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Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 53, No. 4

[ Article ]
Journal of Korea Technical Association of the Pulp and Paper Industry - Vol. 53, No. 3, pp.3-14
Abbreviation: J. Korea TAPPI
ISSN: 0253-3200 (Print)
Print publication date 30 Jun 2021
Received 03 May 2021 Revised 02 Jun 2021 Accepted 04 Jun 2021
DOI: https://doi.org/10.7584/JKTAPPI.2021.06.53.3.3

Introduction to the Novel Plate for Manufacturing Thermomechanical Pulp
Jeong-Heon Ryu1 ; Chul-Hwan Kim2, ; Byeong-Gul Min3 ; Min-Seok Lee1 ; Ho-Kyung Goo1 ; Chang-Yeong Lee1 ; Jin-Hwa Park1
1Department of Forest Products, Gyeongsang National University, Jinju, 52828, Student, Korea
2Major of Environmental Materials Science, IALS, Gyeongsang National University, Jinju, 52828, Professor, Korea
3Representative, Kos1 Co., Gimhae, 50873, Korea

Correspondence to : E-mail: jameskim@gnu.ac.kr (Address: IALS, Gyeongsang National University, Jinju, 52828, Korea)

Funding Information ▼

Abstract

Refiner plates for manufacturing thermomechanical pulp (TMP) exhibit heavy and blunt-bar edges because they are manufactured via casting. Thus, replacing a cast plate that has exhausted its service life is time-consuming and a large amount of energy is required to reach the target freeness. To overcome the limitations of conventional cast plates for manufacturing TMP and chemi-TMP (CTMP), new dissimilar metal-joining technology was applied to lighten the TMP plate. A lightweight aluminum alloy was used as the base of the plate, and stainless steel alloy exhibiting excellent wear resistance was used as the bars. The lightweight plate, which was manufactured by inserting the bars into the base plate, can reduce the weight of the plate by up to a quarter compared with the cast plate. The refining performance of the lightweight plate was also similar or better than those of the cast plate. Particularly, the target freeness can be reached faster because the bar edge of the lightweight plate of a vertical bar shape was sharper than those of cast bars with a draft angle. This lightweight TMP plate obtained better results than the cast plates regarding the raw material throughput, shives content, and strength property development. However, the brightness of TMP and CTMP in the lightweight plate were slightly lower compared with those in the cast plate.


Keywords: Thermomechanical pulp plate, dissimilar metal joining technology, lightweight plate, cast plate, vertical bar shape

1. Introduction

Mechanical pulping (MP) requires a considerable amount of electrical energy.1) Over the years, many efforts have been made toward reducing the amount of required electrical energy.2) The primary approach was using refiner MP (RMP), which uses hot steam to soften the lignin holding the wood fibers together. Further, wood chips were exposed to pressurized hot steam to accelerate lignin softening, which is thermomechanical pulping (TMP).3,4)

The TMP process has gradually evolved over the years. After preheating the wood chips with hot steam, they are refined in a pressurized refiner and subsequently refined in a secondary refiner. However, pressurized refiners are used in both stages. However, the TMP process notably contributes to longer fibers, fewer shives and fines, and stronger pulp compared with RMP, it did not achieve energy conservation.4-13) The refining process accounts for 67% of the total energy consumption of TMP. This is because the refining process relies more on mechanical action rather than chemical routes to separate the wood fibers from the wood chips and develop their fiber properties; this is a major drawback in TMP.

The strategies for reducing the energy consumption of the TMP process include the pretreatment of the wood chips, control of the refiner operation, plate design, and concentration during refining.9-16) The pretreatment of wood chips with hot steam is a well-known strategy for conserving energy through lignin softening. An example of energy saving through refining control is increasing the revolution per minute (RPM) of the refiner motor for high-intensity refining.6,15) Unfortunately, energy reduction during TMP has been reported to be largely achieved by compromising pulp strength.12) Therefore, TMP plants worldwide are exploring strategies for reducing energy consumptions while maintaining pulp qualities.

