Low Consistency Refining of Wood Shavings

Transcription

1 Mechanical Pulping T22 Low Consistency Refining of Wood Shavings By T. Kang, G. Soong, J.A. Olson, and D.M. Martinez Abstract: This study examines the possibility of low consistency (LC) refining of wood shavings to replace the energy intensive high consistency (HC) refining of chips. The study demonstrated that LC refining of wood shavings required significantly less energy to reach the same freeness and paper strength as high consistency refining of wood chips, but the maximum paper strength of LC refined wood shavings was lower. Further advantages of using LC refining of shavings are increased light scattering coefficient and brightness of paper. A s electrical energy costs continue to rise with increasing demand and with the increasing shortage of high quality, long-fibred softwood chips, the future of thermo-mechanical pulping (TMP) using energy-intensive conventional, high consistency disc refiners is far from certain. In the last 30 years, since mechanical pulp strength has been sufficient to completely replace kraft pulp in most high quality paper grades, there have been a large number of studies that have looked at means of producing high strength pulp using lower energy. However, the average specific energy used to make mechanical pulps has typically increased over this time. One strategy of reducing energy is the increased use of low consistency (LC) refiners. Conventional TMP chip refining is done at high consistencies of 25% to 40%, while conventional post refining is done at various consistencies ranging from 3 % to 40 %, but is most commonly done at low consistency in the 3-4% range. It is well known that the LC pulp refining consumes less energy than high consistency (HC) refining to achieve the same quality change [1-3]. As energy costs become higher, LC refining is gaining more attention as possible means of replacing some of HC refining energy used in the manufacture of high quality papermaking fibres [4-6]. The results of these attempts have shown the reduction of energy consumption up to 30%. Wood chips are the main raw material in HC mechanical pulping processes, and saw mill residues such as sawdust and planar shavings can be mixed with wood chips for the pulping process [7]. Even wood shavings alone can be used in HC TMP pulping [8], which showed about 25 % reduction in refining energy. Initial fibre length of these wood shavings was longer than that of conventional wood chips, but the final fibre length of wood shavings after HC refining were shorter than wood chips. Wood shavings are thinner and lighter than wood chips and can be fed directly through an LC refiner without HC refining. In effect much of the defibering is done in the veneering or shaving process. Chemical pretreatment such as sulfonation [9-10] or alkaline peroxide pretreatment (APMP) [11-14] might be needed to soften the lignin resulting in easier fibre separation during LC refining, reducing refining energy. APMP seems to be the better pretreatment due to better pulp quality, sulphur free environment and lower energy consumption over conventional sulfonation [11, 15]. In this research, we explore the use of readily available wood materials and novel mechanical pulping methods to complete the initial defibering of wood to produce a feed material that is suitable for the more energy efficient LC refiner. The objectives of the study were to evaluate whether wood shavings can be used for LC refining at the primary stage after chemical pretreatment, and to examine the potential energy savings with wood shavings compared to wood chips. methods and materials Raw material Conventional wood chips and shavings (85% pine, 10% spruce, 5% fir) were obtained from a pulp mill in the interior of British Columbia. Thicknesses of wood shavings are approximately 0.2 to 0.5 mm, and thicknesses of wood chips are about 3 to 5 mm. Fig.1 shows wood shavings and wood chips used in the study. T. Kang formerly Pulp and Paper Centre, University of British Columbia, Now Winstone Pulp International. Ohakune, New Zealand G. Soong J.A. Olson D.M. Martinez pulpandpapercanada.com March/April 2010 Pulp & Paper Canada 25

