.39
4.05
84.03
85.33
84.28
84.20
92.51
92.75
94.59
94.73
5.3.1.1 Brightness and Opacity
Table 5.3 and Fig. 5.2 show the effect of fungal treatment and cooking time on the
brightness of the finally bleached pulps. Generally brightness value of unbleached pulps
is lower than bleached pulps. The brightness value depends on pulping processes and
pulping conditions because they can reduce and remove lignin content in wood and non-
wood fiber. The residual lignin, remains in unbleached pulp, has major effect on
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brightness value. If a comparison is made between NDC, DC, NDT, DT bleached pulps
brightness was increased in following order.
NDC < DC < NDT < DT
The final brightness of NDC bleached pulps when compared with NDT showed a
different pattern. In case of NDC pulps, brightness increased in pulps cooked from 30
minutes to 120 minutes. The maximum brightness was 74.76% in the bleached pulp
cooked for 120 minutes. Whereas in case of NDT pulp, highest brightness observed was
80.06% in the bleached pulp cooked upto 60 minutes cooking time. At the same time
when the brightness is compared with NDC, the brightness is comparatively 5.98 units
more in NDT.
In case of DC and DT, when brightness values were compared a different pattern
was observed. Brightness values of DC pulps increased upto 90 minutes. After 90
minutes a slight decrease in brightness was observed. In case of DT maximum brightness
was observed in the pulps of 60 minutes cooking time period. However decrease in
brightness was observed from 60 to 120 minutes cooking. The highest observed
brightness was 85.33% in DT pulp of 60 minutes and 76.43% in DC pulp of 90 minutes
cooking.
If a comparison is made between NDC and DT, brightness properties of DT are
higher than NDC. At the optimum cooking time period i.e. 30 minutes, when the
brightness value of NDC is compared with DT, the brightness is comparatively 10.01
units more in DT. The lower kappa number and higher brightness in DT pulp indicates
that fungal treatment makes a chemical change in lignin, which helps remove the lignin
easily in the subsequent bleaching stages (Dube and Kothari, 1983; Kirk and Shimada,
1985; Akhtar et al., 1993).
Table 5.3 and Fig. 5.3 show the effect of fungal treatment and cooking time on the
opacity of the finally bleached pulps. No significant effect of fungal treatment was
observed on opacity of NDC, NDT, DC and DT pulps at all the cooking time periods.
Whereas opacity values were slightly lower in DT pulp sheets compared to NDC pulp
sheets. In case of DT pulp samples, fibers are degraded slightly more than NDC pulp
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samples. This is due to harsh treatment by chemicals during cooking where the treated
fibers are exposed for longer time.
5.3.1.2 Tensile Index
The variations in tensile index for NDC, DC, NDT and DT bleached pulps are
shown in Table 5.3 and Fig. 5.4. When tensile index of NDC compared with NDT
bleached pulp samples for all the four cooking time periods, it showed increase in tensile
index upto 90 minutes cooking. However reduction in tensile properties of both bleached
NDC and NDT pulps were observed after 90 minutes cooking. The maximum tensile
obtained were 70.74 N.m/g in NDC and 69.54 N.m/g in NDT bleached pulp of 90
minutes cooking.
Tensile index of DC when compared with DT show decreasing pattern from
bleached pulp of 30 minutes to 120 minutes cooking. Tensile index decreased from 68.69
N.m/g to 59.42 N.m/g in case of DC and 61.19 N.m/g to 58.11 N.m/g in case of DT with
increasing cooking time.
A comparison between NDC and DT pulps shows 5.21 N.m/g higher tensile in
DT pulp than NDC pulp at optimum cooking time period i.e. 30 minutes.
5.3.1.3 Tear Index
The variation in tear index after bleaching shown in Table 5.3 and Fig. 5.5, for
NDC, NDT, DC and DT pulp samples, is explained below:
Tear index of NDC when compared with NDT did not show much difference in
the pulps cooked from 30 to 120 minutes. In NDC maximum tear index obtained was
19.96 mNm2/g in 90 minutes cooked pulp. Whereas; in case of NDT maximum tear index
was 18.72 mNm2/g for 60 minutes cooked pulp. In case of NDT there was a decrease in
tear index of bleached pulps cooked for 90 and 120 minutes subsequently.