The refiner plate largely consists of two main regions, depending on the manufacturer’s design.6) The first region is the breaker-bar section in which the wood chips are broken into small fragments, and the second region is the intermediate section where the wood fragments are converted into fiber bundles. Additionally, the third region is the refining section in which the fiber bundles are separated through bending, crushing, pulling, and pushing. The quality of the resulting mechanical pulp, which is a measure of its papermaking properties, depends on how well energy is transferred to the fiber during the refining. Thus, the design of the refiner plate is crucial in determining pulp quality. 15) Some factors, such as the width, grooves, and height of the bars, as well as the dams between them, must be considered when designing the filling of the refiner bar plate because they affect refining.8)

Refiner plates are cast from ferrous alloys that exhibit wear and corrosion resistance, and castability. 17) The cast plate is designed such that the bars, which are formed on the base plate, exhibit a draft angle since the bars must be exhibit a tapered shape to prevent the damage to the sand mold. These tapered bars can reduce the flow rate of raw materials during the refining process and negatively impact the TMP production. Additionally, as the TMP plate with a diameter of ~2 m weighs >25 kg, replacing it is time-consuming. Therefore, increasing the flow rate of raw materials and lightening the plate by manufacturing a bar plate without a draft angle is desirable, thereby improving productivity, reducing cost, and improving the efficiency of plate replacement in the TMP process.

Most local mechanical-pulp-manufacturing companies use imported refiner plates, which are expensive. Therefore, localizing refiner plate production for mechanical-pulp production is urgent. Further, because TMP refining is an energy-intensive process; a new concept of a refiner plate must be used to increase energy efficiency and reduce the energy consumption of the TMP process. Regarding the refining of chemical pulp (kraft pulp), using vertical bar plates positively affects the stock throughput, paper quality, and energy efficiency has been confirmed.18,19) In this study, the possibility of manufacturing a lightweight with vertical bars to increase the sharpness of the bar edge and flow rates of raw materials, was investigated. The production of such a lightweight, vertical-bar TMP plate in Korea is expected to enhance competition with other TMP manufacturers.


2. Materials and Methods
2.1 Raw material

Pine chips (Pinus densiflora, Jeonju Paper Co., Ltd.) were used to manufacture the thermomechanical pulp. The wood chips were washed with water to remove impurities before the production of pulp. The chemical compositions of the wood chips are listed in Table 1.

Table 1. 
Chemical characteristics of Pinus densiflora
Lignin Hot extractives Cold extractives Organic extractives Ash
Pinus densiflora 32.2 3.4 2.7 4.5 0.2

2.2 Manufacture of bar plates

Conventional refiner plates have been manufactured via sand casting, which required draft angles of 2°–5° to remove the sand mold and achieve the design pattern before closing and pouring (Fig. 1). The sharp corners in the patterns have resulted in local structural weaknesses, such as shrinkage, cracks, tears, and draws.20) Refiner plates exhibiting draft angles can minimize the open area of the flow zones between the bars, stock throughput, and plate life.18,21-23)


Fig. 1. 
Schematic of sand casting.

Conventional bar plates are manufactured from metals in the sand casting, which uses sand as the mold material. Regarding lightweight bar plates, an aluminum alloy was used as the raw material to reduce the weight of bar plates (Fig. 1). The first step toward manufacturing a lightweight bar plate is producing a plate mold with dams. Then, a flask, which is packed with resin-coated sand, is placed on the plate mold. The resin-coated sand was treated at 200℃–500℃ using a liquefied petroleum gas (LPG) torch. When the flask containing the cured sand was removed from the plate mold, the bottom side of the bar was exposed upward. Additionally, after the flask holding the bars was combined with the other flask corresponding to the plate base, the resin-coated sand in the flask was completely cured again using the LPG heat source. The empty spaces were filled with molten aluminum alloy by carefully pouring the molten metal solution into the inlet of the clamped flask. Following the cooling of the aluminum alloy, the flask containing the sand was removed. Thereafter, the obtained refiner bar plate was ground to control the height of the bar and remove the irregular part of the grooves.

