Comparative Analysis of Ultra-Fine Bar Refining and Valley Beating on Softwood and Hardwood Kraft Pulps: Implications for Fiber Integrity and Paper Strength Enhancement

Abstract

This study compared the effects of refining with an ultra-fine bar plate and Valley beating on softwood (SwBKP) and hardwood (HwBKP) bleached kraft pulps. Ultra-fine bar refining reduced pulp freeness from 700 mL to 200 mL CSF in just 10 min for both SwBKP and HwBKP, whereas Valley beating required over 60 min to reach comparable freeness levels. At equivalent freeness (e.g., 300 mL CSF), refining resulted in a tensile strength increase of approximately 35% for SwBKP and 20% for HwBKP compared to beating. In addition, refining delivered higher burst and tear strengths across all tested freeness levels, while better preserving fiber length. The results clearly demonstrate that refining with an ultra-fine bar plate achieves faster development of desirable paper properties and superior strength enhancement relative to Valley beating for both softwood and hardwood kraft pulps. These findings emphasize the importance of direct refining trials and plate optimization for improving pulp quality and energy efficiency in papermaking.

1. Introduction

Mechanical treatment of pulp is a pivotal step in the papermaking industry, profoundly influencing the properties of the final paper product. In this study, mechanical treatment refers specifically to the processes of beating and refining, which are applied to alter pulp fiber morphology and enhance paper performance [1,2]. This research utilizes two types of mechanical treatment equipment: the Valley beater, a laboratory-scale device that enables controlled and reproducible fiber modification, and a disc refiner equipped with an ultra-fine bar plate, designed for industrial-scale refining with enhanced energy efficiency and precision.

Beating and refining are essential mechanical processes for modifying pulp fiber properties and ultimately improving paper quality [1]. Both processes expose pulp fibers to a combination of shear, compression, and bending forces, resulting in external and internal fibrillation as well as fiber shortening [3,4], while shear forces mainly cause fiber cutting, compression, and bending enhance fibrillation, thereby increasing the surface area available for inter-fiber bonding and improving the strength properties of the resulting paper. However, the magnitude and outcome of these effects are highly dependent on the pulp source. Softwood kraft pulp (SwBKP) and hardwood kraft pulp (HwBKP), two widely used industrial pulps, differ markedly in fiber length, coarseness, and structure, which in turn shape their responses to mechanical treatment. Softwood fibers tend to impart strength and durability, while hardwood fibers improve surface smoothness and printability [2,5].

Previous studies have examined the effects of mechanical treatment on different pulp types. For instance, it has been reported that increased refining intensity improves the tensile strength of softwood pulp but at the cost of higher energy consumption [6,7]. In contrast, hardwood pulp requires less energy to achieve comparable levels of fibrillation, highlighting differences in refining efficiency between pulp types [8,9]. Some research on mixed pulp stocks has demonstrated that optimal mixing ratios can enhance both the strength and smoothness of paper products [10,11]. However, these studies also note the complexity of mechanical treatment effects on mixed pulps, which are influenced by factors such as fiber morphology and treatment intensity.

The Valley beater, a laboratory-scale adaptation of the Hollander beater, is widely used for its ability to simulate industrial refining processes with high precision and control. It consists of a cylindrical roll fitted with metal bars or knives and a movable bedplate, which together apply shear, compression, and bending forces to pulp fibers, resulting in effective fibrillation and fiber shortening [3,5]. This equipment is particularly valuable for research and development due to its reproducibility and controllability.

In contrast, industrial refiners—such as disc and conical refiners—are essential for large-scale papermaking due to their efficiency and scalability [1]. Disc refiners, which include single, twin, and double-disc configurations, use rotating discs with bars and grooves to impart mechanical forces to the pulp [5]. Conical refiners operate similarly but utilize a conical rotor and stator. Recent advancements in refiner technology have focused on improving energy efficiency and refining performance. For example, the development of a lightweight refiner plate for hardwood kraft pulp, resulting in enhanced refining efficiency and reduced energy consumption [11,12]. Similarly, the introduction of a novel lightweight vertical bar plate for softwood pulp further improves refining performance and contributes to energy savings [13]. In this study, an ultra-fine bar plate equipped with closely spaced, narrow bars was employed in the disc refiner to provide precise and controlled mechanical treatment. This design maximizes fiber-to-fiber contact while minimizing excessive cutting, promoting effective fibrillation and fiber bonding. The ultra-fine bar plate’s refined bar pattern allows for intensified refining intensity at low energy consumption, making it highly suitable for optimizing pulp properties in both softwood and hardwood kraft pulps.

