CHAPTER 4 Kraft Pulping and Characterization of Strength ...shodhganga.inflibnet.ac.in/bitstream/10603/6212/9/09_chapter 4.pdf · 124 CHAPTER 4 Kraft Pulping and Characterization

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CHAPTER

4

Kraft Pulping and Characterization of Strength Properties of

Fungal Treated and Untreated Bamboo Pulps

4.1 Introduction

Kraft pulping is one of the major chemical pulping processes used for production

of pulp from complex wood based raw materials (Grace and Malcolm, 1989;

Gullichsen and Fogetholm, 2000). In order to make paper from wood or annual plants,

the fibers are released either mechanically or chemically (Sjostrom and Westermark,

1993; Biermann, 1996) and among all pulping methods, kraft pulping is the most widely

used process. About 95% chemical pulp worldwide is produced using this method (FAO,

1998; Sixta, 2006; Sjodahl, 2006). This process depends largely on chemical

components, chemical concentration, chemical charge, chip cleanliness, dimensions, time

of cooking, temperature, pressure and proper combination of chemicals and energy is

vital for uniform cooking (Smook, 1992). Global environmental concerns for abatement

of pollution from pulp and paper mills have resulted in exploration of alternative methods

leading to better delignification by using environmentally compatible chemicals and

micro-organisms. Biological processes offers potential opportunities for making the

industry more environment friendly as removal of lignin can be achieved by use of white

rot fungi to degrade the complex recalcitrant lignin molecules in the lignocellulosic raw

materials. White rot fungi alter the lignin polymers in the cell walls of the wood, which

therefore "softens" the wood chips (Hunt et al., 2004; Talaeipour et al., 2010; Kasmani

et al., 2011). The main challenge in the biological delignification is long duration of

fungal development on the wood substrates and less delignification. The reason ascribed

for slow growth of fungi is that the fungal hyphae can not penetrate to the core of chips

and only surface phenomenon occurs during fungal treatment stage. When the chips are

exposed for longer duration it has been observed that cellulose degradation starts, which

is required for growth of the fungi (Krik et al., 1980; Zodrazil and Brunnert, 1981;

Odier and Monties, 1983). Therefore for growth of the fungi without significant

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cellulose/ hemicellulose degradation, it is necessary that proper substrate modification

should be made to grow fungi within minimum time periods. In view of this bamboo

chips were mechanically destructured for opening the fiber structure, thereby increasing

the surface area and reducing the density. The bamboo chips subjected to mechanical

destructuring were treated with white rot fungi for effective biodelignification and these

pretreated samples were subsequently cooked for different time periods to find out the

effect of biodelignification on strength properties of the pulp. The present chapter is

focused on the chemical kraft pulping process of fungal treated bamboo destructured and

non destructured samples.

4.1.1 Kraft Pulping

C.F. Dahl is usually credited with the development of the kraft process in 1879.

He first substituted the make-up chemical, sodium carbonate, often used to replace the

alkali consumed in the soda cooking, by sodium sulphate. In 1884, Dahl obtained a

patent for this process. It was found that the addition of sulphide (obtained in the

recovery boiler from the sulphate) to the cook accelerated the delignification and gave a

stronger pulp than the soda process. The active chemicals in the kraft process are

hydroxide and hydrogen sulfide ions in the form of NaOH and Na2S, also known as white

liquor. White liquor is reacted with the raw materials in a large pressure vessel called a

digester. The white liquor and the chips are heated to a cooking temperature of about

1600C and are allowed to cook at that temperature for about two hours. This cooking

temperature and time is usually maintained for hardwood fibers. For non wood fibers,

usually lower temperature and cooking time is required. During the pulping, the

hydroxide and hydrosulfide anions react with the lignin, causing the polymer to fragment

into smaller water/alkali-soluble fragments (Hamidi, 2006).

4.1.2 Kraft Pulping Chemistry

During kraft cooking, the lignin, which essentially glues the fibers together, is

degraded by the active cooking species, the hydroxide and hydrogen sulphide ions. The

degradation products are dissolved and the fibers are separated from each other. The de-

polymerization of lignin in an alkaline process is mainly due to the cleavage of β-aryl

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ether linkages. The cleavage of the β-O-4 linkages is increased by reactions involving

hydrogen sulphide ions, and this in turn increases the rate of delignification in the kraft

cook. After degradation, the lignin is dissolved in the cooking liquor. Depending on the

sodium hydroxide ion concentration and on other parameters affecting the solubility, the

lignin can precipitate back onto the fibers. Alkalinity is the most important driving force

for kraft pulping reaction and large amount of chemical is needed to carry the cooking

reaction to completion (Christensen, 1991). Sulphidity enhances the rate of

delignification without affecting the carbohydrates. The initial pH lies between 13 to 14

due to sodium hydroxide, which decreases with time as the organic acids are liberated

from wood during cooking (Islam, 2004).

The delignification process during kraft cooking is known to be divided into three

dominating phases (Fig. 4.2) in the production of pulp for bleached grades. These phases

are known as the initial, bulk and residual delignification phases (Clayton, 1963; Olm

and Tistad, 1979; Wilder and Daleski, 1965; Kleinert, 1966; LeMon and Teder,

1973). Mainly low molecular weight lignin dissolves during the initial phase. As the kraft

cook continues, lignin fractions of higher molecular weight are dissolved (Gellerstedt et

al., 1984; Soderhjelm, 1986). According to the studies of Wilder and Daleski (1965)

23% of the lignin in the wood chip is removed during the first five minutes of pulping at

1500C thereafter the delignification slows down remarkably (Chakar and Ragauskas,

2004). Most of the delignification process, accounting for about 70% of the lignin, occurs

during the slower bulk phase. The residual phase, which is less selective towards lignin

degradation, starts to dominate after about 90-95% delignification. The further pulping

could result in significant degradation of carbohydrates. The remaining or residual lignin,

typically 4–5% (by weight) at the end of a conventional softwood kraft cook, is removed via

bleaching techniques. It has been suggested that the poor selectivity in the residual

delignification phase may be attributed to: (i) the low reactivity of the residual lignin towards

the pulping chemicals results in more resistance to delignification; (ii) the residual lignin

being chemically linked to carbohydrates is resistant to delignification (Hamidi, 2006).

The carbohydrate degradation is, however, better divided into two different

phases (Lindgren, 1997) involving peeling, chain cleavage and dissolution of short

carbohydrate chains. A significant part, 40-70% of the hemicelluloses in softwood mainly

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glucomannan is dissolved during the first phase (Aurell and Hartler, 1965). The

removal of glucomannan is independent of the alkali concentration while the degradation

of xylan increases by 50% when the hydroxide concentration is increased from 1.0 to 1.5

mol/dm3 (Aurell, 1963). The selectivity of the kraft cook, i.e. the carbohydrate yield at a

given lignin content in the pulp, decreases when the slow residual phase delignification

starts to dominate, since the carbohydrate degradation rate is the same as when the faster

bulk delignification phase is dominating.

4.1.3 Black Liquor

Black liquor, the spent pulping liquor, is a dark viscous liquid that is separated

from the pulp after the cooking process. Black liquor contains dissolved lignin and other

alkali soluble organic matters as well as inorganic components of white liquor. Black

liquor is concentrated and incinerated in a specially designed recovery boiler to recover

cooking chemicals and generate steam from combustion of organic matters. Modern

recovery boilers produce steam at high pressure (45 to 60 kg/cm2) and this steam is used

to generate electricity by passing through steam turbines (Testova, 2006). The steam

extracted from the turbines at lower pressures (10 kg/cm2) is used to meet the process

heating requirements in pulp mill. A modern pulp mill is self sufficient in terms of the

steam and power needs provided it has an efficient chemical recovery island. The physic-

chemical properties of black liquor are very important from the economical point of view

of the mill. The inorganic and organic constituents present in the black liquor determine

suitability of the pulping method for further processing in a recovery island. The changes

in composition of the raw material after fungal treatment can significantly change the

characteristics of the black liquor, therefore it is necessary that evaluation of black liquor

characteristics should be studied to find out the effect of fungal pretreatment on various

black liquor properties.

