Effect of Industrial Grade MgO with Different Particle Sizes on the

Article

Effect of Industrial Grade MgO with Different Particle Sizes on the Bleaching of Poplar Chemi-Thermomechanical Pulp Junhua Zhang, Wei Liu, Song Han, Qingxi Hou,* Yang Li, Yang Wang, and Zhaoxia Long Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, China ABSTRACT: Magnesium-based alkalis have become an attractive alkali source in the peroxide bleaching process of high-yield pulps. The objective of this work is to evaluate the effect of industrial grade MgO with different particle sizes as the alkali source in the peroxide bleaching process of poplar chemi-thermomechanical pulp (CTMP). The effect of MgO with different particle sizes on pulp properties including strength and optical properties was investigated. With the particle size of MgO decreasing, the peroxide bleaching process produced a bleached CTMP with higher tensile and tear indices, whereas the bulk and light-scattering coefficients were less. The maximum ISO brightness (73.2%) was obtained when the particle size of MgO was 4.3 μm. The possible reason for the brightness change with the variation of particle size of MgO was discussed.

1. INTRODUCTION Poplar is a fast-growing hardwood species and a good raw material for chemi-thermomechanical pulping, because of its characteristics such as a shallow wood color and a good liquid absorbency. Bleached chemi-thermomechanical pulp (BCTMP) with a freeness of 100−600 mL and an ISO brightness of 60− 85% has been widely used in the production of specialty paper, newsprint, and the intermediate layer of the coated paperboard as one of the main high-yield pulps (HYPs).1−3 Sodium hydroxide is a traditional alkali source in the alkaline peroxide bleaching process. However, many drawbacks have been discovered, including the dissolving of the wood chemical compositions, a decrease in the bleaching yield, an increase in the chemical oxygen demand (COD) of the effluent, and so on.4−6 Researchers7 proposed using magnesium-based alkalis to replace sodium hydroxide in peroxide bleaching of mechanical pulps at early time. Subsequently, other researchers8−13 continued to explore the use of divalent alkalis for peroxide bleaching of mechanical pulps. The advantages of magnesium-based alkalis used as the bleaching alkali source in the pulp and paper industry were reported in some researches and applications, e.g., higher bulk, light-scattering coefficient and bleaching yield, lower COD and cationic demand, decreased bleaching cost, and oxalate scaling.4,14−17 Thus far the studies on the CTMP peroxide bleaching process have mainly focused on the magnesium-based alkalis partially substituting NaOH or the different magnesium-based alkalis with different particle sizes. Wong et al.18 evaluated peroxide bleaching using two kinds of industrial grade MgO with different particle sizes. The somewhat better response was obtained when the industrial grade MgO with a smaller particle size was used in softwood thermomechanical pulp (TMP) peroxide bleaching process. Chi19 used Mg(OH)2 which was allowed to pass through different screens having different mesh sizes, such as 100, 200, and 400-mesh, as the alkali source in the CTMP H2O2 bleaching process. The maximum ISO brightness was obtained by using Mg(OH)2 passing through a 200-mesh screen but retaining on a 400-mesh screen. However, the influence of MgO/ Mg(OH)2 particle size on the strength properties, opacity, and light-scattering coefficient except the brightness of the resultant © 2013 American Chemical Society

pulps has not been investigated in detail. Also, little systematic study in the literature about the effect of the same kind of industrial grade MgO with different particle sizes on the peroxide bleaching process is available. Industrial grade MgO may have a greater application potential in the pulp industry due to its much lower price, especially in China, because China is rich in magnesium mine resources in the world. In this work, an industrial grade MgO was partially used in the alkaline peroxide bleaching process of poplar CTMP to get an optimal substitution percentage. Subsequently, the MgO with different particle sizes from being ground for different times was used in the alkaline peroxide bleaching process of poplar CTMP under the condition of the obtained optimal substitution percentage. The aim is to investigate the effect of the industrial grade MgO with different particle sizes on the bleaching of poplar CTMP.

