Delignification of bamboo (Bambusa procera acher)

Industrial Crops and Products 19 (2004) 49–57

Delignification of bamboo (Bambusa procera acher) Part 1. Kraft pulping and the subsequent oxygen delignification to pulp with a low kappa number Thi Hong Mˆan Vu, Hannu Pakkanen, Raimo Alén∗ Laboratory of Applied Chemistry, University of Jyväskylä, P.O. Box 35, FIN-40014, Jyväskylä, Finland Received 11 April 2002; accepted 25 July 2003

Abstract Delignification of bamboo (Bambusa procera acher) was carried out by conventional kraft and soda pulping under varying conditions to determine the relationships between selected cooking parameters (EA 14–20%, sulfidity 0–45%, maximum temperature 165 and 170 ◦ C, and time at maximum temperature 30–95 min) and pulp properties (kappa number, viscosity, and yield). Results indicated that in order to obtain relatively low kappa numbers (17–24), high sulfidity (35–45%) at lower EA (14–16%) increased both pulp viscosity and yield compared to the case of low sulfidity (0–15%) at higher EA (16–18%). Pulp with lower kappa numbers (11–15) and acceptable viscosities (1080–1190 ml/g) can be obtained at a sulfidity level of 25–35% with 18% EA or at a sulfidity level of 15–35% with 20% EA. It was further shown that bamboo kraft pulp was easily delignified by oxygen–alkali delignification to a low kappa number (7–9) without any significant loss in viscosity. © 2003 Elsevier B.V. All rights reserved. Keywords: Bamboo; Kraft pulping; Oxygen delignification; Kappa number; Pulp viscosity; Pulp yield

1. Introduction Bamboo belongs to the family of grasses and is one of the non-wood raw materials most widely used for the production of paper and paperboard in Asia (Assumpcao, 1992; Atchison, 1998). In general, bamboo can be considered as a long-fibered or semilong-fibered fibrous material as the length of fibers in many bamboo species is comparable to that of softwood fibers, average values being in the range ∗ Corresponding author. Tel.: +358-14-2602562; fax: +358-14-2602581. E-mail address: [email protected] (R. Al´en).

1.5–4.4 mm (Ilvessalo-Pfäffli, 1995). For this reason, bamboo chemical pulps have attracted increasing attention in other countries like Brazil, as a reinforcement pulp. Due to the high strength of bamboo pulps, these pulps can be used for more versatile paper products than the majority of other non-wood pulps. For bamboo, kraft pulping is generally preferred to soda pulping when preparing chemical pulp (Rydholm, 1965; Misra, 1980). The kraft process provides satisfactory delignification as well as high yield and viscosity. The main reason for this is that the fiber dimensions and main chemical constituents of bamboo (i.e. the structure of bamboo) typically bear a close resemblance to those of woods. For this

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reason, the large-scale delignification of bamboo is conventionally based on techniques similar to those generally applied to wood pulping. Depending on the subsequent bleaching sequence and the target paper grade, unbleached kraft pulps with kappa number of 18–30 are normally made from bamboo (Bhargava, 1987). However, elemental chlorine is still commonly used in the bleaching of bamboo pulps, although in some mills chlorine has partly been substituted by chlorine dioxide. PHOENIX Pulp & Paper Public Co., Ltd. in Thailand pioneered the manufacture bamboo market kraft pulp using elemental chlorine-free (ECF) bleaching for oxygen-delignified pulp (Mittal and Maheshwari, 1996). A few studies (Gomide et al., 1991; Kishore et al., 1995; Singh et al., 1995) have also reported on the oxygen delignification of bamboo kraft pulp. However, no study has explicitly aimed at achieving a very low kappa number for bamboo pulp. An effective decrease in the content of residual lignin in kraft pulp without any negative effects on the pulp strength properties would be of great importance with respect, on the one hand, to environmental protection and, on the other hand, to reducing bleaching costs. The aim of this study was to investigate the possibilities of producing bamboo chemical pulp with a low residual lignin content for ECF or totally chlorine-free (TCF) bleaching by means of conventional kraft pulping followed by oxygen delignification. The study is part of a larger research project aiming, in addition to cooking experiments with non-wood raw materials, at the detailed characterization of the corresponding spent liquors from cooking, oxygen delignification, and bleaching.

