Appl. Sci. 2025 , 15 , 9160
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( a ) ( b ) Figure8. ( a ) Comparison of the length-weighted fiber length of SwBKP and HwBKP according to beating and refining time; ( b ) Comparison of the length-weighted fiber length of SwBKP and HwBKP according to freeness. This outcome is closely related to the fundamental differences in mechanical action and design between the two refining systems [30]. Disk refiners, especially those equipped with ultra-fine bar patterns, apply more precise and controlled mechanical action to the fibers. The uniform bar and groove dimensions, along with consistent gap clearance, promote frequent and effective fiber-bar interactions, which enhance fiber modification, primarily through fibrillation, while minimizing excessive fiber cutting and shortening [26]. This enables rapid freeness reduction without a proportional loss in fiber length, allowing the refiner to maintain longer fibers even as the pulp becomes more refined [24,26]. In contrast, the Valley beater operates with wider grooves and longer bars, resulting in less controlled and more variable mechanical action. This can lead to fiber bundles passing through the beater without adequate treatment, while others may be subjected to excessive mechanical forces, increasing the risk of fiber cutting and shortening. As a result, the beater tends to produce shorter fibers at comparable freeness levels, and the reduction in fiber length occurs more gradually [24]. The difference in fiber length outcomes between the two refining methods is more pronounced for SwBKP than for HwBKP. Softwood fibers, being longer and more flexible, are more susceptible to both the beneficial and detrimental effects of refining. They can be effectively modified by the precise action of the refiner but are also at greater risk of shortening if subjected to uncontrolled mechanical forces. Hardwood fibers, which are shorter and stiffer, show less pronounced differences between the two refining methods, likely due to their lower propensity for fibrillation and cutting [24]. These findings are consistent with previous studies, which have reported that disk refiners generally induce less fiber shortening than Valley beaters, especially at moderate refining intensities [25,26]. Severe fiber cutting and a rapid decrease in fiber length are more likely with Valley beater refining, particularly at low pulp consistencies or with low- strength fibers. The refiner’s ability to deliver rapid freeness reduction while preserving fiber length underscores its advantage for efficient and effective fiber modification in pulp processing. In summary, the structural and operational differences between the refiner and the beater significantly influence fiber length outcomes. The disk refiner with an ultra-fine bar plate demonstrates a superior capacity for maintaining fiber length during rapid freeness reduction, making it a preferred choice for applications where fiber length preservation is critical [24].
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