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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 52 , No. 3

[ Original ]
Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 52, No. 2, pp.32-42
Abbreviation: J. Korea TAPPI
ISSN: 0253-3200 (Print)
Print publication date 30 Apr 2020
Received 02 Jan 2020 Revised 26 Mar 2020 Accepted 30 Mar 2020
DOI: https://doi.org/10.7584/JKTAPPI.2020.04.52.2.32

Refining Mechanism of Combined Refining Plates with Different Bar Angles
Xiya Guo1, ; Jixian Dong2, ; Huan Liu1 ; Chuanwu Duan1 ; Ruifan Yang1 ; Kai Qi1 ; Lingbo Kong2,
1College of Mechanical and Electrical Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, 710021, People’s Republic of China, Student
2College of Mechanical and Electrical Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, 710021, People’s Republic of China, Professor

Correspondence to : E-mail: guoxiyaya@163.com (Address: College of Mechanical and Electrical Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, 710021, People’s Republic of China)
Co-corresponding Author : E-mail: djx@sust.edu.cn, lbkong@sust.edu.cn (Address: College of Mechanical and Electrical Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, 710021, People’s Republic of China)

Funding Information ▼

Abstract

The refining plate is a key component of the disc refiner, which plays an important role in improving fiber morphology and its properties during pulp refining. In this study, the mixed pulps of bleached softwood and unbleached hardwood pulp was refined by the combined refining plates with different bar angles. Beating degree, fiber length, fiber width, fiber length distribution and fiber fibrillation were analyzed to explore the influences of different combinations of refining plates on the improvement of pulp and fiber properties. The results indicated that the combined refining plate of 0° and 5° had the more significant changes in fiber length distribution, beating degree and fiber morphology than that of the combined refining plate of 0° and 0° as refining proceeded. The influences of the combined refining plate of 0° and 39° on the fiber length components, beating degree and fiber morphology were weakest compared to the remaining two. This study is of positive significance for the design and the selection of refining plates.


Keywords: Papermaking, disc refiner, combined refining plate, bar angle, refining mechanism

1. Introduction

According to the previous studies, whether producing cultural paper, tissue or special paper, mixed pulps composed of softwood pulp and hardwood pulp were used by most enterprises for refining and bar profiles of rotor and stator segments are the same.1-3) Chauhan et al.1) believed that excellent paper was produced from a mixture of softwood and hardwood pulp. The “Treatment of Flocs” hypothesis, proposed by Banks,4) showed that long fibers would form tightly large flocs to protect the short fibers from crushing and shearing, which resulted in the uneven treatment of short fibers during the mixed refining system. Many scientists had conducted researches on the “Treatment of Flocs” hypothesis,5-9) it can be seen that separate or mixed refining of softwood and hardwood pulp had no significant differences on the pulp quality, while the mixed pulp refining could reduce energy consumption, improve bonding performance of fiber and tensile strength of paper.10) Chauhan et al.1) concluded that separate and mixed pulp refining had almost no effect on fiber length distribution and its beating degree in pulping process, but mixed pulp refining could promote various properties of paper, such as tensile strength and tear strength. Meanwhile, it can be concluded that bar profile, rotation speed and gap clearance had significant effects on fiber morphology and its properties.11-13)

The bar profile of refining plates mainly includes structural parameters such as bar width, groove width, bar height, bar angle and dam. As one of the most important parameters of refining plates, the bar angle is usually expressed by the angle between the bar and the radial direction of refining plates. Its magnitude and direction will affect the flow rate and quality of pulp.14) Wang and Wang15) achieved that bar angle was the main factor which affects the speed and direction of pulp motion, and different bar angles would have a great influences on refining quality and energy consumption. In order to obtain high-quality fibers, Computational Fluid Dynamics (CFD) was applied to analyze the role of bar angle and circumferential dam setting on pulp flow during the refining process.16) Liu and Zhu17) obtained that the plate bar angle affected the refining quality and pumping effect of pulp by refining plates design of medium consistency refining process and experiment studies. It was shown that the time for fiber separation is shorter and a higher refining efficiency would be reached when the pulp was refining by plates with smaller bar angle.18) Su and Wang19) pointed out that throughput and refining quality of disc refiner were related to the size of bar angle and its rotation direction. It can be found that the fiber cutting rate was larger when the plate with smaller bar angle was used in refining through the study of the fiber cutting of isometric straight refining plate with different bar angles.20) Elahimehr21) derived that bar intersecting length was inversely proportional to fiber length. Lumiainen22) used the bar intersecting angle and the bar width to calculate the refining intensity. A new refining dynamics model, which regarded bar angle as one of the parameters, was established to predict fiber cutting during refining, which was correctly proved by pilot test.23) Hammar et al.24) verified the changing of refining conditions can save energy at least 15%, and maintain the pulp properties if combined conical refining plates with different bar angles were used in low consistency refining of mechanical pulp. Mikko25) found there is a significant effect of bar angle on the refining quality.