2. EXPERIMENTAL SECTION 2.1. Materials. The poplar CTMP (a Schopper-Riegler beating degree (°SR) of 15, initial ISO brightness of 44.5%, bulk of 2.87 cm3/g, tensile index of 14.6 N·m/g, and tear index of 1.42 mN·m2/g) was collected from a paper mill in China. The collected pulp was then stored in a cold room at 4 °C. Industrial grade MgO was obtained from a MgO production mill in Liaoning province, China. All other chemicals used in the experiment were analytical grade products. Table 1 presents the properties of the industrial grade MgO applied in the study. 2.2. Grinding and Particle Size Testing of Magnesium Oxide. The industrial grade MgO which had been dried to a constant weight in advance at 105 °C was ground in a basket mill (model SA II-0.75, Shanghai Suowei Mechanical and Electrical Equipment Co. Ltd., China) for different times at 10% consistency and 520 rpm rotating speed. Then the particle size Received: Revised: Accepted: Published: 7645

January 13, 2013 May 9, 2013 May 14, 2013 May 14, 2013 dx.doi.org/10.1021/ie400140z | Ind. Eng. Chem. Res. 2013, 52, 7645−7650

Industrial & Engineering Chemistry Research

Article

2.5. Pulp Washing. The bleached poplar CTMP was put in a Büchner funnel with a 200-mesh screen and then washed with deionized water. The first filtrate obtained was recycled once to maintain the fines in the pulp. The bleached poplar CTMP was washed thoroughly until the filtrate was clear. Finally the washed pulp was collected in a polyethylene bag for later use. 2.6. Pulp Handsheets Preparation and Testing. Pulp handsheets of 60 ± 2 g/m2 (oven-dry) were prepared and formed according to the standard methods of ISO 5263 (2004) and ISO 5269-1 (2005), respectively. In addition, physical properties of the handsheets were determined according to ISO 5270 (1998). 2.7. SEM Analysis of Magnesium Oxide Morphology. A Hitachi SU1510 scanning electronic microscope (SEM) was used to examine the magnesium oxide morphology. A low acceleration voltage (5.0 kV) was used to protect the magnesium oxide from being damaged, and a magnification of 5000× was selected.

Table 1. Properties of the Applied Industrial Grade Magnesium Oxide component

content

MgO, % CaO, % Fe, % Mn, % Cu, ppm particle size, μm

87.07 1.59 0.38 0.09 0.7 17.4

of the ground MgO was tested using a laser diffraction particle size analyzer (model LS 13 320, Beckman Coulter, U.S.A.). 2.3. Peroxide Bleaching Process. The bleaching experiments were conducted in polyethylene bags using a water bath according to the following conditions: 60 g of the oven-dried poplar CTMP; 3.0% total alkali charge (on NaOH), of which 0%, 10%, 25%, 35%, 50%, and 75% of NaOH was replaced with MgO (molar ratio); 0.2% diethylenetriaminepentaacetic acid (DTPA); 2.0% Na2SiO3; 6.0% H2O2; 20% of pulp consistency; 90 °C; and 150 min. All of these chemical dosages were based on the weight of the oven-dried pulp. In the case of the peroxide bleaching process, NaOH, MgO, part of the deionized water, and DTPA were mixed well first with the CTMP by kneading the polyethylene bag by hand. The prepared bleaching liquor (i.e., the mixture of remaining deionized water, Na2SiO3, and H2O2) was then added into the bag. After the pulp and chemicals were mixed well by kneading constantly, the polyethylene bag was sealed and placed into the water bath at a temperature of 90 °C. The bleaching duration time began to count as soon as the temperature of the pulp suspension reached 90 °C. The polyethylene bag was kneaded once every 20 min in order to make the bleaching reaction uniform. At the completion of the bleaching process, the pulp sample was cooled with cold running water to room temperature promptly. 2.4. Collection of Bleach Effluent and Preparation of the Test Sample. After bleaching was complete, some bleached poplar CTMP (equivalent to 5 g of oven-dried pulp) was collected from the polyethylene bag and diluted into 1% pulp consistency with deionized water. The well-mixed pulp suspension was then filtered in a Büchner funnel with a 200mesh screen. The filtrate was recycled once to go through the fiber mat to collect the fines. The resultant filtrate was further filtered with a medium-fast filter paper to remove the residual fines and then used for determining the chemical oxygen demand (CODCr). The CODCr was determined according to the USEPA Reactor Digestion Method using a DRB 200 COD instrument (HACH Co. Ltd., U.S.A.) and a DR 1010 COD instrument (HACH Co. Ltd., U.S.A.).

