Effluent Characteristics and Pulp Properties Changes with the Partially

ARTICLE pubs.acs.org/IECR

Effluent Characteristics and Pulp Properties Changes with the Partially Substituting MgO for NaOH in the High-Consistency Retention Stage of Triploid Poplar P-RC APMP W. Liu, Q. X. Hou,* B. Yang, W. Yuan, J. P. Zhao, R. X. Zhang, and J. M. Liu Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, China ABSTRACT: Magnesium-based alkali is another attractive alkali source in the peroxide bleaching process of high-yield pulps. This work investigated the effluent characteristics in the high-consistency retention stage and pulp properties of triploid poplar P-RC APMP (that is, so-called preconditioning followed by refining chemical treatment, alkaline peroxide mechanical pulp) by partially substituting MgO for NaOH. The results showed that the pH, dissolved and colloidal substances (DCS), chemical oxygen demand (CODCr), cationic demand, and conductivity of the effluent in the high-consistency retention stage decreased with the increase of the substitution percentage of MgO for NaOH. Meanwhile, both the light-scattering coefficient and the bulk of the pulps increased, but the tensile index and internal bond decreased. The tensile index and internal bond had a good linear relationship with bulk.

’ INTRODUCTION Bleached chem-thermo-mechanical pulp (BCTMP) and preconditioning followed by refining chemical treatment, alkaline peroxide mechanical pulp (P-RC APMP), as the main high-yield pulps (HYPs), have been widely used in the pulp and paper industry.1,2 Sodium hydroxide is the dominant alkali source in the peroxide bleaching of HYPs. The strong alkalinity of sodium hydroxide can increase the noneffective decomposition of peroxide and cause the decomposition of partial carbohydrates in pulp.3 Consequently, the bleaching yield will decrease and the effluent chemical oxygen demand (COD) load will increase.4,5 Meanwhile, the strong alkalinity of NaOH can still lead to the formation of new chromogenic groups in lignin structure units, i.e., the alkali-induced darkening, so the brightness of the bleached HYP is decreased.6 Some benefits of the magnesium-based alkali (MgO or Mg(OH)2) used as the bleaching alkali source in the pulp and paper industry were reported in some research and applications, e.g., lower COD and cationic demand, decreased bleaching cost and oxalate scaling, and higher bleaching yield.4,7-12 Thus far, most of the research related to the substitution of magnesium-based alkali for sodium-based alkali has focused on the peroxide bleaching processes of thermo-mechanical pulp (TMP), pressurized ground wood (PGW), and chem-thermo-mechanical pulp (CTMP), e.g., maple CTMP, maple PGW, and spruce TMP.8,10,13,14 Wong et al. evaluated the alkalis of magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) for peroxide bleaching of softwood TMP. They found that the pulp produced had a higher yield and bulk, and the effluent had a lower biological oxygen demand (BOD), dissolved solids, and cationic demand.12 He et al. found that the tensile and burst strength properties of spruce TMP remained unchanged or decreased slightly in the Mg(OH)2-based peroxide bleaching process. For maple PGW and CTMP, both the Mg(OH)2-based and NaOH-based peroxide processes could produce the bleached pulp with higher strength properties than the unbleached pulp; the effect of the Mg(OH)2based process was more remarkable than the NaOH-based process.13 r 2011 American Chemical Society

The advantages of magnesium-based alkali are very obvious in the bleaching process. P-RC APMP can optimize the potential chemical and mechanical effects in the pulping process. Peroxide will take effect through the whole pulping process. Both chips and resultant pulps can be bleached, which is the most significant difference between BCTMP and P-RC APMP. Thus, the bleaching process of P-RC APMP is more complex than that of BCTMP. Can the magnesium-based alkali be used in the P-RC APMP? How much magnesium-based alkali can be used in this process? These two questions are worth studying to extend the application of magnesium-based alkali. Previous works showed that partial MgO substitution for NaOH in the second-stage impregnation resulted in a lower pH of residual bleaching liquor and a higher treated wood chips yield than those of the NaOH-based process. In addition, dissolution of benzene alcohol extractives, lignin, pentosans, and carbohydrates were all decreased. The resultantly yielded P-RC APMP pulp has a higher bulk, opacity, and light-scattering coefficient but lower strength properties and a slightly lower brightness.15 The effluent had good characteristics, and the resultant pulp had acceptable physical properties while selecting 50% substitution MgO for NaOH at the second-stage impregnation.16 In this study, the substitution percentage of MgO for NaOH in the second-stage impregnation was fixed at 50% and the MgO was partially used in the high-consistency retention stage with different substitution percentages under two different levels of hydrogen peroxide. The aim of this study was to investigate the effects of the substitution percentage of MgO for NaOH in the high-consistency retention stage on the changes of the effluent characteristics and the pulp properties of the P-RC APMP. Received: September 16, 2010 Accepted: December 19, 2010 Revised: December 9, 2010 Published: January 18, 2011 1860

dx.doi.org/10.1021/ie1019084 | Ind. Eng. Chem. Res. 2011, 50, 1860–1865

Industrial & Engineering Chemistry Research

ARTICLE

Figure 1. Flow sheet of the P-RC APMP experiment.

