# Study of Refining Effect of Mixed Pulps Using Refiner Plates with Different Bar Patterns
## 3 Results and Discussion  
### Fiber Properties  
Refining is easily assessed using the drainage rate of the pulp fibers, which is closely related to the external and internal fibrillation of the pulp fibers, delamination of the fiber wall, and shortening of the fibers [22]. A high and fast drainage rate means a high freeness. Figure 6 compares the refining energy required to achieve the target freeness (approximately 180 mL CSF) when the mixed pulps were individually refined by the two plates. As the mixing ratio of hardwood pulp fibers increased, the refining energy required to reach the target freeness decreased irrespective of the bar fillings of the plates. In the case of Sw-BKP, which was composed of long fibers, the most refining energy was consumed until the target freeness was reached irrespective of plate filling, but in the case of PB97, the refining energy was reduced by about 50% compared to PA55. When the refining energy consumed up to the target freeness was compared, it was confirmed that the energy consumption of PA55 was significantly reduced, as Hw-BKP was mixed. It is worth noting that the refining energy was remarkably decreased when Hw-BKP was mixed with Sw-BKP compared to Hw-BKP alone. PA55 has a wide groove between the bars, so short hardwood fibers are more likely to pass unrefined. However, as hardwood fibers were mixed with long softwood fibers, the time for the mixed fibers to pass through the grooves was delayed, so it was considered that refining was appropriately performed (refer to Figure 7a). As the long softwood fibers held the short fibers to form large flocs, these flocs were more easily caught on the bar edges of the plate and appeared to be refined quickly. In the end, it was confirmed that, as the CEL of the refiner plate became smaller, it was more advantageous to refine the mixed pulps with long and short pulp fibers than to refine a single pulp. Figure 6: Effect of CEL on freeness drop of the mixed pulps: (**a**) PA55; (**b**) PB97. However, PB97 showed different refining behavior from PA55. As the mixing ratio of Hw-BKP increased, the refining energy required up to the target freeness decreased sequentially. In PB97, with narrower grooves, it was considered that more softwood fibers were mixed to form larger or harder flocs, which took more time to pass through the plate grooves, and consequently, more refining energy was consumed [6]. On the other hand, the soft flocs with short fibers passed through the plate grooves more easily and caught on the bar edges so that they were refined quickly. Figure 8 shows the CEL effect of the refiner plates on the mean fiber length and fines content of the mixed pulps refined up to about 180 mL CSF. It is well known that high-intensity refining tends to induce fiber shortening [23]. When only Sw-BKP was refined, PA55 with smaller CEL generated more fiber cutting than PB97 with higher CEL, leading to a shorter mean fiber length. However, there was no significant difference in mean fiber length and fines content between PA55 and PB97 even though the hardwood fibers were mixed. Figure 9 is a graph comparing the WRV of the mixed pulps refined up to about 180 mL CSF. When only Sw-BKP and Hw-BKP were refined independently, there was no remarkable WRV difference in both PA55 and PB97. However, as Sw-BKP and Hw-BKP were mixed, the WRVs of the mixed pulps were greater than those of the single pulp,  
Figure 8: Effect of CEL on mean fiber length and fines content of the mixed pulps: (**a**) PA55; (**b**) PB97. Figure 7: Effect of CEL on fiber flow during refining: (**a**) small flocs quickly pass through the groove but larger flocs are caught on the bar edges; (**b**) while small flocs are readily caught on the bar edge between the narrow grooves, larger flocs take more time to pass through the grooves. regardless of CEL. As Sw-BKP was mixed with Hw-BKP, long fibers formed flocs with short fibers during refining, and the flocs absorbed the physical impacts from the bar edges [1; 6].