In case of DC and DT, when the tear values of bleached pulps of all cooking time
periods are compared, a different pattern is observed. In DT pulps, highest observed tear
index was 6.73 mNm2/g of 30 minutes pulp. However a drop in tear values was then
observed. If a comparison is made between DC and DT, tear index of DC are higher than
DT. At the same time when the tear index values are compared with corresponding
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brightness, the drop in brightness is comparatively more in DC than DT. The variation in
tear index was due to harsh treatment of bleaching chemical.
Tear index of NDC bleached pulps when compared with DT bleached pulps, of all
the four cooking time periods, show more drop in tear properties. In NDC, maximum tear
index obtained was 19.96 mNm2/g of 90 minutes pulp. However, in case of DT
maximum tear index observed was 6.73 mNm2/g of 30 minutes pulp. The tear index was
lower in DT pulp sheets than NDC pulp sheets.
5.3.1.4 Burst index
Table 5.3 and Fig. 5.6 show the effect of bleaching chemicals on burst index of all
the pulp samples. Burst index of NDC bleached pulp observed at 30 minutes of cooking
time was 3.97 K.Pa.m2/g, whereas this value increased to 4.28 K.Pa.m
2/g in the case of
NDT. In case of NDC the maximum burst index observed was 4.91 K.Pa.m2/g in the
bleached pulp of 60 minutes cooking time periods. However, reduction in burst index
properties was observed in the pulps after 60 minutes. In case of NDT, burst index
increased from 4.28 K.Pa.m2/g to 4.97 K.Pa.m
2/g in 30 to 60 minutes. There is no
significant difference in the burst properties of NDT and NDC bleached pulps. However
brightness of NDT is more than NDC.
In case of DC and DT, when burst values are compared, a different pattern is
observed in all bleached pulps of different cooking time periods. In DC, highest burst
observed was 4.45 K.Pa.m2/g in the pulp of 90 minutes. Whereas in DT, highest burst
observed was 4.64 K.Pa.m2/g in the pulp of 30 minutes. However a drop in burst values
was then observed in both the pulp samples. This is mainly due to exposure of the fibers
to bleaching chemicals for longer time. When a comparison is made in between NDC and
DT bleached pulps of 30 minutes cooking time period, less burst value is observed in
NDC than DT. Whereas other burst values are more in NDC than DT bleached pulp. This
is due to harsh treatment of bleaching chemical.
5.4 Conclusion
To achieve the desired brightness, all pulp samples extracted at four different
cooking times i.e. 30 minutes, 60 minutes, 90 minutes and 120 minutes, were bleached
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using CEHH sequences. Bleaching chemical charge was calculated on the basis of kappa
number and kappa factor. The Kappa factor for each pulp sample was 0.22. On the basis of
kappa number at same kappa factor it was observed that saving of bleaching chemical was
5kg/t in destructured control (DC), 12kg/t in non destructured treated (NDT), 23kg/t in
destructured treated (DT), when compared from non destructured control (NDC). Although
conventional bleaching of chemical pulp using chlorine is the most economic one, it is not
possible to achieve higher brightness without sacrificing pulp strength. Hypochlorite
extensively degrades the cellulose. With conventional CEHH sequences, the target
brightness is achieved at the cost of drastic reduction in strength properties (Ates et al.,
2008). The investigation has shown that brightness of destructured treated samples was
10.01% ISO points more than non destructured control pulp. Fungal treatment makes a
chemical change in lignin, which helps to remove the lignin easily in the subsequent
bleaching stages. Opacity of all pulp samples was more than 90% and there was a little
effect of fungal treatment on opacity observed. The strength properties of the pulp samples
did not show much difference among them. If a comparison is made between strength
properties of unbleached sheets with bleached sheets, tensile, tear and burst properties
shows 8-10% reduction due to harsh chemical treatments given for bleaching purpose.
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(a) (b)
d
(c) (d)
Fig. 5.1: Bleached Pulp Sheets of (a) NDC, (b) DC, (c) NDT and (d) DT.
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Fig. 5.2: Brightness of Bleached NDC, DC, NDT and DT Pulps in Relation to
Cooking Time (Minutes).
Fig. 5.3: Opacity of Bleached NDC, DC, NDT and DT Pulps in Relation to Cooking
Time (Minutes).
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Fig. 5.4: Tensile Index of Bleached NDC, DC, NDT and DT Pulps in Relation to
Cooking Time (Minutes).
Fig. 5.5: Tear Index of Bleached NDC, DC, NDT and DT Pulps in Relation to
Cooking Time (Minutes).
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Fig. 5.6: Brust Index of Bleached NDC, DC, NDT and DT Pulps in Relation to
Cooking Time (Minutes).
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