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Alkaline pulping with additives of kenaf from Sudan
Industrial Crops and Products 15 (2002) 229– 235 www.elsevier.com/locate/indcrop
Alkaline pulping with additives of kenaf from Sudan P. Khristova a, O. Kordsachia b,*, R. Patt b, T. Khider c, I. Karrar a a
b
Uni6ersity of Khartoum, People’s Hall 11113, P.O. Box 6272, Khartoum, Sudan Institute of Wood Chemistry, Technologie des Holzes, Leuschnerstr. 91, 2107 Hamburg, Germany c Faculty of Applied and Industrial Sciences, Uni6ersity of Juba, Sudan Accepted 3 December 2001
Abstract Soda-anthraquinone (soda-AQ), alkaline sulfite-anthraquinone (AS-AQ) and ASAM (alkaline sulfite-anthraquinone-methanol) cooking of kenaf core, bark and whole stalk from Sudan was carried out under different conditions and pulps with good to very good yields and mechanical properties were obtained. The AS-AQ process was particularly well suited for kenaf whole stalk and bark with yield, viscosity, brightness and strength properties superior to those of soda and soda-AQ pulping. ASAM pulping provided a further selective delignification with higher yield, lower k number and superior viscosity and brightness at high pulp strength level. Blending 90 and 70% kenaf core pulp with pulp from kenaf bark improved all the strength and optical properties, with the 70% blends better. The application of a totally chlorine free (TCF) bleaching sequence of OQ1(PO)Q2P on whole kenaf stalk soda-AQ and ASAM pulps gave pulp very high brightness and strength properties. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hibiscus cannabinus-kenaf; Papermaking; Fiber morphology; Chemical composition; Alkaline pulping with additives; Blending; Bleaching
1. Introduction Kenaf (Hibiscus cannabinus) is a herbaceous annual plant grown in many parts of the tropics and in some sub-tropical and warm temperate areas for its bark fibers used as a substitute for jute in cordage and sacking. In Sudan, kenaf is grown extensively as a cordage fiber in Abu-Namaa area (Central Sudan), characterized by high
* Corresponding author. Tel./fax: + 49-40-73962-502. E-mail address: [email protected] (O. Kordsachia).
savanna conditions with heavy rains. Kenaf can produce two crops per year. Yields are 16–19 air-dry metric tons/ha per year. Sufficient rainfall or irrigation and the use of fertilizers are necessary for optimum yield. The kenaf plant contains two distinct fiber components, bark and core. The bark (bast) fibers constitute 35–40% and the core (woody) fibers 60–65% by weight of the stalk (Touzinski et al., 1973; James and McCamley, 1981; Kaldor et al., 1990; Zhou et al., 1997). The separation of the bark and core by simple mechanical and screening treatments has made possible the use of bark alone to produce a high quality long fiber pulp (Watson and Gartside,
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1976; James and McCamley, 1981). The yield and strength properties of soda and sulfate pulps from kenaf bark were approximately equivalent under identical cooking conditions. Soda pulp consumed higher bleaching chemical, but showed less degradation than sulfate pulp (Sharma et al., 1985). Cold soda and alkaline sulfite processes gave high-yield (72–88%) pulps with satisfactory strength properties, brightness and opacity. Blending of these pulps with commercial bleached bamboo pulps further improved the strength properties and brightness (Sharma et al., 1984). Kenaf bast fibers cooked by kraft, kraft-AQ, soda, soda-AQ processes gave pulp yields in the range of 51–60%. Kenaf core fibers treated with the same chemical pulping processes gave pulp yields of 40–54% (Hart et al., 1991). Kraft and soda pulping of kenaf gave similar yields and papermaking properties. The purpose of the present work was to evaluate the suitability for papermaking of the Sudanese varieties of kenaf by the soda and alkaline sulfite methods with additives separately for the bark, core and the whole stalk. Blending trials should show whether the strength properties of kenaf core pulp could be considerably improved by blending with kenaf core pulp.
2. Materials and methods Kenaf stalks were collected from Abu Namaa scheme (Central Sudan), grown on fertile clay soil under high savanna conditions. The 2– 3 m high kenaf stalks were 3.5–6.0 cm in diameter at the centre of the stalk. The stalks were chopped to 3– 5 cm and the kenaf bark and core were manually separated. The soda-AQ pulp fibers were used for fiber measurement after dispersion in water and staining with 1% aqueous safranine solution and mounted on slides (Horn, 1978). Fiber measurements were microscopically carried out at × 300 and ×400 magnification. A star mill ground representative sample of the stalks 40– 60 mesh fraction was used for chemical analyses by the Tappi test methods, except the Kurschner– Hoffer cellulose (Obolenskaya et al., 1965). The pulping trials with soda-AQ (Holton, 1977), AS-AQ and
ASAM (Kordsachia and Patt, 1988) methods against a reference soda cooking with different levels of alkali charge were carried out in a 7-l electrically heated rotary digester. A Jokro mill was used for beating. Pulps characteristics and strength properties were determined according to Tappi and German Zellcheming standards. Bleaching was carried out with ASAM and sodaAQ whole kenaf stalk pulps with an oxygen delignification (O)-chelation (Q)-pressurized peroxide (PO)-chelation (Q)-peroxide (P) sequence (Khider, 2001).