Conventional bar plates without dams and lightweight bar plates with dams, which were embedded in the lab-scale single-disk refiner (KOS1, Gimhae, Korea), were used for the refining process (Fig. 2, Table 2, and Table 3). The two bar plates were manufactured by KOS1 (Gimhae, Korea).


Fig. 2. 
The bar shape inserted in the base plate.

Table 2. 
Comparison of two different plates
Cast TMP plate Lightweight TMP plate
Plate image
Segment No. 3 (×120°) 1
Breaking and intermediate sections Bar width (mm) 6.5 6.5
Groove height (mm) 4 4.8
Refining section Bar width (mm) 2.4 2
Groove width (mm) 2 2
Groove height (mm) 4 4.8
Dam width (mm) Not available 2×3.5
Plate weight (kg) 3.1 2.3

Table 3. 
Comparison of CELs and SELs of two different TMP plates
Cast TMP plates Lightweight TMP plates
CEL (km/s) 14.7 61.5
SEL (J/m)–TMP 0.8573 0.1979
SEL (J/m)–CTMP 0.4 0.1

New manufacturing technology was developed to remove the draft angle that was formed on the bar of the refiner plate. The new bar-plate-manufacturing technology, which is a double-joining method for heterogeneous metals, can eliminate the draft angle of the bar and dramatically reduce the weight of the bar segment. A stainless steel alloy exhibiting excellent wear resistance was used. The bar, which was inserted into the base plate, was cut into a wedge shape by a laser to prevent it from falling during the refining process (Fig. 2). The precut bars were heat-treated at ~1100℃ to improve their wear resistance.

To lighten the plate base, an aluminum alloy was used as the main material for the base plates. Thus, the overall weight of the plate could be reduced by almost half of that of the sand-casting plate.

The mold (named an assembly jig) was manufactured based on the 3D CAD drawing of the TMP plate (Fig. 3). The first step toward manufacturing a lightweight TMP plate was to insert the precut metal bars into the assembly jig with the buried part facing downward. Then, the flask was placed on the bar-inserted assembly jig and packed with resin-coated sand that was cured at 200℃–500℃ by a high-pressure LPG torch. When the flask containing the cured sand was lifted from the assembly jig, the bottom of the bars, which would be inserted into the base of the plate, was exposed upward. After combining the bar-containing flask with the other flask corresponding to the base of the plate, the resin-coated sand in the flask was cured again by the LPG heat source. The molten aluminum alloy filled the empty spaces between the bars and the plate base when it was carefully poured into the inlet of the clamped flask (Fig. 4). After the aluminum alloy was completely cooled, the sand-containing flask was removed to obtain only the refiner plate segments. A post-treatment process was performed to achieve uniform bar height and remove irregularities in the grooves.


Fig. 3. 
Three-dimensional image and the lightweight TMP plate.


Fig. 4. 
Drawn and real sand molds filled with molten aluminum.

Fig. 5 shows the two types of TMP plates with different patterns. Fig. 5(a) shows that the conventional plate, which was manufactured via a casting, consisted of the breaking, intermediate refining, and refining zones (No dam was present between the bars). Fig. 5(b) shows that the lightweight TMP plate comprised only two zones (the breaker and refining zones); the dams were regularly placed between the bars to prevent the easy exit of the fiber bundles from the refining zone.


Fig. 5. 
Comparison of the patterns of the two plates.

