Bioresource Technology 192 (2015) 501–506

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Mechanical pretreatment improving hemicelluloses removal from cellulosic fibers during cold caustic extraction Jianguo Li a,b, Yishan Liu b,c, Chao Duan a,b, Hongjie Zhang a, Yonghao Ni a,b,⇑ a

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada c Institute of Paper Science and Technology, Sichuan University of Science and Engineering, Zigong, Sichuan 643000, China b

h i g h l i g h t s  A concept of combined refining and CCE for hemicelluloses removal was proposed.  Refining changed fiber morphologies, facilitating hemicelluloses removal in CCE.  The combined process decreased the alkali dosage/concentration in subsequent CCE.  The combined process led to other advantages, such as high Fock reactivity.

a r t i c l e

i n f o

Article history: Received 20 May 2015 Received in revised form 2 June 2015 Accepted 3 June 2015 Available online 9 June 2015 Keywords: Mechanical refining Fiber morphology Hemicelluloses removal Cold caustic extraction Dissolving pulp

a b s t r a c t Hemicelluloses removal is a prerequisite for the production of high-quality cellulose (also known as dissolving pulp), and further recovery and utilization of hemicelluloses, which can be considered as a typical Integrated Forest Biorefinery concept. In this paper, a process of combined mechanical refining and cold caustic extraction (CCE), which was applied to a softwood sulfite sample, was investigated. The results showed that the hemicelluloses removal efficiency and selectivity were higher for the combined treatment than that for the CCE alone. The combined treatment can thus decrease the alkali concentration (from 8% to 4%) to achieve a similar hemicelluloses removal. The improved results were due to the fact that the mechanical refining resulted in increases in pore volume and diameter, water retention value (WRV) and specific surface area (SSA), all of which can make positive contributions to the hemicelluloses removal in the subsequent CCE process. Ó 2015 Published by Elsevier Ltd.

1. Introduction Lignocellulosic feed stocks for chemicals, materials and fuel have been the focus in recent years (Dashtban et al., 2014; Fatehi et al., 2013). The wood-based regenerated cellulose products which can be manufactured from dissolving pulps include viscose rayon, cellulose nitrate, and cellulose acetate, among others (Gübitz et al., 1997; Schild and Sixta, 2011). Also, the dissolving pulps can be superior raw materials for the production of nanofibrillation cellulose (NFC), nanocrystalline celluloses (NCC) and microcrystalline cellulose (MCC) (Wang et al., 2015; Sixta et al., 2013).

⇑ Corresponding author at: Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada. Tel.: +1 506 451 6857; fax: +1 506 453 4767. E-mail address: [email protected] (Y. Ni). http://dx.doi.org/10.1016/j.biortech.2015.06.011 0960-8524/Ó 2015 Published by Elsevier Ltd.

The well-commercialized dissolving pulps are derived from softwood, hardwood or mixtures of them based on the acid sulfite or pre-hydrolysis kraft (PHK) pulping process. A low hemicelluloses content is one of the important quality parameters for the dissolving pulps (Schild and Sixta, 2011; Miao et al., 2014). There have been many studies reporting the negative impact of hemicelluloses on the manufacturing processes of various products and the properties of resulting products, from dissolving pulps (Sixta et al., 2013; Wang et al., 2014). Moreover, the removed hemicelluloses can be recovered and further converted into other products, such as bio-ethanol, furfural, and xylitol (Shi et al., 2011; Shen et al., 2013; Panthapulakkal et al., 2015; Liu et al., 2013a). In the literature, many studies have been devoted to the development of relevant technologies on the hemicelluloses removal. For example, Gehmayr and Sixta (2012) investigated the hemicelluloses removal from a softwood sulfite pulp by using cold caustic extraction (CCE). Liu et al. (2013b) examined the effect of alkaline treatment on the pentosan content of a sugarcane bagasse. Hakala