Table 1. Properties of the Magnesium Oxide component

Table 2. Experimental Conditions content component

second-stage impregnation

high-consistency retention stage

MgO, %

g98.0

Fe, %

e0.005

NaOH, %

3.0

3.0

Cu, %

e0.001

H2O2, %

3.5

3.5, 4.0

heavy metal, % on Pb

e0.003

substitution percentage of

50

0, 25, 50, 75

8.7

MgO for NaOH, %

particle size, μm

’ EXPERIMENTAL SECTION The flow sheet of the P-RC APMP experiment is shown in Figure 1. Materials. Wood chips used in the experiment were collected from a pulp mill in Henan province, China. The chips were subjected to treatments of presteaming, first-stage squeezing extrusion, and first-stage impregnation in the pulp mill. The chips obtained were stored in a cool room with a temperature of about 4 °C. The atmospheric presteaming was carried out at 60 °C for 20 min. The screw compression ratio in the first-stage squeezing extrusion was 4:1. The conditions of the first-stage impregnation were as follows: 0.3% NaOH, 0.1% DTPA, and a pressure of 0.5 MPa. All chemicals used in the experiment were of analytical grade. Table1 lists the components of the magnesium oxide used in the study. Methods. The wood chips obtained in the pulp mill were immersed in hot water with a temperature of about 90 °C for 20 min. Then, the chips were extruded in a screw extruder. The extruded chips and bleaching chemicals were put into a 3 L bleaching reactor for implementing the second-stage impregnation. The first-stage refining was performed using a KRK refiner with a gap of 0.8 mm between refining surfaces at 15% refining consistency. After the refining stage, the refined chips were thickened and air dried under room temperature. Then, these air-dried chips were kept in a plastic bag to balance the moisture to about 60%. The highconsistency retention treatment was still carried out in the same bleaching reactor. The chemicals were mixed thoroughly with the refined chips before both of them were transferred into the reactor. The reaction conditions of this stage are shown in Table 2. Following the high-consistency retention stage, the second-stage refining was carried out using the same refiner with a 0.10 mm plate gap at 15% consistency. The refined pulp was collected for treatment of latency at 4% consistency, 95 °C for 40 min. After the latency treatment was finished, the pulp was screened using the screener (model 535, Lorentzen & Wettre Co. Ltd., Sweden) with a 0.15 mm slot size. The pulp beating was conducted in the PFI standard refiner at 10% consistency until the beating degree reached 45°SR. The handsheets making and physical testing were measured according to the methods in the literature.17

Na2SiO3, %

3.0

3.0

DTPA, % pulp consistency, %

0.05 30

0.05 30

temperature, °C

90

90

reaction time, min

90

90

In order to collect the effluent in the high-consistency retention stage for future analysis, after treatment in the high-consistency retention stage, about 5 g (oven dry) of the resultant pulp sample was collected and diluted into 1% pulp consistency with distilled water. Then, the fiber suspension was stirred thoroughly for 30 min at a stirring speed of 500 rpm. The well-mixed fiber suspension was filtrated in a B€uchner funnel with a 200-mesh screen, and the filtrate was recycled once to remove the fines. The filtrate was further filtered with a piece of medium-fast filter paper and collected for determination of pH, CODCr, conductivity, and cationic demand. After the high-consistency retention stage, 10 g (oven dry) of the resultant pulp was collected and diluted into 2% pulp consistency with distilled water. Then, the fiber suspension was stirred thoroughly (60 min, 60 °C, 500 rpm), centrifuged (2000 rpm, 30 min), and deposited for a moment until the supernatant [i.e., dissolved and colloidal substances (DCS) water samples] can be taken easily. The DCS water samples were filtrated through a Millipore membrane (0.22 μm). The filtrates obtained were the DS water samples. After 10 mL of DCS and DS water samples were dried to a constant in the oven at 105 °C, the solids obtained were DCS and DS, respectively. In addition, the colloidal substances (CS) content was calculated from the contents of DCS and DS, i.e., CS = DCS - DS.18 The CODCr was determined according to the method in the literature.17 The conductivity and cationic demand were measured using a HI98303 conductivity instrument (HANNA Instrument Co., Ltd., Italy) and a M€uteck PCD-03 charge analyzer (BTG Co., Ltd., Germany), respectively. Before determination of cationic demand, 0.10 mol/L of sulfuric acid was prepared for adjusting the pH value to 6.8 ( 0.1.13 An AA6800 atomic absorption spectrophotometer (Shimazu Co., Ltd., Japan) was used to measure the content of Na and Mg in the effluent. The 84-92 lab master Z-Directional tensile tester (TMI Group of Companies, USA) was used to measure the internal bond 1861

dx.doi.org/10.1021/ie1019084 |Ind. Eng. Chem. Res. 2011, 50, 1860–1865


Figure 2. Effect of substitution of MgO for NaOH on the pH of the effluent from the high-consistency retention stage of the P-RC APMP process.