3. Results and discussion The kenaf bast fibers average length (2.4 mm) was in the range of softwoods and bamboo (1–7 mm), whilst the woody portion fiber length (0.7 mm) was similar to that of hardwoods (0.7–1.5 mm). The quite wide core fibers (28.7 mm) had medium thick walls (4.2 mm) in contrast to less wide (19.9 mm) but almost of the same thickness bast fibers. The ash contents of the bark (KB), core (KC) and whole kenaf stalk (KW) were rather high with 2.9–4.0% (Table 1), but typical for tropical non-woody plants. However, the silica contents were B 0.2% (0.06–0.13%). The hot water extractives (4.4–9.5%), organic solvents extractives (2.0–3.0%) and the 1% NaOH solubles (26.1– 29.3%) were rather high due to the presence of many soluble polysaccharides and phenolic compounds. The good Kurschner–Hoffer cellulose contents for kenaf bark and whole kenaf stalk implied good pulp yields to be expected. The total lignin content of kenaf bark (12.8%) was low and moderate for the whole kenaf stalk (15.6%) and the kenaf core (19.6%).
3.1. Pulping of kenaf bark In soda pulping of kenaf bark (KB3), carried out as reference cook with an active alkali level of 17% as Na2O, a k number of 26.8 was attained (Table 2). The addition of 0.1% anthraquinone to the cooking liquor (KB2) at lower effective alkali charge (15%) gave a much higher degree of delig-
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loses, which is an essential advantage with regard to the fiber bonding properties. The strength properties of the unbleached kenaf bark pulps (Table 2) were quite high, especially for the AS-AQ pulp. The tensile and burst strengths of the soda-AQ were lower than for the soda reference, which might be attributed to some carbohydrate degradation associated with the more intense delignification.
nification (k number 12.5) and an increase in yield by 3.9% at a lower reject content. Moreover, brightness and pulp viscosity were improved. The alkaline sulfite-anthraquinone (AS-AQ) (KB1) pulping of kenaf bark seemed attractive from many points. A very high yield of almost 60%, high viscosity of 1218 ml/g and comparatively high initial ISO brightness of 34% were achieved. An alkali ratio (Na2SO3/NaOH) of 70:30 was used at high cooking temperature of 175 °C and high active alkali charge of 17% as Na2O on oven-dry pulp (odp). Although the AS-AQ pulping needed a higher alkali charge (17%) and a higher cooking temperature (175 °C), its pulp had much higher viscosity compared to those of soda and soda-AQ pulps (854 and 857 ml/g). The comparatively easy delignification with the AS-AQ process makes it well suited for production of pulps rich in hemicellu-
3.2. Pulping of kenaf core Due to its much higher lignin content, kenaf core is more difficult to pulp than kenaf bark and the k number-yield relationship is less favorable. Thus, a k number B 20 was not achieved with the conditions chosen (Table 3). The ASAM process resulted in the lowest k number of 21.9 at the highest pulp viscosity and brightness. The
Table 1 Chemical composition of kenaf core and bark from Sudan compared with data from other locations (all values expressed as percent oven-dry solubles or components on oven-dry raw material) Component
Hibiscus cannabinus
Origin, reference
Sudan Bark
Ash Silica Solubles in: Hot water 1% NaOH Alcohol-benzene (1/2) Total extractives Cellulose, Kurschner–Hoffer Holocellulose a-Cellulose Pentosans Lignina Acid-soluble lignin Cellulose/lignin ratio a
Corrected for ash. Alcohol-cyclohexane (1/2). * Khristova et al. (1998). ** CTFT (1987). *** Touzinski et al. (1973). n/a, Not available. b
Sudan* Core
4 0.06
2.9 0.1
6.7 26.1 2.0b 9.3 53.8 N/a N/a N/a 8.1 4.7 4.2
4.4 29.3 3.0b 7.3 45.7 n/a n/a n/a 19.6 3.5 2.3
France**
USA***
Whole stalk 3.4 0.13 9.5 28.4 2.9 n/a 49.1 78.9 49.5 18.6 15.6 n/a 3.1
5.9 0.03 9.3 32.2 5.6 n/a 47.6 n/a n/a n/a 19.3 n/a 3.3
4.1 n/a 10.2 54.4 3.1 54.4 n/a n/a 37.4 n/a 19.7 n/a 4.1
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Table 2 Hibiscus cannabinus-kenaf bark (KB): pulping conditions and results Cooking process Pulp code Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-bark ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
Soda KB3
Soda-AQ KB2
AS-AQ KB1
17
15
4 165
0.1 4 165
60
60
70
120
120
120
50.2 1.2 26.8 834 16.5
54.1 0.3 12.5 857 22.5
59.6 0.9 19.1 1218 34.7
Handsheet properties at 40 °SR Tear index (mN/m2/g) 9.2 Tensile index (N/m/g) 69.9 Burst index (kPa/m2/g) 4.6 Runnability factor 8.1 Light scattering 19.7 coefficient (m2/kg)
11.6 62.8 4.2 9.0 21.1
11.9 79.9 5.1 9.9 19.8
Pulping results Total yield (%) Rejects (%) k Number CED viscosity (ml/g) ISO brightness (%)
17 70:30 0.1 4 175
screened yield was only slightly lower than for AS-AQ cooking, which gave a much higher k number of 42.9. Low yields were obtained with the soda process, especially in the case of the control cooking without AQ addition. The viscosities were also significantly lower as for the alkaline sulfite pulp. Comparison of the strength properties of the different kenaf core pulps indicated, in general, the superior strength properties of the ASAM pulps. The high tensile strength, which is mainly based on the good bonding ability of the fibers, results from the high carbohydrate content of the ASAM pulps due to the high stability of xylan and cellulose in the outer cell wall layers. However, the AS-AQ kenaf core pulp had an even higher tensile strength compared to ASAM
pulp, which can be explained by the much lower delignification rate. The soda pulps showed much lower tensile strength, but the tear resistance was slightly higher. The runnability factor indicates that the ASAM pulp had the best overall strength among the pulps tested. From Tables 2 and 3 it is obvious that the kenaf bark and core behave very differently in cooking and the pulps differ considerably in their properties, particularly with regard to the tear–tensile relationship. Therefore, it was of interest to conduct pulping trials with whole kenaf stalk.