Combining softwood and hardwood pulps in various proportions is a common industrial practice to balance paper strength and quality [2]. However, the effects of beating and refining on mixed pulp stocks are complex and can be influenced by fiber morphology, refining intensity, and the specific equipment used [14,15]. Understanding these comparative effects is crucial for optimizing paper properties while minimizing energy consumption and production costs.

This study aims to compare the effects of Valley beating and ultra-fine bar refining on softwood and hardwood kraft pulps, both individually and in mixed stocks. By analyzing changes in fiber morphology, paper strength, and related properties, this research seeks to identify optimal treatment conditions for achieving desired paper qualities. Additionally, the study will assess the energy implications of each process, providing insights for enhancing the sustainability of pulp and paper production. By elucidating the interplay between pulp type, mechanical treatment, and paper properties, this research contributes to a deeper understanding of the papermaking process. The findings are expected to inform industry practices and support the development of more efficient and sustainable papermaking technologies.

2. Materials and Methods

2.1. Raw Materials

Hardwood bleached kraft pulp (HwBKP) and softwood bleached kraft pulp (SwBKP) were supplied from Moorim Paper Co., Ltd. in Jinju, Republic of Korea. They were torn into small pieces and soaked in distilled water for over 4 h before disintegration to ensure proper hydration and ease of fiber separation. This step also helps reduce energy consumption during disintegration and maintains the intrinsic properties of the fibers [16].

The fiber characteristics of these two raw materials are summarized in

Table 1. Fiber characteristics of HwBKP and SwBKP used for beating and refining.

	Arithmetic Mean Fiber Length (mm)	Length-Weighted Mean Fiber Length (mm)	Arithmetic Fines Contents (≤0.2 mm) (%)	Coarseness (mg/m)
HwBKP	0.5	0.7	31	5.8
SwBKP	1.3	2.3	28	15.5

. While softwood and hardwood kraft pulps exhibit more pronounced differences in fiber morphology than in chemical composition, both types of data were examined in this study.

Table 2. Chemical composition of HwBKP and SwBKP used for beating and refining.

	Cellulose (%)	Hemi-Cellulose (%)	Lignin (%)	Extractives (%)
HwBKP	83 ± 1.24	20 ± 0.91	0.3 ± 0.13	0.3 ± 0.13
SwBKP	86 ± 1.81	13 ± 0.45	0.3 ± 0.12	0.1 ± 0.11

shows the chemical compositions of HwBKP and SwBKP. However, since these differences had little effect on the beating and refining results of SwBKP and HwBKP, the data in Table 2 were not further discussed in this study.

2.2. Beating and Refining

Before beating, the soaked pulp specimen underwent disintegration. This was accomplished using a Valley beater (Daeil Machinery Co., Inc., Daejeon, Republic of Korea) without any applied load, maintaining a pulp consistency of 1.57 ± 0.04% for at least 3 to 5 min, following ISO 5264-1 [17]. After disintegration, the pulp was beaten within the Valley beater (refer to Figure 1) with a 5.5 kg load at around 500 rpm until the target refining level of around 190 mL CSF.

The specific dimensions of the Valley beater are shown in Figure 2 and

Table 3. Specific dimensions of the laboratory Valley beater.

Roll with Flybars	Bedplate
Roll diameter with fly bars: 193.8 mm	Number of bars: 7
Roll width: 4.8 mm	Bar width: 3.2 mm
Roll length: 152.4 mm	Bar height: 159 mm
Number of flybars: 32	Groove width: 2.4 mm
Flybar height: 10 mm	Angle between the roll axis and bed plate: 5°

. The width of the flybars is 4.8 mm, and the width of the bars on the bedplate is 3.2 mm. The groove width between the flybars is 12.25 mm, and the groove width between the bars on the bedplate is 2.4 mm.

