Study of Mixed Refining Behaviors of Softwood Kraft Pulps and Hardwood Kraft Pulps Using Different Bar Fillings

Study of Mixed Refining Behaviors of Softwood Kraft Pulps and Hardwood Kraft Pulps Using Different Bar Fillings

Ji-Young Lee, Chul-Hwan Kim

, Sol Kwon, Hyeung-Hun Park, Hyun-Tek Yim, Ho-Gyeong Gu and Byung-Gul Min

Received September 6, 2018; Received in revised form October 4, 2018; Accepted October 5, 2018

ABSTRACT

Mixed refining using a narrow and coarse bar filling was applied to the mixing furnishes of long fibers and short fibers. According to different bar fillings, pure HwBKP in the narrow filling showed a faster drop in freeness than the mixed furnishes, and pure SwBKP in the coarse filling showed the slowest drop, differently from the mixed furnish.

The mixed furnishes with more long fibers caused their mean fiber lengths to be shorter in both plate fillings. The increase of water retention values in the mixing furnishes was more affected by the amount of short fibers in the mixed furnish, not by the plate filling types. For physical properties of paper according to co-refining, better improvement was acquired in the coarse filling rather than in the narrow filling. In particular, the amount of long fibers in the mixed furnish contributed to the better improvement of physical properties in the same refining energy regardless of the plate fillings. Finally, it could be concluded that refining of mixed furnishes preferred the coarser bar filling to the nar- rower bar filling in developing fiber and paper properties.

Keywords: Co-refining, coarse filling, narrow filling, physical properties, refining energy

• Major of Environmental Material Science in the Division of Environmental Forest Sciences, IALS, Gyeongsang National University, Jinju, 52828, Republic of Korea

1 Koswon Co. Ltd., 396beon-gil (62-31) Seobu-ro, Gimhae, 50870, Republic of Korea

† Corresponding Author: E-mail: [email protected]

Printed in Korea http://dx.doi.org/10.7584/JKTAPPI.2018.10.50.5.31

1. Introduction

It has been well recognized that refining is an un- avoidable process in papermaking. The choice of the refining system stems from the end products, raw materials, and expected capacity range.

In gen- eral, separate refining lines for different pulps are

used for new big paper machines. However, for older

smaller paper machines, different pulps are mixed

together before refining is done. Both separate and

mixed refining systems have their intrinsic advan-

tages.

For some pulp blends, separate refining

can expect better strength at lower energy con-

sumption than a mixed refining, but for some pulp

blends, mixed refining may lead to better results.

This means that requirements for targeted refining results for different pulps must be established con- sidering end products and production capacity.

Refining effects of the blended pulps are greatly crucial to achieve the desired freeness and strength properties which will be different according their mixing ratios. Chuhan et al. (2011) studied the ef- fect of separate and mixed refining of two different pulps on paper properties using PFI mill. They concluded that the strength and other properties of paper prepared from mixed refining in high con- sistency were either better and comparable than those of separated refined pulps.

Gao and his co-workers (2009) showed that, for a given free- ness, separate BCTMP and hardwood KP refining in PFI mill required more energy than their co-re- fining.

However, while stock consistency is one major parameter to affect refining effects on pulp fibers, the previous workers did not consider re- fining in lower consistency of 3-5%. High consis- tency refining such as PFI mill displays different behaviors from low consistency refining. In many cases, high consistency refining had been replaced by the low consistency refining (Ghost et al., 2003).

Therefore, it is required that refining ef- fects on blended pulps will be investigated at a low consistency. Another important factor to affect refining results is raw materials to be blended.

Writing and printing papers mostly use kraft pulps for softwood and hardwood as raw materials. That is, the blend of long fibers and short fibers must be investigated regarding co-refining.

When pulp furnishes are blended, the mixing ra-

tio of long-fibered softwood pulp and short-fi- bered hardwood pulp may affect refining behaviors in a refining zone. Therefore, for successful mixed refining, characteristics of each pulp component must be thoroughly understood before refining.

Long-fibered softwood fibers have high ability of flocculation to build strong flocs that do not easily break during refining. On the contrary, short-fi- bered hardwood pulps have difficulty in getting stapled on the bar edges, which leads to easy break of weak flocs during refining. During refining, fi- ber flocs are formed and broken up continuously under the shear force that exists in the grooves and in the refining zone.

Based on the above facts, it becomes evident that refining results will be changed according to mix- ing ratios of softwood pulp and hardwood pulps. It will therefore be interesting to evaluate how the mixed pulps are treated in a refining zone when blending ratios of pulps with different fiber length are changed.

In this study, co-refining using long-fibered softwood pulp and short-fibered hardwood pulp was applied to a disk refiner under the different mixing ratios and different refining steps. Finally, energy consumption, fiber properties and paper strength were compared to explore the optimal co-refining condition.

