Effect of eucalyptus wood characteristics on alkaline peroxide mechanical pulping (APMP) energy consumption and pulp properties

Transcription

1 Effect of eucalyptus wood characteristics on alkaline peroxide mechanical pulping (APMP) energy consumption and pulp properties Marcelo C. dos S. Muguet: Ph.D. Student,Aalto University, Finland, Jorge L. Colodette : Full Professor, UFV-LCP, Brazil, colodett@ufv.br Anna-Stiina Jääskeläinen: Research Fellow, Aalto University, Finland, tiina.jaaskelainen@aalto.fi Abstract Eucalyptus trees grow very fast in Brazil and are mainly used for chemical pulp production. Some studies revealed the potential of eucalyptus wood regarding mechanical pulps. Mechanical pulps are important to the paper industry due to their high yield, low environmental impact, high bulk, opacity and lightscattering coefficient. On the other hand, the high refining energy consumption significantly contributes to the overall process costs. Chemical treatments are applied in order to reduce refining energy consumption allied with pulp properties gains and APMP showed to be an interesting process. Thus the objective of this study was to evaluate the APMP process of different hybrids of eucalyptus and to link the wood structure, properties and composition with refining energy consumption and pulp properties. The chemical treatments were made with two impregnation stages. The first one consisted of chelant and alkali treatments and the second one, the alkaline peroxide with stabilizers. Following, the wood chips were refined in a wing defibrator and the energy consumption was measured. Wood characterization, the pre-refining chemical treatments and refining specific energy consumption did not show too many differences, but the development of pulp properties among the samples were quite distinct, with the most promising results for the hybrid A. Keywords:Alkaline peroxide mechanical pulping; energy consumption; eucalyptus; pulp properties. Introduction Eucalyptus is becoming the most important fiber source for papermaking worldwide. The major interest in eucalyptus wood comes from its low production cost in certain regions, mainly because of high forest productivity and the increasing understanding of its application in various paper grades made eucalyptus the preferred fibers worldwide [1]. Eucalyptus woods have been used already in mechanical pulping, but in terms of process design and post-pulping bleachability, only two processes are suitable for high brightness paper grades and packaging. The alkaline peroxide mechanical pulping (APMP) and alkaline sulfite mechanical pulping (CTMP) [2]. The advantages of the APMP process are that the wood chips can be fully bleached prior to refining [3], high flexibility in processing different types of woods and also the delivery of fibers with higher density, tear and tensile strength when compared to CTMP fibers [2]. Mechanical pulping is an energy intensive process and ways of reducing it are already known, such as application of enzymes[4,5], reduction of raw materials variability [6] and the use of chemicals, e.g. alkaline peroxide. However, it is obvious that wood composition and ultrastructure have distinct impact on the energy consumption. Thus, the objective of this study was to evaluate the feasibility of top-level Eucalyptushybrid clones for APMP process, evaluating the effect of wood characteristics on energy consumption and pulp properties. Experimental Four different hybrids of Brazilian grown Eucalyptus trees derived from a genetic breeding program were used in this study, and respectively coded as A, B, C and REF. The hybrid REF was chosen to be the reference since it is already planted as industrial source. The others are being evaluated for their potential as wood source for the pulp and paper industries. The carbohydrates composition was analyzed by HPLC- PAD after acid hydrolysis following the procedure described by Wallis et al. [7]. Total uronic acids were evaluated according to Scott [8].Insoluble and soluble lignins were measured according to Gomide and Demuner [9] and Goldsdchimid [10], respectively. Lignin content was defined as the sum of insoluble (Klason) and soluble lignin as described by Jääskeläinen et al.[11].syringyl/guaiacyl ratio (S/G) was

