Surface Mechanical Treatment of Tmp Pulp

SURFACE MECHANICAL TREATMENT OF bars is relatively small, resulting in the sliding of wood chips TMP PULP FIBERS USING GRIT MATERIAL and fibers off the bars, and thus less treatment.

Several researchers have attempted to overcome these prob- Phichit Somboon and Hannu Paulapuro lems by applying a combination of grinding and refining, us- Helsinki University of Technology ing a modified refiner plate with an abrasive surface [11-14]. Laboratory of Paper and Printing Technology This technique has shown the potential for reducing the en- P.O. Box 6300, FIN-02015 TKK, Finland ergy consumption. However, it has not been successful in practical applications because of problems with the modifica- ABSTRACT tion of the segments, the operation of refiners and the inten- Surface mechanical treatment of pulp fibers using grit mate- sive destruction of pulp fibers. To make it possible to apply rial in thermomechanical pulp (TMP) refining after the first- the grinding technique to wood chip refining, it is necessary to stage refining and the subsequent refining of the treated pulp determine where this technique should be applied, and how were studied. The surface mechanical treatment was per- the fibers can be efficiently broken down and fibrillated. formed using an ultra-fine friction grinder. The grit size of the grinding stone, the intensity of treatment and the rotational In the present research project, the focus was on reducing the speed were optimized to accomplish fast development and to energy consumption in the fibrillation stage (the second stage minimize the shortening of pulp fibers. The subsequent refin- of refining). The research hypothesis was the elastic work can ing was carried out using a wing defibrator operated under be reduced by increasing the disruption and opening the fiber typical TMP refining conditions. According to the results, sur- wall structure during the defibration stage by applying grit face mechanical treatment using a grinding stone with a grit material through the grinding method, thereby promoting the diameter of 297-420 µm, operated at a contact point of the development of pulp fibers and reducing the energy consump- stones and a high rotational speed of 1500 rpm, provided an tion. In a previous study [15], high-freeness TMP pulp from a efficient disruption of pulp fibers with minimized cutting. Dis- reject line was disrupted with grit material and subsequently ruption of the pulp about 20% of total energy consumption refined under TMP refining conditions. The results showed produced a promising fracture of fiber cell wall for the further the potential for reducing the energy consumption. However, development. In the subsequent refining, the disrupted pulp the pulp fibers were weakened and shortened during grit was found to result in faster development of pulp freeness, treatment and refining. To solve these problems, a deeper un- while requiring 37% less energy. Laboratory sheets showed derstanding must be gained of the parameters involved in the no significant differences in properties between the disrupted use of grit material and appropriate raw materials. and non-disrupted pulps at a given freeness. This study was designed to gain a better understanding of mechanical treatment using grit material of the first-stage TMP INTRODUCTION pulp fibers, with the aim to achieve efficient disruption of the In the refining of wood chips, the underlying mechanism of fiber wall structure while minimizing the degradation of fiber the development of fibers proceeds in two stages: in the initial quality. Another aim was to evaluate the potential for reducing stage, called the defibration stage, the wood chips are broken the energy consumption in the second stage of treatment, in- down into coarse fibers. In the second stage, called the fibril- cluding disruption and refining. lation stage, they are further developed, e.g., delaminated, peeled off, and fibrillated, to the extent necessary for papermaking. These processes consume over 90% of the total elec- tric energy used in mechanical pulp production [1, 2]. Theo- retically, the energy input required in refining is relatively low [3-6]. It has been addressed that the high energy consumption in refining is the result of inefficient work during the defibration and fibrillation stages, potentially related to the nature of the wood raw material.