2 T23 Mechanical Pulping table i. Pulp properties. Raw Refining Net CSF Fibre Bulk Bauer Bauer Bauer material consistency refining (ml) length (cm 3 /g) McNett McNett McNett (%) energy (mm) fractions fractions fractions (kwh/t) (%) (%) (%) R48 R200 P200 Wood 22% chips Wood 5% shavings Wood 21% shavings Fig. 1. Wood shavings (top) and wood chips (bottom). Scale bar of 10 mm. Chemical Pretreatment Both wood chips and shavings are separately impregnated with 0.5% diethylenetriaminepentaacetic acid (DTPA) for 60 min at 60 C at a consistency of 10%. After washing, they were atmospherically steamed for 20 min, and then they were impregnated with 6% sodium hydroxide (NaOH), 4% hydrogen peroxide (H 2 O 2 ), 0.05% magnesium sulphate (MgSO 4 ), 1% sodium silicate (Na 2 SiO 3 ) and 0.5% DTPA for 60 min at 70 C at a consistency of 20%. After chemical pretreatment, the ph was The pretreated wood shavings and wood chips were then directly fed through a refiner. Refining A Sprout-Waldron 305 mm atmospheric single disc refiner (Koppers Co. Inc., Muncy, USA) equipped with D2A507 Ni Hard plates was used for all refining runs. Wood chips and wood shavings were separately refined at 21-22% consistency (referred to as HC refining), and the shavings were also refined at 5% consistency (referred to as LC refining). After refining, the pulp discharged from the refiner was neutralized with H 2 SO 4. Measurements The pulps were hot disintegrated and screened on a screen (0.15 mm slots) before pulp testing. All pulp and paper samples were tested according to standard methods. Fibre length and coarseness were measured using the fibre quality analyzer (OpTest Equipment, Hawkesbury, Canada). Coarseness measurements of the mechanical pulp samples were performed on the P14/R28 Bauer McNett fractions. Results and discussion The initial fibre length of wood shavings obtained from the pulp mill is quite shorter than that from wood chips. This is shown in Fig. 2. A shorter fibre length was expected because of the smaller dimensions of wood shavings. A longer fibre length of wood shavings can be achieved using a specially designed knife [8, 16], and fibres of these shavings were deformed highly without reducing fibre length. Fibre length of HC refined wood chips was reduced by 22% at a refining energy of 4163 kwh/t, while that of HC refined wood shavings was reduced by 26% at a refining energy of 4312 kwh/t. The fibre length of LC refined wood shavings was assumed to be similar to that of HC refined wood shavings at a given refining energy level. Although the relative initial fibre length of wood shavings is shorter in this study, the reduction ratio of fibre length by refining is similar regardless of raw materials and refining consistency. If the initial fibre length of wood shavings is longer or similar to that of wood chips, the final fibre length may be similar to that of wood chips. This implies that a specialized shaving process designed to preserve fibre length may yield an LC refined pulp with much longer fibre length. The freeness of both wood shavings and wood chips decreased as refining energy increased, as shown in Fig. 3 and Table I, but at a given freeness, there was a significant difference in refining energy between wood shavings and wood chips. The freeness of HC refined wood shavings reaches 96 ml at 4312 kwh/t, while that of HC refined wood chips reaches to 552 ml at a similar refining energy level. In other words, wood chips required more energy to reach the same freeness than wood shavings. A similar response to refining of both wood chips and wood shavings for HC refining was shown earlier by Viforr & Salmén [8]. For wood shavings, there was no difference in freeness and energy relationship between LC and HC refining although LC refining was expected to require lower energy than HC refining. Fig. 4 shows the changes in the coarseness of the fibre fraction (P14R28) as a function of refining energy. Fibre coarseness of both HC and LC refined wood shavings is higher in the lower range of refining energy, and fibre separation from wood shavings does not seem to be as efficient as wood chips. The reduction of coarseness with refining energy is similar for both LC and HC refined wood shavings. A higher coarseness of wood shavings with large middle fractions both from HC and LC refining (Table I) indicates that fibre cutting seems to be a dominant mechanism in refining of wood shavings, 26 Pulp & Paper Canada March/April 2010 pulpandpapercanada.com

