Intensified and Short Catalytic Bleaching of Eucalyptus Kraft Pulp

Article pubs.acs.org/IECR

Intensified and Short Catalytic Bleaching of Eucalyptus Kraft Pulp Ghazaleh Afsahi,*,† Naveen Kumar Chenna,† and Tapani Vuorinen† †

Department of Forest Products Technology, Aalto University School of Chemical Technology, P.O. Box 16300, 00076 Espoo, Finland ABSTRACT: Bleaching processes remove residual lignin and hexenuronic acid from cellulosic pulps. The reactions taking place in several stages are slow and consume large amounts of chemicals. The present study demonstrates a novel three-stage pulp bleaching sequence that combines a tertiary amine-catalyzed hypochlorite oxidation with subsequent treatments with ozone and hydrogen peroxide. With this sequence, the residual lignin and hexenuronic acid contents of an oxygen-delignified eucalyptus pulp were decreased by ≫90%, and a full brightness (88% ISO) was achieved. The total reaction time was only ∼1 h, which is one-fifth of the retention time of the current industrial pulp bleaching sequences. The chemical need was low, and the viscosity of the pulp remained high. This study may open new doors to future pulp bleaching with fewer and smaller bleaching towers and diminished use of chemicals.

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INTRODUCTION Bleaching is one of the key processes in the pulp industry and has been the focus of many studies since the 18th century.1−4 Increasing the brightness and its stability through removal of hexenuronic acid (HexA) and lignin are the main goals of pulp bleaching. The bleaching process is influenced by the applied chemicals, the overall reaction time, and the bleaching sequence. The application of multistage bleaching sequences has been proven to be the most effective method in industry.3 From the beginning, a combination of bleaching and extraction treatments has been used for bleaching chemical pulps. The bleaching chemicals and the order in which they are used make up the “bleaching sequence”. Bleaching processes generally contain two phases within each sequence: (1) a delignification segment, whose main function is to remove most of the residual lignin; and (2) a brightening segment, whose principal function is to increase the brightness of the pulp.3 To improve the overall brightness of the pulp, it is important to apply chemicals in each stage while taking into account the time period of each reaction. In addition, the use of chemicals that minimize the environmental impact is highly appreciated. Therefore, as an important milestone, the elemental chlorine free bleaching (ECF), based on use of chlorine dioxide (D), was introduced to replace the elemental chlorine bleaching, which was producing high amounts of organochlorine compounds.2 ECF bleaching is today the dominant process in the industry, and more than 70% of all bleached chemical pulp is produced with this technology.3 However, several factors, such as huge investments in bleaching towers and long retention times form the downside of ECF bleaching processes.3,4 Totally chlorine-free (TCF) bleaching processes that were claimed to be less toxic5 were introduced later.4,5 In a typical TCF bleaching process, the dominant reagents are oxygen-based chemicals, such as ozone (Z), oxygen (O), and hydrogen peroxide (P).6−9 Typical ECF and TCF bleaching processes need approximately 4−8 h2 to reach an ∼90% pulp brightness. Chemical pulp bleaching is based on oxidation chemistry, and the process is considered selective when the oxidants react © 2015 American Chemical Society