2 evaluated according to Lin and Dence[12]. Acetyl groups were evaluated according to Solar et al. [13]. Wood density was evaluated according to LCP property method. The APMP process was carried out with a prevaporization stage of 20 minutes and a double-stage chemicals impregnation (Table 1). The first impregnation stage consisted of the application of sodium hydroxide and DTPA and it was carried in a press apparatus, under room temperature for 20 minutes, Liquor-to-wood ratio (L/W) 4:1. The liquor was then pressed out of the chips and collected for further analysis. The second impregnation stage consisted of the application of alkaline hydrogen peroxide and stabilizers and it was carried out under 60 o C, in water bath, for 60 minutes, and L/W 4:1 and after, the liquor was then pressed out of the chips and collected for further analysis.further, the refinings were carried out in a wing defibrator, in four levels: 4, 7, 10 and 13 minutes, with consistency around 43 %, and temperature around 130 o C. The liquors obtained after each impregnation stage were subjected to ph measurementand hydrogen peroxide concentration by iodometrictitration. The pulps physical, mechanical and optical properties were evaluated according to TAPPI standards (T220 sp- 06, T410 om-08, T551 om-06, T414 om-04, T494 om-06, T519 om-06) Table 1.Chemicals charges (kg/odt) applied during double-staged impregnation Chemicals 1 st impregnation 2 nd impregnation Total H 2 O DTPA NaOH Na 2 SiO MgSO Results and Discussion Wood Characterization The four hybrids showed distinct chemical and physical characteristics (Table 2). Differences of more than 4% and 1%, respectively can be seen for glucans and xylans content among the samples. Such differences can play interesting roles regarding mechanical properties of the pulps produced. Lignin content, did not change that much, with exception of the reference (REF), with around 3% less than the other samples. Density followed the same trend of lignin, with again IP-REF with slightly lower density than the others. Table 2. Wood chemical and physical characterization Carbohydrates (%) Lignin (%) Gluc. Xyl. Gal. Man. Arab. Ins. Sol. Total S/G Acetyl (%) UA (%) Dens. (Kg/m 3 ) A B C REF APMP Experiments Double-stage Impregnation In the APMP process the wood chips are impregnated in two stages: first using chelating agents to remove metals followed by the second impregnation with hydrogen peroxide to bleach the wood chips. No significant differences could be seen among the residual liquors from both stages, regarding final ph and hydrogen peroxide residual (Table 3). That could be an indication that the pre-treatment was equally successful for all samples, indicating that differences in pulp properties would be mostly due to the wood differences between themselves.

3 Freeness (CSF) Table 3. Characterization of liquors from both pre-impregnation stages Sample ph 1 st Impreg. ph 2 nd Impreg. Resid. H 2 O 2 (% of applied) A B C REF Refinings High refining energy consumption significantly contributes to the overall process costs of a mill, and ways of reducing it are always being searched. The use of chemicals are said to reduce it [2], and also the finding of a suitable wood for such process. It can be seen in Figure 1 that all four Eucalyptus hybrids needed similar energies to be refined, what can be explained by such small differences, especially in wood density[6]and chips size[14] Refining Time (minutes) Figure 1.Refining time vs. specific energy consumption (S.E.C.) for all 4 Eucalyptus hybrids pulp samples. Although the wood characterization, the pre-refining chemical treatments and refining specific energy consumption did not show too many differences, the development of drainage properties among the samples were quite distinct. It is worth noting that differences in freeness (CSF) are connected especially to the capability of fibers to form a consolidated mat, avoiding then water drainage. It can be seen that G1xUGL and IP-REF had the best refinability while the worst performance was from the sample DGxU2, which means that the mat was better formed and consolidated for A and REF than C (Fig. 2). Such differences can cause huge differences in paper properties Figure 2. Specific energy consumption (S.E.C.) vs. Freeness for all 4 Eucalyptus hybrids pulp samples.

4 Opacity (%) Sheet Density (kg/m³) Sheet Bulk (cm³/g) Pulp physical and opticalproperties An ideal mechanical pulp produces a paper sheet with high opacity, brightness, bulk and smoothness, and a suitable pore structure at low grammage and without excessive use of reinforcement pulp [15]. As could be seen in Figure 2, relevant differences in refinability could be noticed among the foureucalyptus hybrids. Such differences can play an important role regarding paper properties. It can be seen in Figure 3 that, as expected, the density of paper sheets increases (Fig. 3a), with longer refinings, and obviously bulk decreases (Fig. 3b), since they are inversely proportional. One suitable explanation is that the fibers tend to be more collapsed or broken during longer refinings, differently fromshorterrefinings, when the fibers are either still in shives form or not so collapsed. A B 300 5, ,5 4,0 3, ,0 Figure 3. Specific energy consumption (S.E.C.) vs. Density (a) and Bulk (b) for all 4 Eucalyptus hybrids pulp samples. One good comparison to be done is the relationship between opacity and bulk, since both properties are desired for mechanical pulps[15]. It can be seen infigure 4 that there is a tendency of decreasing opacity with increasing bulk ,0 3,5 4,0 4,5 5,0 Sheet Bulk (cm³/g) Figure 4.Bulk vs. opacity for all 4 Eucalyptus hybrids pulp samples. Brightness is another important parameter regarding chemimechanical pulps, since part of the chemicals used are applied for brightness gains. It can be seen in Figure 5 that there was no trend across the refining for all samples, and that can be explained by the heterogeneous impregnation of all chips, which will be the matter of future studies.