4.2 Pulping After Biological Pretreatment: Related Literature Review

A large number of research group have carried out studies on pulping and

bleaching of fungal treated lignocellulosic raw materials. A systematic literature search

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was carried out to review various studies carried out in this field and a brief review of

literature is presented below.

Akamatsu et al. (1984) demonstrated that poplar chips treated with 10 species of

fungi reduced refining energy during thermomechanical pulping: three fungi (Trametes

sanguinea, T. coccinea, and Coriolous hirsutus) improved breaking length and tear

strength properties.

Akhtar et al. (1992a) have shown that fungal treatment of aspen and loblolly pine

with Ceriporiopsis subvermispora prior to refiner mechanical pulping shows promising

results in terms of electrical energy saving, improving paper strength properties and

reducing the environmental impact of pulping. They found that aspen wood required 47%

less energy. Burst and tear indices increased by 22% and 19% respectively. With loblolly

pine, energy saving amounted to 37%, burst and tear indices increased by 41% and 45%

respectively. Fungal treatment caused 6% weight loss in aspen and 5% in loblolly pine.

Beth wall et al. (1993) found that pretreatment of wood with white rot fungus,

Phanerochaete chrysosporium could successfully biopulp softwood, save 33% energy

and improved 39% tear index. Subsequently, Pilon et al. (1982) reported an increase in

paper strength properties when refiner mechanical pulp was treated with Polyporus

versicolor.

In a study by Akhtar et al. (1992b) loblolly pine chips were treated with five

different strains of the white-rot fungus Ceriporiopsis subvermispora (FP-90031-sp, FP-

105752, L-14807, L-15225, and FP-104027) for four weeks prior to refiner mechanical

pulping. Weight loss of the chips during fungal treatment ranged from 4% to 7%. The

electricity consumed during fiberizing and refining of the treated chips was 21-37% less

than control chips. All five fungal strains improved burst (33-46%) and tear indices (47-

60%) over the untreated control. Fungal treatment had no effect on tensile properties.

Hand sheets prepared from treated pulps had lower brightness and light scattering

coefficient. Treatment had no apparent effect on opacity. Based on energy savings and

improvement in strength properties, strain FP-90031-sp appeared to be superior to the

other strains.

Pretreatment of aspen wood chips with three different strains of the white-rot

fungus Ceriporiopsis subvermispora (FP-90031-sp, L-6133-sp, and CZ-3) for four weeks

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prior to refiner mechanical pulping saved electrical energy (40-48%) and improved burst

(23-40 %), tear (13l–l62%), tensile (17–27%), and tensile energy absorption (13–25%)

indices. All strains decreased density (12–14%), brightness (18–21%), and light

scattering coefficient (34–37%) over the untreated control. Based on energy savings and

strength improvements, strain CZ-3 appeared to be superior to the other (Akhtar, 1994).

Two strain of Ceriporiopsis subvermispora (CZ-3 and SS-3) were used for

treatment of loblolly pine chips prior to sodium and calcium based sulfite pulping. Scott

et al. (1998) found that during sodium-bisulfite pulping, the fungal pretreatments reduced

the kappa number by 27% with slightly lower pulp yield compared to the control.

However during calcium based sulfite pulping, strains CZ-3 and SS-3 reduced kappa

number by 48% and 21% but had the same pulp yield as that of the control.

Mosai et al. (1999) has shown that among 10 fungal species and strains,

Ceriporiopsis subvermispora SS-3 was found most promising for studies of biosulfite

pulping of E. grandis. After 10 days treatment, Ceriporiopsis subvermispora SS-3

decreased kappa number by 29% and increased brightness by 12% with 5% lost of pulp

yield. When the inoculation time was reduced to 5 days, 5% reduction of kappa number,

11% increase in brightness and no loss of pulp yield were observed.

Sabharwal et al. (1996) investigated that Ceriporiopsis subvermispora saves

electrical energy (8-10%) during biomechanical pulping and also improves strength

properties in non woody plant such as jute and kenef.

Biomechanical pulping of loblolly and red pine logs pretreated with Phlebiopsis

gigantea were investigated by Blanchette et al. (1998) and Behrendt et al. (2000).

Results showed that P. gigantea was able to colonize 90 to 100% of the freshly cut logs

after 8 weeks. Up to a 59% decrease in resinous wood extractives was observed in

loblolly and red pine logs inoculated with P. gigantea as compared to non-inoculated

logs. Simon’s staining was used to evaluate cell wall changes in mechanically refined

pulp fibers during biological pulping processes, and showed 55 to 77% of the fibers from

treated logs stained, while 25 to 58% of the fibers from aged control logs stained. Refined

wood from inoculated logs required less energy (9 to 27%) to reach a freeness of 100

CSF (control). Pretreatment of red pine logs with P. gigantea also resulted in a 17%,

20%, and 13% increase in burst, tear, and tensile strength properties respectively.

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Suitability of 278 strains of South African wood decay fungi for the pretreatment

of softwood chips for kraft pulping was assessed on the basis of kappa number, yield and

strength properties of pulp. A number of these strains were more efficient in reducing

kappa number than the frequently used strains of Phanerochaete chrysosporium, and

Ceriporiopsis subvermispora. Six strains of Stereum hirsutum and a strain of an

unidentified species were able to reduce the kappa number significantly without a

significant influence on the pulp yield (Wolfaardt et al., 2004).

The soda–AQ pulps were made from the reed by Fu et al. (2004) after

pretreatment by white rot fungi Panus conchatus, Cyathus stercoreus and Pleurotus

florida respectively. The results showed that kappa number of the pulps decreased from

11% and 24%, the brightness increased from 10% and 33% and the viscosity increased

from 8% and 5% respectively by Pleurotus florida and Panus conchatus. The alkali

consumptions were decreased substantially and pulp yields were increased by 6% and

30% respectively. But it was reversed for the pulp from reed treated by Cyathus stercoreus.

Cyathus stercoreus was preferential to degrade cellulose, which was not good for

biopulping.

Scott et al. (1995) examined the effect of fungal pretreatment prior to sodium and

calcium based sulfite pulping. The pretreatment involved a 2-week incubation of the

chips with two strains of Ceriporiopsis subvermispora (CZ-3 and SS-3). The results from

this experiment indicated that during sodium bisulfite pulping kappa number reduced

27% with slightly lower pulp yield but during calcium based sulfite pulping strains CZ-3

and SS-3 reduced the kappa number by 48% and 21%.

Young et al. (2003) treated poplar wood chips in a rotary bioreactor for 10 days

with the white-rot fungus Phanerochaete chrysosporium KCCM 34740 prior to kraft

cooking. It was observed that fungal pretreatment increased pulp yield and reduced the

kappa number of the hardwood pulps. Additionally, lower initial pulp freenesses were

observed. They also observed that the energy required for beating (to achieve target

freeness) was significantly lower in fungal pretreated wood, without any deleterious

influence on paper quality. These findings clearly suggested that fungal pretreatment of

hardwood chips with P. chrysosporium KCCM 34740 could reduce the chemical loading

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and energy consumption during kraft pulping, and concurrently reduce the energy

required during pulp refining.