3.3. Pulping of whole kenaf stalk ASAM pulping of whole kenaf stalk gave lowest k number (15.5), highest screened yield (52.1%), viscosity (1191 ml/g) and brightness (41.9), while those parameters were the lowest for the soda process (Table 4). Soda-AQ pulping also gave a fairly good k number–yield relationship, but much lower viscosity of 814 ml/g and ISO brightness of 22.2% were attained. The soda and AS-AQ cooks were carried out at higher alkali charge of 19%, nevertheless delignification was not sufficient and high reject contents were obtained in both cases. Concerning pulp strength, ASAM and soda-AQ pulping gave similar results but with higher tear strength for the soda-AQ and higher tensile strength for ASAM pulp. Comparison of kenaf bark, kenaf core and whole kenaf stalk as separate raw materials shows that kenaf bark was easy to cook and gave good yields of pulp with high tear resistance, while kenaf core was much more difficult to pulp and gave significantly lower yield at higher k number. Kenaf core pulps showed high bonding ability and thus, high tensile strength. For whole kenaf stalks intermediate results were obtained. Although kenaf bark pulp seems the most suitable raw material for blending with shortfibered raw materials to improve their physical properties, especially the tear resistance, it is obviously possible and attractive to use the whole kenaf stalk pulp for the same purpose. In partic-
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ular, kenaf can easily be cooked with the ASAM process yielding pulps with low k number, high viscosity and initial brightness.
3.4. Blending kenaf bark with kenaf core pulps The kenaf core soda-AQ and AS-AQ pulps were blended in ratios of 90/10 and 70/30 with the corresponding kenaf bark pulps (Table 5). With both ratios the tear index, the overall strength (runnability factor) and light scattering coefficient were improved for both kenaf core pulps. In the case of soda-AQ pulps, the tensile index was also improved, but for AS-AQ blends, the values were somewhat lower than for the 100% kenaf core pulp (Table 3). The highest tensile index was reached for the 70% AS-AQ blends. The benefits of blending pulps of kenaf core with kenaf bark at different ratios are evident. Compared with the whole kenaf stalk pulps with higher share of bark fibres (Table 4), the AS-AQ blends had higher and the soda-AQ blends somewhat lower mechanical strength properties.
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3.5. Bleaching of whole kenaf stalk pulp TCF bleaching of the soda-AQ and the ASAM whole kenaf pulp applying the sequence OQ1(OP)Q2P resulted in a final brightness of 84% for soda-AQ and 88% for the ASAM pulp (Table 6). In total, the peroxide charge was 4% for both pulps. The bleached ASAM pulp had a lower k number of 2.7 and a better viscosity of 994 ml/g compared to the soda-AQ pulp (4.3 and 737 ml/g, respectively). In oxygen delignification of whole kenaf stalk pulp, a nearly 50% delignification was achieved. The k number of the soda-AQ pulp was reduced from 20.4 to 10.5 and the brightness was improved by 38% with only small losses in viscosity. The k number of ASAM pulp dropped from 15.5 to 7.8 and the viscosity from 1191 to 1073 ml/g at a considerable gain in brightness of 18%. As the degree of polymerization at given k number is regarded as a measure of selectivity of the delignification (Gevert and Lohmander, 1997), the ASAM pulp showed better bleaching selectivity than the soda-AQ pulp. The chelation stages with
Table 3 Hibiscus cannabinus-kenaf core (KC): pulping conditions and results Cooking process Pulp code
Soda KC4
Soda-AQ KC2
AS-AQ KC1
ASAM KC3
17 70: 30 0.1 4 175 70 120
17 70:30 0.1 4 175 70 120
Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-core ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
19
17
4 165 60 120
0.1 4 165 60 120
Cooking results Total yield (%) Rejects (%) k Number CED viscosity (ml/g) ISO brightness (%)
43.3 2.9 25.4 765 29.8
47.8 1.9 22.3 717 25.6
51.3 3.3 42.9 1064 29.6
48.9 1.6 21.9 1073 38.1
67.2 5.4 3.5 6.1 30.1
61.1 5.4 3.4 5.8 28.0
96.5 4.3 5.0 6.5 n/a
82.3 5.4 4.6 6.7 19.1
Handsheet properties at 40 °SR Tensile index (N/m/g) Tear index (mN/m2/g) Burst index (kPa/m2/g) Runnability factor Light scattering coefficient (m2/kg)
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Table 4 Hibiscus cannabinus-whole kenaf stalk (KW): pulping conditions and results Cooking process Pulp code
Soda KW4
Soda-AQ KW1
AS-AQ KW2
ASAM KW3
19 70: 30 0.1 4 175 70 120
17 70: 30 0.1 4 175 70 120
Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-stalks ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
19
17
4 165 60 120
0.1 4 165 60 120
Pulping results Total yield (%) Rejects (%) k No. CED viscosity (ml/g) ISO brightness (%)
45.1 4.4 26.5 803 21.3
49.9 0.4 20.4 814 22.2
54.9 12.8 32.2 1187 29.3
52.6 0.5 15.5 1191 41.9
72.5 9.3 5.5 8.3 23.6
96.9 10.2 5.6 10.1 20.4
96.3 8.2 6.1 9.0 17.1
101.9 9.4 7.4 10.1 16.9
Handsheet properties at 40 °SR Tensile index (N/m/g) Tear index (mN/m2/g) Burst index (kPa/m2/g) Runnability factor LSC (m2/kg)
EDTA addition prior the PO and P stages gave a remarkable gain in brightness of : 13 and 5% ISO in the first and second Q stage. The rather high tensile strength and good tear resistance of the unbleached whole kenaf stalk pulps were decreased after TCF bleaching (Tables 4 and 6). As expected, the tensile index of the soda-AQ pulp was lower than that of the ASAM pulp, which also had higher final viscosity. It seems that the fiber weakening indicated by the viscosity reduction during bleaching was fully Table 5 Blending kenaf core (KC) with kenaf bark (KB) unbleached pulps Cooking process Cook code Blending ratio
Soda-AQ KC2+KB2 90/10 70/30
AS-AQ KC1+KB1 90/10 70/30
Tear index (mN/m2/g) Tensile index (N/m/g) Burst index (kPa/m2/g) Runnability factor LSC (m2/kg)
6.7 71.7 3.9 7.0 26.7
6.5 87.2 5.8 7.6 17.3
8.3 69.6 4.1 7.7 27.4
6.4 95 6.1 7.9 16.5
compensated by the improved bonding ability of the bleached fibers. In general, the bleaching process led to a certain leveling of the differences in the tensile strength that existed between the two unbleached whole kenaf stalk pulps. 4. Conclusions The fiber dimensions are specific for each part of the kenaf stalk, with the bast fibers similar to Table 6 Whole kenaf stalk (KW) bleached pulp evaluation Pulping process Pulp code
Soda/AQ KW1
ASAM KW3
ISO brightness (% ISO) CED viscosity (ml/g) k No.
