Low Consistency Refining of Wood Shavings - Pulp and Paper ...

Mechanical PulpingT22Low ConsistencyRefining ofWood ShavingsBy T. Kang, G. Soong, J.A. Olson, and D.M. MartinezAbstract: This study examines the possibility of low consistency (LC) refining of wood shavings to replacethe energy intensive high consistency (HC) refining of chips. The study demonstrated that LC refining ofwood shavings required significantly less energy to reach the same freeness and paper strength as high consistencyrefining 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 brightnessof paper.As electrical energy costscontinue to rise with increasingdemand and with the increasingshortage of high quality, long-fibredsoftwood chips, the future of thermo-mechanicalpulping (TMP) using energy-intensive conventional,high consistency disc refiners is far fromcertain. In the last 30 years, since mechanicalpulp strength has been sufficient to completelyreplace kraft pulp in most high quality papergrades, there have been a large number of studiesthat have looked at means of producing highstrength pulp using lower energy. However, theaverage specific energy used to make mechanicalpulps has typically increased over this time.One strategy of reducing energy is theincreased use of low consistency (LC) refiners.Conventional TMP chip refining is done at highconsistencies of 25% to 40%, while conventionalpost refining is done at various consistenciesranging from 3 % to 40 %, but is most commonlydone at low consistency in the 3-4% range. It iswell known that the LC pulp refining consumesless energy than high consistency (HC) refiningto achieve the same quality change [1-3].As energy costs become higher, LC refiningis gaining more attention as possible means ofreplacing some of HC refining energy used in themanufacture of high quality papermaking fibres[4-6]. The results of these attempts have shownthe reduction of energy consumption up to 30%.Wood chips are the main raw material inHC mechanical pulping processes, and saw millresidues such as sawdust and planar shavings canbe mixed with wood chips for the pulping process[7]. Even wood shavings alone can be usedin HC TMP pulping [8], which showed about25 % reduction in refining energy. Initial fibrelength of these wood shavings was longer thanthat of conventional wood chips, but the finalfibre length of wood shavings after HC refiningwere shorter than wood chips.Wood shavings are thinner and lighter thanwood chips and can be fed directly through anLC refiner without HC refining. In effect muchof the defibering is done in the veneering or shavingprocess. Chemical pretreatment such as sulfonation[9-10] or alkaline peroxide pretreatment(APMP) [11-14] might be needed to soften thelignin resulting in easier fibre separation duringLC refining, reducing refining energy. APMPseems to be the better pretreatment due to betterpulp quality, sulphur free environment and lowerenergy consumption over conventional sulfonation[11, 15].In this research, we explore the use of readilyavailable wood materials and novel mechanicalpulping methods to complete the initial defiberingof wood to produce a feed material that issuitable for the more energy efficient LC refiner.The objectives of the study were to evaluatewhether wood shavings can be used for LCrefining at the primary stage after chemical pretreatment,and to examine the potential energysavings with wood shavings compared to woodchips.methods and materialsRaw materialConventional wood chips and shavings (85%pine, 10% spruce, 5% fir) were obtained froma pulp mill in the interior of British Columbia.Thicknesses of wood shavings are approximately0.2 to 0.5 mm, and thicknesses of wood chips areabout 3 to 5 mm. Fig.1 shows wood shavings andwood chips used in the study.T. Kangformerly Pulp and PaperCentre, University of BritishColumbia, Vancouver B.C.Now Winstone Pulp International.Ohakune, NewZealandG. SoongPulp and Paper Centre, Universityof British Columbia,Vancouver B.C.J.A. OlsonPulp and Paper Centre, Universityof British Columbia,Vancouver B.C.D.M. MartinezPulp and Paper Centre, Universityof British Columbia,Vancouver B.C.pulpandpapercanada.com March/April 2010 Pulp & Paper Canada 25