with the removable components, especially HexA and lignin, without attacking cellulose. The use of oxidants in hardwood kraft pulp bleaching can be minimized by applying an acid pretreatment stage (A) that selectively hydrolyzes HexA at pH 3−4.10 This treatment, however, requires a long retention time (at least 2 h) at a high temperature (85−100 °C). In modern eucalyptus pulp mills, the bleaching sequences include a hot acid stage, for example, A/D−EOP−D−P or D/A−EOP−D−P.10 Ozone is one of the most efficient oxidants to attack lignin and HexA;11 however, the use of ozone may also lead to oxidization of carbohydrates, forming carbonyl and carboxylic acid structures within the polysaccharide chain.11,12 These groups may initiate further degradation, chain cleavage, and loss in pulp viscosity. Therefore, utilization of ozone in the beginning of pulp bleaching where the degradation of cellulose is suppressed over the reactions of lignin that is still present in high contents has been suggested. Despite its limitations, ozone has a certain position in industrial pulp bleaching.13 Ozone has been a key oxidant in development of TCF bleaching sequences, such as A−Ze−P, EOP−Z−P, etc. Ozone has also been used in light ECF bleaching sequences, where it partly replaces chlorine dioxide, for example, ZD−EOP−D, Ze−D−P, D−Ze−DP, etc.14 Because of its high reactivity, ozone bleaching does not require a high temperature and partial pressure.15 The reaction time is very short, and therefore, large bleaching towers are not needed for ozone.11 It is stable at low temperature but decomposes slowly at higher temperatures.12 In contrast to other bleaching chemicals, including chlorine dioxide, ozone does not form chromophores, such as quinones as reaction products.15 Ozone reacts with HexA and lignin through the kinds of reactions that are illustrated in Scheme 1.15 Ozone reacts through its strong electrophilic character with carbon−carbon double bonds in the lignin remaining after Received: Revised: Accepted: Published: 8417

May 8, August August August

2015 14, 2015 14, 2015 14, 2015 DOI: 10.1021/acs.iecr.5b01725 Ind. Eng. Chem. Res. 2015, 54, 8417−8421

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Industrial & Engineering Chemistry Research

chlorine was measured before every experiment by iodometric titration as described in the standard method SCAN-C 29:72. 1,4-diazabicyclo[2.2. 2]octane (DABCO, 98%) was purchased from Sigma-Aldrich. KI solution, 0.2 M Na2S2O3 (99.9%, VWR), and 2 M HCl (Merck) were used for quantification of ozone by iodometric titration. The concentration of aqueous solution of H2O2 (5%) was measured before every experiment by iodometric titration as described in the standard method SCAN-C 29:72. Methods. The catalytic oxidations of the pulp were performed at 3% consistency in a stirred Büchiglasuster batch reactor (2 L) made of borosilicate glass.23 DABCO (0.1% on pulp) was added to the pulp suspension (1 L) which was then neutralized (pH 7) with 1 M H2SO4. The reactor vessel was thermostated at 25 °C and NaOCl (1% on pulp) was added. The concentration of residual active chlorine was monitored during the treatment by taking 5 mL aliquots from the solution and titrating them according to SCAN-C 29:72. The bleaching experiment was interrupted after a reaction time of 10 min by filtration and washing with cold, deionized water in a Büchner funnel. A 10 mL aliquot of the undiluted filtrate was analyzed for the residual active chlorine, and the measurement was repeated twice to reduce the experimental error. The catalytically oxidized pulp was then treated with ozone under conditions resembling those in conventional highconsistency ozone bleaching. To acidify the pulp suspension, its consistency was decreased to 3%, then the acidity of the suspension was adjusted to pH 3 with a few drops of 1 M H2SO4. After that, the consistency was increased to ∼30% through centrifugation, and the ozonation was performed in a specially designed rotating vessel. The setup included an ozone generator, a flow meter, and a round-bottomed rotating glass reactor (1 L).24 Ozone was produced from oxygen gas by an electric discharge method using a WEDECO GSO30 system. Next, the acidified pulp after the first stage (20 g dry matter content) was added to the reactor, and the gas from the ozone generator was flowed (2.5 L/min) through the reactor for 1−3 min to get a target dosage of 0.3−0.5% ozone. The ozone content of the gas inflow and outflow was measured by iodometric titration. The ozone consumption was calculated as the difference of the inflow and outflow. The ozone (Z) step was performed at 0.3%, 0.4%, and 0.5% ozone dosages (o.d.p). After the ozone treatment, the pulp was washed first with hot water and then with cold deionized water to stop the reaction. The pulp was then centrifuged and stored in cold (5 °C) for the next step. The final brightening was conducted with hydrogen peroxide at 10% consistency in plastic bags placed in a water bath. First, the pulp suspension was mixed with MgSO4·7H2O (0.2%) and H2O2 (0.3−0.6%), after which 1 M NaOH was added to adjust the pH to 11.2. The plastic bags were kept in a water bath at 85 °C for 1 h. During the bleaching, the pulp in the plastic bag was kneaded every 15 min. In the end, the pulp fibers were separated using a Büchner funnel and washed with cold deionized water to stop the chemical reactions. Finally, the pulp was centrifuged to ∼30% consistency, homogenized, and stored in cold. Additional bleaching experiments were carried out by varying the temperature (60−85 °C) and time (30−60 min). Pulp sheets of each stage were prepared according to TAPPI standard method T205, and kappa number (2 parallel measurements) and viscosity (3 parallel measurements) of the pulps were determined according to standard methods SCAN-C 1:00 and SCAN-CM 15:99, respectively. The content