2.3 Refining

The wood chips were washed with water to remove contaminants, after which they were soaked in water for ~12 h at 40℃ to maintain the constant moisture content of ~50%. The soaked wood chips were presteamed in a laboratory digester for ~10 min at 120℃ in a liquor-to-wood ratio of 2:1. Particularly, to prepare the chemi-thermomechanical pulp (CTMP), the wood chips were impregnated with a liquor containing NaOH (3%) and Na2SO4 (3%) using an oven-dried weight of the wood chips in a liquor-to-wood ratio of 2:1. The laboratory digester was used for the impregnation of the wood chips, and the impregnation temperature and time were set to 120℃ and 60 min, respectively.

Fig. 6 shows that the presteamed or impregnated wood chips were fed into the single-disk refiner (KOS1, Korea), which was fitted with the TMP plates, to prepare TMP and CTMP at 1500 rpm. Two types of refiner plates with different bar patterns were used (Table 2).


Fig. 6. 
Single-disk refiner fitted with the TMP plate.

The fiber bundles, which were discharged from the refiner, were steamed for ~10 min at 120℃ in the digester, after which they were fed back into the refiner. The passing frequency of the fiber bundles through the refiner was 4 times. The TMP fibers, which were prepared in the single-disk refiner, were finally beaten for 40 min in a Valley beater (Daeil Machinery Co., Korea; 5.5 kg weight on the lever arm and 1.57% stock concentration). Finally, the Sommerville screen (Daeil Machinery Co., Korea) was used to remove shives in the TMP fibers; the shives content was calculated by measuring the oven-dried weight of the fiber bundles that were left on the screen:

Shives content%=Dried fiber weight%left on screenDried fiber weight%before screening×100[1] 
2.4 Measurement of TMP properties

The rate of dewatering a dilute suspension of the TMP fibers was measured by the Canadian Standard Freeness Test [according to ISO5267-2 (2001)]. The handsheets (basis weight=60 g/m2) for measuring the physical and optical properties were designed according to ISO 5269-1 (2005). The tensile strength was measured according to ISO 1924-1 (1992). Further, the optical properties were determined by an Elrepho Spectrophotometer (Lorentzen & Wettre, Sweden). The fiber length and fine contents of TMP were measured using an FQA-360 fiber quality analyzer (OpTest Equipment Inc., Canada).


3. Results and Discussion
3.1 Dimension and weights of two different plates

The dimensions of the cast and lightweight plates are compared in Table 2. The cast and lightweight plates consist of three and one segments, respectively, and their bar and groove widths were similar, but the depth of the bars in the lightweight plate was higher.

Although the lightweight plate contained more bars, its weight was ~26% lighter than that of the cast plate (Fig. 7). Dissimilar to refiner plates for stock preparation, TMP plates are large and heavy (diameter=>60 inches), making their replacement challenging. For example, if one cast segment of a 68-inches TMP plate weighs ~20.6 kg, the weight of a segment of the same size could be reduced to ~15 kg (a quarter of the casting segment when it was manufactured as a lightweight plate). Notably, this can shorten the replacement time for the worn plates.


Fig. 7. 
Comparison of the weights of the cast and lightweight plates for producing TMP.

3.2 Cutting edge length (CEL), specific edge load (SEL), and throughput

The TMP process allows the paper to develop appropriate physical properties by inducing structural changes through the mechanical decomposition of the fiber-constituting wood chips. Dissimilar to other MPs, TMP requires higher amounts of energy, but it affords longer fibers, fewer shives, and fewer fines. Concurrently, the refining effect varies depending on the bar patterns. The most widely used theories to predict the refining effects are CEL and SEL. The SEL for each bar plate was calculated based on ISO/TR 11371 (2013) as follows:

SEL=Pt-P0Zr×Zst×l×n=Pt-P0CLF×n=Pt-P0CEL[2] 

where SEL is in J/m; Pt is the total load power (kW); P0 is the no-load power (kW); n is the rotation speed (rev/s); Zr and Zst are the number of rotor and stator bars, respectively; l is the length of the bar (km); CLF is the cutting length factor (km/rev); and CEL is in km/s.