To sum up, the refining of mixed pulp is common in the production of enterprises, but refining plates with the same bar angle were always used. It would be interesting to clarify the effect of different combined refining plates of rotor and stator segments on refining characteristics, especially for mixed pulp refining. At present, there is few studies on the effect of plate bar angle, especially for combined refining plates with different bar angle. In this paper, three groups of combined refining plates were used in low consistency refining by an experimental disc refiner, and the beating degree, fiber length, fiber width, fiber distribution and fiber fibrillation were measured to explore effects of different combined refining plates on properties of mixed pulp. It is conducive to understand refining mechanism of combined refining plates with different bar angles, and provide ideas for study of the influence of refining plates on the refining quality, and give a basis of designing or selecting of suitable refining plates.


2. Experiments and Methods
2.1 Fiber raw materials

In this study, mixed pulp of bleached hardwood pulp and unbleached softwood pulp was used in the ratio of four to one, which was soaked in water for 4 hours before refining. The detailed parameters of the pulp properties were shown in Table 1.

Table 1. 
Detail parameters of raw materials used in refining
Pulp Length-weighted mean fiber length(mm) Fines content(%) Beating degree(°SR)
Bleached hardwood pulp 0.65-0.85 8.0 13
Unbleached softwood pulp 2.00-3.00 7.5 13

2.2 Refining

Before refining, the consistency of the mixed pulp was adjusted to 3%, and MD3000 laboratory disc refiner (Regmed, Osasco, Brazil) was used for pulp refining, as shown in Fig. 1. The refining was conducted by using three combined refining plates with different bar angle at a constant speed of 1,460 rpm. The bar profile of them were shown in Tables 2 and 3. The straight bar plate with bar angle of 0° was fixed as the rotor, and different combinations of refining plates achieved by changing the refining plates with different bar angle, such as 0°, 5°, and 39°. Among them, the combined refining plate with stator bar angle of 0° was regard as trial Disc 1, the combined refining plate with stator bar angle of 5° was trial Disc 2, and the last one with stator bar angle of 39° was trial Disc 3. All plates were directly processed by Nantong Huayan Casting Co., Ltd. (Nantong, China) through the computer numerical control machine tools (CNC) machine (Shenzhen Dima Co., Ltd., Shenzhen, China).


Fig. 1. 
The MD3000 SD refiner.

Table 2. 
The bar parameters of three stator plates
Disc 1 Disc 2 Disc 3
Plate
Bar angle of stator plate 39°

Table 3. 
Common bar parameters of all plates
Bar width Groove width Bar height Inner diameter Outer diameter Field angle
2 mm 3 mm 4 mm 82.5 mm 203 mm 40°

In the process of refining, the soaked pulp were disintegrated at gap of 5 mm for 4 minutes, and then the sample pulp was gained and named 0 after adjusting the disc gap to 2 mm for 4 minutes. After that, the gap clearance was adjusted to 0.1 mm quickly, and 10 pulp samples were collected at intervals of 2 min. The samples collected at different times were centrifuged and dried by an electric blast drying oven 101 (Beijing Zhongxing Weiye Instrument Co., Ltd., Beijing, China) to calculate the moisture content of the pulp for subsequent experiments.

2.3 Measurement of pulp and fiber properties

Length-weighted average fiber length, fiber length distribution, fines content, fiber width, fiber coarseness and fibrillation of the pulp samples were analyzed by a FS5 fiber quality analyzer (Valmet, Espoo, Finland), beating degree of the pulp samples was measured by a DFR-05 draining freeness retention (BTG, Herrsching, Germany) according to GB/T3332-2004. Pulp properties and fiber morphology were analyzed to study the mechanism of the mixed pulp refining by combined refining plates with the different bar angles.