Wood is a viscoelastic material [3, 7, 8]. The mechanical breakdown of the structure of the wood matrix in refining fundamentally begins from the application of cyclic stresses to the wood matrix. The repeated viscoelastic deformation caused by cyclic stresses results in plastic deformation, which continues until the breaking point of the structure is reached, as shown in Figure 1. The repeated viscoelastic deformation consumes a high amount of energy without producing any development of wood fibers [2, 3, 8, 9]. Figure 1. Transformation of wood material from vis- In addition, the friction of fibers over the refiner bars plays an coelastic to plastic deformations under cyclically important role for the energy loss. According to Sundholm constant stress [3]. [10], the friction force between the wood material and refiner

1 EXPERIMENTAL Second-stage refining The experiments were divided into two parts. The first part Feed pulps of the second-stage refining were prepared, dis- was designed to find out how to achieve efficient disruption of rupted with grit material under optimized conditions. The de- pulp fibers, while minimizing fiber shortening. The second grees of grit treatment were targeted at 10, 15, and 20% of the part of the experiment was intended to evaluate the potential total refining energy consumption. After the disruption, all for reducing the energy consumption, and to examine the pulp disrupted pulps were thickened to high consistency and further and paper properties of the disrupted pulp produced in the refined under typical TMP refining conditions, as shown in subsequent refining. Figure 2.

Raw materials The raw material was the first-stage TMP pulp made from Norway spruce ( Picea abies L. Karst. ) with a CSF of 580 ml produced at Stora Enso’s Summa mill in Finland.

Surface mechanical treatment The mechanical treatment of the surface of TMP pulp fibers was carried out using an ultra-fine friction grinder [15]. In the beginning of the study, the key process parameters of the grinder were analyzed for optimizing the treatment in order to achieve fast disruption of pulp fibers, while minimizing fiber shortening. The analysis was based on a statistical model of a single replication of a 2 3 factorial design [16], as shown in Table 1. The intensity of treatment, rotational speed and grit Figure 2. Experimental schematic of the second- size of the grinding stone were considered. stage treatment of TMP pulp with a combination of disruption and refining. Table 1. A 2 3 factorial experiment for analysis of surface mechanical treatment of the first-stage TMP Second-stage refining was carried out using a wing defibrator pulp fibers using grit material. at Helsinki University of Technology [15]. The feed pulps were controlled at a consistency of 23% and a dry weight of 150 g. The peripheral speed of the defibrator was set to 750 RUN A B C LABELS rpm. The pulps were refined at a temperature of 130 °C without preheating and under various specific energy consump- 1 - - - (1) tions from 1 to 5 MWh/t. After refining, pulp samples were 2 + - - a taken for testing fiber and paper properties. The specific en- 3 - + - b ergy consumption in the second stage of treatment, including 4 + + - ab disruption and refining, was evaluated. 5 - - + c 6 + - + ac Sample testing The drainability of pulp fibers and laboratory sheets was 7 - + + bc tested with the whole pulp according to SCAN and ISO stan- 8 + + + abc dards. Drainability was analyzed using a Canadian standard A - Grinding position 30 µm below the contact of stones freeness tester. Laboratory sheets were formed with white wa- A + Grinding position 5 µm below the contact of stones ter circulation, and dried with a drying plate in a conditioning B - Rotational speed 1200 rpm B + Rotational speed 1500 rpm room at 23 °C and 50% RH. The physical properties of labo- C - Grinding stone No. 80, grit diameter of 149-210 µm ratory sheets were determined according to ISO standards. C + Grinding stone No. 46, grit diameter of 297-420 µm Fiber length and coarseness were measured with a Kajaani Fi- The intensity of treatment was based on the relative position berLab apparatus according to TAPPI standards. Fiber length of grinding stones. The position was controlled at below the was measured with the whole pulp. Fiber coarseness was ana- contact point of the stones in the motion stage, at 5 µm (low lyzed from fractionated pulp using a Bauer-McNett classifier intensity) and 30 µm (high intensity) [15]. The peripheral with the screen number 30 (R30). speed of the grinding stone was adjusted to 1200 rpm and 1500 rpm. The impact of the grit size was analyzed by using a The wet strength of long fibers (R30) was determined based on derivation of the breaking stress of wet paper strips at a stone No. 80 with a grit diameter of 149-210 µm and a stone zero span and the number of fibers bearing the load [17]. No. 46 with a grit diameter of 297-420 µm. The pulp slurry feed was controlled at a low consistency of 4% and circulated Breaks in the wall structure of fibers were measured based on through the grinder with four passes. After treatment, the the micropore volume in the cell wall of fractionated fibers pulps were sampled for measuring pulp drainability, fiber (R30). The measurement was made at the Helsinki University length and fiber coarseness for the factorial analysis. of Technology using a differential scanning calorimeter based

