Chemithermomechanical pulp (CTMP) is often used in central layers of multiply paperboards due to its high bulk and strength. Such a CTMP should consist of well-separated undamaged fibres with sufficient bonding capacity. The basic objective of this work is to optimize process conditions in low-consistency (LC) refining, i. e. to select or ultimately develop new optimal LC refiner filling patterns, in order to produce fibrillar fines and improve the separation of fibres from each other while preserving the natural fibre morphology as much as possible. Furthermore, the aim is to evaluate if this type of work can be done at laboratory-scale or if it is necessary to run trials in pilot- or mill-scale in order to get relevant answers. First stage CTMP made from Norway spruce (Picea abies) was LC refined in mill-, pilot- and laboratory-scale trials and with different filling patterns. The results show that an LR1 laboratory refiner can favourably be used instead of larger refiners in order to characterize CTMP with regard to tensile index and z-strength versus bulk. A fine filling pattern resulted in CTMP with higher tensile index, z-strength and energy efficiency at maintained bulk compared to a standard filling pattern.
Chemithermomechanical pulp (CTMP) is often used in central layers, as a mixture together with broke and softwood kraft pulp, of multiply paperboards due to its high bulk and strength. The present study emanated from a project where the key research challenge was to find ways to produce paperboards with higher bulk and with as good strength properties as the standard material. The aim of the project was to find synergetic effects by joint studies in three different technology areas: chip refining, low consistency (LC) refining and fibre surface chemistry. In chip refining, the fibres should be well separated with preserved stiffness. This is achieved if conditions affecting wood chip softening are optimized and refiner fillings giving a favourable processing are utilized (Persson et al. 2019). The present study is based on results from the LC refining technology area and its objective is to produce fibrillar fines, improve fibre surface area and improve separation of fibres from one another while preserving the fibre stiffness as much as possible. This would be achieved by gentle mechanical treatment, i. e. low intensity refining of the fibre surface. The strength of CTMP can then be further improved by introducing cationic and anionic functional groups together with an organic catalyst, enhancing fibre-to-fibre adhesion (Alimohammadzadeh et al. 2019). The impact of possible improvements in the three technology areas is modelled in a fourth parallel synergetic project both with regard to process modifications and with regard to material properties as strength and bulk. These models are then validated by means of mill scale trials.
In a previous study, Rusu et al. (2011) found that internal fibrillation, i. e. kinks and micro-compressions, was decisive for fibre stiffness, as assessed by FiberMaster bendability measurements (Karlsson et al. 1999). Their finding was supported by the fact that the pulp with the lowest specific refining energy (SRE) also was found to be the stiffest. Pettersson et al. (2015) pointed out that fibre stiffness is important in achieving a bulky paper with sufficiently high strength, and that fibre length is less important. They performed laboratory refining trials with CTMP, of different freeness and fibre length, reinforced by kraft pulp. Klinga et al. (2008) reported similar results from trials with CTMP in pilot-scale refiners. Moreover, they found that stiff fibres formed stronger bonds if they were pressed together during drying in a Rapid Köthen sheet former (100 kPa and 93 °C) compared to a standard sheet former (400 kPa and 23 °C). Elahimehr (2014) showed that bulk correlates to SRE and freeness for different rotational speeds and filling patterns. He did not find any difference in the relationship between tensile development and change in bulk in pilot trials where three different rotational speeds and a wide range of filling patterns were used. This is to some extent contradictory to the findings reported by Klinga et al. (2008). However, Elahimehr (2014) used a standard sheet former. In the present work testing of pulp strength and bulk is performed on Rapid Köthen sheets, and that is assumed to shed light to these contradictory results.
LC refining would make it possible to produce more fibrillar fines also called secondary fines (Odabas et al. 2016) and thereby further increase the network strength at maintained bulk (Ljungqvist et al. 2014, Moberg et al. 2014, Hafrén et al. 2014). Also Motamedian et al. (2019) found that an increased number of fibrillar fines increases the strength of paper. This was explained by the ability of fibrillar fines to reinforce the bonds in the fibre network.
