Papermaking pulps from the fibrous fraction of Miscanthus x Giganteus

Transcription

1 Industrial Crops and Products 11 (2000) Papermaking pulps from the fibrous fraction of Miscanthus x Giganteus P. Cappelletto a, F. Mongardini a, B. Barberi a, M. Sannibale a, M. Brizzi a, *, V. Pignatelli b a Istituto Poligrafico e Zecca dello Stato, Centro Tecnico Industriale, ia Salaria, Rome, Italy b ENEA INN BIOAG, C.R. Casaccia, Via Anguillarese 301, S.M. di Galeria Rome, Italy Accepted 8 October 1999 Abstract The use of non-wood raw material could be an effective mean of backing up worldwide wood supply, especially in countries with insufficient forest resources. An example of such non-wood plants is miscanthus (Miscanthus spp.), a crop adapted to European growing conditions and that has been extensively investigated as an important source of biomass for energy production, for building materials, geotextiles and substrates in nurseries and greenhouses. This crop is also considered a possible raw material for the paper industry. In Italy, miscanthus biomass productivity ranges from 15 t ha 1 per year to t ha 1 per year. In the present study, the possibility of utilising miscanthus fibres, as a reinforcing material in paper furnishes based on recycled fibres, was investigated. In order to transform a heterogeneous material into several concentrated and uniform fractions, miscanthus stalks prior to pulping underwent a mechanical cleaning and separation treatment in dry conditions. Two high yield pulping processes (CTMP and TMP) were applied only to the fibrous fraction. Subsequently, the miscanthus pulps were mixed, at different ratio, with wastepaper (old newspapers). Results show that miscanthus pulps, especially CTMP, are suitable to reinforce recycled fibres and to improve the mechanical characteristics of papers containing secondary fibres Elsevier Science B.V. All rights reserved. Keywords: Miscanthus x giganteus; Non-wood fibres; High yield pulping; Paper industry 1. Introduction The use of non-wood raw material could be an effective mean of backing up world-wide wood * Corresponding author. Tel.: ; fax: address: brizzi@polig.ipzs.it (M. Brizzi) supply, especially in countries with insufficient forest resources. Non-wood pulp capacity worldwide was 6.9% of total papermaking pulp capacity in 1997 and is estimated to have gone to 11.2% in 1998 (Atchinson, 1998). The non-wood pulp is produced mainly in developing countries but also in Europe; the raw materials most widely used are straw, bagasse and bamboo (Oggiano et al., 1997) /00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