After disintegrating the pulp fibers using the Valley beater, excess water was removed from the pulp slurry to adjust the concentration to 4–5% for refining according to the ISO/TS 11371 guideline (https://www.iso.org/standard/84109.html, accessed on 1 April 2025). The thick stock was then fed into a single disk refiner (KOSWON Co., Gimhae City, Republic of Korea), designed with a specified disk pattern for ultra-low intensity refining (see Figure 3). The disk plate was fabricated by assembling two distinct metal alloys, resulting in a vertical bar configuration that differed from conventional cast plates with a draft angle [18,19,20]. The precise dimensions of the bars used for ultra-low intensity refining are illustrated in Figure 4. With a plate diameter of 12 inches and a cutting edge length (CEL) of 97 km/s, the refiner operated at 800 rpm. The primary goal of this refining process was to modify the mixed pulp stocks to achieve a Canadian freeness level of around 190 mL, by ISO 5267-1 [21]. In practice, due to the high efficiency of the ultra-fine bar plate, refining produced some samples with freeness values below 190 mL CSF. Therefore, results are presented for the full range of observed freeness values, including those below the target.

2.3. Analysis of Pulp and Paper Properties

The mean fiber length of pulp fibers was measured using the Fiber Quality Analyzer FQA-360 (Optest Equipment Inc., Hawkesbury, ON, Canada). For assessing the physical properties of paper, handsheets of 70 g/m² were prepared, conditioned, and tested following ISO 5269-1 [22]. The physical properties, specifically tensile, burst, and tear strength, were measured by ISO 5270 [23].

Microscopic observations of fiber morphology and structural changes were conducted using an Olympus BX51 optical microscope (BX51, OLYMPUS, Tokyo, Japan). Samples of softwood and hardwood bleached kraft pulps before and after mechanical treatment were prepared by placing diluted pulp suspensions on glass slides and observed under magnifications of 100× and 200×. The imaging allowed qualitative assessment of fiber length, fibrillation, and surface characteristics to clarify the effects of Valley beating and ultra-fine bar refining on fiber structure.

3. Results

3.1. Freeness

Figure 5 compares the freeness drop rate of pulp stocks during beating and refining. The results show that the refiner plate with the ultra-fine bar pattern induced a much faster reduction in freeness for both SwBKP and HwBKP compared to beating. This efficiency arises from several factors inherent to the refiner design: the disk refiner achieves highly controlled, intense fiber treatment due to its consistent bar and groove pattern and uniform gap clearance, maximizing the frequency and effectiveness of fiber-bar interactions. This promotes both internal and external fibrillation and enables rapid freeness reduction.

In contrast, the beating curve for HwBKP shows a noticeable plateau after approximately 40 min, where the rate of freeness reduction slows significantly. This plateau can be attributed to the gradual exhaustion of readily accessible fiber surfaces for fibrillation and swelling, meaning that further mechanical action produces diminishing changes in pulp drainage characteristics. As beating continues, additional treatment has a limited effect on freeness due to the increased resistance of more fully fibrillated and shortened fibers, which is a typical feature seen with hardwood pulps under prolonged beating conditions.

The disk refiner, particularly when fitted with ultra-fine bar patterns, applies highly controlled and intense mechanical action to the pulp fibers. As shown in Figure 6, its consistent bar and groove dimensions, along with uniform gap clearance, enhance the frequency and effectiveness of fiber-bar interactions [24]. This design maximizes the contact surface area and increases the number of bar impacts per unit time, thereby promoting both internal and external fibrillation of fibers, which is critical for rapid freeness reduction [25,26,27].

The disk refiner, particularly when fitted with ultra-fine bar patterns, applies highly controlled and intense mechanical action to the pulp fibers. Its consistent bar and groove dimensions, along with uniform gap clearance, enhance the frequency and effectiveness of fiber-bar interactions [24], as illustrated in Figure 6. This design, characterized by uniform bar dimensions, enhances refining efficiency by maximizing the contact surface area and increasing the frequency of bar impacts against fiber bundles captured by the bar edges [25,27]. Consequently, this leads to more effective fiber modification, including both internal and external fibrillation, which is crucial for rapidly developing fiber properties. These factors contributed to the refiner’s ability to reach the desired freeness level more quickly than the beater.

In contrast, the Valley beater operates with wider flybars and larger grooves, resulting in less precise mechanical action at lower rotational speeds. The beater relies on repeated compression and shearing to gradually increase fiber flexibility and surface area through fibrillation. However, as illustrated in Figure 7, the less controlled geometry means that some fiber bundles may pass through the beater without adequate modification, necessitating longer processing times to achieve similar freeness levels [28,29]. Additionally, fibers that become trapped in the grooves between flybars are not efficiently beaten, further reducing the overall efficiency of the process [28].