83.9 737 4.3
88.0 994 2.70
8.2 89.4 5.1 24.3
8.2 97.8 5.5 18.7
Handsheet properties at 40 °SR Tear index (mN/m2/g) Tensile index (N/m/g) Burst index (kPa/m2/g) LSC (m2/kg)
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softwoods and the woody core similar to hardwoods. The chemical composition is characterized by low to moderate lignin content and good carbohydrate content. The reference soda pulping of all raw materials yielded \40% of bleachable grade pulp with satisfactory strength characteristics. The addition of AQ in soda cooking accelerated the delignification, reduced alkali consumption, k number and brightness, increased yield and viscosity and gave pulps with properties superior to those of reference soda pulping. The AS-AQ process was particularly well suited for pulping of kenaf bark, but not for kenaf core, which behaved like hardwood. With kenaf bark yield, viscosity, brightness and the strength properties were superior to those of soda and soda-AQ pulps. In ASAM pulping, the synergistic effect of AQ and methanol provided a far-reaching selective delignification. Higher yield, lower k numbers and superior viscosity and brightness were obtained at high pulp strength level. The use of whole kenaf stalk without separation of core and bark gives pulps with very good quality a high yield, especially with the ASAM process. Thus, it seems possible to avoid the high cost of separation, which would represent a problem for commercialization of this raw material. The application of a TCF bleaching sequence gave pulp very high brightness and strength properties, especially with the ASAM pulps. Blending of kenaf core and bark pulps might enhance the commercial utilization of these raw materials for improved paper end product. In this respect, kenaf bark is a strong contender to meet part of the additional fiber supply requirement as a high quality, high yield option.
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References CTFT, 1987. Annual Report of Cellulose and Chemistry Division of CTFT, Nogent-sur-Marne, France. Gevert, B., Lohmander, S., 1997. Influence of sulphur compounds, manganese and magnesium on oxygen bleaching of kraft pulp. Tappi J. 80 (10), 263 – 268. Hart, P.W., Brogdon, B.N., Hsieh, J.S., 1991. Anthraquinone pulping of non-wood species. Nonwood Plant Fibers. Progress Report No. 21, 183 – 191. Holton, H.H., 1977. Soda additive softwood pulping a major new process. Pulp Paper Can. 78, T218 – T223. Horn, R.A., 1978. Morphology of Pulp Fiber from Hardwoods and Influence on Paper Strength. Forest Products Lab Report 312, Madison. James, K.R., McCamley, J., 1981. Separation of core and bark from forage harvested kenaf. In: I.M. Wood, G.A. Stuart (Eds.), Proceedings of the Kenaf Conference (Kenaf as a Potential Source of Pulp in Australia), Brisbane, Queensland, pp. 69– 72. Kaldor, A.F., Karlgren, C., Verwest, H., 1990. Kenaf, a fast growing fiber source for papermaking. Tappi J. 72, 205 – 209. Khider, T., 2001. Pulping Potentialities of Some Sudanese Nonwood Species. Ph.D. Thesis, University of Khartoum. Khristova, P., Bentchgeva, S., Karar, I., 1998. Soda-AQ pulping of four Acacias from Sudan. Cell. Chem. Technol. 32, 299 – 307. Kordsachia, O., Patt, R., 1988. Full bleaching of ASAM pulps without chlorine compounds. Holzforschung 42, 203 – 209. Obolenskaya, A.V., Tshegolev, V.P., Akim, G.L., Kossovitcz, N.C., Emelyanova, I.Z., 1965. Prakticheskie Raboti po Himii Drevesini I Tzelulozi. Lesprom, Moscow 411 pp. Sharma, Y.K., Dhawan, R., Karira, B.G., 1984. High yield pulps from kenaf. Ind. Forest. 110, 401 – 406. Sharma, Y.K., Dhawan, R., Karira, B.G., 1985. Pulping kenaf bark — soda vs. sulfate. Ind. Forest. 111, 832 – 836. Touzinski, G.F., Clark, T.F., Tallant, W.H., Knolek, W.F., 1973. Soda pulps from kenaf bark and from core. In: Non-wood Plant Fibre Pulping. CA Report No. 52. Tappi, Atlanta. Watson, A.J., Gartside, B.G., 1976. Using woody fibre from agricultural crops. Aus. For. 39 (1), 16 – 22. Zhou, C., Ohtani, Y., Sameshima, K., Zhen, M., 1997. Selection of kenaf (Hibiscus cannabinus L.) varieties for papermaking on arid hillside land in China. Mok. Gakkais. 43 (9), 770 – 777.