Alkaline sulfite/anthraquinone pulping of pine wood chips biotreated with

Ceriporiopsis subvermispora was studied by Mendonça et al. (2004). Pine wood chips

were treated for 30 days with Cemporiopsis subvermispora in bioreactors. The biotreated

wood chips and undecayed controls were subjected to modified alkaline

sulfite/anthraquinone (ASA) cooking at 170°C or 175°C applying varying cooking times

ranging from 30 to 270 min. In all cases, the residual lignin content of the pulps prepared

from biologically pretreated wood chips was lower than that of the control pulp. With

increasing cooking time, however, the differences in kappa number became smaller.

Wood chips cooked for a short time required mechanical refining for fiber liberation. A

disk-refining step resulted in pulps with low reject content (0.4%) and high screened

yield (56-60%). In this case, the use of biotreated wood chips provided pulps with

significantly lower kappa numbers than for the control pulp (71 and 83, respectively).The

pulp from biologically pretreated wood fibrillated rapidly, reaching 20°SR in only 38 min

beating time in a Jokro mill, while the control pulp required 56 min. to reach the same

beating degree. Although easier to beat, the biopretreated pulps showed tensile and burst

indices similar to those of the control samples. However, their tear indices were always

lower. Easier delignification after wood biotreatment was not observed for the reactions

performed at long cooking times. Oxygen delignification of biotreated and conventional

ASA pulps with low kappa numbers reduced kappa number and improved brightness

considerably with the biotreated pulps being favored by a better preservation of viscosity.

Bio-modification of eucalyptus chemithermomechanical pulp with different white

rot fungi was investigated by Yang et al. (2007). In this work eucalyptus CTMP was

treated with three different types of white rot fungi Phanerochaete chrysosporium (P.c-

1767), Trametes hirsute 19-6 (T.h-19-6) and Trametes hirsute 19-6w (T.h-19-6w), under

a stationary culture condition. Pulp total weight loss, lignin loss, cellulose loss were

determined to compare the different enzymes secreted by the three fungal strains. Pulp

physical strengths, optical properties and bleachability after the fungal treatment were

investigated to compare the effect of fungal treatment on the pulp quality improvement.

The results have shown that lignin reduction by both T.h-19-6 and T.h-19-6w was about

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twice as much as that by P.c-1767. However, the selectivity of T.h-19-6w towards lignin

over cellulose was only 0.82, while that of T.h-19-6 was as high as 4.43. After Trametes

hirsute 19-6 treatment, pulp tensile, tear and internal bonding strength increased by about

27%, 38% and 40% respectively.

Myers et al. (2000)

biopulped small-diameter softwood and aspen, for

comparative purposes, using wild-type isolates of Ceriporiopsis subvermispora and

Phanerochaete chrysosporium and a genetically altered strain of Phanerochaete

chrysosporium. Results showed that refiner mechanical pulping of pine and aspen chips

treated with wild-type or genetically modified strains generally saved electrical energy,

improved pulp properties, and improved paper strength properties. Genetic manipulation

of P. chrysosporium substantially enhanced biopulping performance relative to the

parental wild-type strain.

In a study by Sabharwal et al. (1996) untreated and fungal treated jute and kenaf

bast was studied for the production of mechanical pulp. They determined energy

consumption at each stage of refining under various freeness levels. The investigation

showed 8-10% considerable energy saving and improved strength properties compare to

refining mechanical pulping.

Idarraga et al. (2001) reported that white rot fungi improved 22-66% mechanical

properties of biologically pretreated sisal and reduced 39% energy consumption.

Bajpai et al. (2004) studied the pretreatment of wheat straw with lignin-degrading

fungi and its effect on chemical pulping. Ceriporiopsis subvermispora strains, which

preferentially attack the lignin, were used for biochemical pulping of bagasse. Treatment

of depithed bagasse with different strains of C. subvermispora reduced the kappa number

by 10-15% and increased unbleached pulp brightness by 1.1-2.0 ISO points on chemical

pulping at the same alkali charge. Bleaching of biopulps at the same chemical charge

increased final brightness by 4.7-5.6 ISO points and whiteness by 10.2-11.4 ISO points.

Fungal treatment did not result in any adverse effect on the strength properties of pulp.

Agricultural residues such as rice, wheat and barley straw were pretreated with

white rot fungi, Ceriporiopsis subvermispora prior to biochemical pulping by Yaghoubi

et al. (2008). Biological treatment of rice, wheat and barley straw samples resulted in

decrease of the kappa number of these straws by 34%, 21% and 19%, respectively, as

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compared with control samples. The tensile strength and burst factor of hand sheets

produced from rice straw were increased by 51% and 33% as compared with the control

straws. The tensile strength and burst factor of hand sheets produced from wheat straws

were improved by 67% and 36%, these variables for barely straws were 36.7% and 45%,

respectively. They said, although the delignification of wheat and barley straws were not

as efficient as chemical process, but the quality of papers produced by biochemical

pulping of straws were excellent.

Many research groups around the world have studied white rot fungi for

biological delignification in the pulp and paper industry. Studies have been shown that

pretreatment of wood chips with lignin degrading fungi ameliorates some of the problems

associated with pulping processes. A proper penetration or diffusion of alkali into wood

chips is essential for all alkaline pulping processes. The diffusion of alkali in the wood

components results in swelling and softening of the wood and subsequently the chemical

reaction of alkali. Traditionally, in alkaline chemical pulping impregnation is conducted

at temperatures above 1000C. If the time of impregnation is short, this leads to mostly

unreacted chips. In the present chapter impregnation with fungus and variation of the

cooking time has been presented to study the effect in terms of some parameters viz.

kappa number, pulp yield, energy requirement chemical consumption to indicate the

effectiveness of fungal pretreatment and subsequent pulping on pulp properties.

4.3 Materials and Methods

Production of pulp from chemical pulping process generally utilizes conventional

ways to digest raw material under specified chemical charge, bath ratio, time and

temperature. In the present study the bamboo (Dendrocalamus strictus) samples were

biologically pretreated with and without destructuring the chips, prior to chemical

pulping process. The details of the materials, procedures and test methods used are

discussed below. Fig. 4.1 shows the overall cooking procedure with instruments.

4.3.1 Preparation of Biologically Pretreated Samples

As discussed in the preceding chapter, better delignification with consideration of

minimum cellulose loss, was found with T. versicolor in comparison to S. commune and

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F. flavous. Therefore bamboo destructured and non destructured samples were pretreated

with Trametes versicolor at optimum conditions for determining the subsequent pulping

parameters. Treatment of the samples was performed in conical flasks (1000 ml) for both

the samples. Each conical flask contained 50 g (O.D. basis) sample either destructured or

non destructured with 60% moisture level in non destructured and 80% moisture level in

destructured sample, 4% molasses concentration with pH 5.5 was added to both the raw

materials and mixed well to obtain uniform samples. Each flask was sterilized by

autoclave at 1210C for 20 min. After sterilization samples were inoculated with a

mycelium suspension of Trametes versicolor. The rate of mycelium application was same

as before i.e. 0.003 g (O.D. basis). After receiving inocula, the conical flasks were mixed

well for uniform inoculation. Each flask with sufficient aeration was placed in an

incubator at 250C for 21 Days. Thus all the incubation conditions were kept, as optimized

earlier. Untreated samples were taken as control.