2. Materials and Methods

2.1 Raw materials

Hardwood bleached kraft pulp (HwBKP) and

Table 1. Fiber dimension of softwood pulp used for refining Arithmetic mean fiber

length (mm)

Length-weighted mean fiber length (mm)

Fiber width (μm)

Coarseness (mg/100 m)

HwBKP 0.53 0.73 13.6 5.8

SwBKP 1.25 2.34 34.4 15.5

softwood bleached kraft pulp (SwBKP) were sup- plied from Moorim Paper mill in Jinju, Korea. They were torn in small pieces and soaked in distilled water over 4 hours before disintegration.

2.2 Refining

Before refining, soaked pulp specimens were dis- integrated by Valley beater with no load at a con- sistency of 1.57±0.04% for at least 3-5 minutes.

When pulp was properly disintegrated, extra water in the pulp stock was removed to adjust its consis- tency to around 4-5%. Disintegrated SwBKP and HwBKP were blended before refining, as shown in Table 2.

Refining was conducted with the laboratory single disk refiner (Fig. 1, KOSWON Co., Korea) with two different plate fillings, shown in Table 3. The mixed pulp stocks were refined to achieve a Cana- dian freeness of 190 mL.

2.3 Measurement of pulp and paper properties

Mean fiber length, fines contents (≤0.2 mm), fi- ber width and coarseness (mg/100 m) were deter- mined by FQA-360 (Optest Equipment Inc., Can- dada). Handsheets for measuring physical proper- ties of paper were made, conditioned and tested

Table 2. Blending ratios of SwBKP and HwBKP before refining

Pulp types Blending ratio (%) based on OD pulp weight

SwBKP 100 70 30 0

HwBKP 0 30 70 100

Table 3. Different plate fillings used for refining HwBKP Refiner plate for harsh refining with coarse filling

Refiner plate for gentle refining with narrow filling

Plate segment

Bar dimension (mm)

Bar number 108 186

Cutting edge length

(m/rev) 354.5 1,466.8

Fig. 1. Laboratory single disk refiner used for re-

fining.

according to TAPPI Standard Test Method T 205.

Their physical properties including tensile and tear strength were measured based on TAPPI Standard Test Method T 220. Paper bulk was calculated us- ing basis weight of each sheet and thickness mea- sured by L&W caliper test (Micrometer, Sweden).

Water retention value (WRV, g/g) was measured based on ISO 23714:2014.

3. Results and Discussion

3.1 Change of freeness and fiber length

Refining of blended pulps are sometimes used for older smaller paper machines. Independent refin- ing produces better strength at lower energy con- sumption than mixed refining, but co-refining can be a better choice for some pulp blends. Therefore, fiber characteristics must be well understood be- fore optimal refining types are chosen.

Fig. 2 shows the effect of mixed refining with two different plate patterns on pulp freeness. As refin- ing proceeded, freeness dropped differently ac- cording to bar fillings. For the narrow filling in Fig. 2(a), HwBKP itself decreased faster than mixed pulps with long fibers. While, more SwBKP in the mixed stock caused to the slower decrease of freeness. Therefore, it was assumed that long-fi-

bered softwood pulp protected short-fibered hard- wood pulps on the bar edge during refining. It is well known that longer fibers have a great ten- dency to contact themselves, leading to the in- crease of floc size in the grooves. Strong flocs may cover up short fibers which are not directly ex- posed to the impact of the coming bars in the nar- row filling. Therefore, more SwBKP in the mixed stock required more refining energy to achieve the desired freeness level.

For coarse filling in Fig. 2(b), when short-fibered SwBKP was more mixed in a stock, freeness also dropped a little faster than the stock containing more HwBKP during refining. SwBKP itself dis- played the slowest response on freeness drop but consistent impact on long fibers led to a sharp drop in freeness, differently from the mixed furnish.

It is helpful to compare length-weighted mean fiber length in understanding refining behavior of mixed pulps in the two different plate fillings. Ir- respective of bar fillings and mixing ratios, fiber length was decreased as refining energy started to be consumed (refer to Fig. 3). It could be shown that fiber cutting and fibrillation simultaneously contributed to the decrease of fiber length. Differ- ently from pure SwBKP, pure HwBKP displayed faster increase in fines contents on the same re- fining energy because the short fibers were ex-

Fig. 2. Effect of plate fillings in co-refining on freeness drop of mixed pulps.