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et al. (2013) studied the enzyme-aided caustic extraction of xylan from a hardwood kraft pulp. It is well known that CCE is very effective in removing the hemicelluloses, with a typical alkali concentration of 8–10% (Sixta, 2006). The CCE process in removing the residual hemicelluloses from pulp fibers can be divided into two stages: (1) physical interaction between fiber and aqueous sodium hydroxide resulting in fibers swelling; (2) diffusion of hemicelluloses, from fibers interior to exterior, and finally into bulk phase, through the pores in the fiber wall (Sixta, 2006). However, during a typical CCE stage (with its NaOH concentration of 8% or higher), the gradual transition of cellulose lattice from cellulose I (native cellulose) converting into Na-cellulose, and finally into cellulose II (regenerated cellulose), can take place (Schild and Sixta, 2011; Sixta et al., 2013). Thus, upon drying, the reactivity of cellulose decreases due to the very compact fiber structure of cellulose II (with extensive inter-molecule hydrogen bonding). In addition, it was known that it was a challenge in removing residual alkali from the highly swollen pulp after a CCE process, which consequently leads to high capital investment (effective multiple post-washers would have to be installed), and high chemical consumption (Gehmayr and Sixta, 2012). In fact, CCE is a very selective process in removing hemicelluloses from pulp fibers (Gehmayr and Sixta, 2012; Sixta, 2006), due to the excellent swelling of fibers and solubility of hemicelluloses under the conditions. The hemicelluloses removal in the CCE process is mainly influenced by the fiber morphology/structure, which can be modified effectively by mechanical treatments, such as pulp refining, as reported in the literature. For example, it was shown that the accessibility of lignocellulosic materials to macromolecules, such as enzymes, can be improved by using pulp refining (Suurnäkki et al., 1996). In this study, the performance of combined mechanical refining and CCE in removing the hemicelluloses from a softwood sulfite pulp was investigated. Firstly, the mechanical refining was applied to pulp fibers by using a PFI refiner. The changes in fiber morphologies, including pore volume, pore size, specific surface area (SSA) and water retention value (WRV), were determined. Subsequently, the efficiency and selectivity of hemicelluloses removal from the mechanically refined pulp during the CCE process were investigated. 2. Methods

Scallan, 1968a,b). The solute molecules used were a glucose (alpha-D-glucose) and a series of dextran fractions from Sigma– Aldrich Chemical Co., USA. The relationships between molecular diameter and molecular weight for glucose and dextran were listed in Table 1 (Hui et al., 2009). The experimental procedures and data analyses method followed Stone and Scallan (1968a). The total pore volume was the inaccessible pore volume for the largest probe molecule (dextran with the diameter of 56 nm) used in this study, which was as Vinac,56. The median pore diameter was the pore size at which one half of the pore volume was contained in larger pores and one half in smaller pores (Stone and Scallan, 1968b). The accessible pore volume for a given probe molecule, Vac,m, was simply that the largest inaccessible pore volume, Vinac,56 (in this study), subtracted the inaccessible pore volume, Vinac,m (Wang et al., 2012). Therefore,

V ac;m ¼ V inac;56  V inac;m

ð1Þ

2.3.2. Water retention value The water retention value (WRV) of samples was determined based on a method in the literature (Hui et al., 2009), by using a laboratory centrifuge at 900g for 30 min. 2.3.3. Specific surface area The specific surface area (SSA) of samples was determined based on the nitrogen adsorption method by using a Belsorp-Max volumetric gas adsorption instrument (Bel Japan, Inc., Osaka, Japan) (Tian et al., 2014). The samples were diluted to 0.5% pulp consistency to ensure in an adequate dispersion of pulp fibers, subsequently freeze–dried prior to the measurement. 2.3.4. Hemicelluloses content The hemicelluloses content of samples, namely, the dissolved carbohydrates content in the 18% NaOH solution (known as the S18), was determined by following Tappi T 235 cm-09. 2.3.5. Pulp yield and hemicelluloses removal selectivity The samples were collected and weighted before and after a caustic treatment. The mass ratio of samples before and after a caustic treatment was calculated as the pulp yield. After a caustic treatment, the hemicelluloses removal selectivity was calculated as followed:

Hemicelluloses removal selectivity ð%Þ 2.1. Materials

¼ A softwood acid sulfite pulp was provided from a mill in Canada. A Bauer-McNett fiber classifier (MC Tec Co., Ltd., Giessen, Netherlands) was used to obtain those that were retained on the 30-mesh screen, and they were the raw material used in this study.

mi  C i  mf  C f  100 mi  mf

ð2Þ

where mi and Ci are the initial sample weight and hemicelluloses content, respectively, and mf and Cf are the sample weight and hemicelluloses content after a caustic treatment, respectively (Li et al., 2015).

2.2. Mechanical treatment and cold caustic extraction The mechanical treatment was performed by using a PFI refiner according to Tappi standard T 248 sp-08. A 30 g (equivalent to oven dried) pulp was refined at a 10% pulp consistency for 10,000 revolutions. Subsequently, a cold caustic extraction (CCE) was carried out. A 20 g (equivalent to oven dried) pulp was treated at a 4% or 8% NaOH concentration with other conditions being 10% pulp consistency, 25 °C and 30 min in polyethylene bags. 2.3. Analyses 2.3.1. Pore volume and diameter The pore volume and median pore diameter of samples were measured by using the solute exclusion method (Stone and

2.3.6. Fock reactivity The Fock reactivity of samples was determined based on an improved method (Tian et al., 2013). The samples were air-dried prior to the measurement.