T23Mechanical Pulpingtable i. Pulp properties.Raw Refining Net CSF Fibre Bulk Bauer Bauer Bauermaterial consistency refining (ml) length (cm 3 /g) McNett McNett McNett(%) energy (mm) fractions fractions fractions(kWh/t) (%) (%) (%)R48 R200 P200Wood 22% 0 – 2.70 – – – –chips 4163 552 2.10 3.4 68.5 12.2 19.35600 484 2.07 3.1 – – –6297 321 2.03 2.9 66.3 13.7 20.07040 218 1.95 2.5 60.8 14.0 25.2Wood 5% 1657 246 1.16 2.9 49.2 30.1 20.7shavings 2520 177 1.10 2.8 50.2 30.0 19.92904 163 1.12 2.8 56.5 26.7 16.9Wood 21% 0 – 1.49 – – – –shavings 1148 290 1.07 3.0 49.9 27.6 22.52042 229 1.13 2.9 – – –2852 143 1.14 2.7 45.5 31.7 22.84312 96 1.10 2.4 39.0 30.2 30.8Fig. 1. Wood shavings (top) and woodchips (bottom). Scale bar of 10 mm.Chemical PretreatmentBoth wood chips and shavings are separatelyimpregnated with 0.5% diethylenetriaminepentaaceticacid (DTPA) for60 min at 60°C at a consistency of 10%.After washing, they were atmosphericallysteamed for 20 min, and then they wereimpregnated with 6% sodium hydroxide(NaOH), 4% hydrogen peroxide (H 2O 2),0.05% magnesium sulphate (MgSO 4),1% sodium silicate (Na 2SiO 3) and 0.5%DTPA for 60 min at 70°C at a consistencyof 20%. After chemical pretreatment,the pH was 11.1-11.5. The pretreatedwood shavings and wood chips were thendirectly fed through a refiner.RefiningA Sprout-Waldron 305 mm atmosphericsingle disc refiner (Koppers Co. Inc.,Muncy, USA) equipped with D2A507Ni Hard plates was used for all refiningruns. Wood chips and wood shavingswere 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 fromthe refiner was neutralized with H 2SO 4.MeasurementsThe pulps were hot disintegrated andscreened on a screen (0.15 mm slots)before pulp testing. All pulp and papersamples were tested according to standardmethods. Fibre length and coarseness weremeasured using the fibre quality analyzer(OpTest Equipment, Hawkesbury,Canada). Coarseness measurements of themechanical pulp samples were performedon the P14/R28 Bauer McNett fractions.Results and discussionThe initial fibre length of wood shavingsobtained from the pulp mill is quiteshorter than that from wood chips. This isshown in Fig. 2. A shorter fibre length wasexpected because of the smaller dimensionsof wood shavings. A longer fibrelength of wood shavings can be achievedusing a specially designed knife [8, 16],and fibres of these shavings were deformedhighly without reducing fibre length. Fibrelength of HC refined wood chips wasreduced by 22% at a refining energy of4163 kWh/t, while that of HC refinedwood shavings was reduced by 26% ata refining energy of 4312 kWh/t. Thefibre length of LC refined wood shavingswas assumed to be similar to that of HCrefined wood shavings at a given refiningenergy level. Although the relative initialfibre length of wood shavings is shorterin this study, the reduction ratio of fibrelength by refining is similar regardless ofraw materials and refining consistency. Ifthe initial fibre length of wood shavings islonger or similar to that of wood chips, thefinal fibre length may be similar to that ofwood chips. This implies that a specializedshaving process designed to preserve fibrelength may yield an LC refined pulp withmuch longer fibre length.The freeness of both wood shavingsand wood chips decreased as refiningenergy increased, as shown in Fig. 3 andTable I, but at a given freeness, there wasa significant difference in refining energybetween wood shavings and wood chips.The freeness of HC refined wood shavingsreaches 96 ml at 4312 kWh/t, while that ofHC refined wood chips reaches to 552 mlat a similar refining energy level. In otherwords, wood chips required more energyto reach the same freeness than woodshavings. A similar response to refining ofboth wood chips and wood shavings forHC refining was shown earlier by Viforr& Salmén [8]. For wood shavings, therewas no difference in freeness and energyrelationship between LC and HC refiningalthough LC refining was expected torequire lower energy than HC refining.Fig. 4 shows the changes in the coarsenessof the fibre fraction (P14R28) as afunction of refining energy. Fibre coarsenessof both HC and LC refined woodshavings is higher in the lower range ofrefining energy, and fibre separation fromwood shavings does not seem to be asefficient as wood chips. The reduction ofcoarseness with refining energy is similarfor both LC and HC refined wood shavings.A higher coarseness of wood shavingswith large middle fractions both fromHC and LC refining (Table I) indicatesthat fibre cutting seems to be a dominantmechanism in refining of wood shavings,26 Pulp & Paper Canada March/April 2010 pulpandpapercanada.com