Alkaline pulping with additives of kenaf from Sudan P. Khristova a, O. Kordsachia b,*, R. Patt b, T. Khider c, I. Karrar a a
b
Uni6ersity of Khartoum, People’s Hall 11113, P.O. Box 6272, Khartoum, Sudan Institute of Wood Chemistry, Technologie des Holzes, Leuschnerstr. 91, 2107 Hamburg, Germany c Faculty of Applied and Industrial Sciences, Uni6ersity of Juba, Sudan Accepted 3 December 2001
Abstract Soda-anthraquinone (soda-AQ), alkaline sulfite-anthraquinone (AS-AQ) and ASAM (alkaline sulfite-anthraquinone-methanol) cooking of kenaf core, bark and whole stalk from Sudan was carried out under different conditions and pulps with good to very good yields and mechanical properties were obtained. The AS-AQ process was particularly well suited for kenaf whole stalk and bark with yield, viscosity, brightness and strength properties superior to those of soda and soda-AQ pulping. ASAM pulping provided a further selective delignification with higher yield, lower k number and superior viscosity and brightness at high pulp strength level. Blending 90 and 70% kenaf core pulp with pulp from kenaf bark improved all the strength and optical properties, with the 70% blends better. The application of a totally chlorine free (TCF) bleaching sequence of OQ1(PO)Q2P on whole kenaf stalk soda-AQ and ASAM pulps gave pulp very high brightness and strength properties. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hibiscus cannabinus-kenaf; Papermaking; Fiber morphology; Chemical composition; Alkaline pulping with additives; Blending; Bleaching
1. Introduction Kenaf (Hibiscus cannabinus) is a herbaceous annual plant grown in many parts of the tropics and in some sub-tropical and warm temperate areas for its bark fibers used as a substitute for jute in cordage and sacking. In Sudan, kenaf is grown extensively as a cordage fiber in Abu-Namaa area (Central Sudan), characterized by high
* Corresponding author. Tel./fax: + 49-40-73962-502. E-mail address: [email protected] (O. Kordsachia).
savanna conditions with heavy rains. Kenaf can produce two crops per year. Yields are 16–19 air-dry metric tons/ha per year. Sufficient rainfall or irrigation and the use of fertilizers are necessary for optimum yield. The kenaf plant contains two distinct fiber components, bark and core. The bark (bast) fibers constitute 35–40% and the core (woody) fibers 60–65% by weight of the stalk (Touzinski et al., 1973; James and McCamley, 1981; Kaldor et al., 1990; Zhou et al., 1997). The separation of the bark and core by simple mechanical and screening treatments has made possible the use of bark alone to produce a high quality long fiber pulp (Watson and Gartside,
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1976; James and McCamley, 1981). The yield and strength properties of soda and sulfate pulps from kenaf bark were approximately equivalent under identical cooking conditions. Soda pulp consumed higher bleaching chemical, but showed less degradation than sulfate pulp (Sharma et al., 1985). Cold soda and alkaline sulfite processes gave high-yield (72–88%) pulps with satisfactory strength properties, brightness and opacity. Blending of these pulps with commercial bleached bamboo pulps further improved the strength properties and brightness (Sharma et al., 1984). Kenaf bast fibers cooked by kraft, kraft-AQ, soda, soda-AQ processes gave pulp yields in the range of 51–60%. Kenaf core fibers treated with the same chemical pulping processes gave pulp yields of 40–54% (Hart et al., 1991). Kraft and soda pulping of kenaf gave similar yields and papermaking properties. The purpose of the present work was to evaluate the suitability for papermaking of the Sudanese varieties of kenaf by the soda and alkaline sulfite methods with additives separately for the bark, core and the whole stalk. Blending trials should show whether the strength properties of kenaf core pulp could be considerably improved by blending with kenaf core pulp.
2. Materials and methods Kenaf stalks were collected from Abu Namaa scheme (Central Sudan), grown on fertile clay soil under high savanna conditions. The 2– 3 m high kenaf stalks were 3.5–6.0 cm in diameter at the centre of the stalk. The stalks were chopped to 3– 5 cm and the kenaf bark and core were manually separated. The soda-AQ pulp fibers were used for fiber measurement after dispersion in water and staining with 1% aqueous safranine solution and mounted on slides (Horn, 1978). Fiber measurements were microscopically carried out at × 300 and ×400 magnification. A star mill ground representative sample of the stalks 40– 60 mesh fraction was used for chemical analyses by the Tappi test methods, except the Kurschner– Hoffer cellulose (Obolenskaya et al., 1965). The pulping trials with soda-AQ (Holton, 1977), AS-AQ and
ASAM (Kordsachia and Patt, 1988) methods against a reference soda cooking with different levels of alkali charge were carried out in a 7-l electrically heated rotary digester. A Jokro mill was used for beating. Pulps characteristics and strength properties were determined according to Tappi and German Zellcheming standards. Bleaching was carried out with ASAM and sodaAQ whole kenaf stalk pulps with an oxygen delignification (O)-chelation (Q)-pressurized peroxide (PO)-chelation (Q)-peroxide (P) sequence (Khider, 2001).
3. Results and discussion The kenaf bast fibers average length (2.4 mm) was in the range of softwoods and bamboo (1–7 mm), whilst the woody portion fiber length (0.7 mm) was similar to that of hardwoods (0.7–1.5 mm). The quite wide core fibers (28.7 mm) had medium thick walls (4.2 mm) in contrast to less wide (19.9 mm) but almost of the same thickness bast fibers. The ash contents of the bark (KB), core (KC) and whole kenaf stalk (KW) were rather high with 2.9–4.0% (Table 1), but typical for tropical non-woody plants. However, the silica contents were B 0.2% (0.06–0.13%). The hot water extractives (4.4–9.5%), organic solvents extractives (2.0–3.0%) and the 1% NaOH solubles (26.1– 29.3%) were rather high due to the presence of many soluble polysaccharides and phenolic compounds. The good Kurschner–Hoffer cellulose contents for kenaf bark and whole kenaf stalk implied good pulp yields to be expected. The total lignin content of kenaf bark (12.8%) was low and moderate for the whole kenaf stalk (15.6%) and the kenaf core (19.6%).