5 Tensile Index (kn.m/kg) Brightness (%ISO) Figure 5.Specific energy consumption (S.E.C.) vs. brightness for all 4 Eucalyptus hybrids pulp samples. Pulp mechanical properties Some mechanical pulps are not only used as bulking agents, but also as material for packaging. The use of bleached mechanical pulps, in the case of this study APMP, can be taken a bit further, for example, the use of such pulps for printing and writing (P&W) paper. High quality P&W paper grades require pulps of high tensile strength to withstand the forces the paper undergoes during manufacturing in high speed machines and their use [16]. Tensile index can show how the paper will behave on its manufacturing process, and can be highly affected by bonding strength between fibers, fiber length and stiffness [17]. It is shown in Figure 6 that in general, all four hybrids produced weaker sheets, when comparing to chemical fibers [18]. Mechanical pulps can have higher tensile index than chemical pulps, at same sheet density [19], but in this study the sheets density were quite low, because of the not so intensive refining (Fig. 2), but such results are in accordance to results obtained byxu and Sauborin [2]. Other explanation is that in chemical pulps the fiber surface is mainly comprised by carbohydrates, differently from chemimechanical pulps, where massive extent of the surface area is covered with lignin[20], avoiding and/or disturbing hydrogen bonds formation between the fiber surface carbohydrates. Despite all that, still eminent differences can be seen among all four hybrids Figure 6. Specific energy consumption (S.E.C.) vs. tensile index for all 4 Eucalyptus hybrids pulp samples. Increased fiber strength, other things being equal, inevitably causes an increase in tear value[21]. By that, it can be seen in Figure 7, that fibers from A seem to be much stronger than C. The tear index is also very dependent on the details of how and how many fibers are pulled out of the network during propagation of the failure line [21]. The effect of increasing fiber bonding is important, what can vanish the effect of fiber length [22]. As it was seen before (Fig. 6) the bonding of fibers in DGxU2 is not eminent,

6 Tear Index (mn.m²/g) Tear Index (mn.m²/g) corroborating the results found for tear index (Fig. 7). 2,0 1,7 1,4 1,1 0,8 0,5 Figure 7. Specific energy consumption (S.E.C.) vs. tear index for all 4 Eucalyptus hybrids pulp samples. Castanho and Oliveira [23] suggested that tear index is better interpreted when correlated to tensile index (Fig. 8). It can be concluded that fiber bonding, as well as fiber strength is much higher to pulps produced from A than from C. 2,0 1,7 1,4 1,1 0,8 0, Tensile Index (kn.m/kg) Figure 8. Tensile index vs. tear index for all 4 Eucalyptus hybrids pulp samples. The low glucans and xylans content allied to high lignin content with low S/G ratio of C can be the reasons of such poor behavior in mechanical properties. Contrarily, for A, the high xylan content can be important in fiber bonds formation and low lignin S/G ratio in better delignification, thus delivering moreintact and strong fibers,but in-depth studies regarding these matters are being carried out in our laboratories. Conclusions With these preliminary results, it can be concluded that wood chemical and physical characteristics are distinct among hybrids. The energy consumption was similar to all hybrids, but resulted in pulps with totally distinct drainage properties. The optical properties were influence by heterogeneous impregnation during pre-refining stages and physical and mechanical properties were quite distinct, demonstratingthe hybrid A as a promising raw material, although more in-depth studies still needed in order to explain such differences. References 1. Magaton, A.S., Colodette, J.L., Gouvêa, A.F.G., Gomide, J.L., Muguet, M.C.S., Pedrazzi, C., Eucalyptus wood quality and its impact on kraft pulp production and use. Tappi J. 8, 32-39, (2009).