Table 1 Some characteristics of Vietnamese bamboo (B. procera acher) Component

Composition of bamboo dry solids (%)

Carbohydratesa

68.6 25.8 0.8 2.6 2.2 0.7

Lignin Extractives Proteins Ashb Silica a b

Calculated by difference. Contains silica.

tent by multiplying it by a factor of 6.25. Nitrogen content was obtained from an elemental analysis, which was performed in a micro elemental analyzer (LECO CHN-600). Ash content was determined at 525 ◦ C (TAPPI T 211 om-93). Silica content was measured by a VARIAN SpectrAA 220/FS atomic absorption spectrometer (AAS). For this analysis, bamboo chips were ground in a CYCLOTEC 1093 Sample Mill and the milled material was completely ashed at 550 ◦ C. The bamboo ash was first fluxed with a mixture of potassium carbonate–sodium carbonate (anhydrous, 1.25:1) after which the fused sample was dissolved in 2.5 M hydrochloric acid. The dissolved sample was diluted with UHQ water for AAS analysis. For pulping, bamboo chips were screened in a laboratory chip screen according to SCAN-CM 40:94 standard with the exception that, in addition to the fractions obtained from the 7 and 13 mm hole screens, fraction from the 3 mm hole screen was also accepted. The chips were air dried and stored at about 92% dry solids content. 2.2. Kraft pulping

2. Material and methods 2.1. Raw material The bamboo material used was Bambusa procera acher from the central part of Vietnam. Some of the characteristics of this material are shown in Table 1. Lignin content was determined as a sum of acid-insoluble Klason lignin and acid-soluble lignin (TAPPI T-222 om 98). Extractives content was determined by acetone extraction (TAPPI T 280 pm-99). Protein content was calculated from the nitrogen con-

A CRS Autoclave System CAS 420 oil bath digester system equipped with 1.25 l stainless steel autoclaves was used. Each autoclave was charged with 200 g of oven-dried (o.d.) chips. The cooking conditions are shown in Tables 2 and 3. At the end of each cook, the autoclaves were removed from the oil bath and cooled rapidly in cold water. The black liquor was separated from the pulp by pressing in a nylon washing bag. The pulp in this bag was then thoroughly washed using a water shower. The washed pulp was dewatered in a centrifuge and homogenized, and the total yield was determined.

T.H. Mˆan Vu et al. / Industrial Crops and Products 19 (2004) 49–57 Table 2 Cooking conditions of the effective alkali (EA) and sulfidity studies EA, on o.d. bamboo (as NaOH) (%) Sulfidity (%) Liquor-to-bamboo ratio (l/kg) Cooking temperature (◦ C) Time from 80 ◦ C to maximum temperature (min) Time at maximum temperature (min)

14, 16, 18, and 20 0, 15, 25, 35, and 45 4 165 85 95

Table 3 Cooking conditions of the H-factor study EA on o.d. bamboo (as NaOH) (%) Sulfidity (%) Liquor-to-bamboo ratio (l/kg) Cooking temperature (◦ C) Time from 80 ◦ C to maximum temperature (min) Time at maximum temperaturea (min) a

14 and 16 25 4 165 and 170 85 30–95

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were applied to the different pulps. For pulps with a kappa number higher than 23, a higher sodium hydroxide charge was also used. At the end of each delignification, the liquor was separated from the pulp by pressing in a nylon washing bag. The pulp was washed by diluting it to a consistency of 2–3% with deionized water and this mixture was then carefully dewatered by pressing. This washing procedure was repeated four times. Prior to analysis, the washed pulp was thickened in a centrifuge and homogenized. The kappa number and viscosity of the oxygen-delignified pulps were determined according to the same standard test methods as used for the cooked pulps.

3. Results and discussions 3.1. Effect of EA and sulfidity

Depended on the H-factor (600–1100).