(a) Narrow filling (b) Coarse filling

posed to the impacts of refiner bar edges. Contin- uous impacts at the bar edge transferred great re- fining energy to fiber flocs, and thereafter accu- mulated energy led to physical dismantle of fiber structure even though long fibers might cover over short fibers within the flocs. In particular, severer reduction in mean fiber length of the furnish with more long fibers actually happened at the same refining energy, which was the same with the nar- row and coarse filling. It might be due to the fact that long fibers had a rather greater favor to meet bar edges than short fibers did. The repeated con- tacts/impacts between long fibers and the bar edges in the single or the mixed furnish contrib- uted to the faster reduction of fiber length.

The influence of water retention value (WRV) on refiner plate fillings was investigated, as shown in Fig. 4. WRV is the important parameter to indicate the amount of the water retained in the fiber wall

and bound to fines. In the end, WRV estimates the ability of fibers and fines to swell. Refining con- tributed to the increase of WRV irrespective of the plate filling types. However, the different plate fillings differently affected the change of WRV for the mixed pulps. In the narrow filling, more short fibers adapted themselves to refining and therefore their WRVs increased faster than WRV of the mixed pulp with more long fibers. In the coarse filling, the mixed pulps differently responded to the change of WRV. The stock with more long fi- bers had less tendency to increase WRV rapidly with prolonged refining. Even if short fibers are not well treated when they are blended with long fibers before refining, they can be a key factor to increase swelling capability of fiber furnish after refining. This equally occurs in the narrow and coarse filling.

Finally it became clear that short fiber fractions (a) Narrow filling

Fig. 3. Effect of plate fillings in co-refining on mean fiber length of mixed pulps.

(b) Coarse filling

in the mixed pulps greatly contributed to the in- crease of WRV regardless of the difference of plate fillings.

3.2 Bulk and physical properties of paper

Paper bulk is affected by refining in the sense that it co-varies with mechanical properties. Un- fortunately, refining is not a good treatment to maintain acceptable paper bulk. Fig. 5 shows the effect of plate fillings on paper bulk of mixed pulps. As one single pulp such as SwBKP and HwBKP was refined, paper bulk sharply decreased no matter what the plate fillings are. However, co-refining of HwBKP and SwBKP delayed the rate of bulk reduction as refining proceeded. When short fibers were more blended in a stock, refining

negatively influenced paper bulk. It was important to note that co-refining of HwBKP and SwBKP played a better role in the narrow filling for slow- ing down bulk sacrifice.

The change in tensile strength of sheets after re- fining of mixed pulps are compared in Fig. 6. The improvements were observed according to the du- ration of refining, and the more long fibers led to greater increase in tensile strength regardless of different plate fillings. However, the increasing rate of tensile strength was much faster in the coarse filling than in the narrow filling. It could be concluded that co-refining of mixed pulps requires a coarser bar pattern for better development of strength.

As refining in the narrow and coarse filling in- (a) Narrow filling

Fig. 5. Effect of plate fillings on paper bulk of mixed pulps.

(b) Coarse filling (a) Narrow filling

Fig. 4. Effect of plate fillings in co-refining on water retention value of mixed pulps.

(b) Coarse filling

creased, tear resistance passed through a maxi- mum, and then began to decrease (refer to Fig. 7).

Likewise, the coarse filling contributed to the faster and greater increase in tear resistance than the narrow filling. However, after the maximum in tear resistance, the sharp decrease in strength of SwBKP occurred in the coarse filling. It could be assumed that blended pulps might be useful to preserve greater tear resistance in the same refin- ing energy.

4. Conclusions

The effects of co-refining of the mixed pulps with short-fibered HwBKP and long-fibered SwBKP

(a) Narrow filling

Fig. 6. Effect of plate fillings on tensile strength of mixed pulps.

(b) Coarse filling

were investigated with two different plate fillings.

Regardless of the plate fillings, two single fur- nishes of HwBKP and SwBKP showed a faster drop in freeness than the mixed furnishes. The fur- nishes with more long fibers displayed faster re- duction in mean fiber length. For swelling behavior of fibers walls, the different plate filling patterns did not affect the increasing rate of WRV for the mixed furnish, but the mixing amount of short fi- bers did affect it more positively. For physical properties of paper according to co-refining, the coarse filling contributed to better improvement than the narrow filling did. In particular, the amount of long fibers mixed in the furnish had more contribution to the increase of physical prop- erties in the same refining energy. In conclusion, (a) Narrow filling

Fig. 7. Effect of plate fillings on tear resistance of mixed pulps.

(b) Coarse filling

when the fiber furnish was composed of the mixing raw materials of SwBKP and HwBKP, the coarser bar pattern was preferable in developing fiber and paper properties. It must be also kept in mind that, even if the furnish is mixed, refining results may be changed according to plate fillings.

Acknowledgement

This research was supported by Basic Science Re- search Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant number: 2017R1D1A3B04027967).

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