3. Results and Discussion
3.1 Beating degree

Beating degree is one of the main characteristics of pulp draining performance, which can comprehensively reflect the degree of fiber cutting, swelling and fibrillation, The beating degree is easily affected by temperature and consistency.26) Therefore, the laboratory temperature was maintained at about 20℃ during the experiments, and the pulp consistency was controlled at 0.2%. As refining proceeded, the beating degree of pulp samples, which were refined by different combined refining plates with three kinds of bar angle, increased differently according to the analysis of results, as shown in Fig. 2. The impact of the three combined refining plates on beating degree of mixed pulp samples was relatively clear, and the changing trend of it over time was significantly different. The beating degree of pulp samples refined by the Disc 2 (0°*5°) increased faster than that of the Disc 1 (0°*0°) when the pulp was refined for 10 min, while that of the Disc 3 (0°*39°) was the slowest among them. After the refining time was beyond 12 minutes, the Disc 1 (0°*0°) and Disc 2 (0°*5°) caused to the slower increase of beating degree, while the Disc 3 (0°*39°) contributed to the faster increase of beating degree. The beating degree of pulp samples when it was refined by the Disc 2 (0° *5°) for 20 min was 85°SR, however, that of the Disc 1 (0°*0°) and Disc 3 (0°*39°) were was 82°SR and 77°SR. It can be concluded that the changing ability of the Disc 2 (0°*5°) on beating degree is greatly compared to that of the others.


Fig. 2. 
The beating degree changes of samples refined by Disc 1, Disc 2 and Disc 3 with the refining time.

3.2 Fiber length
3.2.1 Average fiber length and fines content

Average fiber length is one of the main parameters of pulp fibers, and it is also an important indicator for measuring the bonding strength between fibers which directly affects physical properties of paper. Therefore, average fiber length is regarded as one of references for estimating pulp quality and determining papermaking process.26)

It can be seen from Fig. 3 that average fiber length of the pulp samples refined by the Disc 1 (0°*0°) dropped faster than that of the Disc 2 (0°*5°) during refining, while the average fiber length of the samples refined by the Disc 3 (0°*39°) displayed the slowest reducing trend, which indicated the greater bar angle was, the weaker fibers were cut. The conclusion was coincided with the results through CFD simulations by Liu and Hua.18) The fiber length change rate was defined as λ in order to clarify cutting degree of the three combined refining plates, which can be expressed as


Fig. 3. 
The length-weighted average fiber length changes of sampled fibers of Disc 1, Disc 2 and Disc 3 during refining.

λ=L0-LL[1] 

Where L0 represented the average fiber length of raw materials, and L represented the average fiber length after 20 minutes for refining. The experimental results showed that the Disc 1 (0°*0°) was beneficial to fiber cutting, with a change rate of 66%, and those of other refining plates were 59% and 54%.

During refining, microfibers and cut fiber particles were detached from the fibers, which would be the sources of fines. Fines filled the space between the fibers to increase fiber bonding strength during the papermaking process. Therefore, analysis of fines content was not only helpful to clarify the refining mechanism of different refining plates, but also to obtain the pulp with excellent properties.

In this experiment, fines content of the pulp samples refined by three combined refining plates continuously increased as the increase of refining time, while its variation trend of pulp refined by them was different, as shown in Fig. 4. The fines content changing of pulp samples refined by the Disc 1 (0°*0°) and Disc 3 (0°*39°), respectively about 12% and 10%, were lower compared to that of the Disc 2, 65.94%. Fiber bonding strength becomes strong with the increase of fines content.27) After refining, the fiber fines of the sampled pulps refined by the Disc 1 (0°*0°) , Disc 2 (0°*5°) and Disc 3 (0°*39°) seriously contributed to the performance of pulp and formed paper, but more microfibers and cut fiber particles were detached by the Disc 2 (0°*5°).


Fig. 4. 
The fines content changes of samples refined by Disc 1,Disc 2 and Disc 3 with the refining time.

3.2.2 Fiber length distribution

Fiber length continuously changes under the shear force and normal force applied by the plate. The fiber length distribution reflects the fiber proportion in different fiber length range. In this study, the fiber length distribution of pulp samples refined by three combined refining plates were analyzed to clarify the fiber cutting mechanism of combined refining plates with different bar angle.