4.3.2 Cooking Liquor Preparation (White Liquor)

The cooking liquor was prepared in the laboratory from NaOH and industrial

grade Na2S. The detailed method of preparation of NaOH and Na2S solutions is presented

below.

4.3.2.1 Preparation and Analysis of Sodium Hydroxide Solution

Laboratory grade sodium hydroxide pellets were dissolved in distilled water.

After the solution cooled down, the clear liquid was decanted and titrated as briefed

below:

10 ml solution of NaOH was pipetted into a 250 ml conical flask. 20 ml 10%

BaCl2 solution was added to it and total volume was made up to 250 ml by distilled

water. After the precipitate had settled, 50 ml of the solution was pipette out and titrated

against standard 1N HCl using phenolphthalein as indicator. Consumption of HCl was

noted.

HCl consumption (ml) x N x 31

Concentration of NaOH as Na2O (g/l) =

Sample Taken (ml)

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4.3.2.2 Preparation and Analysis of Sodium Sulfide Solution

Laboratory grade sodium sulfide (Na2S) was dissolved in distilled water with

continuous stirring. After the solution cooled down, the clear liquid was decanted and

titrated as follows:

10 ml of sodium sulfide solution was made up to 250 ml in a volumetric flask. 10

ml of this solution was pipetted into a conical flask containing 20 ml of 0.1N iodine

solution and 20 ml of 2N H2SO4 and was titrated with standard 0.2N Na2S2O3 solution.

The volume of Na2S2O3 consumed was noted (x ml). A blank titration with 10 ml of

iodine and 20 ml of 2N H2SO4 solution was also carried out. The volume of Na2S2O3

solution consumed for 10 ml of iodine solution was also noted (y ml).

Y ml Na2S2O3 = 10 ml I2

10 X

X ml Na2S2O3 =

Y

(20 – 10 X)

I2 consumed by Na2S =

Y

(20 – 10 X) × 2.5 × 3.1 × N

Concentration of Na2S (as Na2O) g/l =

Y

Where N = Normality of the iodine solution

4.3.2.3 Preparation and Analysis of White Liquor

In the laboratory the cooking liquor or white liquor is usually prepared by

dissolving separately calculated amount of sodium hydroxide and sodium sulfide in

water, and then mixing the two in appropriate amount to get a white liquor of the desired

sulfidity. The sulfidity of the liquor is usually kept around 25%.

The white liquor was thus prepared from the solution prepared as A and B.

Volumes equivalent to 750 g of NaOH as Na2O and 250 g of Na2S as Na2O were mixed

together and the total volume made up to 10 liters. This solution was stored in a plastic

container. The white liquor was analyzed as follows:

25 ml of the white liquor was pipetted into a 250 ml volumetric flask. 70 ml 20%

BaCl2 solution was added to it and total volume was made up to 250 ml by distilled

water. After the precipitate had settled, 50 ml of the solution was pipetted and titrated

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against standard 1N HCl using phenolphthalein as indicator. Consumption of HCl was

noted (A ml). A few drops of methyl orange were then added to the solution and the

titration continued till the red color appeared. The consumption of HCl was noted (B ml).

Total active alkali as Na2O g/l = B × 6.2

Na2S as Na2O g/l = 2 (B - A) × 6.2

2 (B - A) × 100

Sulfidity % =

B

4.3.3 Cooking Process and Conditions

The treated and untreated bamboo non destructured and destructured samples

were cooked by kraft pulping process in a laboratory digester consisting of six autoclaves

rotating in an electrically heated poly-ethylene glycol (PEG) bath. Glycol bath is a large

vessel with an electric heater, glycol circulation pump and a rotating shaft. The shaft has

two vertical disks with special housings for the autoclaves, which are installed at an angle

of approximately 25°C to the shaft. All six autoclaves can be run simultaneously. The

bath can be charged with a lesser number of the autoclaves, but balance should be

maintained. When the shaft is rotating the autoclaves are immersed into glycol in turns.

At the same time mixing occurs inside the autoclaves. The glycol bath has a range of

working temperatures between 80°C and 170° C.

Before cooking the moisture contents of the treated and untreated bamboo

samples (non destructured and destructured) were carefully determined using a

representative samples. A known weight of bamboo samples (200g O.D.) was charged in

each autoclave with appropriate amount of white liquor of 25% sulfidity and 16% active

alkalinity at a liquid to raw material ratio of 4:1. The autoclaves were then tightly closed

and placed into the heated glycol bath and the rotation was started. Calculation of H-

factor was started when the content of autoclaves heated up. The schedule of digester

heating consisted of 30 minutes for heating from ambient temperature to 100°C, 90

minutes for heating from 100°C to 160°C. The cooking time at 160°C was 30 minutes, 60

minutes, 90 minutes and 120 minutes respectively. Table 4.1 shows the cooking

conditions of treated/untreated non destructured and destructured bamboo samples.

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Table 4.1: Cooking Conditions for Treated/Untreated Non Destructured and

Destructured Bamboo Samples.

Cooking Conditions

Amount of samples (O.D.) 200 g

Liquor to samples ratio 4:1

Cooking temperature 1600C

Cooking time from 1000C to 160

0C 90 minutes

Cooking time at 1600C 30 minutes, 60 minutes, 90 minutes, 120 minutes

Sulfidity, (%) 25

Active alkali, (%) 16

4.3.3.1 Washing

After each cooking, the autoclaves were cooled by spraying cold water on

autoclaves for 5 min. Black liquor was collected from the autoclaves and pulps were

taken out and washed in a cylindrical vessel with wire at the bottom. Washing was

carried out with warm water and followed by mechanical disintegrator to disintegrate the

pulp samples. The washing process continued until the color of the water remained

unchanged. After washing the pulps were centrifuged until water came out and then

homogenized. Dry matter contents were determined and the total yield of pulps was

determined.

4.3.3.2 Pulp Yield Determination

The pulp yield was determined by weighing the wet pulps (W) and taking two

representative samples each of 50 g for determining the dryness. The samples were

weighed and dried in an oven at 105°C for overnight. The dried samples were weighed

and the yield was determined as follow:

A × 100

Dryness % =

B

W × Dryness of pulp

Total pulp yield % =

O.D. weight of sample taken

A = O.D. weight of pulp taken for dryness

B = A.D. weight of the pulp taken for dryness (50 g)

W = Total A.D. weight of the pulp.

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4.3.3.3 Screening

After calculating yield, pulps from each experiment were disintegrated for 10 min

before screening. Pulps were screened on flat 0.20 mm slotted screen, in order to separate

the undesired materials from the pulps. Pulps were centrifuged and homogenized. The

rejects were collected to determine the percentage of pulps reject and screened pulps

yield.

O.D. weight of reject × 100

Rejects % =

Amount of raw material cooked (O.D. Basis)

4.3.3.4 Kappa Number

The kappa number is defined as the number of milliliters of 0.1 N KMn04 solution

consumed by per gram of moisture free pulp under standardized conditions. It shows the

degree of delignification of pulp and is proportional to the lignin content of pulp. The

kappa number of pulps was measured by using TAPPI standard method T236 cm-76. The

method is briefly described below.

10 g of each pulp (dry weight) was mixed and made into a pad by filtering on a

Buchner funnel, avoiding any loss of fibers. The pads were air dried and torn into small

pieces. Small torn pieces from the samples were weighed to 2.0 g. The test specimen was

disintegrated in 500 ml or less of distilled water until free of fiber bundles. The

disintegrated test specimen was transferred to a 2000 ml reaction beaker with proper

rinsing adding enough distilled water to bring the total volume to 700 ml. The

temperature of distilled water was 25.0±0.2°C. 100 ml potassium permanganate solution

(0.1N) and 100 ml of the sulfuric acid solution (4N) was pipetted in a 250 ml beaker. The

mixture was brought to 25°C quickly and was added immediately to the disintegrated test

specimen, simultaneously started a stopwatch. The beaker was rinsed using not more than

5 ml of distilled water and added to the reaction mixture. The total volume was made to

1000ml.