Scheme 1. Oxidation of Lignin with Ozone

pulping and other delignification processes.16 During ozone treatment, new functional groups are formed in lignin; as a consequence, its macromolecular structure gets altered. Most of the phenolic groups in lignin are oxidized, and the hydrophilicity of the polymer increases, making it soluble in water under neutral condition.16 Ozone seems to be more efficient than some other oxidants, such as chlorine dioxide, and hydrogen peroxide to eliminate condensed phenolic structures in residual kraft lignin.16 Recently, it was postulated that the depolymerization of cellulose in an ozone treatment of pulp is due to secondary reaction of the reactive oxygen species formed in the oxidation of HexA and lignin.17 Thus, the viscosity of the pulp stayed at a higher level when most of HexA was removed with an acid treatment (A) prior to the ozone stage. Hydrogen peroxide (H2O2) in alkali is an effective oxidant that removes the partially oxidized components formed in the preceding bleaching stages. H2O2 also reacts with chromophoric compounds such as quinones and oxidizes them to gain the brightness of the pulp.18 The formation of the perhydroxyl anion, a highly nucleophilic intermediate, is responsible for the oxidation of chromophores through the cleavage of the side chains.19 Several parameters, such as chemical usage, bleaching yield, water consumption, effluent load, treatability, brightness stability, refinability, and strength of the pulp, together determine the choice of the bleaching technology. Although the chemical costs form one of the most important criteria, lower investment costs may favor sequences with fewer bleaching stages. In general, the order of using the various chemicals is detrimental for the overall efficiency and costs. For instance, the use of the strongest chemicals in the initial bleaching may reduce the residual lignin and HexA contents to such a low level that only one additional stage is needed to reach full brightness.20 Although it is possible to bleach pulp in either one or two stages, this does not deliver full brightness.21,22 Recently, we reported on initial bleaching of a eucalyptus kraft pulp with hypochlorite and a tertiary amine as a catalyst.23 Most of the HexA and a significant part of the residual lignin were removed very rapidly with a small amount of the oxidant under mild conditions. The present work deals with subsequent bleaching of the catalytically oxidized pulp with ozone and hydrogen peroxide. The performance of the short bleaching sequence is discussed, and the influence of bleaching parameters in each stage is evaluated.

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EXPERIMENTAL SECTION Materials. Oxygen-delignified eucalyptus pulp was obtained from a Brazilian kraft pulp mill. The kappa number of the pulp was 11.4; HexA content, 66 mmol/kg; and viscosity, 1130 mL/ g. An aqueous solution of NaOCl (3.5%) was purchased from VWR. The concentration of NaOCl expressed as active 8418

DOI: 10.1021/acs.iecr.5b01725 Ind. Eng. Chem. Res. 2015, 54, 8417−8421

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Table 1. Changes in Kappa Number, HexA, and Lignin Contents, Brightness and Viscosity of Oxygen Delignified Kraft Pulp during Hcat−Z−P Bleaching Sequence O3 (%)a ref Hcat Hcat−Z Hcat−Z Hcat−Z Hcat−Z−P Hcat−Z−P Hcat−Z−P Hcat−Z−P Hcat−Z−P a

0.3 (0.28) 0.4 (0.32) 0.5 (0.46) 0.3 0.3 0.3 0.5 0.5

H2O2 (%)

0.6 0.6 0.6 0.3 0.6

pH 7 2.5 2.5 2.5 11.2 11.2 11.2 11.2 11.2

T (°C) 25 25 25 25 60 60 85 85 85

t (min)