Analysis of the sampled fibers were performed after refining, and the fibers, wider than 10 μm, were concerned when measuring their fiber length distributions, as shown in Figs. 5-9. The fiber length was defined by five groups, F1 (0-0.2 mm), F2 (0.2-0.6 mm), F3 (0.6-1.2 mm), F4 (1.2-2.0 mm) and F5 (2.0-3.2 mm). The fiber percentage in F1 and F2 continuously increased when refined by three combined refining plates. While, fiber content of F1 refined by the Disc 2 (0°*5°) was significantly higher than other two combined refining plates, which indicated that fiber cutting effects of the Disc 2 (0°*5°) was strongest compared to the others. The fiber content of F2 refined by three combined refining plates was approximate through the analysis of the sampled pulps. The Disc 2 (0°*5°) was a better choice for acquiring a sharp reduction of fiber in F3, and F3 was reduced from 50.9% to 43%, while that of the Disc 1 (0°*0°) and Disc 3 (0°*39°) were 43.6% and 45.7%. Fiber percentage of pulp samples refined by the Disc 1 (0°*0°), Disc 2 (0°*5°) and Disc 3 (0°*39°) in F4 were firstly increased and then decreased, the reason is why long fiber with a length of longer than 2.0 mm in raw pulps was easily trapped and treated in the beginning of refining. The F4 turning refining time of pulp samples refined by the Disc 2 (0°*5°) was 4 minutes, while that of the Disc 1 (0°*0°) and Disc 3 (0°*39°) were 8 minutes and 12 minutes. It can be concluded that fiber percentage of the pulp samples in F5 gradually decreased through the analysis of pulp samples refined by the Disc 1 (0°*0°), Disc 2 (0°*5°) and Disc 3 (0°*39°). The Disc 2 (0°*5°) could quickly cut the long fiber component of the pulp samples compared to the Disc 1 (0°*0°) and Disc 3 (0°*39°), which indicated a strong fiber cutting effect of the Disc 2 (0°*5°). The fiber percentage in the range of 0.6-1.2mm was over 40% after refining by three combined refining plates over the refining time of 20 min. Meanwhile, the ratio of the short and long fiber component was about four to one, which shows that fiber cutting of short fibers was affected by long fiber component during refining, and it was consistent with the conclusion proposed by Page et al.5)


Fig. 5. 
The F1 (0-0.2mm) distribution changes of sampled fibers by Disc 1, Disc 2 and Disc 3 during refining.


Fig. 6. 
The F2 (0.2-0.6mm) distribution changes of sampled fibers by Disc 1, Disc 2 and Disc 3 during refining.


Fig. 7. 
The F3 (0.6-1.2mm) distribution changes of sampled fibers by Disc 1, Disc 2 and Disc 3 with the refining time.


Fig. 8. 
The F4 (1.2-2.0mm) distribution changes of sampled fibers by Disc 1, Disc 2 and Disc 3 during refining.


Fig. 9. 
The F5 (2.0-3.2mm) distribution changes of sampled fibers by Disc 1, Disc 2 and Disc 3 during refining.

3.3 Other important parameters of fibers

In addition to beating degree, fiber length and its distribution, and fines content, other important parameters of fiber properties also include fiber width, ratio of length to width, fiber coarseness and fibrillation. These parameters are also an important basis for evaluating the pulp quality for papermaking, among them, fiber coarseness is the index of judging the properties of pulp and formed paper in Europe.14) In this study, the fiber width of pulp samples refined by different combined refining plates with different bar angles were analyzed, as shown in Fig. 10, and it was found that the effect of combined refining plates bar angle on fiber width was relatively small and there was almost no difference in changing of fiber width. Fiber width of pulp samples dropped differently according to the bar angle, and the fiber width of pulp samples refined by the Disc 2 (0°*5°) quickly decreased when the refining time was shorter than 12 min, and then the reduction rate of fiber width begins to decrease. There was an obvious difference in fiber width of the pulp samples refined by the Disc 1 (0°*0°) and Disc 3 (0°*39°) when the refining time was shorter than 10 min, while the fiber width of pulp samples refined by both of them remained almost the same when the refining time was beyond 10 min. Fiber widths of the untreated pulps which was planned to be refined by the Disc 1 (0°*0°), Disc 2 (0°*5°) and Disc 3 (0°*39°) were 27.12 μm, 27.5 μm and 26.66 μm, while fiber widths of pulp samples refined by three combined refining plates for 20 min were 25.29 μm, 24.95 μm and 25.32 μm after refining, which can be concluded that the Disc 2 (0°*5°) has a strong changing ability of fiber width. Judging from the refining results, the ratio of length to width had the significant decrease during refining, as shown in Fig. 11. It can be obtained that the reduction trend of that ratio were similar through analysis of the pulp samples refined by the Disc 1 (0°*0°), Disc 2 (0°*5°) and Disc 3 (0°*39°), while the changing ability of the Disc 1 (0°*0°) on the rate of length to width was strongest.