peer reviewedT24Fig. 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. P14R28fraction used.which is not an ideal mechanism for fibredevelopment.Tensile strength improvement is proportionalto the refining energy applied.This is shown in Fig. 5. Regardless ofthe differences in refining consistency, allpoints of tensile strength of wood shavingsare placed almost on the same straight lineas a function of refining energy, but thereis a difference in tensile strength improvementbetween wood shavings and woodchips. At a given refining energy, tensilestrength of wood shavings is higher thanthat of wood chips, but the maximumtensile strength of wood shavings is limitedprobably due to shorter fibre length. Asimilar trend is seen for tensile stiffness asshown in Fig. 6.Fig. 7 shows that light scattering coefficientfor wood shavings is higher thanwood chips as tensile strength increases.Refining of both wood chips and woodshavings at higher consistency increasesthe light scattering coefficient slowly, whileLC refining of wood shavings increaseslight scattering coefficient faster. The differencebetween wood shavings and woodchips in HC refining can be explained bythe amount of fines and middle fractions.The decrease in bulk as shown in Table Ican be the reason for their slower increasein light scattering coefficient. For LCrefined wood shavings, the bulk remainsalmost constant, while the amount of theFig. 5. Tensile strength as a function of refining energy.Error bars represent the 95% confidence interval.middle fraction is higher than that of woodchips.Brightness of wood shavings is muchhigher than wood chips as shown inFig. 8. Thinner thickness of shavings willlikely be favourable for alkaline peroxidepretreatment. It was shown that chemicalpenetration and diffusion rate is a functionof chip thickness [17-18]. For wood shavings,bleaching efficiency is better for LCrefining. This may be due to more efficientmixing between H 2O 2and wood shavingsat lower consistency.Chip dimensions, such as chip thickness,are an important parameter in determiningenergy consumption in refining.More energy is required to fracture a woodpulpandpapercanada.com March/April 2010 Pulp & Paper Canada 27

T25Mechanical PulpingFig. 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 foundto be as low as 5 kWh/t while that for wood chips is in the rangebetween 15-25 kWh/t [16].Wood shavings have several advantages over conventionalwood chips. Firstly, less energy is required to refine and bleachwood shavings having thinner thickness compared to conventionalwood chips. Secondly, fibres deformed during producingwood shavings would improve fibre separation and fibre flexibilityduring refining. This preliminary study indicates thatreadily available wood material such as wood shavings used inLC refining has the potential to save both energy and bleachingcosts. However, commercially available wood shavings used inthis study have a shorter fibre length, and fibre cutting caused byLC refining was inevitable. Fibre length of wood shavings can bepreserved using a specially designed knife and shaving geometry.Alternative pretreatments such as steam explosion or microwaveor biological treatment should be considered as a means to softenwood shavings for LC refining. Future work will also look at producingdifferent types of wood shavings designed for LC refining.ConclusionsWood shavings pretreated with alkaline peroxide were directlyfed through an atmospheric refiner at a consistency of 5%. HCand LC refining of wood shavings showed a better reduction inrefining energy at a given freeness than HC refining of woodchips, and no difference in the relationship of refining energyand freeness was found between LC and HC refining of woodshavings. LC refining of wood shavings was found to reduce themean fibre length of shavings in a similar ratio with HC refiningof shavings and wood chips. The relatively high coarseness ofLC refined wood shavings was reduced slightly as a function ofrefining energy.The maximum obtainable tensile strength for LC refinedshavings is lower than that of HC refined chips. At a given tensilestrength, lower refining energy was required for LC and HCFig. 8. ISO brightness as a function of refining energy.refining of wood shavings compared to HC refining of woodchips. A great advantage of using LC refining of shavings overwood chips is increased light scattering and brightness of paper.ACKNOWLEDGEMENTSThis work was funded by the Natural Sciences and EngineeringResearch Council of Canada through the Collaborative Researchand Development program and through the support of our partnersBC Hydro, Paprican, Catalyst Paper, Howe Sound Pulpand Paper, West Fraser Quesnel River Pulp, Canfor, Andritz,Arkema, Honeywell, WestCan Engineering, Advanced FiberTechnologies, Ontario Power Authority and CEATI international.LITERATURE1. MUSSELMAN, R, LETARTE, D., SIMARD, R. & LACHANCE, C., Third stagelow consistency refining of TMP for newsprint/directory grades, Proc. AppitaConf., p.363-368 (1996).28 Pulp & Paper Canada March/April 2010 pulpandpapercanada.com