7 2. Xu, E.C. and Sabourin, M.J., Evaluation of APMP and BCTMP for market pulps from South American eucalyptus. Tappi J. 82, 75-82, (1999). 3. Cort, C.J., Bohn, W.L., Alkaline peroxide mechanical pulping of hardwoods. Tappi J. 6, 79-84, (1991). 4. Hart, P.W., Waite, D.M., Thibault, L., Tomashek, J., Rousseau, M.-E., Hill, C., Sabourin, M.J, Refining energy reduction and pulp characteristic modification of alkaline peroxide mechanical pulp (APMP) through enzyme application. Tappi J. 5, 19-25, (2009a). 5. Hart, P.W., Waite, D.M., Thibault, L., Tomashek, J., Rousseau, M.-E., Hill, C., Sabourin, M.J, Selective enzyme impregnation of chips to reduce specific energy in alkaline peroxide mechanical pulping. Holzforschung 63, , (2009b). 6. Duncar, E., Laperrière, L., Ding, F., Decreasing specific energy of thermomechanical pulps from reduction of raw materials variability. Tappi J. 9, 23-29, (2009). 7. Wallis, A.F.A., Wearne, R.H., Wright, P.J., Chemical analysis of polysaccharides in plantation eucalypt woods and pulps. Appita J. 49, , (1996). 8. Scott, R.W. Colometric determination of hexuronic acids in plant materials. Analytical Chemistry, n.7, p , (1979). 9. Gomide, J.L., Demuner, B.J., Determinação do teor de lignin em material lenhoso: métodoklasonmodificado. O papel 47, 36-38, (1986). 10. Goldschimid, O., Ultraviolet spectra. In: Lignins, Occurrence, formation, structure and reactions (Ed: K. V. Sarkanen, and C. H. Ludwig), John Wiley and Sons, New York, USA, pp , (1971). 11. Jääskeläinen, A.-S., Saariaho, A.-M., Vuorinen, T., Quantification of lignin and hexenuronic acid in bleached hardwood kraft pulps: A new calibration method fo UVRR spectroscopy and evaluation of the conventional methods. J Wood Chem. Technol. 25, 51-65, (2005). 12. Lin, S.Y.; Dence, C.W. Methods in lignin chemistry. Berlin: Springer-Verlag,. A8p, (1992). 13. Solar, R., F. Kacik, and I. Melcer, Simple Semimicro Method for the Determination of O-Acetyl Groups in Wood and Related Materials. Nordic Pulp and Paper Research Journal, 4: , (1987). 14. Jones, T. G., Song, G.G., Richardson, J.D. Effect of chipper setting on chip size distribution and mechanical pulp properties. Appita J. 58, 56-63, (2005). 15. Lönnberg, B., Idea of mechanical pulping, in: Lönnberg, B. (Ed.), Mechanical Pulping. Paperijapuuoy, Helsinki, pp , (2009). 16. Muguet, M.C.S., Colodette, J.L., Pedrazzi, C., Oliveira, R.C., Behavior of deposited xylans during oxygen delignification across pulp bleaching and refining, and their influence on paper properties. In: V International Colloquium on Eucalyptus Pulp, Porto Seguro, Brazil, (2011). 17. Nazhad, M.M., Harris, E.J., Dodson, C.T.J., Kerekes, R.J., The influence of formation on tensile strength of paper made from mechanical pulps. Tappi J. 83, 1-9, (2000). 18. Pedrazzi, C., Colodette, J.L., Oliveira, R.C., Muguet, M.C.S., Gomide. J.L., Physical-mechanical properties evaluation of pulps produced with novel bleaching sequences. CiênciaFlorestal. 20, , (2010). 19. Xu, E.C., Zhou.,Y., Synergistic effect between chemical mechanical pulps and chemical pulps from hardwoods. Tappi J. 11, 4-9, (2007). 20. Li, K., Tan, X., Yan, D., The middle lamella remainders on the surface of various mechanical pulp fibres. Surf. Interface Anal. 38, , (2006). 21. Cowan, W.F., Explaining handsheet tensile and tear in terms of fiber-quality numbers. Tappi J. 78, , (1995). 22. Kärenlampi, P.P., The effect of pulp fiber properties on the tearing work of paper. Tappi J. 79, , (1996). 23. Castanho, C.G., Oliveira, R.C., Estudos de aproveitamento de rejeitofibroso industrial da polpaçãokraft de eucaliptonaprodução de papéisreciclados. In: CongressoAnual de Celulose e Papel, ABTCP, São Paulo, Brazil, (2000). Acknowledgements The authors would like to acknowledge the financial support from the Academy of Finland (Effect of fiber wall chemistry on energy demand in wood defibration, Defibre, ) and CNPq Brazil. The authors would also like to acknowledge José MaurícioLino, for the wood chemical characterization,heikkitulokas, for the help during refining experiments and Kyösti Ruuttunen for inspiring

8 discussions.