At the end of exactly 10 min., the reaction was stopped by adding 20 ml of the

potassium iodide solution (1.0N) from a graduated cylinder and immediately after

mixing, the free iodine was titrated with the sodium thiosulfate solution (0.2N), adding a

139

few drops of the starch indicator towards the end of reaction. A blank determination was

carried out using exactly the same method as above without taking any pulp sample.

p × f

K = and

w

(b-a) N

p =

0.1

K = Kappa number

f = Factor for correction to a 50% permanganate consumption, dependent on the value of

p.

w = Weight of moisture free pulp, (g)

p = Amount of 0.1N permanganate actually consumed by the specimen, (ml)

b = Amount of the thiosulfate consumed in blank titration (without adding pulp), (ml)

a = Amount of the thiosulfate consumed by the test specimen, (ml)

N = Normality of the thiosulfate solution.

4.3.4 Hand Sheets Preparation and Testing

Hand sheets were prepared of both treated and untreated bamboo pulps with the

help of laboratory sheet former. The following steps were conducted for preparation and

testing of pulp sheets.

4.3.4.1 Pulp Beating

Beating of pulps was performed in a PFI mill. The advantage of this kind of

equipment is the small amount of pulp needed for the beating procedure. PFI mill

consists principally of a roll with bars, housing with smooth surface and a loading device

to provide the pressure. Both roll and housing rotate in the same direction, but at different

peripheral speeds, beating is thus provided by the shear force. The pulps in amount of 30

g and concentration of 10% were placed in the gap between the roll and the housing and

were spread evenly on the wall of the housing. The number of the revolutions of the PFI

mill were counted and used to control the process. After the 5000 revolutions the beating

was discontinued by removing the beating pressure. The motors were switched off and

140

the roll and beater housing were stopped. Cover and the roll were removed. The stocks

were transferred into measuring cylinder of at least 2 liters in volume. The stocks were

diluted with water to 2000 ml. The pulps suspensions were homogenized by

disintegrating it at 10000 revolutions in the disintegrator.

4.3.4.2 Determination of Drainability

Drainability or beating degree of pulp can be determined using Canadian Standard

Freeness Method (CSF). A CSF tester is a vessel with a drainage chamber, screen plate

and rate measuring funnel. Drainage chamber is a metal cylinder. The upper end of this

cylinder is closed by similar lid, attached to the shelf bracket in which the cylinder is held

when in use. The hinge and latching mechanisms provides an air tight closure by means

of a rubber gasket on the inside of lid. An air cock in the center of upper lid allows the

admission of air into the cylinder. The rate measuring funnel has two outlets. One of

them is placed axially directly in the bottom, while the other outlet is shifted to a side.

The suspensions of 3 g O.D. pulps with 1000 ml water were let to drain through

the drainage chamber. The volumes of water that leaves the vessel through the side outlet

were measured. The higher drainage rate, the more water leaves the vessel through the

side outlet, the lower the value of beating degree.

4.3.4.3 Sheet Preparation

Laboratory sheets were prepared in a laboratory sheet former, where pulp

suspension was drained through a wire. The sheet former was connected to a vacuum

system to provide better water removal. After formation the sheets were separated from

the wire and pressed with dry blotter on one side of a sheet and smooth metal plate on the

other with pressure of 410±10 kPa for totally 7 minutes in two stages. First stage pressing

was completed in 5 minutes and second stage pressing was completed in 2 minutes after

pressing the sheets were dried on the metal plates at room temperature for a day. The

target basis weight of the sheets was chosen to be 60 g/m2.

The hand sheets of each condition were measured for strength properties such as

basis weight or grammage, tensile index, tear index, and burst index.

141

4.3.4.4 Sheet Testing

Tear Strength: The tear strength of the sheets was determined using L &W Tear tester

(ISO 1924-2; 1994(E)) to measures the internal tearing resistance of paper sheets. The

results were reported as tear index obtained by dividing the tearing resistance measured

in units of millinewtons (mN) by the grammage of the paper in units of grams per square

meter (g/m2).

Tensile Strength: The tensile strength of the sheets was determined using L &W Tensile

tester (ISO 1974; 1990(E)).

Bursting Strength: The bursting strength of the sheets was determined using Bursting

strength tester (ISO 2758; 2001(E)). As it is more meaningful to compare bursting

strength of papers of differing grammages in terms of burst index, therefore burst index

was obtained by dividing the bursting strength in kPa by the grammage of the paper in

g/m2.

4.3.5 Black Liquor Analysis

4.3.5.1 Residual Active Alkali

It is the measure of free NaOH + Na2S content remaining in the black liquor after

pulping. Certain level of residual active alkali is essential to keep the black liquor

colloidally stable. Liquor with low levels of RAA may precipitate at low solid

concentration during evaporation, which eventually will increase the black liquor

viscosity.

25 ml of black liquor were pipetted into 250 ml volumetric flasks. 20 ml of 20%

BaCl2 were added and stirred; the contents of flasks were diluted to the mark, mixed well

and allowed to settle. 50 ml of the supernatant liquid from each volumetric flask was

pipetted to 150 ml beaker equipped with magnetic stirrer. 5 ml of 40% neutral

formaldehyde was added and titrated immediately with 0.1N HCl to a pH about 7.0.

142

Mill equivalent of acid × 0.1 × 31

R.A.A. g/l as Na2O =

ml of black liquor taken

4.3.5.2 Total Solids in Black Liquor

Total solids of black liquor represent the sum of total dissolved solids and total

suspended solids present in the black liquor. The given method can be used to calibrate

routine control parameters in chemical recovery. This method measures the solids

remaining after removal of water and other non aqueous volatile materials normally lost

in evaporation section. 10 ml black liquor were placed in pre weighted china dishes and

kept in the oven at 1050C for overnight.

W2 - W1 x 100

Total solid % =

Sample weight

W1 = Empty weight of china dish

W2= O.D. weight of china dish with sample

4.4 Results and Discussion

4.4.1 Effect of Cooking Time on Pulp Characteristics

Table 4.2 summarizes the results of kraft pulping experiments that were

conducted to investigate the effect of cooking time at 16% chemical charge as Na2O, 1:4

bath ratio and 25% sulfidity at 160°C temperature, in treated and untreated bamboo

samples (destructured and non destructured). The effect of pulping time on pulp yield and

kappa number in each sample was studied and evaluated for best pulping conditions.

4.4.1.1 Kappa Number and Pulp Yield

The kappa number is an index used by the pulp and paper industry to express the

residual lignin content in unbleached pulp after cooking. Lignin is responsible for the

brown coloration of paper and residual lignin is removed in the subsequent bleaching

stage to obtain desired brightness. Therefore, the lignin content must be well known, so

that only a minimum amount of bleach chemicals are used. Higher the lignin content

143

more is the kappa no. The pulp having high lignin content is termed as hard cooked pulp

and the pulp with low lignin content as soft cooked pulp. The hard cooked pulp requires

more bleaching chemicals to attain particular brightness compared to soft cooked pulp.

Table 4.2 shows the effect of cooking time on kappa number, reject (%) and pulp yield

(%) of treated/untreated bamboo non destructured and destructured samples.