Figure 3. Effect of substitution of MgO for NaOH on the DCS content from the high-consistency retention stage of the P-RC APMP process.

strength of handsheets following the testing method of TAPPI T541.

’ RESULTS AND DISCUSSION Effect of Substitution of MgO for NaOH on the pH of the Effluent from the High-Consistency Retention Stage. The

pH value of the effluent from the high-consistency retention stage decreased as the substitution percentage of MgO for NaOH increased, as shown in Figure 2. It had a good linear relationship between the pH values and the substitution percentages of MgO for NaOH. It indicated that addition of MgO could make the alkalinity of the whole bleaching system weak. This is mainly due to the low solubility of MgO in water. The solubility is 0.086 g/L at 60 °C and 0.062 g/L at 80 °C, lower than that of NaOH.15 Effect of Substitution of MgO for NaOH on the DCS, CODCr, and Cationic Demand of the Effluent from the High-Consistency Retention Stage. The DCS would be released during mechanical pulping and bleaching and dissolved into the paper mill process water. These components are composed of hemicelluloses, lipophilic extractives, and lignin-like compounds.19 Mild bleaching condition, e.g., a lower amount of alkali and weaker alkali, can reduce anionic trash obviously and therefore improve the pulp cleanliness.5 The DCS and DS contents were measured under the condition of 3.5% H2O2 charge in the high-consistency retention stage. The DCS was decreased obviously with addition of MgO, as shown in Figure 3.

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Figure 4. Effect of substitution of MgO for NaOH on the CODCr and cationic demand from the high-consistency retention stage of the P-RC APMP process.

The cationic demand (CD) can be used to indicate the anionic trash content and solid content of DCS. The CD was measured under the condition of 3.5% H2O2 charge in the high-consistency retention stage. The results showed that the cationic demand of the effluent from the high-consistency retention stage decreased greatly with the increase of the substitution percentage of MgO for NaOH, as shown in Figure 4. The weak alkalinity of magnesium hydroxide and the presence of Mg2þ ions in the system can contribute to the observed decrease of anionic trash. The Mg2þ ions can bind with polygalacturonic acids, oxidized lignin, and resin acids, causing them to be deposited onto pulp fibers by neutralizing their anionic charges.20 The results demonstrated that MgO was useful in decreasing the amount of anionic trash in the pulping process. This can be mainly proved by the measurement of the solid content of DCS and CD. Thus, application of MgO may provide promise to the well runability of a paper machine, especially with the closed recycle enhancement of white water in the modern papermaking process. The CODCr was an important index of effluent load. The CODCr was measured under the condition of 3.5% H2O2 charge in the high-consistency retention stage. The results illustrated that CODCr of the effluent from the high-consistency retention stage decreased greatly with the increase of the substitution percentage of MgO for NaOH, as shown in Figure 4. The CODCr decreased nearly 80% when the substitution percentage of MgO for NaOH increased from 0% to 75%. The dramatic change mainly resulted from the weak alkalinity of MgO, which could reduce the dissolution of lignin, acetic acid, and methanol and then lead to the decrease of COD load.7 CODCr may come from dissolved carbohydrates, lignin, and other organic substances in the bleaching filtrate.7 Effect of Substitution of MgO for NaOH on the Conductivity of the Effluent from the High-Consistency Retention Stage. With the increase of the substitution percentage of MgO for NaOH, the conductivity decreased greatly. The conductivity could decrease nearly 49% when the substitution percentage increased from 0% to 75%, as shown in Figure 5. Some organic substances in the peroxide bleaching process, e.g., oxidized lignin and polygalacturonic acid, can contribute to the conductivity of the effluent.7 The pH of the bleaching system changed with the substitution of MgO for NaOH, so the amount of these organic substances decreased, which can be reflected by the changes of DCS, as shown in Figure 3. On the other hand, the amounts of Mg2þ and Naþ in the bleaching system changed 1862



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Figure 5. Effect of substitution of MgO for NaOH on the conductivity of the effluent from the high-consistency retention stage of the P-RC APMP process.

Figure 7. Effect of substitution of MgO for NaOH on the bulk of the handsheets made of the P-RC APMP.