2 on the thermoporosimetry method with an isothermal step To achieve efficient disruption of pulp fibers and to minimize melting technique [18]. their shortening, it was suggested that the disruption of the first-stage TMP pulp should be performed using a grinding The morphological changes in fiber cell walls were observed stone with a grit diameter of 297-420 µm. The grinder should in accordance with the KCL method. External fibrillation and be operated at a high rotational speed of 1500 rpm and a splitting of long fibers (R30) were analyzed. grinding position of 5 µm below a contact point of the grinding stones (a low intensity of disruption). The images of long fibers were captured using a scanning electron microscope (SEM) at the Institute of Biotechnology Effects of surface mechanical treatment on proper- of the University of Helsinki. The samples were dehydrated ties of fibers through a series of graded ethanol concentrations and dried A promising raw material for analyzing the disruption of fiber using a critical point dryer before taking images. wall structure is intact TMP pulp fibers. In practice, the sepa- ration and fibrillation stages overlap, proceeding concurrently RESULTS AND DISCUSSION in the refiner. Therefore, the pulp fibers taken from first-stage Optimization of surface mechanical treatment refining were some extent developed [1]. Figure 5 shows the Figures 3 and 4 show the main effects [16] of the disrupting surface morphology of the long-fiber fraction (R30) of the parameter on pulp freeness and fiber length. Grinding stone first-stage TMP pulp observed with a scanning electron mi- No. 46 with a grit diameter of 149-210 µm was found to pro- croscope. The pulp fibers were somewhat fibrillated, and the duce fast development of pulp freeness, while maintaining fi- outer surface clearly consisted of the middle lamella, the pri- ber length. A low peripheral speed of 1200 rpm and grinding mary wall, and the secondary S1 layer, related to the separa- position of 30 µm below the contact point of the stones (high tion zone of TMP fibers in the wood matrix [1, 2, 19, 20]. intensity of treatment) result in faster development of pulp When the grit material was applied to the pulp fibers, varying freeness, but cause more cutting of fibers. the specific energy consumption from 10 to 20% of total refining energy consumption, the cell wall structure of fibers was found to be modified with less degradation of fiber properties.

Figures 6, 7 and Table 2 show the effects of the grit treatment on the properties of fibers. Pulp freeness was reduced from 580 ml to 360 ml. External fibrillation, splitting of fibers and the pore volume of the fiber cell wall were found to increase. The average length of the whole pulp fibers did not change. However, based on the fractionation analysis, the long-fiber fraction (R30) decreased by about 10 %. The strength properties of the long-fiber fraction were not severely degraded.

Table 2. Effects of surface mechanical treatment on fiber properties.

Figure 3. Average main effects (disrupting intensity, Percentage of disrupting energy of total energy consumption rotational speed and grit size of grinding stone) on in second-stage refining pulp freeness. 0 % 10 % 15% 20% Freeness* (ml) 580 480 420 360

Fiber length* (mm) 1.96 1.96 2.03 2.02

Pore volume** (µl/g) 646 648 662 672

Fibrillated fibers** (%) 34 43 42 49

Fiber splitting** (%) 17 15 14 21

Fiber coarseness** (mg/m) 0.635 0.356 0.390 0.408

Fiber strength** (mN) 279 161 160 193 * Whole pulp ** Fractionated pulp-R30

Figure 4. Average main effects (disrupting intensity, rotational speed and grit size of a grinding stone) on fiber length. 3

(a) (b)

Figure 5. First-stage TMP pulp fibers having CSF of 580 ml (R30).

(a) (b)

Figure 6. First-stage TMP pulp fibers disrupted using a grit material to CSF of 360 ml (R30).

4 According to these results, disruption and opening of the fiber Second-stage refining cell wall, with minimized shortening and weakening of the At the beginning of pulp development, from a pulp freeness of pulp fibers before fibrillation, can be achieved with an abra- 580 to 360 ml, disruption and refining consumed the same sive stone with a grit diameter of 297-420 µm, operated at low amount of energy as shown in Figure 8 and Table 4. When the intensity (approximately a contact point of the stones), and pulps were further refined, the freeness trend of the disrupted high rotational speed of 1500 rpm. The disruption can be per- pulps began to slope more steeply than those of non-disrupted formed from pulp freeness of 580 to 360 ml. pulp. This indicates that the target freeness will be achieved faster, while consuming less energy.