Table 4.2: Effect of Cooking Time on Pulp yield, Kappa number and Reject of

Bamboo Treated and Untreated Samples.

S. No. Pulp Samples Cooking Time

(minutes)

Pulp Yield (%) Kappa Number Rejects (%)

1 NDC* 1

NDC 2

NDC 3

NDC 4

30

60

90

120

46.97

46.18

45.32

44.57

25.10±0.12a

18.29±0.31b

15.26±0.21c

13.89±0.26d

0.4415

0.1080

0.0820

0.0548

2 DC **1

DC 2

DC 3

DC 4

30

60

90

120

49.94

49.83

47.43

46.24

22.86±0.23a

17.65±0.20b

14.68±0.29c

12.54±0.31d

0.2398

0.1008

0.0982

0.0840

3 NDT #1

NDT 2

NDT 3

NDT 4

30

60

90

120

46.72

45.20

44.96

40.13

19.67±0.34a

15.58±0.36b

13.31±0.21c

12.74±0.25d

0.1661

0.0915

0.0221

0.0560

4 DT##1

DT 2

DT 3

DT 4

30

60

90

120

47.19

46.53

45.03

43.92

14.88±0.07a

12.16±0.14b

10.25±0.28c

9.86±0.03d

0.1417

0.0525

0.0194

0.0165

* NDC- Non destructured control, **DC- Destructured control, #NDT- Non destructured treated and ##DT- Destructured treated.

The results in Table 4.2 show that with increase in cooking time, kappa number

and yield of pulp is reduced in case of both treated and untreated samples. In case of non

destructured control (NDC) the lowest kappa number (13.89) was obtained after pulping

upto 120 minutes, whereas with destructured control (DC) sample it reduced to 12.54

kappa number after 120 minutes of cooking. After fungal treatment non-destructured and

destructured samples (NDT & DT) showed further reduction in kappa number & yield.

Kappa number obtained in case of NDT was 12.74 and for DT 9.86 after 120 minutes of

144

pulping. ANOVA showed significant difference between kappa number reduction with

varying cooking time in NDC, DC, NDT and DT pulp samples. Results of DMRT

revealed significant difference (p<0.05) between all the treatments means. Table 4.3

shows results of ANOVA main effects and interaction of samples and cooking time on

kappa number reduction. The main effects and interaction between samples and cooking

times were significant. The pulping results indicated that highest pulp yield was obtained

after 30 minutes cooking, however kappa number was found to be high in this case.

Reduction in kappa number observed after 60 and 90 minutes was relatively rapid

however reduction in pulp yield observed was in the range of 1-2%. The screening rejects

were also high after 30 minutes of cooking. The investigation has shown that rate of

delignification in destructured treated sample (DT) was high which was represented by

decrease in kappa number. The relative rate of delignification for the samples can be

presented as shown below:

Non-destructured control sample (NDC) ˃ destructured control sample (DC) ˃

non destructured treated sample (NDT) ˃ destructured treated sample (DT).

Table 4.3: ANOVA Results for Kappa Number Reduction.

Source Sum of Squares df Mean Square F Sig.

Sample 273.709 3 91.236 1.509E3 .000

Cooking Time 498.071 3 166.024 2.746E3 .000

Sample * Cooking Time 43.128 9 4.792 79.253 .000

Error 1.935 32 .060

Total 12416.522 48

4.4.1.2 Optimization of Pulping Conditions for Uniform Pulp Properties from Un-

treated / Treated, Non Destructured and Destructured Bamboo Samples

The commercial kraft cooking conditions in paper mills for bamboo chips are 90

minutes heating at 160°C maintaining the constant H-factor. By maintaining these

conditions during the pulping of non destructured treated sample (NDC), pulp of kappa

number 15.26 was obtained. Kappa number of pulp was therefore set at 15 as target value

for optimization of the pulping conditions and in order to obtain uniform pulp quality.

145

The results obtained after pulping of various samples at different conditions were

analyzed and it was observed that the kappa number close to the targeted kappa number

was obtained after pulping of destructured control pulp (DC), non destructured treated

pulp (NDT) and destructured treated pulp samples after 90 minutes, 60 minutes and 30

minutes at 1600C respectively, without affecting the yield at the same chemical charge. In

this study, it was observed that without fungal treatment, rejects were around 0.8% but it

decreased to less than 0.11% after the fungal treatment.

The yield and kappa number in the non destructured control (NDC) and non

destructured treated (NDT) samples were studied by increasing the cooking time. In this

case a sharp decrease in kappa number was observed in treated samples with

simultaneous lowering of reject % with increasing cooking time period. This can be

explained by facilitating swelling and loosening of cell wall structures after pretreatment.

The cell porosity occurs early in the colonization process by lignin-degrading fungi

(Nishida et al., 1988; Akhtar et al., 1992a&b; Fujita et al., 1993). Also, these fungi

remove and/or modify lignin in wood cell walls that might be removed easily during kraft

pulping (Reddy, 1984; Messner et al., 1998). This higher porosity leads to the

production of lower amount of rejects. Scott et al. (1995) reported that fungi treatment

can reduce the yield slightly. This decrease may be due to both the dissolution of lignin

and the concurrent attack on the carbohydrates (Messner et al., 1998).

After destructuring of the treated sample, pulping results indicated that cooking

for 30 min. at top temperature (1600C) is the most optimum cooking time for DT

samples. At optimum cooking time period kappa number was 10.22 point less in DT

when compared with NDC pulp. The destructured samples (DT) would reduce pulping

energy requirement (both electrical and thermal energy) needed for the cooking of raw

material without affecting the pulp properties. In case of destructured samples saving of

heat energy was observed after fungal treatment as the heating time required for pulping

reduced from 90 min. to 30 min., thereby resulting in saving of 60 min. heating at 1600C

temperature. Fig. 4.3 and 4.4 show the effect of cooking time on kappa number and pulp

yield (%) for NDC, DC, NDT and DC respectively. It is evident that the kappa number

below target value 15 is obtained by pulping of the treated destructured samples without

higher rejects and at reasonable yield.

146

4.4.2 Analysis of Black Liquor after Different Cooking Times

Physico-chemical properties of spent pulping liquor represent the effect of

pulping conditions on delignification and indirectly on the pulp properties. Experiments

were conducted to investigate the effect of cooking time on physico-chemical properties

of black liquor after pulping with 16% charge as Na2O, 1:4 bath ratio and 25% sulfidity

at 1600C temperature. The black liquor samples collected after the pulping of NDC, DC,

NDT and DT for different pulping durations were analyzed to represent the effect of

variables. Results of black liquor characteristics after different cooking time periods are

presented in Table 4.4.

Table 4.4: Spent Black Liquor Characteristics of Non Destructured Control,

Destructured Control, Non Destructured Treated and Destructured Treated

Bamboo Samples at Different Cooking Time Periods.

S.

No.