3. REUSULTS AND DISCUSSION 3.1. Effect of Substitution of MgO for NaOH on Properties of the BCTMP and Bleach Effluent. The effect of substitution of MgO for NaOH on physical and optical properties of the BCTMP and bleach effluent was investigated. Table 2 indicates that when the substitution percentage of MgO for NaOH increased, the bulk, opacity, and light-scattering coefficient of the BCTMP increased and COD of the bleach effluent had a substantial decrease. However, the use of industrial grade MgO in the peroxide bleaching process had a negative effect on the brightness, tensile, and tear index of the BCTMP. The weaker alkalinity of the bleaching system would be the main reason for these above changes.20 In this study, the ISO brightness of all of the yielded bleached CTMPs could reach 70% at different substitution percentages of MgO for NaOH. It can meet the requirement to mix with other kinds of pulps to make specialty paper.21 At the substitution percentage of 50%, the bulk, opacity, and light-scattering coefficient of the BCTMP increased by 9.13%, 2.68%, and 8.39%, respectively, whereas the CODCr of the bleach effluent decreased by 37.5%. After considering making the BCTMP featured a high bulk and lightscattering coefficient, appropriate strength properties, and a low effluent load and bleaching production cost, the substitution percentage of 50% of MgO for NaOH was chosen as the optimal condition for the subsequent experiments. 3.2. Effect of Grinding Time on Particle Size of the MgO. The change of particle size of industrial grade MgO ground for different times is presented in Figure 1. The initial particle size of the industrial grade MgO used in this study was 17.4 μm (Table 1). When the grinding duration time reached 60 min, the particle size of the MgO would decrease to 2.9 μm.

Table 2. Properties of the BCTMP and Bleach Effluent substitution percentage of MgO for NaOH (%)

bulk (cm3·g−1)

brightness (% ISO)

light-scattering coefficient (m2·kg−1)

opacity (%)

tensile index (N·m·g−1)

tear index (mN·m2·g−1)

CODCr (mg·L−1)

unbleached pulp 0 10 25 35 50 75

2.87 2.41 2.49 2.56 2.58 2.63 2.75

44.5 75.2 74.3 73.8 73.0 72.5 71.8

58.2 46.5 47.0 48.3 48.6 50.4 51.1

97.8 85.7 86.3 86.7 87.3 88.0 88.5

14.6 32.8 30.3 26.8 25.3 23.5 21.0

1.42 3.00 2.85 2.75 2.70 2.66 2.47

811 733 625 601 507 410

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dx.doi.org/10.1021/ie400140z | Ind. Eng. Chem. Res. 2013, 52, 7645−7650


Article

In addition, it also can be seen in Figure 4 that the smaller the MgO particle size, the greater the effect on strength properties of the BCTMP. When the particle size decreased from 4.3 to 2.9 μm, the tensile index would increase by around 2 N·m/g, 80% of the overall increment. It can be supposed that as the MgO particle size decreased with the grinding time extending, the corresponding specific surface area of MgO would increase, leading to an acceleration of the hydration rate of MgO to form Mg(OH)2, as shown in eq 1. In other words, under the condition of the same mass weight, the MgO with a smaller particle size would produce more Mg(OH)2 in water. Consequently, there would be more HOO− produced in the peroxide bleaching process (eq 2). Some hydrophobic materials, such as lignin and benzene alcohol extractives, could be subsequently removed to some extent from pulp fibers,20 and the carboxyl groups in the fiber surface were also increased, thus, causing in more fibers swelling.14,24 The flexible fibers would decrease the bulk of pulp and increase the interfiber bonds, leading to an increase in tensile and tear indices of the bleached pulp.25

Figure 1. Effect of grinding time on particle size of MgO.

