Molecules 2019 , 24 , 1800
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2.3. E ff ect of CNF on Retention and Drainage Process The e ff ect of the addition of CNF was assessed on the retention and drainage process of recycled ONP and recycled OCC with and without a retention system. In both cases, drainage e ff ect was studied only with CNF oxidized with 10 mmol of NaClO / g of pulp before the homogenization. Figure 8 shows the drainage curves of experiments performed with and without TRDS for recycled ONP (Figure 8a) and recycled OCC (Figure 8b). To compare drainage results, drainage time was calculated when 300 g of water were drained (W300). Figure 9 shows the e ff ect of CNF dose on the W300 time, the total solid retention and ash retention in recycled ONP and OCC pulps (Figure 9a,b, respectively) using TRDS.
Figure 8. E ff ect of CNF dose and three-component retention and drainage system on the drainage process. ( a )ONPand ( b )OCC.
Figure 9. E ff ect of drainage time (W300), solid retention, and ash retention of the handsheets with di ff erent doses of CNF and TRDS. ( a ) Recycled ONP pulp and ( b ) recycled OCC pulp.
In general, the drainage time of the recycled ONP pulp was lower than OCC in all conditions studied, probably due to the higher amount of fines that the recycled ONP pulp has compared to the recycled OCC pulp (Table 1). Cellulose fines usually consist of a very complex and heterogeneous set of materials, thus a certain fraction of fines is similar to cellulose microfibers [19]. The drainage results obtained in this study are according to Taipale et al. (2010) [19] and Johnson et al. (2016) [34], which demonstrated that lower content of fines in the pulp decreased the dependence of the drainage time with the CNF content. In both recycled pulps, higher doses of CNF absorbed more water in the pulp, making more di ffi cult the drainage process, thus increasing the W300 by 334% and 77% when 3 wt.% CNF from recycled ONP and recycled OCC were added into the pulp in absence of TRDS. However, this e ff ect can be reduced by 48% and 30%, respectively with the incorporation of TRDS as retention agent. These results are according to several authors, which demonstrated that the addition of CNF into a pulp suspension gets worse the drainage rate but this e ff ect can be counteracted by the addition of di ff erent retention systems [19,21,32,34].
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