Fig. 10. 
The fiber width changes of sampled fibers by Disc 1, Disc 2 and Disc 3 with the time of refining.


Fig. 11. 
The ratio of fiber length to width changes of samples treated by Disc 1, Disc 2 and Disc 3 during refining.

Fiber coarseness refers to fiber weight per meter, which affects the bearing forces on fibers and the strength of paper.14) During refining, the fibers absorb water and swelled, and the fraction of water in fibers would decrease with the increase of temperature. The fiber coarseness would changes constantly due to the stretching and shearing effect of refining plate. Fiber coarseness of the sampled fibers refined by the Disc 1 (0°*0°), Disc 2 (0°*5°) and Disc 3 (0°*39°) were analyzed, as shown in Fig. 12, fiber coarseness of pulp samples refined by the Disc 2 (0°*5°) increased faster than that of the Disc 1 (0°*0°) during refining, however, the final fiber coarseness was very similar at the end of refining. The final fiber coarseness of pulp sample refined by the Disc 1 (0°*0°) was 0.14 mg/m, and that of the Disc 2 (0°*5°) was 0.138 mg/m. Meanwhile, the Disc 3 (0°*39°) had a small effect on fiber coarseness, and the final fiber coarseness differed from that of untreated pulp sample by 0.042 mg/m.


Fig. 12. 
The fiber coarseness changes of sampled pulps by Disc 1, Disc 2 and Disc 3 with the time of refining.

Fibrillation, including internal fibrillation and external fibrillation, means the longitudinal cracking of fibers, and the producing of fibrils due to complex mechanical effects of refining plates.14) The internal fibrillation can make fibers more plastic and soft, while the external fibrillation can enhance the bonding strength between fibers and finally improve strength and density of paper. It is important to compare the fiber fibrillation of pulp samples refined by the different combined refining plates, as shown in Fig. 13. Fiber fibrillation of pulp samples refined by different plates went up continuously with the increasing of refining time. Differently from the effect of the Disc 2 (0°*5°) and Disc 3 (0°*39°) on fiber fibrillation, the Disc 1 (0°*0°) had a strong ability on increasing of fiber fibrillation compared to others, which meant the refining plates with the small bar angle benefited the change of fiber fibrillation.


Fig. 13. 
The fibrillation changes of samples refined by Disc 1, Disc 2 and Disc 3 during refining.


4. Conclusions

1) Beating degree of the mixed pulps composed of unbleached softwood pulp and bleached hardwood pulp could be greatly improved by the combined refining plates with proper bar angles. The plate with the small bar angle could fast increase the beating degree and significantly affect drainability of pulp.

2) When bar angle of the rotor was fixed, the smaller the bar angle of the stator, the faster drop the fiber average length of the mixed pulps. At the same time, fiber length distribution of the mixed pulps changed significantly by the greater increase of fines content, which means that the fiber cutting rate and refining mechanism of different combined refining plates were different.

3) Fiber parameters of the mixed pulps, such as fiber coarseness, fines contents and ratio of fiber length to width, were obviously changed by the combined refining plates with different bar angles during low consistency refining. The combined refining plates with small bar angle contributed to the faster increase of fiber coarseness and fines content, and the sharp decrease of the ratio of fiber length to width of the mixed pulps. It can be shown that the combined refining plates with the smaller bar angle had distinct effects on the properties of mixed pulps.


Acknowledgments

The authors sincerely thank the funding by the National Natural Science Foundation (Grant No. 50745048) and Shaanxi Provincial key research and development Project (Fund) (Grant No. 2020GY-105). The authors give a special thanks to Henan Cigarette Industry Sheet Co., Ltd. and Nantong Huayan Casting Co., Ltd.


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