3 peer reviewed T24 Fig. 2. Fibre length as a function of refining energy. Fig. 3. Freeness as a function of refining energy. Fig. 4. Coarseness as a function of refining energy. P14R28 fraction used. which is not an ideal mechanism for fibre development. Tensile strength improvement is proportional to the refining energy applied. This is shown in Fig. 5. Regardless of the differences in refining consistency, all points of tensile strength of wood shavings are placed almost on the same straight line as a function of refining energy, but there is a difference in tensile strength improvement between wood shavings and wood chips. At a given refining energy, tensile strength of wood shavings is higher than that of wood chips, but the maximum tensile strength of wood shavings is limited probably due to shorter fibre length. A similar trend is seen for tensile stiffness as shown in Fig. 6. Fig. 7 shows that light scattering coefficient for wood shavings is higher than wood chips as tensile strength increases. Refining of both wood chips and wood shavings at higher consistency increases the light scattering coefficient slowly, while LC refining of wood shavings increases light scattering coefficient faster. The difference between wood shavings and wood chips in HC refining can be explained by the amount of fines and middle fractions. The decrease in bulk as shown in Table I can be the reason for their slower increase in light scattering coefficient. For LC refined wood shavings, the bulk remains almost constant, while the amount of the Fig. 5. Tensile strength as a function of refining energy. Error bars represent the 95% confidence interval. middle fraction is higher than that of wood chips. Brightness of wood shavings is much higher than wood chips as shown in Fig. 8. Thinner thickness of shavings will likely be favourable for alkaline peroxide pretreatment. It was shown that chemical penetration and diffusion rate is a function of chip thickness [17-18]. For wood shavings, bleaching efficiency is better for LC refining. This may be due to more efficient mixing between H 2 O 2 and wood shavings at lower consistency. Chip dimensions, such as chip thickness, are an important parameter in determining energy consumption in refining. More energy is required to fracture a wood pulpandpapercanada.com March/April 2010 Pulp & Paper Canada 27

4 T25 Mechanical Pulping Fig. 6. Tensile stiffness as a function of refining energy. Error bars represent the 95% confidence interval. Fig. 7. Light scattering coefficient vs. tensile strength. chip with a higher thickness, and to refine thicker chips [19-20]. The energy consumption for producing wood shavings was found to be as low as 5 kwh/t while that for wood chips is in the range between kwh/t [16]. Wood shavings have several advantages over conventional wood chips. Firstly, less energy is required to refine and bleach wood shavings having thinner thickness compared to conventional wood chips. Secondly, fibres deformed during producing wood shavings would improve fibre separation and fibre flexibility during refining. This preliminary study indicates that readily available wood material such as wood shavings used in LC refining has the potential to save both energy and bleaching costs. However, commercially available wood shavings used in this study have a shorter fibre length, and fibre cutting caused by LC refining was inevitable. Fibre length of wood shavings can be preserved using a specially designed knife and shaving geometry. Alternative pretreatments such as steam explosion or microwave or biological treatment should be considered as a means to soften wood shavings for LC refining. Future work will also look at producing different types of wood shavings designed for LC refining. Conclusions Wood shavings pretreated with alkaline peroxide were directly fed through an atmospheric refiner at a consistency of 5%. HC and LC refining of wood shavings showed a better reduction in refining energy at a given freeness than HC refining of wood chips, and no difference in the relationship of refining energy and freeness was found between LC and HC refining of wood shavings. LC refining of wood shavings was found to reduce the mean fibre length of shavings in a similar ratio with HC refining of shavings and wood chips. The relatively high coarseness of LC refined wood shavings was reduced slightly as a function of refining energy. The maximum obtainable tensile strength for LC refined shavings is lower than that of HC refined chips. At a given tensile strength, lower refining energy was required for LC and HC Fig. 8. ISO brightness as a function of refining energy. refining of wood shavings compared to HC refining of wood chips. A great advantage of using LC refining of shavings over wood chips is increased light scattering and brightness of paper. ACKNOWLEDGEMENTS This work was funded by the Natural Sciences and Engineering Research Council of Canada through the Collaborative Research and Development program and through the support of our partners BC Hydro, Paprican, Catalyst Paper, Howe Sound Pulp and Paper, West Fraser Quesnel River Pulp, Canfor, Andritz, Arkema, Honeywell, WestCan Engineering, Advanced Fiber Technologies, Ontario Power Authority and CEATI international. LITERATURE 1. MUSSELMAN, R, LETARTE, D., SIMARD, R. & LACHANCE, C., Third stage low consistency refining of TMP for newsprint/directory grades, Proc. Appita Conf., p (1996). 28 Pulp & Paper Canada March/April 2010 pulpandpapercanada.com