The basic idea in this work is to optimize process conditions in LC refining, i. e. to select or ultimately develop new optimal LC refiner filling patterns, in order to produce fibrillar fines, improve fibre surface area and to improve the separation of fibres from each other while preserving the inherent stiffness as much as possible. Furthermore, the aim is to evaluate if this type of work can be done at laboratory-scale or if it is necessary to run trials in pilot- or mill-scale in order to obtain meaningful results. One benefit of laboratory- and pilot-scale trials is that a wide variety of pulps and filling patterns can be tested at relatively low cost compared to mill-scale trials. Another benefit is not to jeopardize production losses for the entire mill if a trial should fail.
Materials and methods
First stage CTMP from BillerudKorsnäs’ Rockhammar mill made from Norway spruce (Picea abies) was used in all LC refining trials. Trials were performed at mill-, pilot- and laboratory-scale.
LC refining trials
In October 2016, first stage CTMP was taken from BillerudKorsnäs’ Rockhammar mill. The sampling point, (position 1) is marked with an x symbol in Figure 1a. Subsequently, two pilot trials were performed on that pulp at Valmet Technology Centre, Sundsvall in a low consistency JC00 conical pilot refiner (Valmet, Espoo, Finland). Refining was performed with a filling pattern (type JC00RAD2/JC00SAD2) referred to as “standard fillings” and a filling with a fine pattern (type JC00RMM15Z/JC00SMM15Z). In each trial, the pulp was pumped through the refiner from one chest to another, as shown in Figure 1b and the refining gap was adjusted in order to achieve a certain refining energy and thereby a certain refining intensity. Pulp samples were taken at six different refining gaps.
In May 2017, a reference mill trial was performed at Rockhammar mill. The refining gap in a RF5 conical LC refiner (Valmet, Espoo, Finland), was adjusted in order to achieve three different power loads, P (1.5, 1.96 and 2.34 MW) and thereby three different refining intensities. Normally, the RF5 mill refiner operates close to 1.5 MW which corresponds to 1.0 J/m in specific edge load. The standard fillings in the RF5 were of the type RF5RPLF2SDD2/RF5SLFSDD2P. All samples for the mill trial were taken within one hour in order to avoid variations with varying fibre treatment on rejects; directly after the LC refiner (position 3 in Figure 1a) and for the zero power load directly before the LC refiner (position 2). The mass reject ratio was about 10 %. As the refiner functions as a combined second stage and reject refiner it would have been more correct to evaluate the development of fibre and pulp properties over the combined refining and screening stage, i. e. from position 1 to position 4 (Sandberg et al. 2017). However, this is unworkable due to the long time for the process to stabilise after a change in power load.
In conjunction with the mill trial, first stage CTMP was also taken from position 1 (Figure 1a) for two laboratory trials in a laboratory refiner LR1 (Voith-Sulzer, Heidenheim, Germany) at BillerudKorsnäs’ Karlsborg mill. The refining with LR1 was performed with either a conical filling pattern (type 3/0.5/30) or a flat filling pattern (type 3/0.84/30). In each trial, the laboratory refiner operated with a fixed power load, P. The pulp was recirculated through the refiner, as shown in Figure 1c, and pulp samples were taken at six different net specific refining energies (0, 40, 80, 120, 160 and 200 kWh/ton).
Net specific refining energy, SRE (J/kg) was calculated by:
where (W); (kg pulp suspension/s); (kg fibre/kg pulp suspension).
There is no common definition of refining intensity. Berg et al. (2015) found that specific edge load (Brecht and Siewert 1966), refining gap (Mohlin 2006) and force on fibres (Kerekes and Senger 2006) were adequate measures for refining intensity as they all predicted fibre shortening with approximately equal sufficiency. However, refining gap was not measured with sufficiently high precision in this study and therefore force on fibres could not be calculated properly.
The specific edge load, SEL (J/m) was calculated by:
where (rev/s); (m/rev); and cutting speed, (m/s) was defined as:
Lumiainen (1995) showed that width of bars should also be taken into consideration and introduced the specific surface load, SSL (), which is obtained by dividing specific edge load, SEL by impact length, IL (m):
The refining conditions for all trials are summarised in Table 1.