Figure 6. Absolute content of Na and Mg in the filtrate from the highconsistency retention stage of the P-RC APMP process.

Figure 8. Effect of substitution of MgO for NaOH on the lightscattering coefficient of the handsheets made of the P-RC APMP.

Table 3. Brightness of the Pulps with Different Substitution Percentages of MgO for NaOH in the High-Consistency Retention Stage H2O2, % MgO/NaOH, %

4.0

3.5

0

71.43

70.30

25

71.69

70.34

50

71.96

71.44

75

70.13

68.37

greatly with addition of MgO. These two kinds of ions are main inorganic ions that contribute to the conductivity of the effluent. Figure 6 showed the absolute content of Na and Mg in the effluent from the high-consistency retention stage, which were measured using an atomic absorption spectrophotometer. Effect of Substitution of MgO for NaOH on the Pulp Properties. The effects of substitution of MgO for NaOH on the brightness, light-scattering coefficient, bulk, and some strength properties were investigated, as shown in Table 3 and Figures 7, 8, 9, and 10, respectively. These results indicated that the bulk and light-scattering coefficient would increase with the increase of the substitution percentage of MgO for NaOH, while the brightness would have a slight change. The bulk had a strong relationship with the MgO substitution percentage. These results suggested that a partial MgO substitution for NaOH may be used in the P-RC APMP process to produce pulps with better

Figure 9. Relationship between the tensile index and the bulk of the handsheets made of the P-RC APMP.

light-scattering coefficient and bulk. Meanwhile, the pulp tensile index and internal bond had a good linear relationship with bulk, respectively, as shown in Figures 9 and 10. The strength properties tended to decrease with the bulk increase caused by the partial MgO substitution for NaOH. Alkalinity is a more dominant factor than peroxide charge affecting fiber bonding.21,22 At a low alkali charge, an increase in peroxide charge decreased the tensile index, which may be caused by the neutralizing effect of peroxide.21 The changes of bulk, light scattering, tensile index, and internal bond mainly resulted from the interfiber bonding and fiber flexibility.23 1863



Figure 10. Relationship between the internal bond and the bulk of the handsheets made of the P-RC APMP.

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’ CONCLUSIONS MgO was partially used in the high-consistency retention stage of triploid poplar P-RC APMP. The advantages of the MgObased process on decreasing the load of effluent are obvious. The pH, DCS, CODCr, cationic demand, and conductivity of the effluent in the high-consistency retention stage decreased greatly with the increase of the substitution percentage of MgO for NaOH. The bulk and light-scattering coefficient of the P-RC APMP pulps increased with the increase of substitution percentage of MgO for NaOH in the high-consistency retention stage. Partial substitution of MgO for NaOH in the P-RC APMP process has the potential to produce pulps with a better light-scattering coefficient and bulk. Meanwhile, the tensile index and internal bond of the P-RC APMP pulps decreased with the increase of substitution percentage of MgO for NaOH in the high-consistency retention stage. All these results were mainly caused by the weaker alkalinity of MgO-based alkali than NaOH-based alkali.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-22-60601293. Fax: þ86-22-60600300. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the Sino-Canada International Cooperation Project (2008DFA91290) and the National Natural Science Foundation of China (31070528). ’ REFERENCES Figure 11. SEM of the fiber surface morphology from NaOH-based (left) and MgO-based (right) P-RC APMP processes.

Some research work demonstrated that the carboxylic groups generated on the pulp fibers due to hydrolysis of hemicelluloses and oxidation of lignin may contribute to fiber swelling and bonding strength.24-29 According to these researches, the weaker alkalinity of the MgO-based P-RC APMP process would produce less of this kind of acidic groups, so the fiber swelling and bonding strength were worse than the NaOH-based process. In addition, the former study showed that less hydrophobic materials, such as lignin and benzene alcohol extractives, were removed due to partially substituting MgO for NaOH.15 These more hydrophobic materials retained in pulp would have potentially detrimental effects on the pulp qualities.30 For example, the lignin can hinder formation of hydrogen bonds between fibers, restrict the swelling of fibers, and make them stiff.30 In other words, the multivalent cations, such as Mg2þ, may also decrease the fiber swelling and interfiber bonding.24,28,31 In Figure 11, the surface morphology of pulp fibers from both MgO-based and NaOH-based processes is illustrated. As for the surface morphology of pulp fibers from the NaOH-based process, the length of the pulp fibers is long and the fibers bond much more closely. In contrast, the pulp fibers from the MgO-based process do not bond so closely like the pulp fibers from the NaOH-based process, resulting in higher bulk and light-scattering coefficient and lower pulp physical strength properties. Thus, substitution of MgO for NaOH had a negative effect on the pulp strength properties and a positive effect on the bulk and light-scattering coefficient.

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Effluent Characteristics and Pulp Properties Changes with the Partially

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