3.1. Pulping of kenaf bark In soda pulping of kenaf bark (KB3), carried out as reference cook with an active alkali level of 17% as Na2O, a k number of 26.8 was attained (Table 2). The addition of 0.1% anthraquinone to the cooking liquor (KB2) at lower effective alkali charge (15%) gave a much higher degree of delig-
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loses, which is an essential advantage with regard to the fiber bonding properties. The strength properties of the unbleached kenaf bark pulps (Table 2) were quite high, especially for the AS-AQ pulp. The tensile and burst strengths of the soda-AQ were lower than for the soda reference, which might be attributed to some carbohydrate degradation associated with the more intense delignification.
nification (k number 12.5) and an increase in yield by 3.9% at a lower reject content. Moreover, brightness and pulp viscosity were improved. The alkaline sulfite-anthraquinone (AS-AQ) (KB1) pulping of kenaf bark seemed attractive from many points. A very high yield of almost 60%, high viscosity of 1218 ml/g and comparatively high initial ISO brightness of 34% were achieved. An alkali ratio (Na2SO3/NaOH) of 70:30 was used at high cooking temperature of 175 °C and high active alkali charge of 17% as Na2O on oven-dry pulp (odp). Although the AS-AQ pulping needed a higher alkali charge (17%) and a higher cooking temperature (175 °C), its pulp had much higher viscosity compared to those of soda and soda-AQ pulps (854 and 857 ml/g). The comparatively easy delignification with the AS-AQ process makes it well suited for production of pulps rich in hemicellu-
3.2. Pulping of kenaf core Due to its much higher lignin content, kenaf core is more difficult to pulp than kenaf bark and the k number-yield relationship is less favorable. Thus, a k number B 20 was not achieved with the conditions chosen (Table 3). The ASAM process resulted in the lowest k number of 21.9 at the highest pulp viscosity and brightness. The
Table 1 Chemical composition of kenaf core and bark from Sudan compared with data from other locations (all values expressed as percent oven-dry solubles or components on oven-dry raw material) Component
Hibiscus cannabinus
Origin, reference
Sudan Bark
Ash Silica Solubles in: Hot water 1% NaOH Alcohol-benzene (1/2) Total extractives Cellulose, Kurschner–Hoffer Holocellulose a-Cellulose Pentosans Lignina Acid-soluble lignin Cellulose/lignin ratio a
Corrected for ash. Alcohol-cyclohexane (1/2). * Khristova et al. (1998). ** CTFT (1987). *** Touzinski et al. (1973). n/a, Not available. b
Sudan* Core
4 0.06
2.9 0.1
6.7 26.1 2.0b 9.3 53.8 N/a N/a N/a 8.1 4.7 4.2
4.4 29.3 3.0b 7.3 45.7 n/a n/a n/a 19.6 3.5 2.3
France**
USA***
Whole stalk 3.4 0.13 9.5 28.4 2.9 n/a 49.1 78.9 49.5 18.6 15.6 n/a 3.1
5.9 0.03 9.3 32.2 5.6 n/a 47.6 n/a n/a n/a 19.3 n/a 3.3
4.1 n/a 10.2 54.4 3.1 54.4 n/a n/a 37.4 n/a 19.7 n/a 4.1
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Table 2 Hibiscus cannabinus-kenaf bark (KB): pulping conditions and results Cooking process Pulp code Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-bark ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
Soda KB3
Soda-AQ KB2
AS-AQ KB1
17
15
4 165
0.1 4 165
60
60
70
120
120
120
50.2 1.2 26.8 834 16.5
54.1 0.3 12.5 857 22.5
59.6 0.9 19.1 1218 34.7
Handsheet properties at 40 °SR Tear index (mN/m2/g) 9.2 Tensile index (N/m/g) 69.9 Burst index (kPa/m2/g) 4.6 Runnability factor 8.1 Light scattering 19.7 coefficient (m2/kg)
11.6 62.8 4.2 9.0 21.1
11.9 79.9 5.1 9.9 19.8
Pulping results Total yield (%) Rejects (%) k Number CED viscosity (ml/g) ISO brightness (%)
17 70:30 0.1 4 175
screened yield was only slightly lower than for AS-AQ cooking, which gave a much higher k number of 42.9. Low yields were obtained with the soda process, especially in the case of the control cooking without AQ addition. The viscosities were also significantly lower as for the alkaline sulfite pulp. Comparison of the strength properties of the different kenaf core pulps indicated, in general, the superior strength properties of the ASAM pulps. The high tensile strength, which is mainly based on the good bonding ability of the fibers, results from the high carbohydrate content of the ASAM pulps due to the high stability of xylan and cellulose in the outer cell wall layers. However, the AS-AQ kenaf core pulp had an even higher tensile strength compared to ASAM
pulp, which can be explained by the much lower delignification rate. The soda pulps showed much lower tensile strength, but the tear resistance was slightly higher. The runnability factor indicates that the ASAM pulp had the best overall strength among the pulps tested. From Tables 2 and 3 it is obvious that the kenaf bark and core behave very differently in cooking and the pulps differ considerably in their properties, particularly with regard to the tear–tensile relationship. Therefore, it was of interest to conduct pulping trials with whole kenaf stalk.