5 peer reviewed T26 2. MUENSTER, H., FERRITSIUS, O., LECOURT, M. & PETIT-CONIL, M., Energy savings in TMP by high temperature LC/MC refining, Proc. Intl. Mech. Pulp. Conf., p (2005). 3. SABOURIN, M., Minimizing TMP energy consumption using a combination of chip pre-treatment, RTS and multiple stage low consistency refining, Proc. Intl. Mech. Pulp. Conf., CD-ROM (2007). 4. Hammar, L-Å, Htun, M. & Svensson, B., A two-stage refining process to save energy for mechanical pulps, Proc. Intl. Mech. Pulp. Conf., p (1997). 5. XU, E.C., Koefler, H. & Antensteiner, P., Some latest developments in alkali peroxide mechanical pulping, Part 2: Low consistency secondary refining, Pulp Paper Can. 104(10):47-51 (2003). 6. Eriksen, O. & Hammar, L-Å., Refining mechanisms and development of TMP properties in a lowconsistency refiner, Proc. Intl. Mech. Pulp. Conf., p (2007). 7. Leask, R.A., A potential use of a wider range of raw material in thermomechanical pulping, Tappi J. 60(12):82-87 (1977). 8. Viforr, S. & Salmén, L., From wood shavings to mechanical pulp- a new raw material?, Nord. Pulp Paper Res. J. 20(4): (2005). 9. Atack, A., Heitner, C. & Karnis, A., Ultra-high yield pulping of eastern black spruce, Svensk Papperstidn. 81(5): (1980). 10. Axelson, P. & Simonson, R., Thermomechanical pulping with low addition of sulfite. Part 1. Effects of mild sulfite treatment of spruce chips prior to defibration, Svensk Papperstidn. 85(15):R132-R139 (1982). 11. Bohn, W. & Sferrazza, M., Alkaline peroxide mechanical pulping, a revolution in mechanical pulping, Proc. Intl. Mech. Pulp. Conf., p (1989). 12. Yuan, Z., Heitner, C. & McGarry, P., Evaluation of the APMP process for mature and juvenile loblolly pine, Tappi J. 5(7):24-32 (2006). 13. Bian, Y., Ni, Y., Yuan, Z, Heitner, C. & Beaulieu, S., Improving TMP rejects refining through alkaline peroxide pretreatment for value-added mechanical papers, Tappi J. 6(3):24-32 (2007). 14. Zanuttini, M. & Marzocchi, V., Alkaline chemi-mechanical pulp from poplar. Relationship between chemical state, swelling and papermaking properties, Holzforschung. 57(5): (2003). 15. Xu, E.C. & Sabourin, M.J., Evaluation of APMP and BCTMP for market pulps from South American eucalyptus, Tappi J. 82(12):75-82 (1999). 16. HEDBLOM-Hue, S., Malm, Å & Salmén, L., Shear cut chips for mechanical pulping with lower energy demand, Proc. Intl. Mech. Pulp. Conf., p (2001). 17. Jimenez, G., Chian, D.S., McKean, W.T. & Gustafson, R.R., Experimental and theoretical studies to improve pulp uniformity, Proc. TAPPI Pulp. Conf., p (1990). 18. Svedman, M., Tikka, P. & Luhtanen, M., Effects of softwood morphology and chip thickness on pulping with a displacement kraft batch process, Tappi J. 81(7): (1998). 19. Hoekstra, P.L., Veal, M.A., Lee, P.F. & Sinkey, J.D., The effects of chip size on mechanical pulp properties and energy consumption, Tappi J. 66(9): (1983). 20. Eskelinen, E., Hu, S.H. & Marton, R., Wood mechanics and mechanical pulping, Appita J. 36(1):32-38 (1982). Résumé: La présente étude porte sur la possibilité de raffinage à faible concentration (FC) des rabotures et des copeaux au lieu du raffinage haute concentration (HC) des copeaux à forte consommation d énergie. L étude a démontré que le raffinage FC des rabotures de bois exigeait beaucoup moins d énergie pour obtenir le même indice d égouttage et la même résistance du papier que le raffinage HC des copeaux de bois, mais la résistance maximale du papier fabriqué à partir de rabotures raffinées à faible concentration était moindre. Le raffinage FC concentration des rabotures permet aussi d améliorer le coefficient de diffusion de la lumière et la blancheur du papier. Reference: KANG, T., SOONG, G., OLSON, J.A., MARTINEZ, D.M. Low Consistency Refining of Wood Shavings, Pulp & Paper Canada 111(2):T22-T26 (Mar/Apr 2010). Paper presented at the 27th International Mechanical Pulping Conference, Sundsvall, Sweden, June 1-4, Not to be reproduced without permission of PAPTAC. Manuscript received August 31, Revised manuscript approved for publication by the Review Panel November 9, Keywords: LOW CONSISTENCY REFINING, HIGH CONSISTENCY REFINING, WOOD SHAV- INGS, WOOD CHIPS, ENERGY SAVING pulpandpapercanada.com March/April 2010 Pulp & Paper Canada 29