Table 3 presents the results of the comparison of the CEL and SEL of the cast and lightweight plates. Because the patterns of the two plates were significantly different, their CELs and SELs were different. The CELs of the cast and lightweight plates with three and two zones were 14.7 and 61.5 km/s (~4.2 times larger), respectively. The lightweight plate contained much more bars than the cast plate; thus, it correspondingly exhibited a longer CEL, which resulted in a smaller SEL.

The stock throughput was calculated by dividing the amount of the pulp that was discharged through the refiner when it passed through the refiner once by the corresponding refining time.

3.3 Freeness and fiber length

Freeness is a measure of the rate at which a dilute suspension of the pulp was dewatered under specified conditions. The drainage rate of the pulp fibers was affected by the surface area properties and degree of swelling of the fibers, as well as by internal and external fibrillation. Fig. 8 shows the change in the freeness of TMP and CTMP after they were refined with different bar plate fillings. The lightweight bar plates with dams exerted a much better effect in reducing the freeness of the mechanical pulps. In other words, the lightweight plate was more effective in significantly reducing the energy that was required to achieve the target freeness. Since the lightweight plate exhibited very sharp leading bar edges compared with those of the cast plate, it was advantageous for developing the properties of the fiber during the MP process. The increasing the number of bars on a refiner plate at certain refining energy reduces the refining intensity. The lightweight plate contained a greater number of bars along with dams, which enables low-intensity refining. Refining with a lightweight plate was effective because the dams afforded prolonged residence time to the raw materials to achieve complete fiber separation.


Fig. 8. 
Change in the freeness of TMP and CTMP that were prepared by the two plates with different fillings.

Fig. 9 shows the mean fiber length and fine content of TMP and CTMP that were prepared by different plates under the same refining conditions. Although the wood chips were mechanically pulped with the novel lightweight plate, the mean fiber length and fine content of the TMP and CTMP were not remarkably different from those of the cast plate. The lightweight plate allowed a faster decrease in the freeness while minimizing the compromise of the fiber length. Eventually, lightweight plate can replace the existing cast plate enabling low-intensity refining that can induce further external and internal fibrillation of the fibers was confirmed.


Fig. 9. 
Mean fiber length and fine contents of the TMP and CTMP that were prepared by the two plates with different fillings.

3.4 Shives content

Shives are the particles of fiber bundles that are sufficiently large or exhibit adequate quality to cause setbacks relating to product quality and productivity. 24) Shives in mechanical pulps cause the breakdown of the papermaking machine, linting during offset printing, pick-outs, coater scratches, visual defects, and reduction in the print quality.24) If the shives content increases suddenly after refining, it indicates that the TMP plate has attained the replacement cycle. In this case, the serious deterioration of pulp quality must be avoided by replacing the refiner plate. Fig. 10 shows the shives contents of the TMP and CTMP that were prepared by two different plates under the same conditions. Regardless of the pulp types, the lightweight plate with dams exhibited a lower shives content compared with the cast plate without dams, indicating that the lightweight TMP plate was effective in reducing the shives content of TMP. The lightweight fine bar plate exhibited sharp edges and was more effective in breaking the wood chips and refining the fibers was confirmed. Moreover, there were dams between the grooves to increase the residence time of the raw materials, thereby decreasing the shives content.


Fig. 10. 
Shives contents of the TMP and CTMP that were prepared with the two plates with different fillings.

3.5 Stock throughput

Lightweight refiner plates with a vertical bar for TMP exhibit sharp leading edges, indicating that a low amount of energy could obtain the target freeness. This could correspond to reduced operating costs or the availability of additional power for quality enhancement. However, regarding the raw material throughput, it could be assumed that there would be a difference between those of the lightweight (with a vertical bar shape) and cast (with a trapezoid shape) plates.