peer reviewedT262. MUENSTER, H., FERRITSIUS, O., LECOURT, M.& PETIT-CONIL, M., Energy savings in TMP by hightemperature LC/MC refining, Proc. Intl. Mech. Pulp.Conf., p.213-223 (2005).3. SABOURIN, M., Minimizing TMP energy consumptionusing a combination of chip pre-treatment, RTSand multiple stage low consistency refining, Proc. Intl.Mech. Pulp. Conf., CD-ROM (2007).4. Hammar, L-Å, Htun, M. & Svensson, B., Atwo-stage refining process to save energy for mechanicalpulps, Proc. Intl. Mech. Pulp. Conf., p. 257-262(1997).5. XU, E.C., Koefler, H. & Antensteiner, P.,Some latest developments in alkali peroxide mechanicalpulping, Part 2: Low consistency secondary refining,Pulp Paper Can. 104(10):47-51 (2003).6. Eriksen, O. & Hammar, L-Å., Refining mechanismsand development of TMP properties in a lowconsistencyrefiner, Proc. Intl. Mech. Pulp. Conf., p.62-75 (2007).7. Leask, R.A., A potential use of a wider range ofraw material in thermomechanical pulping, Tappi J.60(12):82-87 (1977).8. Viforr, S. & Salmén, L., From wood shavingsto mechanical pulp- a new raw material?, Nord. PulpPaper Res. J. 20(4):418-422 (2005).9. Atack, A., Heitner, C. & Karnis, A., Ultra-highyield pulping of eastern black spruce, Svensk Papperstidn.81(5):133-141 (1980).10. Axelson, P. & Simonson, R., Thermomechanicalpulping with low addition of sulfite. Part 1.Effects of mild sulfite treatment of spruce chips priorto defibration, Svensk Papperstidn. 85(15):R132-R139(1982).11. Bohn, W. & Sferrazza, M., Alkaline peroxidemechanical pulping, a revolution in mechanical pulping,Proc. Intl. Mech. Pulp. Conf., p. 184-200 (1989).12. Yuan, Z., Heitner, C. & McGarry, P., Evaluationof the APMP process for mature and juvenileloblolly pine, Tappi J. 5(7):24-32 (2006).13. Bian, Y., Ni, Y., Yuan, Z, Heitner, C. & Beaulieu,S., Improving TMP rejects refining throughalkaline peroxide pretreatment for value-addedmechanical papers, Tappi J. 6(3):24-32 (2007).14. Zanuttini, M. & Marzocchi, V., Alkalinechemi-mechanical pulp from poplar. Relationshipbetween chemical state, swelling and papermakingproperties, Holzforschung. 57(5):489-459 (2003).15. Xu, E.C. & Sabourin, M.J., Evaluation of APMPand BCTMP for market pulps from South Americaneucalyptus, Tappi J. 82(12):75-82 (1999).16. HEDBLOM-Hue, S., Malm, Å & Salmén, L.,Shear cut chips for mechanical pulping with lowerenergy demand, Proc. Intl. Mech. Pulp. Conf., p.143-148 (2001).17. Jimenez, G., Chian, D.S., McKean, W.T. &Gustafson, R.R., Experimental and theoreticalstudies to improve pulp uniformity, Proc. TAPPI Pulp.Conf., p. 49-53 (1990).18. Svedman, M., Tikka, P. & Luhtanen, M.,Effects of softwood morphology and chip thicknesson pulping with a displacement kraft batch process,Tappi J. 81(7):157-168 (1998).19. Hoekstra, P.L., Veal, M.A., Lee, P.F. &Sinkey, J.D., The effects of chip size on mechanicalpulp properties and energy consumption, Tappi J.66(9):119-122 (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 rabotureset 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 obtenirle même indice d’égouttage et la même résistance du papier que le raffinage HC des copeaux de bois, mais larésistance maximale du papier fabriqué à partir de rabotures raffinées à faible concentration était moindre. Leraffinage FC concentration des rabotures permet aussi d’améliorer le coefficient de diffusion de la lumière etla blancheur du papier.Reference: KANG, T., SOONG, G., OLSON, J.A., MARTINEZ, D.M. Low Consistency Refiningof Wood Shavings, Pulp & Paper Canada 111(2):T22-T26 (Mar/Apr 2010). Paper presented atthe 27th International Mechanical Pulping Conference, Sundsvall, Sweden, June 1-4, 2009. Notto be reproduced without permission of PAPTAC. Manuscript received August 31, 2009. Revisedmanuscript approved for publication by the Review Panel November 9, 2009.Keywords: LOW CONSISTENCY REFINING, HIGH CONSISTENCY REFINING, WOOD SHAV-INGS, WOOD CHIPS, ENERGY SAVINGpulpandpapercanada.com March/April 2010 Pulp & Paper Canada 29