Table 1 Molecules and their molecular weights and diameters (Hui et al., 2009). Probe molecules

Molecular weight

Diameter (nm)

Dextran Dextran Dextran Dextran Dextran Glucose

2,000,000 500,000 73,000 40,000 9300 180

56 27 12 9.0 4.6 0.8

T2000 T500 T1390 T40 T9260

J. Li et al. / Bioresource Technology 192 (2015) 501–506

3. Results and discussion 3.1. Proposed concept of combined mechanical refining and CCE for improving the hemicelluloses removal During the CCE process, the hemicelluloses removal is a process including the fibers swelling in alkali solution, hemicelluloses diffusion in which the hemicelluloses pass through the pores in the fiber wall and finally hemicelluloses dissolution in the bulk phase. The presence of pores and their sizes in fiber walls are critical factors that can affect the hemicelluloses diffusion process (thus, hemicelluloses removal). Previous work showed that the fibers structure and morphology can strongly affect: (1) the removal of non-cellulosic constituents from pulp fibers, such as lignin (Kerr and Goring, 1975); (2) the accessibility of cellulose to chemicals, such as carbon disulfide (Tian et al., 2014; Miao et al., 2015) during the cellulose xanthation process, or enzymes (Luterbacher et al., 2013) during cellulose enzymatic degradation. The mechanical refining can increase the fiber pore size, pore volume and specific surface area. For example, it was reported that the mechanical refining can result in the breakage of internal bonds and delamination of the fiber wall, thus improving the porosity of the fiber wall (Grönqvist et al., 2014). In another study, Tian et al. (2013) reported that the mechanical refining can lead to: (1) opening/enlarging of pores and voids in the fiber wall; (2) disruption of the cellulose amorphous region; (3) slit fibril aggregation; (4) creation of fines. The concept of combined mechanical refining and CCE for enhancing the hemicelluloses removal, or achieving the same hemicelluloses removal but using milder process conditions (for example, a lower NaOH concentration), was illustrated in Fig. 1. For the conventional CCE process (at 8–10% NaOH) (b in Fig. 1), a strong sodium hydroxide concentration (8–10%) would be needed because it is known that such a condition would give an excellent swelling of cellulosic material, thus, very favorable to the hemicelluloses removal (Sixta, 2006). A lower than the optimum NaOH concentration will lead to a decreased fiber swelling, thus reducing the pore size/porosity, which negatively affects the hemicelluloses diffusion through the fiber wall, thus, the overall hemicelluloses removal. Bui et al. (2008) investigated the effect of alkaline treatment on the reactivity of regenerated cellulosic fibers, and the results indicated that the alkaline treatment can increase the porosity of the fiber wall and accessibility of fibers to NaOH. The mechanical refining can loosen fibril aggregation, break bonds between lamellae and promote fibrillation (c in Fig. 1), which leads to increases in pore volume, pore size and specific surface area (to be presented in Table 1). These changes on fiber morphologies, not only directly improve the hemicelluloses diffusion in the subsequent CCE stage, but also create favorable conditions for the alkali-induced swelling of fibers under the CCE conditions (even at 4% NaOH) (d in Fig. 1). The above can lead to a very effective diffusion of hemicelluloses through the fiber wall, allowing a similar hemicelluloses removal to be achieved at a lower NaOH concentration of the CCE stage (d in Fig. 1).

3.2. Influence of mechanical refining on fiber morphologies As shown in Fig. 2, the mechanical refining led to an increase in the accessible pore volume, which was defined as the bulk volume of pores having larger diameter than the given probe molecule. Evidently, the mechanical refining increased the accessible pore volume of both the micro-pores (pore diameter < 10 nm) and the macro-pores (pore diameter > 10 nm). For example, for the pores with a 4.6 nm diameter, the accessible pore volume increased, from 0.58 to 1.07 ml/g, and for the pores with a 12 nm diameter,