After disintegrating at low consistency the pulp was screened on a vibrating flat screen with 0.3 mm wide slots. The accepted pulp was thickened in centrifuge and again homogenized to determine the screened yield and other pulp properties. The kappa number and viscosity of the screened pulp were determined according to standard test methods SCAN-C 1:77 and SCAN-CM 15:88, respectively. In each case, the total yield, screened yield, and the amount of reject were calculated according to the dry solids content of the chips and pulps.

The effects of effective alkali and sulfidity on kappa number, viscosity, and yield of the pulps are shown in Table 5 and Figs. 1–4. It can be seen that both EA and sulfidity had a significant influence on kappa number (Fig. 1). Thus, either an increase in EA at a constant sulfidity or, on the other hand, an increase in sulfidity at a constant EA resulted in a clear reduction in kappa number. The beneficial effect of sulfur addition (e.g. addition of hydrogen sulfide ions) was also easily observed from these cook series. Without sulfur addition (i.e. soda pulping at a sulfidity level of 0%), it was difficult

2.3. Oxygen delignification One-stage medium-consistency alkali oxygen delignification was carried out in a Quantum high intensity mini mixer separately for the kraft pulps at the different kappa numbers. The same conditions (Table 4) Table 4 Oxygen delignification conditions NaOH, on o.d. pulp (%) MgSO4 , on o.d. pulp (%) Consistency (%) O2 -pressure (bar) Temperature (◦ C) Treatment time (min) a

2, 3a 0.5 10 5 90 60

Only for pulps with kappa number higher than 23.

Fig. 1. Effect of EA and sulfidity on the kappa number of bamboo kraft and soda (sulfidity 0%) pulps.

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Table 5 The effect of EA and sulfidity on pulp properties in the alkaline pulping of bamboo (for other cooking conditions, see Table 2) Cook number

EA (%)

Sulfidity (%)

Kappa number

Viscosity (ml/g)

Total yield (%)

Screened yield (%)

Reject (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

14 14 14 14 14 16 16 16 16 16 18 18 18 18 20 20 20 20

0 15 25 35 45 0 15 25 35 45 0 15 25 35 0 15 25 35

56.3 30.1 25.5 23.6 22.1 45.8 23.3 18.6 16.7 16.8 38.4 18.6 15.5 13.2 33.1 15.2 12.9 11.2

871 1207 1247 1319 1294 930 1195 1230 1272 1311 948 1185 1192 1133 964 1106 1091 1080

58.6 54.2 54.2 53.5 53.8 56.2 52.1 51.5 50.4 51.7 53.9 51.9 50.4 50.5 50.5 48.9 48.7 48.1

54.3 51.5 51.3 49.4 50.2 51.6 48.7 48.4 47.3 47.9 49.4 47.9 46.6 46.7 47.0 46.2 45.5 45.0

0.1 0.1 0.1 0.1 ND 0.1 ND ND ND ND ND ND ND ND ND ND ND ND

ND: not detected.

to delignify bamboo to a kappa number of 30 even though high EA charges were used. In particular, in case of non-sulfur cooks, an intensive alkali-catalyzed degradation of carbohydrates occurred, as indicated by the low viscosity values (<1000 ml/g). On the contrary, at the lowest EA (14%) together with high sulfidity (35–45%) it was possible to delignify bamboo to a kappa number close to 20. These conditions also resulted in a higher pulp viscosity and yield than

those obtained under the conditions of high EA (18 and 20%) and low sulfidity (0 and 15%). From these results, it could be concluded that the selective action of hydrogen sulfide ions in bamboo pulping is similar to that detected in wood pulping (Clayton et al., 1989). The results in Fig. 1 further indicated that the effect of sulfidity on kappa number was significant up to a sulfidity level of 15–25%, after which the decrement in kappa number started to level off. However,

Fig. 2. Effect of EA and sulfidity on the viscosity of bamboo kraft and soda (sulfidity 0%) pulps.

Fig. 3. Pulp viscosity vs. kappa number of bamboo kraft pulp at different sulfidities.