Pulp

Samples

Cooking

Time

(minutes)

pH Residual Active

Alkali

(g/l)

Total

solids

(%)

Alkali

Consumption

(%)

1 NDC 1

NDC 2

NDC 3

NDC 4

30

60

90

120

13.71

13.59

13.55

13.50

8.92

6.57

6.13

5.89

16.284

16.620

16.651

16.739

77.70

83.57

84.67

85.27

2 DC 1

DC 2

DC 3

DC 4

30

60

90

120

13.74

13.61

13.52

13.51

7.56

6.63

6.26

6.20

16.641

14.964

15.069

15.555

81.10

83.35

84.35

84.50

3 NDT 1

NDT 2

NDT 3

NDT 4

30

60

90

120

13.77

13.66

13.57

13.53

9.48

8.18

7.93

6.51

15.813

15.802

17.131

16.060

76.3

79.55

80.17

83.72

4 DT 1

DT 2

DT 3

DT 4

30

60

90

120

13.83

13.80

13.78

13.58

10.04

9.17

7.99

6.26

13.456

13.646

13.566

14.424

74.9

77.07

80.02

84.35

4.4.2.1 Residual Active Alkali (RAA)

An important parameter of the chemical recovery process control is the

concentration of residual active alkali, which is an indication of pulping process

147

effectiveness. According to some published studies, low and well controlled residual

alkali guarantees greater carbohydrate concentration, resulting in process yield benefits

(Ahmed et al., 1999; Anon, 1988) and final product strength (Akhtar et al., 1992a).

Table 4.4 shows a declining pattern in residual active alkali with increase in cooking time

in all the pulp samples. Similar pattern was observed in pH values. Sufficient residual

active alkali must be present at the end of the cook to avoid precipitation of lignin

structures dissolved in the liquor. Precipitated lignin structures affect pulp bleachability

since they are difficult to remove during the bleaching operation. The analysis of residual

active alkali indicates that highest level in treated destructured samples ensures a uniform

alkali profile upto 30 minutes. This in turn results in better pulp properties after 30

minutes of pulping.

4.4.2.2 pH

The liquor pH has to be sufficiently high to guarantee proper wood delignification

and avoid lignin precipitation at the end of the cook. According to Fujita et al. (1993)

and Reddy (1984) under normal pulping conditions, lignin precipitation begins at pH

below 12 and is considerable at pH below 11. The pH in none of the pulping experiments

was lower than 12 and the raw material was therefore properly delignified.

4.4.2.3 Black Liquor Solids

Liquor solids concentration is very important for chemical recovery process. It

depends on various washing conditions. The quantity and concentration of organic and

inorganic solids, particularly in the bulk and residual delignification steps also have

diverse effects on process performance and final product quality. In the laboratory, ideal

conditions were maintained to obtain the black liquor soilds. It is observed that the solids

profile did not undergo abrupt alterations.

4.4.2.4 Alkali Consumption

As can be observed in the Table 4.4, alkali consumption is directly proportional to

the cooking time i.e. consumption increases with the increase in time. In case of treated

samples it is observed that the alkali consumption is lower than the untreated samples.

148

Analysis of alkali consumption was made under the optimum pulping conditions of 30

minutes in all the four samples. The alkali consumption was compared after 30 minutes

pulping in NDC and DT samples. The alkali consumption decreased from 77.70% to

74.9% in case of DT, thus showing a total saving of 2.8% in alkali consumption for

treated destructured samples.

4.4.3 Properties of Pulp Sheets

Unbleached pulps obtained from pulping of NDC, DC, NDT and DT were beaten

and used to prepare paper sheets to evaluate the properties. The properties of pulp sheets

for NDC, NDT and DC & DT were compared to find out the variation in Freeness, Tear

Index, Tensile Index and Burst Index of unbleached sheets.

Table 4.5: The Strength Properties of Unbleached Pulp Sheets Made from Non

Destructured Control, Destructured Control, Non Destructured Treated and

Destructured Treated Bamboo Samples.

S.

No.

Pulp

Samples

Kappa

Number

Basis

Weight,

g/m2

Freeness,

CSF, ml

Tensile

Index, N.m/g

Tear Index,

mNm2/g

Burst Index,

K.Pa.m2/g

1 NDC 1

NDC 2

NDC 3

NDC 4

25.10

18.29

15.26

13.89

69.39

65.52

66.73

66.53

360

360

340

320

64.69

76.00

79.72

74.42

20.91

20.98

20.90

19.83

4.88

5.28

5.45

4.91

2 DC 1

DC 2

DC 3

DC 4

22.86

17.65

14.68

12.54

69.00

66.70

62.30

65.00

350

340

340

310

74.50

73.60

72.20

66.00

12.17

12.14

10.88

10.58

5.17

4.49

4.92

4.46

3 NDT 1

NDT 2

NDT 3

NDT 4

19.67

15.58

13.31

12.74

76.70

62.90

63.50

70.10

330

320

320

310

65.30

70.40

77.80

69.80

20.49

18.90

16.91

16.78

4.94

5.05

5.92

5.34

4 DT 1

DT 2

DT 3

DT 4

14.88

12.16

10.25

9.86

64.65

63.22

64.27

65.52

310

310

300

300

68.76

68.28

68.59

68.37

8.16

7.05

7.49

6.74

4.74

5.25

4.85

4.50

149

Table 4.5 lists the basis weight (g/m2), freeness (CSF, ml), Tensile Index (N.m/g),

Tear Index (mNm2/g) & Burst Index (K.Pa.m

2/g) and corresponding kappa number of the

sheets prepared from treated and untreated bamboo pulps. The freeness of pulps in case

of non-destructured and destructured samples untreated and treated with fungi were

determined after 5000 revolutions in the PFI mill. Fig. 4.5 shows that the freeness

decrease is observed in case of the treated samples and as a result the better sheet

properties will be obtained due to less cutting of the fibers to achieve the lower CSF

values.

4.4.3.1 Tensile Index

Tensile properties describe the dimensions and strength of the individual fibers,

their arrangement, and the extent to which they are bonded to each other. Papers made

with long fibers generally have higher tensile strength properties than paper made of

short fibers. However, the extent of inter fiber bonding is considered the most important

factor contributing to tensile strength properties. The variation in tensile index for NDC

& NDT and DC & DT are presented in Table 4.5 and Fig. 4.6. The results were analyzed

and are discussed below.

To study the effect of cooking time periods on tensile properties of NDC and

NDT, the tensile index of these samples were plotted against cooking time and are shown

in Fig. 4.6. In NDC maximum tensile index obtained was 79.72 N.m/g after 90 minutes

of cooking time. In case of NDT a sharp increase in tensile index was observed when

cooking time was extended upto 90 minutes. The tensile index increased from 65.30 to

77.80 N.m/g. However reduction in tensile index was observed after 90 minutes of

cooking time. The values decreased from 77.80 N.m/g to 69.80 N.m/g after 120 minutes

of cooking.

In case of DC and DT, a different pattern of tensile index with cooking time was

observed. In DC, the tensile index has dropped from 74.50 to 66.00 N.m/g after cooking

from 30 minutes to 120 minutes. Whereas in case of DT pulps, tensile did not show much

difference with increase in cooking time from 30 to 120 minutes. Tensile properties

observed were lower in case of DT than DC. This is mainly due to exposure of the fibers

to cooking chemicals for longer time leading to reduction in kappa number. If a

150

comparison is made between NDC and DT at 30 minutes of cooking time, a sharp

increase in tensile index of DT is observed, when compared with NDC.

4.4.3.2 Tear Index

Internal tearing resistance is a measure of the force perpendicular to the plane of

the paper necessary to tear a single sheet through a specified distance after the tear has

already been started. Edge-tearing strength is a measure of the force needed to initiate a

tear. The force needed to initiate a tear may be several times the force needed to

propagate the tear once it is started. Tear index is obtained by dividing the tearing

resistance measured in units of milli newtons (mN) by the grammage of the paper in

units of grams per square meter (g/m2) (Casey, 1996). The variation in Tear Index

shown in Table 4.5 and Fig. 4.7, for NDC & NDT and DC & DT samples is explained

below:

Tear index of NDC did not show a much difference with increase in cooking time

from 30 to 120 minutes. In NDC maximum tear index obtained was 20.98 mNm2/g after

60 minutes of cooking time. Tear index of NDT show a drop in tear properties at all the

four cooking time periods. There was a decrease of 3.71 mNm2/g in tear of NDT after

pulping the samples upto 120 minutes (from 20.49 mNm2/g to 16.78 mNm

2/g). Tear

index of NDC when compared with NDT, showed a drop in tear properties at all the four

cooking time periods.