These findings are consistent with prior studies, which have shown that disk refiners are generally more effective than beaters in rapidly developing fiber properties and achieving targeted freeness levels [25,27]. The superior performance of the refiner in this study can thus be attributed to its ability to deliver more uniform and intense mechanical treatment, resulting in faster and more efficient freeness reduction. In summary, while both refining and beating are capable of modifying fiber properties, the disk refiner with an ultra-fine bar plate offers significant advantages in process efficiency and control.

3.2. Fiber Length

Figure 8 illustrates the comparative effects of a beater and a refiner on the length-weighted mean fiber length of SwBKP and HwBKP. The results show that both SwBKP and HwBKP experience a faster reduction in fiber length when processed with the refiner compared to the beater; however, at equivalent freeness levels, the refiner consistently preserves longer fiber lengths.

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].

3.3. Physical Properties of Paper

Figure 9 and Figure 10 compare the effects of beating and refining on the tensile and burst strength of SwBKP and HwBKP. The data indicate that the refiner, equipped with an ultra-fine bar plate, facilitates faster development of both tensile and burst strength compared to the beater.

At equivalent freeness levels, both SwBKP and HwBKP processed through the refiner exhibited greater tensile and burst strength than those processed through the beater. This can be attributed to the refiner’s precise and intense mechanical action, which enhances fiber bonding by promoting both internal and external fibrillation without excessive fiber cutting [24,27,31]. The increased fiber bonding potential leads to improved strength properties, as more flexible and fibrillated fibers form stronger inter-fiber bonds [32].

In contrast, the beater’s broader grooves and longer bars result in less effective fiber bonding and a higher likelihood of fiber shortening, which negatively impacts strength characteristics [28,29]. The less controlled and less uniform mechanical action of the Valley beater can also lead to uneven fiber treatment, further reducing the overall development of strength properties.

The Valley beater, although widely used in laboratory settings to predict refining outcomes, has inherent limitations in replicating the precise effects of industrial refining. The scale and mechanical precision of the Valley beater differ significantly from those of industrial disk refiners, resulting in discrepancies in fiber modification and, consequently, in the resulting pulp properties [24,29]. While the Valley beater provides valuable initial insights into pulp fiber response to mechanical treatment, it may not capture the nuanced effects observed in refining, especially for pulps with distinct morphological characteristics such as SwBKP and HwBKP.

Both pulp types demonstrated faster strength development through refining than beating, but the extent of improvement varied. SwBKP, with its longer and more flexible fibers, exhibited a more pronounced improvement in tensile and burst strength, consistent with the literature indicating that fiber morphology significantly influences the response to mechanical treatment [27]. HwBKP also benefited from refining, though to a lesser extent, due to its shorter and stiffer fibers.

These findings suggest that while the Valley beater remains a useful tool for preliminary experiments, its limitations in accurately predicting industrial refining effects, especially for specific pulp types, should be recognized. The structural and mechanical precision of refiners is crucial for optimizing pulp strength properties, emphasizing the need for direct refining trials when predicting refining outcomes based on beater experiments [24,27]. Overall, this study underscores the advantages of disk refiners over beaters in achieving superior tensile and burst strength outcomes and highlights important implications for refining process optimization in pulp quality improvement.

Figure 11 presents the evolution of tear strength in SwBKP and HwBKP as a function of both treatment time (Figure 11a) and freeness (Figure 11b) for beating and refining. Several key trends can be observed: For SwBKP (softwood bleached kraft pulp), tear strength initially increases with both beating and refining, reaches a maximum, and then gradually decreases as the treatment proceeds. The initial rise is attributed to improvements in fiber bonding and network entanglement [24]. As refining or beating continues, excessive fiber shortening and increased bonding density eventually lead to a decline in tear strength, since shorter fibers and denser networks limit the fiber pull-out mechanism that underpins tear resistance.

A noteworthy finding is that, at a comparable low freeness (approximately 200 mL CSF), both SwBKP and HwBKP samples subjected to refining demonstrate higher tear strength than those subjected to beating. This advantage is particularly clear for SwBKP. The superiority of refining at this freeness is due to two primary factors. First, Refining reaches the target freeness rapidly with minimal fiber shortening, as confirmed by higher length-weighted fiber length in Figure 8b. Second, the fibers in refined pulp maintain their ability to bridge tear cracks due to their preserved length, while the generated external fibrillation supports sufficient fiber bonding without excessive densification that would otherwise impede tearing [18,24,27].