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the accessible pore volume increased by 186% from 0.15 to 0.43 ml/g; all of these can be attributed to the mechanical refining which resulted in the looseness in fibril aggregation, breakage in bonds between lamella and increase in fibrillation. From Table 2, it was noted that the mechanical refining also resulted in an increase in the total pore volume of softwood sulfite pulp, which was from 1.26 ml/g of the control sample to 2.09 ml/g of the mechanically refined sample. In addition, the mechanical refining led to an improvement of median pore diameter (from 3.85 to 4.83 nm). The WRV, which was indicative of fibers swelling, had a significant enhancement from 1.30 to 2.20 g/g, as a result of the mechanical refining. The specific surface area (SSA) also, increased from 2.21 m2/g for the control sample to 2.89 m2/g for the mechanically refined sample. These favorable changes in fiber morphologies including the pore volume, pore size, fibers swelling and specific surface area, can have direct positive effects on the hemicelluloses diffusion (thus the removal) from the softwood sulfite pulp during the next CCE process, which will be discussed in the subsequent sections. Previous work on different fiber materials supported the conclusion that the mechanical treatment can modify the fiber morphologies (pore size, WRV and specific surface area). Grönqvist et al. (2014) reported that a shredding treatment of 300 min resulted in increases in the micro-pores volume (from 0.42 to 0.47 ml/g) and total pore volume (from 0.53 to 0.82 ml/g) for a softwood dissolving pulp. Hui et al. (2009) found that the WRV of a mixed hardwood kraft pulp increased by 30%, after a 3000 revolutions PFI refining. In another study, the mechanical treatment improved the SSA of a hardwood kraft-based dissolving pulp, by 22% after a 25,000 revolutions PFI refining (Tian et al., 2014). The mechanical refining can destroy the fiber amorphous zone and lead to the internal fibrillation, which contributed to the formation of new pores and expansion of original pores (Grönqvist et al., 2014). Thus, not only the volumes of micro-pores and macro-pores, but also the pore size significantly increased. The mechanical refining can also collapse and/or break the fiber structure, resulting in the dislocation of matrix and formation of cracks in fibers, which can additionally contribute to the improvement of WRV (Hui et al., 2009). Another function of mechanical refining was to produce more secondary fines which resulted in an increase in the specific surface area (Tian et al., 2014; Miao et al., 2015). In summary, the mechanical refining gives rise to remarkable changes on fiber morphologies, including increases in the pore size, pore volume, specific surface area and WRV of the softwood sulfite pulp, all of which can have positive effects on the hemicelluloses diffusion through the fiber wall, thus the hemicelluloses removal in the subsequent CCE stage, as will be shown in the next section. 3.3. Influence of combined mechanical refining and CCE on the hemicelluloses removal efficiency A typical CCE process is operated at a NaOH concentration of 8– 10%, and it is of practical interest in decreasing the NaOH concentration in the CCE stage. In this project, our objective was to determine if the NaOH concentration can be decreased when the mechanical refining was practiced prior to the CCE in reaching a similar hemicelluloses removal degree. The results were shown in Fig. 3. It can be found that the combined mechanical refining and CCE (at 4% NaOH) process led to a decrease in the hemicelluloses content from 9.5% to 5%, which was compared well with the decrease of hemicelluloses content from 9.5% to 4.6% that was obtained at a 8% NaOH CCE process (conventional CCE process). In contrast, without the mechanical refining, a 4% NaOH CCE alone resulted in a much less hemicelluloses removal (from 9.5% to 7.8%). The above results supported the conclusion that the inclusion of mechanical refining in the treatment process can

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Fig. 1. Schematic of the combined mechanical refining and CCE concept for enhancing the hemicelluloses removal from a softwood sulfite pulp. (a) Original pulp fiber sample, (b) sample after conventional CCE (8–10% NaOH) treatment, such a condition leads to hemicelluloses diffusion and dissolution from the fiber wall, (c) sample after mechanical refining, which can open up the fiber wall, thus facilitating the subsequent alkali-induced swelling, (d) sample after combined mechanical refining and CCE (4% NaOH), the refining pretreatment allows a lower NaOH concentration in the CCE stage to have a similar hemicelluloses removal to that in a conventional CCE process, which, otherwise, would not be possible.

1.4

Control sample Refined sample

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0

5

10

15

20

25

30

Pore diamaeter (nm) Fig. 2. Effect of mechanical refining on the accessible pore volume of softwood sulfite pulp.

facilitate the hemicelluloses removal during the subsequent CCE process, which can decrease the NaOH concentration to obtain a similar degree of hemicelluloses removal. Cold caustic extraction can be effective in removing the hemicelluloses with the prerequisite of a high alkali concentration (Sixta, 2006). On the other hand, the removal of hemicelluloses, when CCE was operated at a low alkali concentration, can be rather limited, as shown in the present study (from 9.5% to 7.8% at 4%

Table 2 Effect of mechanical refining on the fiber morphologies of softwood sulfite pulp.