The potential for reducing the energy consumption can be simply assessed from the differences in the slope of the freeness trend lines, as explained in the previous study [15]. Dis- rupting the pulp using 10, 15, and 20 % of the total energy consumption reduces the energy consumption in the second stage of refining by up to 13, 26, and 37 %, respectively, as shown in Table 5 in the Appendix. The potential for energy reduction can also be calculated by taking into account the energy for disrupting and refining, as shown in Table 6 in the Appendix. However, based on the research hypothesis, disruption and opening of the wall structure of fibers affect their development in subsequent refining. This should be presented based on the changes in the slopes of the freeness trend along the energy consumption axis (Figure 8 and Table 5).

Figure 7. Fractionation of first-stage TMP pulp fibers at different degrees of disruption.

Figure 8. Freeness development as a function of specific energy consumption in second-stage treatment including disruption and refining.

5 Based on the results of the mechanical treatment using grit material and the subsequent refining of disrupted pulp, it is believed that increasing disruption and opening of the fiber wall structure is a promisingly fractured surface of fibers for the further development. This might reduce the work needed for breaking down fiber wall structure and generating internal and external fibrillation in the second stage of refining. Con- sequently, less energy would be required to develop the pulp fibers to the desired quality for papermaking.

Pulp and paper properties Figure 9 shows the pore volume of the cell wall of fractionated fibers (R30) at different levels of grit treatment and further refining. At a given pulp freeness, the treated pulps show a higher pore volume, indicating more disruption of the fiber wall structure. The results imply that disruption and opening of the outer layers of fibers will result in a greater disintegra- tion of the fiber wall structure in further refining. Figure 10. Fiber length (whole pulp) as a function of pulp freeness in second-stage refining. Figure 10 shows the fiber length of disrupted and reference pulps as a function of the freeness in second-stage refining. The disrupted fibers are not severely shortened in proportion to the degree of refining. This postulates that disrupting the fibers by up to 20% of the total refining energy does not cause any harmful effects on the fibers in further refining. However, the disrupted pulps have somewhat lower fiber length than non-disrupted pulp at a given freeness.

The tear strength of disrupted pulp was somewhat lower than that of non-disrupted pulp. However, at a freeness below 100 ml, there were no significant differences in tear strength (Fig- ure 11). Light scattering coefficient, tensile strength, and bonding strength (Scott bond) showed no significant differences between disrupted and non-disrupted pulp at a given level of pulp freeness, as shown in Figure 12-14.

Figure 11. Tear resistance as a function of pulp

freeness in second-stage refining.

Figure 9. Micropore volume of fiber cell wall (R30) in second-stage refining. Figure 12. Light scattering coefficient as a function

of pulp freeness in second-stage refining.

6 port for this work. Research partners, the Finnish Pulp and Paper Research Institute (KCL) and the Technical Research Center of Finland (VTT), are gratefully acknowledged. We would also like to thank Professor Richard J. Kerekes for his technical review.