|Width of bar W, mm||4.0||1.3–1.4||3||3||2.0–4.0|
|Bar angle α, °||18||33–15||30||30||18|
|Cutting edge length CEL, km/rev||1.8||10.1||0.020a||0.0336a||120|
|Specific edge load SEL, J/m||0.68–2.0||0.075–0.29||1.0||1.0||1.0–2.0|
|Impact length IL, mm||4.21||1.53–1.47||3.46||3.46||3.15|
|Specific surface load SSL,||160–480||50–190||290||290||330–620|
|Rotation speed ω, rpm||1200||1200||2000||2000||470|
|Pulp consistency C, %||4.3||4.1||4.0||4.0||3.9|
|Flow through the refiner, l/min||200||200||100||100||7000|
|Idle load, kWh/ton||20||30||–||–||34b|
|Net refining energy SRE, kWh/t||48–142||29–118||40–200||40–200||66–120b|
After hot disintegration (ISO 5263-3), tensile index (ISO 1924-2), z-strength (ISO 15754) and bulk (ISO 5270) were measured on handsheets (grammage 140 ) made with a Rapid Köthen sheet former (Rycobel, Deerlijk, Belgium, ISO 5269-2).
In order to get an indication of fibrillar fines content, a method that uses light sources in the UV (peak sensitivity wave-length around 0.25 µm) and IR wavelength spectrum, the crill method was used. The crill value accounts for fines of about 0.25 µm in diameter (Lundberg et al. 2018) while a typical diameter of fibrillar fines is 0.5 µm (Odabas et al. 2016).
Fibre properties of first stage CTMP that were used in LC refiner trials are shown in Table 2.
|1st stage CTMP||October 2016||May 2017|
|Pilot trials||Lab. and mill trials|
|Fibre length (LWA), mm||1.82||1.60|
|Shives sum, no./g||547||403|
|Crill value (unwashed)||200||278|
Results and discussion
Different filling patterns were compared in a pilot-scale conical JC00 refiner and a laboratory-scale LR1 refiner with regard to tensile index, z-strength and energy efficiency. A reference trial was also performed in a conical LC refiner RF5 at BillerudKorsnäs’ Rockhammar mill.
Pilot trials with two different filling patterns
Figure 2a shows that tensile index was 32.6 Nm/g for fillings with a fine pattern and 30.2 Nm/g for the standard fillings, both evaluated at bulk . Figure 2b shows that the corresponding z-strength was 255 kPa for fillings with a fine pattern and 238 kPa for the standard fillings. The tensile index and z-strength before pilot refining in JC00 were 23.2 Nm/g and 179 kPa respectively. Mean values of standard deviations for tensile index, z-strength and bulk in this study were 1.3 Nm/g, 9 kPa and respectively and are indicated by solid lines placed at regression lines at bulk .
As indicated in Figure 3, net SRE was 92 and 91 kWh/ton at bulk for fine-patterned and standard fillings respectively, the corresponding specific edge loads were 0.22 and 1.3 J/m. Energy efficiency for a refiner is defined as the increase in a pulp property over the refiner divided by applied SRE (Sandberg et al. 2017). In the following, we use net SRE in order to calculate energy efficiency with regard to tensile index and z-strength. The energy efficiency for tensile index was (32.6−23.2)/0.092=102 (Nm/g)/(MWh/ton) and (30.2−23.2)/0.091=77 (Nm/g)/(MWh/ton) for fillings with a fine pattern and standard fillings respectively. The corresponding energy efficiencies for z-strength were (255−179)/0.092=826 kPa/(MWh/ton) and (238−179)/0.091=648 kPa/(MWh/ton).
Additional pulp measurements are shown in Figure 4. The crill value (unwashed) was 206 for fillings with a fine pattern and 198 for the standard fillings as shown in Figure 4d. There was no evident difference on different filling pattern (at bulk ) regarding freeness (600 ml), fibre length (1.80 mm) or shives content (0.29 %), shown in Figure 4a–c.
In summary, the pilot trials showed that gentle LC refining in JC00 with a fine filling pattern result in a pulp with higher tensile index (+2.6 Nm/g) and z-strength (+17 kPa) at bulk , compared to harsher refining with standard fillings. The increase in strengths can be explained by a higher amount of fibrillar fibres which is indicated by the higher crill value (+8). Another explanation can be that a gentle treatment of fibres preserves more of their inherent stiffness. Both these explanations are supported by Hyll et al. (2016) who found increased tensile index and bulk by laboratory refining, though of bleached kraft softwood pulp. They found that the generated fines were more fibrillar and that less fibre shortening and kinks were introduced with a finer filling pattern compared to standard ones.
The refining was also performed with higher energy efficiency (+30 %), with regard to both tensile index and z-strength.