2 206 P. Cappelletto et al. / Industrial Crops and Products 11 (2000) Among the non-wood crops suitable for papermaking, an interesting one is miscanthus. The variety Miscanthus sacchariflorus, (the socalled Amur silver grass ), is one of the main papermaking raw materials in the Republic of China, owing to its fast growing cycle, high biomass production, short cooking time and easy pulping (Wu Yi-Ming and Qin Yu, 1988), (Deng Chang-Jiang and Wu Yi-Ming, 1992). Miscanthus, and particularly its variety Miscanthus x giganteus, has been extensively investigated also throughout Europe as a possible raw material for energy, building materials, geotextiles and substrates in nurseries and greenhouses. This crop is also considered an important source of biomass for papermaking pulps. In Italy, a number of productivity trials of miscanthus crop have been performed during the last years, generally on small-scale plots. As an example, under the framework of the R&D Project Miscanthus Productivity Network, two fouryear agronomic trials for the assessment of miscanthus productivity in Italy were carried out at ENEA Research Centres Brasimone and Trisaia (respectively located in Northern and Southern Italy). The results from such trials-as well as from those carried out at different places throughout Italy-indicate that Miscanthus x giganteus is well adapted under different Italian pedoclimatic conditions and yields satisfactorily, its biomass ranging from 15 to t ha 1 per year. More recently (May 1998), an open-field trial on a 3 ha area has been established at the ENEA Research Centre Brasimone, with the objective of assessing the technical and economic feasibility of cultivating miscanthus on marginal lands and with minimal amounts of input. The propagation material has been rhizomes; they were sowed mechanically by using a commercial potato-sowing machine, with a planting density of about rhizomes m 2 ; no irrigation should be necessary after the phase of plant establishment. Italy is totally dependent on imports for its needs of virgin papermaking pulps and the raw material supply for the Italian paper industry mainly rely on wastepaper (in 1998 the ratio between wastepaper utilisation and paper and paperboard production has been 49.9). In the Italian situation, miscanthus could be an effective mean of supplying virgin fibres, thanks to its high productivity, provided that a viable pulping process could be applied to this non-wood plant and a suitable utilisation of its pulps could be found inside the paper manufacturing chain. Previous researches on miscanthus (Cappelletto et al., 1998) and other non-wood plants (Cappelletto and Mongardini, 1997) showed that a preliminary mechanical treatment is necessary in order to separate the raw material into uniform fractions, concentrate the fibrous material, and eliminate foreign matter (together with leaves and pith), thus decreasing chemical consumption and reducing pollution loads in wastewater. Therefore, the pre-treatment system was designed to perform the following operations: compaction of stalks (if they had not already been conditioned during field harvesting); cutting of raw material (usually delivered as round-bales) by means of a blade mill; transport of cut material by means of fans; separation of light fractions by means of a cyclone; separation of heavy fines; this fraction includes the material that have been reduced to powder during the compression, the cutting and the pneumatic transport operations. With these operations the resulting material will be formed by a higher percentage of fibres, since it will lose most of the foreign matter (dust, sand, pebbles) and useless material (leaves, pith, epithelial and parenchyma cells of stalks). In non-wood plants, especially non-bast species as miscanthus, a large fraction of the biomass (up to 30 40%) is formed by cells (epithelial, sclerenchyma, parenchyma and medullar rays) which do not have the appropriate dimensions for a papermaking utilisation. Furthermore they are not apt to be fibrillated and thus they do not improve the paper characteristics. So it is necessary to partially reduce their amount prior to pulping (dry-mechanical treatment), but it is also necessary to arrange the remaining cells in aggregates in order to include them in the final pulp, thus increasing the pulp yield. For this reason the mechanical compaction of raw material and less severe pulping processes (as high yield process CTM and TM) seemed appropriate.

