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quicker than the 35 ms reported for recycled pulp. Mechanical pulp showed signs of resistance to filtrate removal as proven by an °SR value of 50. Therefore, it had the highest dewatering time constants of 169 ms (− 19 kPa gauge), 47 ms (− 37 kPa gauge) and 38 m s (− 55 kPa gauge) which resulted in the respective low plateau or maximum pulp concentration con- stants of 13, 15 and 16%. From the discussion above, high drainability and low vacuum pressures may be associated with high pulp concentration for all three pulps. This is graphically shown in Figs. 9 and 10, where linear lines are included to visually demon- strate the type of proportionality between the two variables. Figures 10 and 11 show the relationship betwee n pulp water retention value and the vacuum dewater- in g time constants. Although the previous discussion showed that bleached hardwood and recycled pulp had the fastest dewatering rate at all vacuum lev- els, they observed the highest water retention value s o f 1.56 an d 1.39 g/ g when compared to mechanical pulp whose water retention was quantified at 1.16 g/g, all of which is provided in Table 4. Therefore, it
may be implied that the two variables have an indi- rect proportionality. Pulp water retention value is a representation of internal fibrillation undergone by pulps during the refining process and is often used to monitor pulp swelling and thereby fibre flexibil- ity (Singh 1996; Olejnik et al. 2017). High WRV can be associated with high swelling and therefore bet- ter flexibility (Olejnik et al. 2017 ). Bleached hard- wood and recycled pulp have better flexibility when compared to mechanical pulp as they have relatively high WRV, as shown in Table 4. Enhanced flexibility promotes web deformation or compression of pulps. This is a desirable phenomenon as it is one of the mechanisms by which dewatering occurs (Åslund and Vomhoff 2008a). However, extremely flexible fibres are prone to compacting during pulp mat for- mation which causes a phenomenon known as sheet sealing (Sjöstrand et al. 2019). Sheet sealing occurs when drainage channels in pulp mats are blocked as fibres form compact networks at the point of contact with the forming wire thereby trapping water from the mat (Hubbe et al. 2020). It is an undesirable effect that can be further exaggerated by high fines content
Fig. 10 Effect o fWR V on the dewatering time constant
100 120 140 160 180
0 20 40 60 80
1.14
1.24
1.34
1.44
1.54
1.64
WRV (g/g)
taoL 55 kPag -55 kPa gauge
taoL 19 kPag
taoL 37 kPag -37 kPa gauge
-19 kPa gauge
Fig. 11 Effect of WRV on the maximum pulp concen- tration constant
22
20
18
16
14
12
1.14
1.24
1.34
1.44
1.54
1.64
WRV (g/g)
MaxConL 19 kPag -19 kPa gauge
Max 37 L -37 kPa gauge
M- 5a x5 Ck Po an gLa u5 g5 e
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