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ing energy expenditure. Throughput stabilizes near 24 g/s at higher specific energies (~125 kWh/t), indicating consistent performance. In contrast, the ultra-fine bar plate at 97 km/s exhibits a gradual throughput increase, peaking at around 22 g/s near 80 kWh/t, beyond which a plateau is observed. This plateau likely arises from the plate’s narrow grooves, which, while enhancing fiber fibrillation and bonding effectiveness, impose higher flow resistance and restrict stock flow at elevated energy levels [5,7]. Despite this limitation, the ultra-fine bar plate achieves superior fiber quality metrics, such as tensile and tear strength improvements and increased water retention, reflecting a favorable energy-to-quality trade-off. Thus, the selection between conventional and ultra-fine bar plates depends on bal- ancing throughput priorities with desired fiber modifications. The conventional plate maximizes material throughput and energy economy, while the ultra-fine bar plate excels in refining quality, particularly beneficial for recycled fiber enhancement. 3.2. Freeness Figure 2 presents a comparative analysis of stock throughput (g/s) versus specific refining energy (kWh/t) for KOCCs using two different cutting edge lengths (CELs): 37 km/s and 97 km/s. The plate at CEL 37 km/s initially exhibits a higher throughput, reaching approximately 25 g/s at 70 kWh/t, demonstrating a higher throughput at a lower energy input. This is largely attributable to its wider grooves, which reduce flow resistance and facilitate greater stock passage. The throughput stabilizes at around 24 g/s at 125 kWh/t, showcasing its efficiency in achieving high throughput with reduced energy.
Figure2. Effect of refining energy on pulp freeness for KOCCs with different CELs.
Conversely, the ultra-fine bar plate at CEL 97 km/s shows a steady increase in through- put up to 22 g/s at approximately 80 kWh/t, after which throughput plateaus. This plateau is likely caused by the design’s narrower grooves that, while promoting superior fiber fibrillation and bonding, increase flow resistance and limit stock flow at higher energies—a phenomenon reported in previous studies. Despite the lower throughput, the ultra-fine bar plate is more energy-efficient in achieving desired fiber quality improvements, such as tensile strength and tear strength, reflecting a trade-off between throughput and fiber modification effectiveness. Therefore, while conventional plates excel in throughput and energy input efficiency, ultra-fine bar plates provide enhanced fiber quality improvements at a moderate trade-off in throughput. Selecting the appropriate refining plate design is essential for optimizing recycled KOCC refining to balance manufacturing efficiency and paper performance. 3.3. Fiber Length and Fines Figure 3 illustrates the effects of refining energy on mean fiber length and fines content for KOCCs, comparing refiner plates with cutting edge lengths (CELs) of 37 km/s and
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