It can be seen in Figure 1 that there was a sharp reduction of the MgO particle size within the first 20 min of grinding and subsequently a slight decrease in the latter 40 min. This could be explained by the fact that, at the same rotating speed during MgO grinding, the smaller the MgO particle size, the lower the grinding efficiency would. The particle size distribution and morphology of the MgO at grinding times of 0, 20, and 60 min are shown in Figure 2. Before grinding, that is, at a grinding time of 0 min, the MgO particle size was large, and the particle size distribution was nonuniform (Figure 2a). In contrast to the unground MgO, the particle sizes of the MgO ground for 20 and 60 min were smaller and more uniform (Figure 2, panels b and c), resulting in an increase of the specific surface area of the resultant MgO and a faster reaction rate with water. 3.3. Effect of MgO with Different Particle Sizes on Bulk and Strength Properties of the BCTMP. Bulk is an important property of paper, paperboard, or combined board, and variations in bulk are also important especially for paper and paperboards used for mechanical purposes. It is one of the important characteristics that affect flexural stiffness.22 Figure 3 shows that when the particle size of the MgO decreased from 17.4 to 2.9 μm the bulk of the resultant BCTMP decreases from 2.63 to 2.43 cm3/g. Tensile and tear strength are indicative of the strength derived from factors such as fiber strength, fiber length, and bonding. It may be used to deduce information about the factors.23 The effect of the MgO with different particle sizes on tensile and tear indices of the resultant BCTMP was investigated, as shown in Figure 4. Figure 4 indicates that the application of the MgO with a smaller particle size in the peroxide bleaching process had an active effect on improving the tensile and tear indices of the bleached CTMP. When the particle size of the MgO decreased from 17.4 to 2.9 μm, the tensile index of the BCTMP increased from 24.4 to 26.9 N·m/g and the tear index increased from 2.66 to 2.81 mN·m2/g, a 10.2% and 5.6% increase for tensile and tear indices of the resultant BCTMP, respectively.

MgO + H 2O → Mg(OH)2

(1)

HOOH + OH− → HOO− + H 2O

(2)

3.4. Effect of MgO with Different Particle Sizes on Optical Properties of the BCTMP. The effect of the MgO with different particle sizes on brightness, light-scattering coefficient, and opacity of the BCTMP was studied, as shown in Figure 5. A downward trend of the light-scattering coefficient and opacity of the resultant BCTMP would occur as the particle size of the MgO decreased. The probable reason responsible for these may be a higher alkalinity of the peroxide bleaching system by using a smaller particle size of the MgO as the alkali source, which finally made the bleached pulp fibers flexible. The maximum pulp brightness obtained at the particle size of 4.3 μm (ground for 20 min) of the industrial grade MgO reached 73.2% ISO, a 0.7% ISO higher than that obtained using the initial industrial grade MgO. 3.5. Hypothesis of the MgO Particle Size on the Alkaline Peroxide-Bleaching of CTMP. According to eqs 1 and 2 and the relevant analysis above, there is a maximum HOO− content that occurred as the particle size of industrial grade MgO reached 2.9 μm (ground for 60 min). As a result, the highest brightness of the bleached pulp should be seemingly obtained under this condition. However, it was actually obtained at the particle size of 4.3 μm (ground for 20 min). This phenomenon that the highest brightness was not obtained by using the MgO with the smallest particle size is pretty much the same as the result obtained by Chi.19 The content of transition metal ions of the industrial grade MgO was probably a significant influencing

Table 3. Effect of Industrial Grade MgO with Different Particle Sizes on Residual H2O2 sample no.

particle size (μm)

H2O (mL)

H2O2 (mL)