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Fig. 4. Effect of EA and sulfidity on the screened yield of bamboo kraft and soda (sulfidity 0%) pulps.

at higher sulfidities (35–45%, EA 14–16%) a lower kappa number (16.7–23.3) could be obtained with the highest viscosity (1272–1319 ml/g) (cooks 4, 5, 9, and 10). Even in case of a very low kappa number (as low as 13–11 for cooks 14 and 18 in Table 5), viscosity was still at an acceptable level at 35% sulfidity (EA 18–20%). Fig. 2 shows that in the case of low EA (14 and 16%), higher viscosity values were obtained at high sulfidities (over 25%), whereas at higher EA (18 and 20%) lower viscosity values were obtained. These findings were in agreement with the fact that during kraft cooking hydrogen sulfide ions react with lignin, and carbohydrate degradation reactions (i.e. indicated in part by the decrease in pulp viscosity) are only affected by hydroxide ions (Alén, 2000). It should be pointed out that the increase in sulfidity at a certain EA level results in an increase in the number of hydrogen sulfide ions with a simultaneous decrease in the number of hydroxide ions. In these experiments, when a kappa number of about 18 was reached, delignification proceeded too far and, owing to the enhanced carbohydrate degradation, a clear decrease in viscosity was seen (Fig. 3). The relationship between kappa number and viscosity observed for the bamboo kraft pulps (Fig. 3) was, within the same kappa number range, similar to that for wood kraft pulps, although different from that for some common non-wood pulps. For example, it has been noted (Sjöblom, 1996; Gullichsen, 1999) that the viscosities of spruce and pine kraft pulps decrease

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along with further delignification. In contrast, it has been reported (Feng and Alén, 2001a,b) that in case of soda-AQ pulping of wheat straw and reed canary grass, the viscosity is typically higher at lower kappa numbers than at higher kappa numbers. Table 5 indicates that, even at very low kappa numbers, bamboo gave quite high cook yields, corresponding to 45.0–51.5% (screened yield) when the kappa number was in the range 11–30. It should be pointed out that the screened yields plus rejects were 2.3–4.6% lower than the total yields. This was probably caused by the loss of parenchyma cells, abundant in bamboo pulp (Bhargava, 1987; Ilvessalo-Pfäffli, 1995), during the screening process. In general, yield decreased when either EA or sulfidity increased (Fig. 4). However, in order to achieve the same kappa number, higher sulfidity at a lower EA generally gave a 0.5% greater yield than in the case of lower sulfidity at a higher EA (cooks 4 and 7, 8 and 12, and 13 and 16 in Table 5). In particular, yield was about 0.6–0.8% higher with 45% sulfidity than with 35% sulfidity at the same EA (cooks 4 and 5, and 9 and 10 in Table 5). It can be concluded that in order to obtain relatively low kappa numbers (17–25) in the kraft pulping of bamboo, high sulfidity (25–45%) at lower EA (14–16%) increased both pulp viscosity and yield compared to the case of low sulfidity (0–15%) at higher EA (16–18%). Pulps with lower kappa numbers (11–15) and still with acceptable viscosities (1080–1190 ml/g) can be obtained at a sulfidity level of 25–35% with 18% EA or at a sulfidity level of 15–35% with 20% EA. 3.2. Effect of cooking temperature and cooking time The effect of cooking temperature and cooking time (H-factor) on the kappa number, viscosity, and yield of the bamboo kraft pulps are presented in Table 6. In addition, Fig. 5 illustrates the effect of maximum cooking temperature and time on kappa number, and Figs. 6 and 7 show the effect of the H-factor on yield and viscosity, respectively, at a maximum temperature of 170 ◦ C. It was observed that at the same H-factor, lower cooking temperature (165 ◦ C versus 170 ◦ C) resulted in lower kappa number (Fig. 5). The possible reason for this observation was that the longer cooking times

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Table 6 The effect of cooking temperature and cooking time on pulp properties in the kraft pulping of bamboo (for other cooking conditions, see Table 3) Cook number

EA (%)

Cooking temperature (◦ C)

Cooking time (min)

H-factor

Kappa number

Viscosity (ml/g)

Total yield (%)

Screened yield (%)

Reject (%)