In case of DC and DT, when the tear values were compared, a different pattern

was observed with change in cooking time. In DT pulps, highest tear index observed was

after 30 minutes of cooking time i.e. 8.16 mNm2/g. However a drop in tear values was

then observed with increase in cooking time. After 120 minutes of cooking the tear index

declined to 6.74 mNm2/g at kappa number 9.86. Whereas in DC pulp, tear index showed

a decreasing pattern from 30 to 120 minutes of cooking time. If a comparison is made

between DC and DT, tear index of DC pulps are better than DT pulps. At the same time

when the tear values are compared with corresponding kappa number, the drop is

comparatively more than DC. This is mainly due to exposure of the fibers to cooking

chemicals for longer time leading to reduction in kappa number.

151

Tear index of NDC when compared with DT, showed more drop in tear properties

at all the four cooking time periods. In NDC, maximum tear index obtained was 20.98

mNm2/g after 60 minutes cooking. However, maximum tear index observed was 8.16

mNm2/g in case of DT after 30 minutes cooking. The tear index was lower in DT pulp

sheets than NDC pulp sheets. It can be explained by comparing the kappa number of both

the pulps. Kappa number of DT pulp after 30 minutes cooking was 14.88 compared to

25.10 in NDC pulp. The low kappa number in DT pulp indicates that the pulp fibers have

been degraded slightly more, than NDC pulps, due to harsh treatment of cooking

chemicals in which the treated fibers are exposed after fungal treatment.

4.4.3.3 Burst Index

Bursting strength is perhaps the most commonly measured strength property of

paper. The test apparently originated from the old time practice of the papermaker who,

in a hands-on quality control evaluation of paper strength, would attempt to push his

thumb through the sheet (Anon, 1988). It is more meaningful to compare bursting

strength of papers of differing grammages in terms of burst index. Burst index is obtained

by dividing the bursting strength in kPa by the grammage of the paper in g/m2. The

variation in burst index shown in Table 4.5 and Fig. 4.8, for NDC & NDT and DC & DT

samples is explained below:

Burst index observed in NDC after 30 minutes of cooking time was 4.88

K.Pa.m2/g whereas this value increased to 4.94 K.Pa.m2/g in the case of NDT. At all the

cooking time periods, non destructured treated pulp (NDT) showed higher burst index

than non destructured control pulp (NDC). The properties had improved in both NDC and

NDT samples with increase in cooking time. In case of NDC the burst index increased

from 4.88 K.Pa.m2/g to 5.45 K.Pa.m

2/g after extending the cooking time from 30 to 90

minutes respectively. However reduction in burst index properties was observed after 90

minutes of cooking time. The values decreased from 5.45 K.Pa.m2/g to 4.91 K.Pa.m

2/g

after 120 minutes in NDC. In case of NDT burst index increased from 4.94 K.Pa.m2/g to

5.92 K.Pa.m2/g after extending the cooking time from 30 to 90 minutes. The burst index

decreased from 5.92 K.Pa.m2/g to 5.34 K.Pa.m

2/g. Similar pattern was observed without

152

much difference in NDC and NDT. At the same time kappa number reduction was more

in NDT than NDC.

In the case of DC and DT, when the burst values are compared, a different pattern

is observed with change in cooking time. In DC pulp, highest burst observed was 5.17

K.Pa.m2/g after 30 minutes of cooking time. Whereas in DT pulp, highest burst observed

was 5.25 K.Pa.m2/g after 60 minutes of cooking time. However a drop in burst values

was then observed with increase in cooking time in both the pulp samples. This is mainly

due to exposure of the fibers to cooking chemicals for longer time.

Burst index of NDC when compared with DT at 30 minutes of cooking time

period, reduction in burst properties was observed in DT. At the same time kappa number

observed was 25.10 in NDC and 14.88 in DT respectively. Comparison of burst index of

NDC and DT shows that the drop in burst, is relatively much less than the drop observed

in kappa number in the respective treated samples

4.5 Conclusion

Investigation has been carried out in order to characterize the pulp of treated and

untreated samples in two forms of bamboo i.e. destructured and non destructured. To

evaluate the effect of pulping time during the cooking process, the samples were

extracted at four different times i.e. 30 min., 60 min., 90 min. and 120 min. Other

conditions during the pulping experiments were 16% charge as Na2O, 1:4 bath ratio and

25% sulfidity at 1600C temperature.

Results of the experiments were investigated to study the effect of cooking time

on various samples (NDC, NDC, NDT and DT) to extract significant amount of lignin

from wood at different time periods keeping the temperature constant at 1600C. The

investigations have shown that delignification rate of destructured treated (DT) samples

was high which was represented by decrease in kappa number. The relative rate of

delignification for the samples was found to be as follows;

Non-destructured samples ˃ destructured samples ˃ non destructured treated samples ˃

destructured treated samples

In order to optimize the pulping conditions for obtaining uniform pulp quality, the

kappa number was set at 15 as a targeted value, on the basis of cooking conditions

153

maintained at mills. The usual kraft cooking conditions in paper mills for bamboo chips

are 90 min. heating at 160°C. It was observed that the kappa number close to the targeted

kappa number in fungal treated destructured pulp (DT) samples was obtained after

pulping at 30 minutes, without affecting the yield at the same chemical charge. Results

indicated that cooking for 30 min. at top temperature (1600C) is the most optimum

cooking time for DT samples. In case of the destructured samples, less pulping energy is

required (both electrical and thermal energy) for the cooking of raw material without

affecting the pulp properties. In case of destructured samples saving of heat energy was

observed after fungal treatment as the heating time required for pulping reduced from 90

min to 30 min., thereby resulting in saving of 60 min. heating at 1600C temperature. The

strength properties of the pulp samples did not show much difference. However, a sharp

decline was observed in kappa number of treated destructured samples (DT) when

compared with untreated non destructured samples (NDC). This shows requirement of

minimum amount of harsh chemical treatments in the subsequent bleaching stages.

154

Fig. 4.1: Cooking Procedure for Treated and Untreated Bamboo Samples.

Treated and Untreated

Destructured Sample

Treated and Untreated

Non Destructured Sample

Cooking with Six Bomb Oil

Bath Digester

Washing of Pulp Screening of Pulp

Beating of Pulp CSF of Pulp Laboratory Sheet

preparation

Unbleached Sheet Beating of Pulp

155

Fig. 4.2: The Amount of Lignin that Reacts According to the Initial, Bulk and

Residual Phase Delignification (Lindgren and Lindstrom, 1996).

Fig. 4.3: Effect of Cooking Times on Kappa Number of Non Destructured Control,

Destructured Control, Non Destructured Treated and Destructured Treated

Bamboo Samples.

156

Fig. 4.4: Effect of Cooking Times on Total Yield% of Non Destructured Control,

Destructured Control, Non Destructured Treated and Destructured Treated

Bamboo Samples.

Fig. 4.5: CSF in Relation to Cooking Time (minutes).

157

Fig. 4.6: Tensile Index in Relation to Cooking Time (minutes).

Fig. 4.7: Tear Index in Relation to Cooking Time (minutes).

158

Fig. 4.8: Burst Index In Relation to Cooking Time (minutes).

159

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