κ

HexA (mmol/kg)

lignin (%)b

brightness (%)

viscosity (mL/g)

10 1 2 3 30 60 30 60 60

11.4 5.9 3.3 2.8 2.5 1.2 1.1 1 1.3 0.9

66 30 ± 0.1 ≤3 ± 0.12c ≤3 ± 0.12c ≤3 ± 0.12c 2.6 ± 0.1 2.5 ± 0.2 2.3 ± 0.1 1,1 ± 0.2 0,8 ± 0.1

100 64 12 10 10 9 9 9 7 6

57 69 81 82 84 85 86 87 86 88

1129 1103 ± 10

1060 1015 1060 1090 1080

± ± ± ± ±

15 10 12 10 15

Measured consumption in parentheses. bRelative content. cThe exact value could not be measured because of overlapping signals.

of HexA and aromatic lignin were quantified by UV resonance Raman (UVRR) spectroscopy6 (two parallel measurements) using a Renishaw 1000 UV Raman spectrometer (Gloucestershire, U.K.), which was connected to a Leica DMLM microscope (Wetzlar, Germany) and an Innova 90C FreD frequency-doubled Ar+ ion laser (Coherent Inc., Santa Clara, California, U.S.A.). The excitation wavelength of the laser was 244 nm; power output, 10 mW; and measuring transmittance, 25%. The beam was directed through a 40× objective, and the sample was rotated to prevent burning. The spectral resolution was ∼7 cm−1, which is the separation ability for two bands close to one another. A diamond crystal was measured every day prior to sample measurements to ensure the repeatability and comparability of the sample band locations. The Raman spectra were linearly baseline-corrected to zero at two wave numbers (800 and 2000 cm−1) in Grams AI spectroscopy software. The spectra were normalized to cellulose peak at 1094 cm−1. ISO brightness was measured using a Loretzen & Wettre SE 070R Elrepho spectrophotometer.25

Figure 1. UVRR spectra of oxygen-delignified eucalyptus kraft pulp (a) treated first with DABCO and HOCl (Hcat) (b) and then with different amounts of ozone (0.3 (c), 0.4 (d), and 0.5% (e) on pulp).

RESULTS AND DISCUSSION The effects of several reaction parameters, including chemical charges, pH, temperature, and time, on DABCO-catalyzed bleaching of an oxygen delignified eucalyptus kraft pulp were reported earlier.23 In this study, we repeated the catalytic stage (Hcat) in the conditions that led to a maximal decrease in kappa number and the most extensive removal of HexA and lignin from the oxygen delignified pulp.23 The catalytic treatment decreased the kappa number by half and led to a significant decrease in the content of HexA and lignin in the pulp (Table 1 and Figure 1). Chenna et al.23 reported on similar changes in the pulp under similar conditions. The catalytic treatment also increased the brightness of the pulp while the viscosity remained almost unaffected (Table 1), which verified the earlier observed high selectivity of the catalysis. Although the conditions of the initial bleaching probably affect the bleachability in the later stages, this aspect was not investigated in the preliminary work of Chenna et al.23 The lower the content of the residual lignin, the harder it is to remove with oxidants.27 Therefore, we elected to apply ozone, known for its high reactivity, in the second bleaching stage. One of the key parameters of ozone bleaching is the acidity of the medium. Because the delignification in this stage has been reported to be most efficient at pH 2−3,16 we elected to carry out the ozonation at pH 3. Ozonation at both medium and high consistency has been used in the laboratory and in