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10. Sundholm, J., Can we reduce energy consumption in me- CONCLUSIONS chanical pulping? International Mechanical Pulping Con- To achieve fast development of pulp freeness while minimiz- ference. Oslo, Norway, 15-17 June 1993. Technical Asso- ing fiber cutting, mechanical treatment of first-stage TMP ciation of the Norwegian Pulp and Paper Industrial, 1993, pulp using grit material should be performed using the grind- pp.133-142. ing stone with a grit diameter of 297-420 µm, operated at high 11. Miles, K. B. and May, W. D., A new plate for chip rotational speed of 1500 rpm and low intensity of treatment at refining. J. Pulp and Paper Science, vol.10, no. 2, 1984, a contact point of the grinding stones. The treatment was pp. J36-J43. found to develop the pulp fibers from a freeness of 580 to 360 12. Stationwala, M. I., Abrasive refiner plates for the ml, while producing promising disruption of the fiber wall production of mechanical pulp. Tappi J., vol. 70, no. 10, structure for further development. 1987, pp.124-127. 13. Kano, T., Iwamida, T. and Sumi, Y., Energy saving in In subsequent refining, the disrupted pulp was found to result mechanical pulping. 1983 International symposium on in faster development of pulp freeness, while requiring 37% wood and pulping chemistry. Tsukuba Science City, 23- less energy. Laboratory sheets showed no significant differ- 27 May 1983. Japanese Technical Association of the Pulp ences in properties between disrupted and non-disrupted pulps and Paper Industry, Japan, 1983, vol. 2, pp. 5-10. at a given freeness. 14. Pregetter, M., Eichinger, R. and Stark, K., Post refining of mechanical pulp using abrasive surface. 5 th International ACKNOWLEDGEMENTS paper and board industry conference-scientific and The authors would like to thank the National Technology technical advance in refining. Vienna, Austria, 29-30 Agency of Finland (TEKES), Metso Paper, UPM-Kymmene April 1999. Pira International, UK, 1999, 15 p. and Stora Enso for providing funding and inspirational sup- 7 15. Somboon, P., Kang, T., and Paulapuro, H., Disrupting the Table 5. Energy consumption in the refining using a wall structure of high-freeness TMP pulp fiber and its ef- wing defibrator. fect on the energy required in the subsequent refining. rd PAPTAC 93 Annual Meeting. Montreal, Quebec, Can- Refining energy (MWh/t) Refining Energy Disruption ada, 5-9 February 2007. Pulp and Paper Technical Asso- area Non reduction (%) Disrupted (CSF-ml) disrupted (%) ciation of Canada, 2007, pp. A59-A64. pulp 16. Montgomery, D. C., The 2 k factorial design. Design and pulp analysis of experiments, John Wiley & Sons cop., New 0 580-70 - 4.18 - York, 1991, pp 290-341. 10 480-70 3.32 3.81 13

17. Somboon, P. and Paulapuro, H., Measuring wet strength 15 420-70 2.63 3.54 26 of wood fibers with a combination of a zero-span tensile apparatus and an automated optical analyzer. Progress in 20 360-70 2.04 3.23 37 Paper Physics Seminar. Miami University, Oxford, Ohio,

USA, 1-5 October 2006, pp.45-48. Table 6. Energy consumption in the second stage of 18. Maloney, T. C. and Paulapuro, H., The formation of pores treatment including disruption and the refining. in the cell wall. J. Pulp and Paper Science, vol. 25, no. 12,

1999, pp. 430-436. Energy Disruption Specific energy consumption (MWh/t) 19. Fernando, D. and Daniel, G., Micro-morphological reduction (%) observations on spurce TMP fibre fractions with emphasis Disrupting Refining Total* (%)

on fibre cell wall fibrillation and splitting. Nordic Pulp 0 0 4.18 4.18 - Paper Research J., vol. 19, no.3, 2004, pp. 278-285. 20. Stationwala, M.I., Mathieu, J., and Karnis, A., On the in- 10 0.36 3.32 3.68 12 teraction of wood and mechanical pulping equipment. 15 0.55 2.63 3.18 24

Part 1: Fibre development and generation of fines. J. Pulp 20 0.81 2.04 2.85 32 Paper Science, vol. 22, no. 5, May 1996, pp. J155-J159. * Total energy used for developing the pulp from freeness of 580 to 70 ml

APPENDIX

Table 3. Average main effects of grit treatment on fiber properties analyzed using a 2 3 factorial experiment.

Fiber Fiber CSF Analyzed parameters length coarseness* (ml) (mm) (mg/m)

Low 390 1.82 0.383 Treatment intensity High 328 1.78 0.351

Rotational 1200 rpm 310 1.73 0.358 speed 1500 rpm 408 1.87 0.376

149-210 micron 382 1.74 0.372 Grit size

297-420 micron 336 1.86 0.362 * Fiber coarseness-R30

Table 4. Specific energy consumption in the second stage treatment including disruption and refining.

Specific energy consumption (MWh/t) Development of pulp freeness Percentage of Disrupting Refining disruption From CSF 580 to 480 ml 0.36 0.25 10 %

From CSF 580 to 420 ml 0.55 0.56 15 %

From CSF 580 to 360 ml 0.81 0.82 20%

From CSF 580 to 70 ml - 4.18* - * Reference for total energy consumption