3 P. Cappelletto et al. / Industrial Crops and Products 11 (2000) Table 1 Brasimone Centre, pedoclimatic conditions Location Latitude Longitude Altitude Climate Soil type ENEA Brasimone Centre Castiglione dei Pépoli-Bologna, Italy N E m above sea level Rainy climate, cold winters Loam-clay, high potassium contents (218 ppm) Utilising as raw material only the fibrous fraction of miscanthus plants, the objectives of the present study were: (a) to investigate the feasibility of obtaining papermaking pulps with soft and low input pulping processes, (b) to evaluate the differences between a miscanthus TMP (thermomechanical pulp) and a miscanthus CTMP (chemi-thermo-mechanical pulp), (c) to determine the possibility of utilising these pulps as a reinforcing material in paper furnishes based on recycled fibres. 2. Material and methods Miscanthus stalks from a three years crop, established at the above mentioned ENEA Brasimone Centre, were utilised as raw material for the papermaking trials that had been carried out at the Centro Tecnico Industriale of Istituto Poligrafico e Zecca dello Stato. Table 1 shows the pedoclimatic conditions of the Brasimone site. For the pulping trials, a processing technique that includes a preliminary dry-mechanical treatment, a high yield pulping process and one peroxide bleaching sequence, has been applied. In the preliminary treatment, the miscanthus stalks were compressed (simulating the conditioning operation in the field), then chipped by means of a Caravaggi Bio 350 blade mill equipped with a 50 mm mesh and finally separated (stalks, leaves, dust) by means of a rotating screen equipped with a 3 mm mesh. Papermaking fibres are concentrated in the stalks, therefore the pulping treatment was applied only to this fraction, formed by cleaned and uniform fibrids 5 7 cm long. Two different pulping processes were tested: TMP and CTMP. Both are high yield processes and require low chemical and energy inputs. Afterwards the CTM pulps alone have been bleached by one stage peroxide sequence. The pulping process was carried out in two stages (wet pre-treatment and defiberizing) for the TMP and three stages (wet pre-treatment, cooking and defiberizing) for the CTMP. Wet pre-treatment took place in an open vessel, while the cooking took place in an electric heated, 16-l rotating boiler. Then the pulp stock was squeezed by means of an elicoidal press. Defiberizing was carried out in an 8 Bauer disc refiner, where two steam injections were applied in order to reach the best fibre elementarization at the lignin melting temperature. Both pulps were screened in a Werverk sorter and refined in a Hollander Valley apparatus. Table 2 Miscabhus x giganteus: pulping and bleaching conditions TMP CTMP Pulping Wet pre- Open vessel, 60 C for 30 min, 1% H 2 O 2 Open vessel, 60 C for 20 min, 1% NaOH treatment Cooking NO 16 litres boiler, 120 C for 30 min, solid/liquid ratio: 1/7; chemicals: NaOH (5%); H 2 O 2 (1%), EDTA (0.2%) Defiberizing 8 Bauer disc refiner, 3000 rpm, 8 Bauer disc refiner, 3000 rpm, 2 steam injections Bleaching 2 steam injections NO Open vessel at atmospheric pressure 80 C for 120 min, concentration: 12%; chemicals: H 2 O 2 (3%), NaOH (2%), Na 2 SiO 3 (2%), EDTA (0.2%), MgSO 4 (0.05)

4 208 P. Cappelletto et al. / Industrial Crops and Products 11 (2000) Table 3 Miscanthus x giganteus: results of dry-mechanical treatment before pulping and stalks fraction chemical analysis, as% Miscanthus fractions (%) Treatment Chemical analysis (after treatment) (%) yield (%) As it is After treatment Ex in benzole+h 2 O Ashes Lignin Holo-cellulose Cellulose resistant Stalks Leaves Pith Total Table 2 shows the pulping and bleaching conditions. The unbleached CTM pulp was bleached applying a hydrogen peroxide process (Table 2 gives bleaching parameters). The two high yield miscanthus pulps were utilised to prepare papers containing different ratios of miscanthus pulps and wastepaper (these secondary fibres originated from old newspapers): 30% miscanthus+70% wastepaper; 50% miscanthus+50% wastepaper; 70% miscanthus+30% wastepaper; 100% miscanthus; 100% wastepaper. Handsheets for evaluation of physico-mechanical properties were prepared according to TAPPI T 205 om-88. Physico-mechanical characteristics of the handsheets tensile, burst and tear indexes were determined according to ISO 192/ , ISO and ISO methods, respectively. Optical properties, including opacity and brightness, were estimated according to UNI 7623 method. 3. Results and discussion Results of the fractionation treatment are shown in Table 3. The dry mechanical treatment not only allowed obtaining three different uniform fractions of material, but also changed the ratio between these fractions. The percentage of the fibrous fraction over the total increased from 75.9 to The total yield of the treatment was 73.5%; leaves and pith were the substances mostly removed. The chemical analysis of the stalk fraction confirms its fibrous constitution, as shown by the quite high cellulose content (Table 3). The lignin content is in the range of the normal values expected for a graminaceous plant, while the ashes content is very low even for a non-wood species (wheat straw has an average ash content of 10%). Results of pulping yield are shown in Table 4. As expected, the TM process gave a much higher yield than the CTMP. In fact the CTMP includes a chemical attack to the lignin and to other non-cellulosic substances. Furthermore the CTMP pulps were bleached and also the bleaching process reduces the total pulp yield. Anyway the 66.5% yield still allows including this pulp into the high yield category and should not be considered too low. In fact, the pulp yield has been determined on a screened pulp where the 160- mesh screen in the sorter had eliminated some of the parenchyma cells and medullar rays. For the bleached CTM pulp, the COD (Chemical Oxygen Table 4 Miscanthus x giganteus. Pulping yield and fibres composition of TM and CTM pulps (%) TMP Yield Fibrous composition mesh mesh mesh mesh mesh Bleached CTMP

