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Cellulose (2025) 32:1835–1850
et al. (2016) in which they discovered that pulps with high lignin content resisted fibrillation of the cell walls during mechanical treatment. This was attrib- uted to the ability of lignin to shelter the fibre cel l wall dur ing fibr illation (Hoeger et al. 2013 ). There- fore, bleached hardwood will be less resistant to inter- nal fibrillation since it has a comparatively low lignin content when considering mechanical pulp. This i s reflected by its high-water retention value of 1.56 g/g at a refining energy of 113 kWh/ton. The opposite is true for mechanical pulp whose WRV only increased from 1.14 to 1.16 g/g. Mechanical pulp had the low- est water retention value of 1.16 g/g at the high- est refining stage when compared to bleached hard- wood wit h a value of 1.56 g/g (see Table 4 above). An increase in the WRV of recycled pulp from 1.34 to 1.39 g/g was also observed. It is expected for the pulps to retain more water because of internal fibrilla- tion brought about by the refining process. Hypothesis testing through Analysis of Variance (ANOVA) was employed to assess the existence of significant relations that may exist between refining energy and water-fibre morphological traits. Relation- ships were rendered null if P >0.05 thereby implying that no significant interactions were found. Freeness was found to be significantly affected by the increase in refining energy for all three pulps. Pearson’s coef- ficient further revealed that the relationship is positive thereby implying that drainability as expressed i n °S R is inversely related to the effect of refining. The water retention value of bleached hardwood and recycled pulp was greatly affected by refining where P <0.05. The water retention value of mechanical pulp was not significantly affected by the changes in refin- ing energy as the null hypothesis was accepted with P >0.05, which is due to its high lignin contents as previously discussed.
The exponential relationships shown in Fig. 7a–c for vacuum pressures of − 19, − 37 and − 55 kPa gauge, respectively, could be regressed using an exponential decay function, shown in Eq. (4) to best represent the plateau in the dewatering rate of pulps concerning dwell time.
− t
− t 𝜏 )+ C
(4)
e
C = C
1 − e
∞ (
𝜏
0
where C is the predicted pulp concentration (%) , C ∞ an d C 0 are the plateau and initial mass pulp concen- trations (%), respectively. The dwell time and dewa- tering time constant are represented by t and τ in ms, respectively. The results are graphically shown below. The three pulp types were exposed to different hig h vacuum conditions as provided in Table 3 . The dwell time ranged from 30 to 250 ms, to demonstrate the plateau of filtrate removal from pulps, a phenomenon that could not be achieved within a shorter duration of vacuum exposures. Shorter dwell time ranges pre- vented the development of an exponential decay pre- dictive function, which has been proven to adequately characterise the dewatering behaviours of pulps in the forming section (Neun 1994; Ramaswamy 2003; Pujara et al. 2008b). Therefore, longer dwell times were considered since that pattern could not be estab- lished with shorter dwell times for the pulps tested. Furthermore, the longest dwell time of 250 ms was included in the test range to demonstrate the use of the solenoid valve bank without the slotted spindle for vacuum pulse generation to dewater the pulp hand sheets effectively. Bleached hardwood pulp achieved the high- est mass concentration values of 14.1 and 21.8 % at − 19 and − 55 kPa gauge as seen in Fig. 7a, c . Bleached hardwood is derived from chemical pulp- ing, which has been reported to produce pulps tha t are relatively easier to dewater due to their hig h deformation rate which enhances the compression dewatering mechanism during high vacuum dewa- tering (Åslund 2008). However, Fig. 7b shows tha t recycled fibre achieved a dryness level of 19.2 % at − 37 kPa gauge. This value is higher than tha t achieved by bleached hardwood, i.e., 18.7% at − 3 7 kPa gauge. Both pulps have a drainability rate of 3 7 SR o at their last refining stage, as reflected by simi- lar vacuum dewatering behaviour. This is furthe r reflected by comparable although lower pulp con- centration values of 13.81 and 19.8% at pressures
Vacuum dewatering experimental data
Vacuum dewatering of the three pulps was assessed by regressing an exponential decay to the experi- mental data to better visualise the diminishing effect of dwell time on pulp concentration as shown in Fig. 7a–c, i.e., a plateau in the dewatering rate of pulps, which is an effect discussed in multiple high vacuum dewatering studies (Ramaswamy 2003). Each pulp was tested using samples collected at the last refining stage.
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