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

14 14 14 14 14 14 14 14 16 16 16 16 16 16 16 16

165 165 165 165 170 170 170 170 165 165 165 165 170 170 170 170

50 60 80 95 30 40 50 60 50 60 80 95 30 40 50 60

623 725 928 1081 642 796 951 1105 623 725 928 1081 642 796 951 1105

30.2 27.9 26.0 25.5 29.9 28.2 27.3 26.5 22.5 22.1 19.6 18.6 23.9 22.4 20.8 18.7

1251 1252 1269 1247 1287 1313 1327 1319 1268 1271 1284 1230 1227 1253 1276 1257

54.7 53.4 53.3 54.2 55.4 53.8 53.8 52.6 51.9 51.6 51.5 51.5 53.6 51.9 51.9 51.6

50.8 50.3 50.5 51.3 52.1 50.8 50.6 50.3 49.6 49.3 48.7 48.4 50.9 49.3 48.5 48.2

0.4 0.2 0.1 0.1 0.6 0.3 0.3 0.2 ND ND ND ND 0.3 0.1 0.2 0.1

ND: not detected.

favored the dissolution of lignin fragments from the fibers. However, in these experiments, kappa number variations, caused by the difference in maximum cooking temperature were relatively low. Viscosities in all pulps were higher than 1230 ml/g and the variations between the different pulps were relatively small (less than 100 ml/g). In cases of 14% EA at 170 ◦ C (cooks 23–26 in Table 6) viscosities were somewhat higher. An increase in the H-factor within the present experimental range led to

a decrease in kappa number and yield, as shown in Figs. 5 and 6, respectively. Viscosity first rose when the H-factor increased and started to drop when the H-factor increased above about 900 (Fig. 7). A similar phenomenon was observed in the cases of the cooks at 165 ◦ C (cooks 19–22 and 27–30 in Table 6). It is known that during alkaline cooking of wood pulp viscosity loss occurs due to random alkaline-catalyzed cleavage of glycosidic bonds in polysaccharide chains (Alén, 2000). A higher H-factor, which leads to lower

Fig. 5. Effect of cooking temperature and time (H-factor) on kappa number of bamboo kraft pulp.

Fig. 6. Effect of H-factor on the screened yield of bamboo kraft pulp at a cooking temperature of 170 ◦ C.

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celluloses has probably more prominent effect than the alkaline cleavage of polysaccharides on viscosity. Therefore, the viscosity increased as more hemicelluloses were removed from pulp. It can be concluded that for proper delignification of the bamboo variety studied, an H-factor of about 900 was needed to achieve pulp with the highest viscosity. 3.3. Oxygen delignification

Fig. 7. Effect of H-factor on the viscosity of bamboo kraft pulp at a cooking temperature of 170 ◦ C.

kappa number and pulp yield, typically results in a lower pulp viscosity. This was true for an H-factor higher than about 900 in our case. The increase in viscosity below an H-factor value of 900 can be explained by the relative decrease in the hemicellulose content of the pulp (Feng and Alén, 2001a,b). At the beginning of the cook, the dissolution of hemi-

Some of the properties of the kraft pulps before and after oxygen delignification are presented in Table 7. It can be seen that the degree of delignification at the same sodium hydroxide charge was dependent on initial kappa number. For pulps with an initial kappa number of 11–22 (pulps from cooks 5 and 8–18), the degree of delignification was between 44 and 48% and final kappa numbers of between 6 and 12 were obtained. These results indicated that kraft pulps with a low kappa number could also be readily delignified by oxygen delignification. The lower the kappa number after cooking, the lower the kappa number after oxygen delignification. For pulps with an initial kappa number of 23–30 (pulps from cooks 2–4 and

Table 7 Some properties of the kraft pulps before and after oxygen delignification (for treatment conditions, see Table 4) Pulp from cook number

Cooking conditionsa EA (%)

2 3 4 5 7 8 9 10 12 13 14 16 17 18 2b 3b 4b 7b

14 14 14 14 16 16 16 16 18 18 18 20 20 20 14 14 14 16

a b

Sulfidity (%)

Initial kappa number

Final kappa number

Kappa number units

15 25 35 45 15 25 35 45 15 25 35 15 25 35 15 25 35 15

30.1 25.5 23.6 22.1 23.3 18.6 16.7 16.8 18.6 15.5 13.2 15.2 12.9 11.2 30.1 25.5 23.6 23.3

19.2 14.6 13.1 11.9 13.9 10.4 8.8 8.7 10.5 8.3 6.9 8.0 6.7 5.9 16.1 12.2 11.2 11.4

10.9 10.9 10.5 10.2 9.4 8.2 7.9 8.1 8.1 7.2 6.3 7.2 6.2 5.3 14.0 13.3 12.4 11.9

For other cooking conditions, see Table 2. NaOH charge was 3%, for other samples NaOH was 2%.