1

4.3 2.9 4.3 2.9 4.3 2.9 4.3 2.9 4.3 2.9

50

2

2 3 4 5

MgO (g)

Na2SiO3 (g)

DTPA (g)

residual H2O2 (% of applied) 100

0.5168 0.5168

0.12

0.5168 0.5168

7647

0.012 0.12

0.012

50.83 ± 0.14 41.99 ± 0.14 96.41 ± 0.15 95.36 ± 0.13 87.85 ± 0.15 86.19 ± 0.15 98.62 ± 0.15 95.08 ± 0.14



Article

Figure 2. Particle size distribution and SEM images of MgO at different grinding times (a) original MgO, (b) 20 min, and (c) 60 min.

factor. It was supposed that, with the decrease of the particle size of MgO, more transition metal was exposed to the interface,

which caused manganese-catalyzed decomposition of the peroxide used as the bleaching agent and formation of colored 7648



Article

were added, the content of residual H2O2 increased obviously. It indicates that the transition metal ions in the industrial grade MgO could cause H2O2 decomposition. In addition, it is also found that the content of residual H2O2 in the case of MgO with the particle size of 4.3 μm was always higher than that of MgO with the particle size of 2.9 μm. It can be concluded that the particle size of the industrial grade MgO ground for different time could affect the brightness of the bleached CTMP.

4. CONCLUSIONS An industrial grade MgO can be partially used as the alkali source in the peroxide bleaching of poplar CTMP. Considering the optical and strength properties of the bleached CTMPs, the resultant bleach effluent load, and the bleaching cost, the optimal substitution percentage of MgO for NaOH was chosen 50%. Industrial grade MgO with different particle sizes can be obtained by grinding treatment. The particle size of MgO reduces from 17.4 to 2.9 μm when the grinding time increased from 0 to 60 min. The decrease of the MgO particle size had a negative influence on the bulk, opacity, and light-scattering coefficient of the resultant poplar BCTMP but was beneficial to improving the tensile and tear indices of the pulp. When the MgO particle size reduced from 17.4 to 2.9 μm, the tensile and tear indices increased by 10.2% and 5.6%, respectively. Moreover, the smaller the MgO particle size, the greater the effect on the strength properties of the BCTMP. The reduction of the particle size of industrial grade MgO was not always helpful to improve the brightness of the poplar BCTMP. The maximum ISO brightness (73.2%) was obtained while the particle size of MgO was 4.3 μm rather than 2.9 μm. The reason for this may be the change of the transition metal ions content during the grinding treatment for the industrial grade MgO applied in the peroxide bleaching process.

Figure 3. Effect of MgO with different particle sizes on bulk of the resultant BCTMP.

Figure 4. Effect of MgO with different particle sizes on tensile and tear indices of the resultant BCTMP.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 22 60601293. Fax: +86 22 60600300. Notes

The authors declare no competing financial interest.

■

Figure 5. Effect of MgO with different particle sizes on optical properties of the resultant BCTMP.

ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (31070528), Project of Tianjin Scientific Innovation System and Platform Construction (10SYSYJC28000), and Project of China “Twelfth Five-Year” National Science and Technology Supporting Plan (2011BAC11B04).

iron−lignin complexes as well. In order to prove the above hypothesis, the following experiments were conducted by using the same kind of industrial grade MgO with different particle sizes of 4.3 and 2.9 μm, respectively. Table 3 lists the contents of the residual H2O2 under five different technological conditions. The dosages of MgO, Na2SiO3, and DTPA were the same as those in the bleaching process at 50% substitution percentage of MgO for NaOH. The residual H2O2 of all of the test samples was measured after they were kept standing in the conical flask for 30 min. As shown in Table 3, H2O2 would not decompose after the reference sample (sample 1) was just kept standing for 30 min. The decrease of the residual H2O2 in samples 2−5 was attributed to the addition of MgO. When Na2SiO3 or/and DTPA (metal chelating agent)

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Effect of Industrial Grade MgO with Different Particle Sizes on the

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