Laboratory trials with two different filling types
Figure 5a shows that tensile index was about 36 Nm/g at bulk for both filling types in LR1 (actually 36.2 Nm/g for flat and 35.8 Nm/g for conical fillings). The tensile index at zero power load were 32.8 and 32.0 Nm/g for flat and conical filling types respectively. Figure 5b shows that z-strength was 345 kPa at bulk for the flat type of fillings and 312 kPa for the conical ones. The z-strength at zero power load were 275 and 262 kPa for flat and conical filling types respectively. Additional pulp properties as those shown in Figure 4 are unfortunately only available for the pilot trials.
Net SRE was 90 and 74 kWh/ton at bulk for flat and conical filling types respectively as indicated in Figure 6b. The energy efficiency can be calculated as 38 (Nm/g)/(MWh/ton) or 780 kPa/MWh/ton for flat fillings and 51 (Nm/g)/(MWh/ton) or 680 kPa/MWh/ton for conical fillings.
In summary, the laboratory trials showed that LC refining in LR1 with flat fillings resulted in higher z-strength (+33 kPa) at bulk compared to conical fillings. The tensile index was about the same (Figure 6a). Refining with a flat filling resulted in opposite differences in energy efficiency, with regard to tensile index (−25 %) and with regard to z-strength (+15 %). These finding are somewhat in contrast to those by Lumiainen (1997a, 1997b) who indicated that refiners with conical fillings have lower energy consumption and produce better fibre development than flat disc refiners due to geometric differences. However, Lumiainen performed the trials in larger-scale refiners.
Figure 7a–b shows that tensile index was 35.5 Nm/g and z-strength was 318 kPa, both evaluated at bulk . Tensile index and z-strength versus net SRE are shown in Figure 8a–b. Net SRE was 109 kWh/ton at bulk as indicated in Figure 7b, the corresponding specific edge load was 1.8 J/m. The tensile index and z-strength before refining were 30.5 Nm/g and 266 kPa respectively. The energy efficiency can be calculated as 46 (Nm/g)/(MWh/ton) or 480 kPa/MWh/ton.
Summary of all trials
Refining curves from laboratory-, pilot- and mill-scale refining follow a common trend as seen in Figure 9a–b. It is therefore plausible that refining effects due to differences in filling pattern in a laboratory or pilot refiner also could be detected in larger refiners. Refining in a JC00 pilot refiner with fine-patterned fillings (bar width 1.3–1.4 mm) results in higher tensile index and z-strength at specified bulk compared to standard fillings (bar width 4 mm). A difference is also seen in an LR1 laboratory refiner where flat fillings gives higher z-strength compared to conical fillings, both with 3 mm bar width. Pulp properties and refining characteristics evaluated at specified bulk values are put together in Table 3.
|Tensile index, Nm/g||30.2||32.8||36||36||35.5|
|Energy efficiency, (Nm/g)/(MWh/t)||76||102||51||38||46|
|Energy efficiency, kPa/(MWh/t)||650||830||680||780||480|
|Net refining energy, kWh/t||84||90||74||90||109|
|Specific edge load, J/m||1.3||0.22||1.0||1.0||1.8|
|Specific surface load,||310||150||290||290||560|
First stage chemithermomechanical pulp (CTMP) made from Norway spruce were used in low consistency refining trials performed at mill-, pilot- and laboratory-scale. Different filling patterns were compared with regard to tensile index, z-strength and energy efficiency.
A fine-patterned filling in a pilot-scale conical refiner gave higher tensile index and z-strength at maintained bulk than did a standard filling.
Refining with a fine-patterned filling also resulted in higher energy efficiency with regard to both tensile index and z-strength.
Differences in these strengths caused by fillings in pilot and laboratory refiners reflect those found in a mill refiner.
An LR1 laboratory refiner can be employed to characterize mill-scale refining for tensile index and z-strength at a common bulk.
Funding source: Stiftelsen för Kunskaps- och Kompetensutveckling
Award Identifier / Grant number: 20150373
Funding statement: This study was funded by the Knowledge Foundation, number 20150373.
The authors are grateful to the Knowledge Foundation, BillerudKorsnäs AB, PulpEye AB and Valmet AB for the financial support. This work was performed within the research project “Eco-friendly efficient chemimechanical system for sustainable packaging materials - e2cmp” at FSCN, Mid Sweden University.
Conflict of interest: The authors declare no conflicts of interest.
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