Reduction of kappa number Percentage

Initial viscosity (ml/g)

Final viscosity (ml/g)

Reduction of viscosity (ml/g)

36 43 44 46 40 44 47 48 44 46 48 47 48 47 47 52 53 51

1207 1247 1319 1294 1164 1230 1272 1311 1204 1192 1122 1106 1091 1080 1207 1247 1319 1164

1121 1118 1145 1137 1079 1070 1127 1115 1066 1039 1010 951 933 921 1056 1035 1051 985

86 129 174 157 85 160 145 196 138 153 112 155 158 159 151 212 268 179

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7), the degree of delignification was between 36 and 44% and the final kappa numbers were between 13 and 19. When the alkali charge for the higher kappa number pulps (2b –4b and 7b ) was increased, the degree of delignification rose to 47–53% and resulted in a final kappa number of 11–16. The results suggested that the lignin content in one-stage oxygen-delignified pulp could be reduced to a low level (kappa number 11–12) either by reducing the residual lignin during cooking or by increasing the alkali charge at the oxygen delignification stage. However, pulp with lower lignin content (kappa number 7–9) and good viscosity could only be obtained if the amount of residual lignin was sufficiently low after cooking. The viscosities of the oxygen-delignified kraft pulps were above 1000 ml/g except for the pulp cooked with the highest EA (20% EA, pulps from cooks 16–18). At the final kappa number of 8.7–12.2, viscosity was highest in the cases of the pulps from high sulfidity cooks (35–45%) together with a low alkali charge (2%) in the oxygen delignification stage (pulps from cooks 5, 9, and 10). At a very low final kappa number of 6.7–8.3, viscosity was about 80–90 ml/g higher in the cases of cooks with a higher sulfidity and lower EA (pulps from cooks 13 and 14) compared with cooks with a lower sulfidity and higher EA (pulps from cooks 16 and 17). At a final kappa number of about 6 (pulp from cook 18), viscosity was quite low owing to the low viscosity of the cooked pulp. It can be seen from this result that delignification of kraft pulp to a kappa number as low as 6 by one-stage oxygen delignification results in severe pulp strength reduction, as indicated by the low viscosity observed. It thus proved possible to delignify some of the selected bamboo kraft pulps by oxygen delignification to pulps with a very low kappa number (7–9) and a good viscosity (>1000 ml/g). This result is of considerable interest to modern pulp mills, aiming, for example, to utilize the ECF or TCF bleaching techniques. However, more work needs to be done to clarify the bleaching ability, and especially pulp strength properties, of these pulps with a low residual lignin content.