Control sample Refined sample

Total pore volume (ml/g)

Median pore diameter (nm)

WRV (g/g)

SSA (m2/g)

1.26

3.85

1.30

2.21

2.09

4.83

2.20

2.89

NaOH vs. from 9.5% to 4.6% at 8% NaOH). Sixta (2006) found that a hemicelluloses removal was just 24%, when a CCE at 40 g/L NaOH concentration (30 °C, 30 min, 10% pulp consistency) was applied to an Eucalyptus PHK, while it was 52% when the NaOH concentration increased to 80 g/L. Another advantage of using a low NaOH concentration in the combined mechanical refining and CCE process to remove the hemicelluloses for manufacturing high-grade dissolving pulp, is that the resulting products can maintain a good accessibility towards chemicals, such as carbon disulfide (thus, the pulp reactivity) in the fiber plant. It is known that the conversion of cellulose I to cellulose II occurs at a high NaOH concentration, and the latter has a much lower accessibility/reactivity than the former, due to their structural differences (Sixta et al., 2013). As shown in Fig. 4, the Fock reactivity of the sample after the combined mechanical refining and CCE (at 4% NaOH), was 43.8%, which was higher than that of 27.4% for the resulting sample from the conventional CCE process (at 8% NaOH). The mechanical refining itself may be

10 9 8

Hemicelluloses content (%)

Accessible pore volume (ml/g)

1.6

7 6 5 4 3 2 1 0

Control

CCE 1

Refining+CCE 1 CCE 2

Fig. 3. Effect of combined mechanical refining and CCE on the hemicelluloses content of softwood sulfite pulp (CCE1: 4% [NaOH]; CCE2: 8% [NaOH]).

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40

(thus the removal) in the CCE process. An improvement in SSA was observed from the mechanical refining, which can have further positive effects on the hemicelluloses removal during the CCE process.

30

4. Conclusions

50

Fock reactivity (%)

505

20

10

0

Control

CCE 1

Refining+CCE 1 CCE 2

Fig. 4. Effect of combined mechanical refining and CCE on the Fock reactivity of softwood sulfite pulp (CCE1: 4% [NaOH]; CCE2: 8% [NaOH]).

partially responsible for the higher Fock reactivity of the sample from the combined mechanical refining and CCE process, because an early study showed that the mechanical refining can improve the Fock reactivity of a PHK-based dissolving pulp (Tian et al., 2014). Köpcke et al. (2010) reported that a CCE treatment at 7% NaOH concentration (10 min, 20 °C, 4% pulp consistency) for upgrading an Eucalypt kraft pulp to dissolving pulp, resulted in a decrease in the Fock reactivity from 57.5% to 39.5%. 3.4. Influence of combined mechanical refining and CCE on the hemicelluloses removal selectivity The yield and hemicelluloses removal selectivity were listed in Table 3. In the CCE process, the overall yield loss is attributed to: (1) hemicelluloses removal; (2) degraded cellulose (short-chain cellulose) dissolution (Li et al., 2015). The results showed that the yield for the CCE alone was higher than that for the combined mechanical refining and CCE process (96.5% vs. 94.3%), which can be due to the difference in hemicelluloses removal degree (from 9.5% to 7.8% from the former vs. from 9.5% to 5% from the latter). Furthermore, as shown in Table 3, the combined mechanical refining and CCE had a higher hemicelluloses removal selectivity than the CCE alone (84.6% vs. 56.3%). The size of fiber pores has a significant impact on the diffusion of molecules from the fiber interior. This is particularly so in pulping and bleaching whereby a decrease in the particle size of lignin/hemicelluloses and/or an increase in pore size of fiber wall, both of which can facilitate the diffusion of degraded lignin/hemicelluloses through the fiber wall. Kerr and Goring (1975) found a topochemical preference in lignin removal from a white birch kraft pulp, and the results indicated that the large-size pores contributed much more to the overall lignin removal than the small-size pores. Evidently, the mechanical refining prior to the CCE improved the total pore volume (from 1.26 to 2.09 ml/g), increased the average pore diameter (from 3.85 to 4.83 nm) and WRV (from 1.30 to 2.20 g/g); all of them can facilitate the diffusion of hemicelluloses

Table 3 Effect of combined mechanical refining and CCE on the yield and hemicelluloses removal selectivity of softwood sulfite pulp (CCE: 4% [NaOH]).

CCE Refining + CCE

Yield (%)

Hemicelluloses removal selectivity (%)

96.5 94.3

56.3 84.6

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Mechanical pretreatment improving hemicelluloses removal from cellulosic fibers during cold caustic extraction.

Hemicelluloses removal is a prerequisite for the production of high-quality cellulose (also known as dissolving pulp), and further recovery and utiliz...
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