3.3. Pulping of whole kenaf stalk ASAM pulping of whole kenaf stalk gave lowest k number (15.5), highest screened yield (52.1%), viscosity (1191 ml/g) and brightness (41.9), while those parameters were the lowest for the soda process (Table 4). Soda-AQ pulping also gave a fairly good k number–yield relationship, but much lower viscosity of 814 ml/g and ISO brightness of 22.2% were attained. The soda and AS-AQ cooks were carried out at higher alkali charge of 19%, nevertheless delignification was not sufficient and high reject contents were obtained in both cases. Concerning pulp strength, ASAM and soda-AQ pulping gave similar results but with higher tear strength for the soda-AQ and higher tensile strength for ASAM pulp. Comparison of kenaf bark, kenaf core and whole kenaf stalk as separate raw materials shows that kenaf bark was easy to cook and gave good yields of pulp with high tear resistance, while kenaf core was much more difficult to pulp and gave significantly lower yield at higher k number. Kenaf core pulps showed high bonding ability and thus, high tensile strength. For whole kenaf stalks intermediate results were obtained. Although kenaf bark pulp seems the most suitable raw material for blending with shortfibered raw materials to improve their physical properties, especially the tear resistance, it is obviously possible and attractive to use the whole kenaf stalk pulp for the same purpose. In partic-
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ular, kenaf can easily be cooked with the ASAM process yielding pulps with low k number, high viscosity and initial brightness.
3.4. Blending kenaf bark with kenaf core pulps The kenaf core soda-AQ and AS-AQ pulps were blended in ratios of 90/10 and 70/30 with the corresponding kenaf bark pulps (Table 5). With both ratios the tear index, the overall strength (runnability factor) and light scattering coefficient were improved for both kenaf core pulps. In the case of soda-AQ pulps, the tensile index was also improved, but for AS-AQ blends, the values were somewhat lower than for the 100% kenaf core pulp (Table 3). The highest tensile index was reached for the 70% AS-AQ blends. The benefits of blending pulps of kenaf core with kenaf bark at different ratios are evident. Compared with the whole kenaf stalk pulps with higher share of bark fibres (Table 4), the AS-AQ blends had higher and the soda-AQ blends somewhat lower mechanical strength properties.
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3.5. Bleaching of whole kenaf stalk pulp TCF bleaching of the soda-AQ and the ASAM whole kenaf pulp applying the sequence OQ1(OP)Q2P resulted in a final brightness of 84% for soda-AQ and 88% for the ASAM pulp (Table 6). In total, the peroxide charge was 4% for both pulps. The bleached ASAM pulp had a lower k number of 2.7 and a better viscosity of 994 ml/g compared to the soda-AQ pulp (4.3 and 737 ml/g, respectively). In oxygen delignification of whole kenaf stalk pulp, a nearly 50% delignification was achieved. The k number of the soda-AQ pulp was reduced from 20.4 to 10.5 and the brightness was improved by 38% with only small losses in viscosity. The k number of ASAM pulp dropped from 15.5 to 7.8 and the viscosity from 1191 to 1073 ml/g at a considerable gain in brightness of 18%. As the degree of polymerization at given k number is regarded as a measure of selectivity of the delignification (Gevert and Lohmander, 1997), the ASAM pulp showed better bleaching selectivity than the soda-AQ pulp. The chelation stages with
Table 3 Hibiscus cannabinus-kenaf core (KC): pulping conditions and results Cooking process Pulp code
Soda KC4
Soda-AQ KC2
AS-AQ KC1
ASAM KC3
17 70: 30 0.1 4 175 70 120
17 70:30 0.1 4 175 70 120
Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-core ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
19
17
4 165 60 120
0.1 4 165 60 120
Cooking results Total yield (%) Rejects (%) k Number CED viscosity (ml/g) ISO brightness (%)
43.3 2.9 25.4 765 29.8
47.8 1.9 22.3 717 25.6
51.3 3.3 42.9 1064 29.6
48.9 1.6 21.9 1073 38.1
67.2 5.4 3.5 6.1 30.1
61.1 5.4 3.4 5.8 28.0
96.5 4.3 5.0 6.5 n/a
82.3 5.4 4.6 6.7 19.1
Handsheet properties at 40 °SR Tensile index (N/m/g) Tear index (mN/m2/g) Burst index (kPa/m2/g) Runnability factor Light scattering coefficient (m2/kg)
234
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Table 4 Hibiscus cannabinus-whole kenaf stalk (KW): pulping conditions and results Cooking process Pulp code
Soda KW4
Soda-AQ KW1
AS-AQ KW2
ASAM KW3
19 70: 30 0.1 4 175 70 120
17 70: 30 0.1 4 175 70 120
Pulping conditions Active alkali as Na2O (%) NaOH-to-Na2SO3 ratio Anthraquinone (%) Liquor-to-stalks ratio Maximum temperature (°C) Time to maximum temperature (min) Time at maximum temperature (min)
19
17
4 165 60 120
0.1 4 165 60 120
Pulping results Total yield (%) Rejects (%) k No. CED viscosity (ml/g) ISO brightness (%)
45.1 4.4 26.5 803 21.3
49.9 0.4 20.4 814 22.2
54.9 12.8 32.2 1187 29.3
52.6 0.5 15.5 1191 41.9
72.5 9.3 5.5 8.3 23.6
96.9 10.2 5.6 10.1 20.4
96.3 8.2 6.1 9.0 17.1
101.9 9.4 7.4 10.1 16.9
Handsheet properties at 40 °SR Tensile index (N/m/g) Tear index (mN/m2/g) Burst index (kPa/m2/g) Runnability factor LSC (m2/kg)
EDTA addition prior the PO and P stages gave a remarkable gain in brightness of : 13 and 5% ISO in the first and second Q stage. The rather high tensile strength and good tear resistance of the unbleached whole kenaf stalk pulps were decreased after TCF bleaching (Tables 4 and 6). As expected, the tensile index of the soda-AQ pulp was lower than that of the ASAM pulp, which also had higher final viscosity. It seems that the fiber weakening indicated by the viscosity reduction during bleaching was fully Table 5 Blending kenaf core (KC) with kenaf bark (KB) unbleached pulps Cooking process Cook code Blending ratio
Soda-AQ KC2+KB2 90/10 70/30
AS-AQ KC1+KB1 90/10 70/30
Tear index (mN/m2/g) Tensile index (N/m/g) Burst index (kPa/m2/g) Runnability factor LSC (m2/kg)
6.7 71.7 3.9 7.0 26.7
6.5 87.2 5.8 7.6 17.3
8.3 69.6 4.1 7.7 27.4
6.4 95 6.1 7.9 16.5
compensated by the improved bonding ability of the bleached fibers. In general, the bleaching process led to a certain leveling of the differences in the tensile strength that existed between the two unbleached whole kenaf stalk pulps. 4. Conclusions The fiber dimensions are specific for each part of the kenaf stalk, with the bast fibers similar to Table 6 Whole kenaf stalk (KW) bleached pulp evaluation Pulping process Pulp code
Soda/AQ KW1
ASAM KW3
ISO brightness (% ISO) CED viscosity (ml/g) k No.