4. Conclusions In this study, bamboo was delignified by kraft pulping and the subsequent oxygen–alkali delignification

to pulp with a low kappa number. The most important findings from the different delignifying experiments were: Both EA and sulfidity had a significant effect on the kappa number and viscosity of the kraft pulps obtained. High sulfidity (35–45%) with lower EA (14–16%) resulted in both higher pulp viscosity and yield compared with low sulfidity (0–15%) with higher EA (16–18%) at the same degree of delignification. Kraft pulps with a high cooking yield and good viscosity were easily delignified by oxygen delignification to a low kappa number (7–9) without any significant loss (110–200 ml/g) in viscosity. The higher level of viscosity obtained from the high sulfidity and low EA cooks compared to the low sulfidity and high EA cooks was also observed in the oxygen-delignified pulps. Acknowledgements Financial support from the Finnish Ministry of Education within the framework of the International Doctoral Program in Pulp and Paper Science and Technology (PaPSaT), is gratefully acknowledged. The authors also wish to thank the Vietnam Paper Corp. and Dongnai Paper Raw Material Co. for kindly providing the bamboo raw material. References Alén, R., 2000. Basic chemistry of wood delignification. In: Stenius, P., (Ed.), Papermaking Science and Technology. Book 3. Forest Products Chemistry. Fapet Oy, Helsinki, Finland, pp. 59–104. Assumpcao, R.M.V., 1992. UNIDO’s program in non-wood pulping and papermaking. In: Proceedings of the 2nd International Non-wood Fiber Pulping and Papermaking Conference, Shanghai, China, vol. 1, pp. 5–19. Atchison, J.E., 1998. Update on global use of non-wood plant fibers and some prospects for their greater use in the United States. In: Proceedings of the 1998 TAPPI North American Non-Wood Fiber Symposium, pp. 13–42. Bhargava, R.L., 1987. Bamboo. In: Hamilton, F., Leopold, B., (Eds.), Pulp and Paper Manufacture, vol. 3. Secondary Fibers and Non-wood Pulping, third ed. The Joint Textbook Committee of the Paper Industry (TAPPI–CPPA), Atlanta, USA, pp. 71–81. Clayton, D., Easty, D., Einspahr, D., Lonsky, W., Malcolm, E., McDonough, T., Schroeder, L., Thompson, N., 1989. Chemistry

T.H. Mˆan Vu et al. / Industrial Crops and Products 19 (2004) 49–57 of alkaline pulping. In: Grace, T.M., Malcolm, E.W., (Eds.), Pulp and Paper Manufacture, vol. 5. Alkaline Pulping, third ed. The Joint Textbook Committee of the Paper Industry (TAPPI–CPPA), Atlanta, USA, pp. 105–113. Feng, Z., Alén, R.J., 2001a. Soda-AQ pulping of wheat straw. Appita J., 54 (2) (Pulping Suppl.), 217–220. Feng, Z., Alén, R., 2001b. Soda-AQ pulping of reed canary grass. Ind. Crops Prod. 14 (1), 31–39. Gomide, J.L., Colodette, J.L., Campos. A.S., 1991. Kraft pulping and oxygen delignification of bamboo. In: Proceedings of the 1991 Pulping Conference, 3–7 November 1991, Orlando, FL, USA. Book 1. pp. 419–426. Gullichsen, J., 1999. Fiber line operations. In: Gullichsen, J., Fogelholm, C.-J., (Eds.), Papermaking Science and Technology. Book 6A. Chemical Pulping. Fapet Oy, Helsinki, Finland, p. A51. Ilvessalo-Pfäffli, M.-S., 1995. Fiber Atlas—Identification of Papermaking Fibers. Springer, Heidelberg, Germany, p. 316. Kishore, H., Chand, S., Rawat, N., Gupta, M.K., Janade, V.T., Bist, V., Roy, T.K., Pant, R., Jauhari, M.B., 1995. Oxygen

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treatment followed by CEH bleaching: economic and quick solution to the pollution problem of Indian paper mills. In: Proceedings of the 2nd International Seminar on Pulp and Paper Industry “Paperex 95”, New Delhi, India, 9–11 December 1995, pp. 69–83. Misra, D.K., 1980. Pulping and bleaching of nonwood fibers. In: Casey, J.P., (Ed.), Pulp and Paper, Chemistry and Chemical Technology, vol. 1, third ed. Wiley, New York, USA, p. 552. Mittal, S.K., Maheshwari, S., 1996. Mill experience on manufacture of ECF bamboo market pulp. In: Proceedings of the 3rd International Non-wood Fiber Pulping and Papermaking Conference, Beijing, China, vol. 1, pp. 378–392. Rydholm, S.A., 1965. Pulping Processes. Interscience Publishers, Wiley, New York, USA, p. 687. Singh, S.V., Datta, S.K., Rai, A.K., 1995. Delignification of kraft pulp with oxygen-chlorine combinations. IPPTA 7 (3), 57–63. Sjöblom, K., 1996. Extended delignification in kraft cooking through improved selectivity. Part 5. Influence of dissolved lignin on the rate of delignification. Nord. Pulp Pap. Res. J. 11 (3), 177–185, 191.