5 Table 5 Miscanthus x giganteus. Papermaking properties of papers containing different percentage of miscanthus pulps and wastepaper Drenability Density Porosity Tensile Elong. Tear index Burst index Brightness Opacity D.F. ( SR) (g cm 3) (s) index (%) (mn m 2 g 1 ) (kpa m 2 g 1 ) (%) (%) (n) (Nm g 1 ) Bleached CTMP 100% miscanthus % miscanthus, % wastepaper 50% miscanthus, % wastepaper 30% miscanthus, % wastepaper TMP 100% miscanthus % miscanthus, % wastepaper 50% miscanthus, % wastepaper 30% miscanthus, % wastepaper Wastepaper 100% wastepaper Stiffness (mn) P. Cappelletto et al. / Industrial Crops and Products 11 (2000)

6 210 P. Cappelletto et al. / Industrial Crops and Products 11 (2000) demand) of the pre-treatment and cooking wastewater has been determined. The two values of 2850 ppm and ppm, respectively, correlate well with the calculated pulp yield. In the two high yield pulps also the fibre distribution is different; the percentage of the longest fibres (those retained at the 28-mesh screen) is 34.3% for the bleached CTMP and 28.8% for the TMP. In fact the decreased flexibility of fibres after a thermo-mechanical process, reduces fibre length during the refining (Table 4). Table 5 shows the papermaking properties of the different types of handsheets that were prepared. As a general observation, when miscanthus bleached CTMP are added to a furnish based on secondary fibres, the papermaking properties of the paper increase, while the miscanthus TMP, as expected, has no such positive effects. In some blends (50% miscanthus CTMP+50% wastepaper and 70% miscanthus CTMP+30% wastepaper) we noted a possible synergetic effect, i.e. the mixture shows higher values than the papers prepared with the single components; the phenomenon certainly deserves further investigation. The optical properties (brightness and opacity) did not show any improvement neither when CTMP were added nor TMP. 4. Conclusions The study confirms the feasibility of applying a high yield pulping process to miscanthus, provided that a dry-mechanical treatment is previously carried out on raw material. The CTMP pulps improved the strength properties of paper based on recycled fibres originating from old newspapers. It s necessary to find an alternative utilisation for the pith and the leaves that have been removed by the dry-mechanical treatment, in order to exploit the whole biomass and make the miscanthus chain more profitable. In the future, the promising results obtained from this work, as well as those from productivity trials in the field, have to be exploited by realising a demonstration scale whole-chain test, in order to assess the technical and economic feasibility of the Miscanthus crop. References Atchinson, J., Progress in the global use of non-wood plant fibers and prospects for their greater use in the future, Inpaper Intl. April June, Cappelletto, P., Mongardini, F., Industrial systems for preparation of cellulose fibers: IPZS experience. Proceedings of the Flax and other bast plants Symposium. Non- Textile Applications, 30 September 1 October 1997, Poznan, Poland. Cappelletto, P., Mongardini, F., Bilancini, L., Nonwood fibers: Miscanthus sinensis. Industria della Carta 2, Deng Chang-Jiang, Wu Yi-Ming, Study of the chemical structure of hemicellulose extracted from Amur Silver grass. Proceedings of the Shanghai Conference, 6 9 April, 1992, Shanghai, China. Oggiano, N., Angelini, L.G., Cappelletto, P., Pulping and paper properties of some fibre crops. Ind. Crops Prod. 7, Wu Yi-Ming, Qin Yu, The characteristics of Miscanthus sacchariflorus kraft cooking and using of process control. Proceedings of the Beijing Conference, July, 1988, Beijing, China..