Fig. 11 shows a graph comparing the throughputs during the productions of TMP and CTMP using the two plates. Regarding TMP, the cast and lightweight plates exhibited different bar fillings, although they exhibited similar throughput up to the third pass, after which the lightweight plate exhibited ~9% more throughput. During the production of CTMP, the throughput of the raw material of CTMP was different from that of TMP. Regarding CTMP, the difference was even greater (~21% more throughput in the lightweight plate than the cast one).


Fig. 11. 
Stock throughputs of TMP and CTMP that were prepared by the two plates with different fillings.

Fig. 12 shows that the refining zone of the lightweight plate was filled with more bars than the cast plate; however, dissimilar to the cast plate, the groove of the lightweight plate exhibited a rectangular shape, which facilitated the easy flow of the refined fibers. Conversely, the shape of the grooves in the refining zone of the cast plate was trapezoidal, and the flow of the raw material was inevitably reduced compared with in the rectangular shape. Additionally, the fibers in the refining zone of the lightweight plate appeared to be easily discharged through the grooves owing to the much more effective deformation of their structure. When the wood chips were subjected to an alkali treatment during the production of CTMP, the binding force of the lignin holding the wood fibers was weakened, and the fibers were easily separated, thereby facilitating the refining process. Concurrently, more fibers might have passed through the lightweight plate exhibiting a rectangular groove shape than through the cast plate exhibiting a trapezoidal groove shape.19,22)


Fig. 12. 
Comparisons of the groove shapes and dimensions in the refining zones of the cast and lightweight plates.

3.6 Physical properties

Tensile strength is the most representative strengths of a paper and paperboard. It is affected by the degree of refining, bonding between fibers, and fiber length. Fig. 13 shows a graph comparing the tensile strengths of TMP and CTMP that were refined with the cast and lightweight plates exhibiting different fillings. Compared with the cast plate, the lightweight plate exhibited ~4.2 times larger CEL; thus, preparing the pulp under low-intensity refining was possible. Consequently, additional external and internal fibrillations were induced without the severe loss of the length of the fiber (Fig. 9); the interfiber bonding was significantly enhanced, resulting in the increased tensile strengths of TMP and CTMP.


Fig. 13. 
Tensile strengths of the TMP and CTMP that were prepared by the two plates exhibiting different fillings.

3.7 Optical properties

Refining to manufacture MP or refining during stock preparation exerts only a slight effect on the darkening of the pulp. However, refining in chemical pulp has been found to decrease the light scattering coefficient, brightness, and opacity of the pulp. TMP and CTMP exhibit similar behaviors to those of chemical pulp when they are subjected to mechanical treatments, such as refining. Fig. 14 shows that during the preparations of TMP and CTMP, the brightness was more reduced in the refining process using the lightweight plate than that using the cast plate. This was probably because the fiber length was hardly decreased when the lightweight plate was used for the refinement, although the light scattering coefficient decreased as the detachment of the fines from the fiber walls increased.25) Thus, CTMP was brighter than TMP because of the pretreatment of the wood chips with sodium sulfite in the impregnation process.13,26)


Fig. 14. 
Brightness of the TMP and CTMP that were prepared by the two plates exhibiting different fillings.


4. Conclusions

A new lightweight TMP-producing plate was manufactured via dissimilar metal joining technology. The plate was ~26% lighter than a typical cast TMP-producing plate. Regarding TMP and CTMP, the lightweight plate exhibited sharper bar edges than the cast plate, and this caused a rapid decrease in the freeness, but there was no significant difference between their average fiber lengths and fine contents. The lightweight plate obtained better results than the cast plate regarding the shives content, raw material throughput, and tensile strength. However, the lightweight plate exhibited a slightly lower result compared with the cast plate regarding the brightness of TMP and CTMP.


Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education (grant number: 2017R1D1A3B04027967) and Jeonju Paper Co.. The authors also thank the KOS1 officials and Jeonju Paper Co. Ltd. for helping manufacture the refiner plates.


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