Conversely, beating requires a prolonged treatment to achieve the same freeness, resulting in more fiber cutting and network damage. The consequence is shorter fibers and a reduced capacity for reinforcing the paper matrix, producing lower tear strength than refining at the same drainage level.

It is also important to note that, while longer fibers generally confer higher tear strength, this relationship depends on achieving an optimal balance between fiber length, fiber bonding, and network structure. Over-refining or excessive beating, even for softer pulps, can lead to increased bonding but decreased fiber pull-out, resulting in a reduction in tear strength beyond the optimum.

For HwBKP, tear strength is consistently lower than for SwBKP due to the inherently shorter and stiffer nature of hardwood fibers [32]. Still, the refined hardwood samples retain slightly higher tear strength than beaten samples at equivalent low freeness, for the same reasons described above.

In summary, Figure 11 shows that refining with an ultra-fine bar plate not only preserves fiber length but also achieves higher tear strength at low freeness compared to beating. This outcome supports the theory that under optimal conditions, refining is more advantageous than beating for maximizing tear resistance, especially in softwood pulps [24,29].

3.4. Observation of Fiber Morphology Before and After Mechanical Treatment

Figure 12 displays optical microscopy images that illustrate the microstructural evolution of both HwBKP and SwBKP fibers subjected to disintegration, Valley beating, and ultra-fine bar refining. All samples were evaluated at similar freeness levels (approximately 200 mL CSF), enabling a direct comparison of morphological effects due to the different mechanical treatments.

In the disintegrated states ((a) and (d)), both HwBKP and SwBKP fibers appear long and relatively unaltered, with little to no external fibrillation and a scarcity of fines. These images represent the baseline fiber condition prior to mechanical processing.

Following the Valley beating ((b) for HwBKP, (e) for SwBKP), fibers from both pulps exhibit noticeable external fibrillation as well as increased fragmentation, with shorter fibers and a higher abundance of fines and detached fiber fragments. This reflects the tendency of beating to induce not only fibrillation but also significant fiber cutting, particularly visible in the formation of curled fragments and debris.

In contrast, the refined pulp images ((c) for HwBKP, (f) for SwBKP) demonstrate extensive fibrillation along relatively intact and elongated fibers. The ultra-fine bar refiner treatment promotes the development of a dense layer of fibrils on the fiber surfaces, enhancing their ability to form strong inter-fiber bonds while largely preserving fiber length and morphology. Less fiber breakage and fewer fines are observed than in the beaten samples, especially for SwBKP, where the network remains open and fiber integrity is evident.

Taken together, Figure 12 visually confirms that ultra-fine bar refining achieves substantial fibrillation and surface area enlargement without the excessive fiber shortening and fragmentation typical of Valley beating. This structural preservation is especially beneficial for mechanical properties (tensile, burst, and tear strengths) in the resulting paper, supporting the quantitative findings of the study. Moreover, the images highlight that the impact of mechanical treatment on fiber morphology varies with pulp type, with softwood fibers generally retaining more length and flexibility than hardwood fibers, regardless of the process.

4. Conclusions

This study demonstrated that ultra-fine bar plate refining offers substantial advantages over Valley beating for both softwood (SwBKP) and hardwood (HwBKP) bleached kraft pulps. The disc refiner with an ultra-fine bar plate rapidly reduced pulp freeness from ~700 mL to ~200 mL CSF within approximately 10 min, while Valley beating required over 60 min to reach comparable freeness. At similar freeness levels (~200–300 mL CSF), tensile strength for refined SwBKP improved by about 35%, and HwBKP by approximately 20%, compared to beaten fibers. Burst and tear strengths also increased significantly; for instance, the tear strength of refined SwBKP was about 25% higher than after beating. Optical microscopy confirmed that refining effectively enhances fibrillation and fiber bonding while better preserving fiber length than beating, resulting in improved mechanical properties of paper sheets. These quantitative findings underscore the enhanced efficiency and performance of ultra-fine bar refining in pulp treatment and highlight its importance for optimizing paper strength and quality in sustainable papermaking processes.