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Cellulose (2025) 32:1835–1850
Fig. 4 Pressure profile at − 55 kPa gauge for a dwell time of 250 ms
100
90
80
70
60
50
0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 Time (sec)
Table 4 Fibre characterisation data Pul p type
Refining inten- sity (kWh/ton)
WRV (g/g) WRV sample variance
Freenes s (°SR)
Freeness sam- ple variance
Fines con- ten t (%)
Fines content sample vari- ance
Bleached hardwood pulp 0
1.23 1.40 1.56 1.14 1.16 1.34 1.39
0.03
25 32 37 48 50 29 37
33
21.5 22.7 25.0 74.1 74.0 43.5 46.3
3.4
60
113
Mechanical pulp
0
0.0010
8.5
0.4
44
0
Recycled pulp
0.0062
25.8
4.3
70
to be dewatered. Samples of each were collected at different refining stages after which morphological traits were recorded thereby showing the transition o f fibre structure of the pulps because of the refining process. It is important to note that the different pul p types were refined at different levels of energy due t o their different structural properties. Figure 5 below shows the effect of refining energy on the freeness o f all three pulps. A graphical analysis of the dewatering parameters presented above reveals the reason for the differences in vacuum dewatering performance of the three pulp types as presented in the sections that follow. The morphological changes that these pulps undergo due to the process of refining affect their drainability an d therefore vacuum dewatering performance. For exam- ple, mechanical pulp has the lowest drainability o f 50°SR which can be attributed to the high fines con- tent of 74.1% after refining at 44 kWh/ton. It is well known that fines reduce drainage capacity because of its negative effect on permeability through the increase of surface area (Olejnik et al. 2017). There- fore, it is expected for the pulp to have the lowest rate
of dewatering when exposed to high vacuum dewater- ing conditions using the laboratory suction box. The opposite is true for bleached hardwood, which has a relatively higher drainability at each refining stage as represented by the lower average °SR values of 25, 32 and 37 (see Table 4). The bleached pulp only gener- ated a fines content of 25% at the last refining stage at 113 kWh/ton. Bleached hardwood pulp is expected to perform better when exposed to vacuum conditions that simulate the dewatering by suction boxes using the laboratory setup. Although recycled pulp has an identical drainability to bleached hardwood with a value of 37°SR, the pulp’s permeability is much lower since it has a relatively higher fines content o f 46.3% after refining. Consequently, recycled pulp will release less filtrate as compared to bleached hard- wood (Fig. 6). Bleached hardwood pulp has low lignin content (Brancato 2008). It has been previously reported that pulps with high lignin content observe considerable resistance to mechanical treatments such as refining (Małachowska et al. 2020). Similar results were dis- cussed by Hoeger et al. (2013) and Delgado-Aguilar
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