83.9 737 4.3
88.0 994 2.70
8.2 89.4 5.1 24.3
8.2 97.8 5.5 18.7
Handsheet properties at 40 °SR Tear index (mN/m2/g) Tensile index (N/m/g) Burst index (kPa/m2/g) LSC (m2/kg)
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softwoods and the woody core similar to hardwoods. The chemical composition is characterized by low to moderate lignin content and good carbohydrate content. The reference soda pulping of all raw materials yielded \40% of bleachable grade pulp with satisfactory strength characteristics. The addition of AQ in soda cooking accelerated the delignification, reduced alkali consumption, k number and brightness, increased yield and viscosity and gave pulps with properties superior to those of reference soda pulping. The AS-AQ process was particularly well suited for pulping of kenaf bark, but not for kenaf core, which behaved like hardwood. With kenaf bark yield, viscosity, brightness and the strength properties were superior to those of soda and soda-AQ pulps. In ASAM pulping, the synergistic effect of AQ and methanol provided a far-reaching selective delignification. Higher yield, lower k numbers and superior viscosity and brightness were obtained at high pulp strength level. The use of whole kenaf stalk without separation of core and bark gives pulps with very good quality a high yield, especially with the ASAM process. Thus, it seems possible to avoid the high cost of separation, which would represent a problem for commercialization of this raw material. The application of a TCF bleaching sequence gave pulp very high brightness and strength properties, especially with the ASAM pulps. Blending of kenaf core and bark pulps might enhance the commercial utilization of these raw materials for improved paper end product. In this respect, kenaf bark is a strong contender to meet part of the additional fiber supply requirement as a high quality, high yield option.
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References CTFT, 1987. Annual Report of Cellulose and Chemistry Division of CTFT, Nogent-sur-Marne, France. Gevert, B., Lohmander, S., 1997. Influence of sulphur compounds, manganese and magnesium on oxygen bleaching of kraft pulp. Tappi J. 80 (10), 263 – 268. Hart, P.W., Brogdon, B.N., Hsieh, J.S., 1991. Anthraquinone pulping of non-wood species. Nonwood Plant Fibers. Progress Report No. 21, 183 – 191. Holton, H.H., 1977. Soda additive softwood pulping a major new process. Pulp Paper Can. 78, T218 – T223. Horn, R.A., 1978. Morphology of Pulp Fiber from Hardwoods and Influence on Paper Strength. Forest Products Lab Report 312, Madison. James, K.R., McCamley, J., 1981. Separation of core and bark from forage harvested kenaf. In: I.M. Wood, G.A. Stuart (Eds.), Proceedings of the Kenaf Conference (Kenaf as a Potential Source of Pulp in Australia), Brisbane, Queensland, pp. 69– 72. Kaldor, A.F., Karlgren, C., Verwest, H., 1990. Kenaf, a fast growing fiber source for papermaking. Tappi J. 72, 205 – 209. Khider, T., 2001. Pulping Potentialities of Some Sudanese Nonwood Species. Ph.D. Thesis, University of Khartoum. Khristova, P., Bentchgeva, S., Karar, I., 1998. Soda-AQ pulping of four Acacias from Sudan. Cell. Chem. Technol. 32, 299 – 307. Kordsachia, O., Patt, R., 1988. Full bleaching of ASAM pulps without chlorine compounds. Holzforschung 42, 203 – 209. Obolenskaya, A.V., Tshegolev, V.P., Akim, G.L., Kossovitcz, N.C., Emelyanova, I.Z., 1965. Prakticheskie Raboti po Himii Drevesini I Tzelulozi. Lesprom, Moscow 411 pp. Sharma, Y.K., Dhawan, R., Karira, B.G., 1984. High yield pulps from kenaf. Ind. Forest. 110, 401 – 406. Sharma, Y.K., Dhawan, R., Karira, B.G., 1985. Pulping kenaf bark — soda vs. sulfate. Ind. Forest. 111, 832 – 836. Touzinski, G.F., Clark, T.F., Tallant, W.H., Knolek, W.F., 1973. Soda pulps from kenaf bark and from core. In: Non-wood Plant Fibre Pulping. CA Report No. 52. Tappi, Atlanta. Watson, A.J., Gartside, B.G., 1976. Using woody fibre from agricultural crops. Aus. For. 39 (1), 16 – 22. Zhou, C., Ohtani, Y., Sameshima, K., Zhen, M., 1997. Selection of kenaf (Hibiscus cannabinus L.) varieties for papermaking on arid hillside land in China. Mok. Gakkais. 43 (9), 770 – 777.