pulp industry. For practical reasons, we elected to use highconsistency ozonation in the laboratory experiments. It has also been claimed that the high consistency and the acidic conditions lead to minimal degradation of carbohydrates and decomposition of ozone.16 The second bleaching stage lowered the kappa number by half when the ozone charge was 0.4%. The final kappa number depended slightly on the amount of ozone used (Table 1). UVRR spectra of the pulps (Figure 1) confirmed that most of the residual HexA (1655 cm−1) and lignin (∼1600 cm−1) were oxidized in the ozone treatment almost independent of the ozone dosage. An increase in the Raman scattering intensity at ∼1700 cm−1 was an indication of formed oxidized lignin structures. The ISO brightness was increased to 81−84%, depending on the charge of ozone (Table 1). It is known that secondary reactions of the reactive oxygen species formed during ozonation may introduce carbonyl groups into cellulose.16 Because their presence interferes with the CED viscosity measurement, we did not determine the viscosity for the ozonebleached pulps. To reach full brightness, we bleached the pulps further with alkaline peroxide. Hydrogen peroxide acts as a true bleaching agent that destroys many of the chromophores present in pulp.27 The alkaline peroxide also oxidizes the possible carbonyl groups in the pulp, thus stabilizing its viscosity.24−26

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8419

DOI: 10.1021/acs.iecr.5b01725 Ind. Eng. Chem. Res. 2015, 54, 8417−8421

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Industrial & Engineering Chemistry Research Because we could not predict the behavior of the Hcat−Z bleached pulp in the alkaline peroxide treatment beforehand, we varied the charges of alkali and peroxide and reaction temperature and time in the trials (Table 1). The peroxide bleaching stage increased the brightness by 2− 6% units and led to 85−88% final brightness, depending on the conditions in the ozone and peroxide bleaching stages. The peroxide treatment decreased the kappa number by 1−2 units, resulting in a final kappa number of ∼1 (Table 1). The UVRR spectra of the peroxide-bleached pulps were indicative of extremely low residual lignin and HexA contents (Figure 2).

Figure 3. UVRR spectra of oxygen-delignified eucalyptus kraft pulp (a) treated first with DABCO and HOCl (Hcat); then with ozone (0.3%) (c); and finally, with 0.6% peroxide for 30 min at 60 °C (d); for 60 min at 60 °C (e); and for 30 min at 85 °C (f).

technology for eucalyptus and other hardwood pulps. The main benefits could be substantially lowered investment costs because of the fewer stages and smaller reactor sizes and lower operation costs because of the diminished need for heating and chemicals. In the future, the applicability of similar short sequences for bleaching of the less reactive softwood kraft pulps should also be studied.

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Figure 2. UVRR spectra of oxygen-delignified eucalyptus kraft pulp (a) treated first with DABCO and HOCl (Hcat) (b); then with ozone (0.5%) (c); and finally, with 0.3% (d) and 0.6% (e) peroxide for 60 min at 85 °C.

AUTHOR INFORMATION

Corresponding Author

*E-mail: ghazaleh.afsahi@aalto.fi. Notes

The viscosity of the pulp remained almost unchanged throughout the bleaching (Table 1). This finding was somewhat unexpected because both the ozone and peroxide stages are known for their ability to oxidize cellulose to some extent.24 Recently, it was claimed that the viscosity drop in the ozone stage is caused by the reactive oxygen species formed in oxidation of HexA and lignin-derived muconic acids.17 Thus, the stability of cellulose toward ozone after the catalytic bleaching could be explained by the low HexA and residual lignin contents. Although the reaction temperature and time affected the brightness gain in the peroxide stage (Table 1), the residual lignin and HexA contents were hardly affected by these parameters (Table 1 and Figure 3).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by a consortium of Andritz, Kemira, Metsä Fiber, Stora Enso, and UPM. Christian Järnefelt, Mirja Reinikainen and Rita Hatakka are acknowledged for their help with the experiments.

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REFERENCES

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CONCLUSIONS The experimental results presented here show that eucalyptus kraft pulps can be bleached extremely efficiently by combining a tertiary amine-catalyzed hypochlorite prebleaching with ozone bleaching. Both stages are highly reactive, and therefore, only very short reaction times are needed, even at low temperatures. Although the brightness is already high after the ozone stage, full brightness is easy to achieve with an additional treatment with alkaline peroxide. After this short Hcat−Z−P sequence, the residual lignin and HexA contents are on the same or lower level as the best ECF bleached pulps. In addition, the pulp viscosity remains at a high level that is not typical for pulps that have been bleached with ozone and peroxide. The novel bleaching sequence presented here has a high potential to be developed toward an industrial bleaching 8420

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