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Production of corrugating medium paper with secondary fibers from digested deinking sludge
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JIEC-2873; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
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Production of corrugating medium paper with secondary fibers from digested deinking sludge Dexing Yin a,b, Yunqin Lin a,*, Zhihuan Chen a, Jialiang Qiao c, Min Xiao a, Dehan Wang a a
College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510642, China Beijing CIIT Environmental Co., Ltd., Beijing 100016, China c College of Food Science, South China Agricultural University, Guangzhou 510642, China b
A R T I C L E I N F O
Article history: Received 27 August 2015 Received in revised form 30 January 2016 Accepted 14 March 2016 Available online xxx Keywords: Corrugating medium paper Secondary fibers Deinking sludge Anaerobic digestion Fiber loads
A B S T R A C T
This work focused on developing an approach to reusing digested deinking sludge for corrugating medium paper manufacture by recycling secondary fibers. The fiber content in the digested deinking sludge (DDS) was around 41.87%, and the main proportion of fibers (87.6%) was subject to secondary fines with lengths less than 0.2 mm. The qualified corrugating medium paper was obtained when the secondary fiber load from DDS was equal or less than 30%. Corrugating medium paper manufactured by adding DDS could maximize reuse and recycling of deinking sludge via methane and papermaking material production, and reduce the environmental pollution as well. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The pulp and paper industry, which is in rapid development nowadays, has become one of the main industries in China. Simultaneously, waste water and sludge are produced in large quantities in the pulping and papermaking processes. Deinking sludge (DS), which is formed in the process of wastepaper pulping by means of flotation deinking, dominates over 50% of the total sludge yield in the pulp and paper industry and it will reach 70% in the following years because more and more wastepaper is reused as a papermaking raw material [1]. Generally, 80–150 kg dry deinking sludge or 160–500 kg wet deinking sludge is generated when one ton dry pulp is produced [2]. How to appropriately treat the huge amounts of deinking sludge has become a vital issue to be addressed for pulping and papermaking plants. In China, randomly discarding sludge is banned owing to the serious secondary pollution. The representatives of traditional conversion technologies for pulp & paper mill sludge (including deinking sludge and sewage sludge) are incineration and composting, even though some researchers have been trying to look for some different ways for pulp & paper mill sludge reusage, such as cement and brick manufacture, specific sorbent production
* Corresponding author. Tel.: +86 020 85280296; fax: +86 020 85287672. E-mail address: [email protected] (Y. Lin).
and agriculture land application [3–6]. Nevertheless, evident shortcomings of applying both above main technologies have been found. Regarding incineration, the difficulty to dehydrate, the low efficiency of energy generation, the trouble to cleanse the toxic exhausted gas and the overall high investment hinder its application [7]. As regards to composting, it is a widely used bio-conversion technology for sludge. Currently, some plants have been set up and they focused on the pulp & paper mill sludge reutilization by composting in China [8]. However, low nitrogen contents, long time for lignocellulose decomposing, lower contents of humic acid in compost and a high risk of heavy metal pollution existing in compost land application have become the main problems to stagnate the development of sludge composting [9– 11]. Thus, investigating new technologies to reuse pulp & paper mill sludge, especially the deinking sludge which yield is growing with the increasing recycle of wastepaper, is significantly important. As mentioned by many authors, the deinking sludge is rich in polysaccharides, cellulose, hemicellulose, lignin and other inorganic components [12]. On the other hand, anaerobic digestion has been highly concerned in the world as a renewable energy production technology for solid waste from various sources, like agricultural residues, municipal solid waste, green trimmings and sludge. Anaerobic digestion of pulp & paper mill sludge (including deinking sludge and sewage sludge) for methane production was also reported currently. Lin et al. [13,14] studied the possibility of
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anaerobic co-digestion of pulp & paper sewage sludge and monosodium glutamate waste liquor and found the accumulative methane yield could reach 88.82 mL g 1 VS (volatile solid). Some other authors found that by controlling pH, hydraulic retention time and temperature, it was possible to optimize the acidogenic and acetogenic phases in the sludge anaerobic digestion process and took good advantages of the soluble organics produced in the hydrolysis stage [15,16]. Besides, Ratnieks and Gaylarde [17] investigated the effect of systemic pH on anaerobic digestion of paper mill sludge, and found that when the systemic pH was 7, the sludge degradation efficiency could be increased to 60%. Domestic and foreign research results indicate that it is feasible to use pulp & paper mill sludge to generate methane for heat or electricity recovery. In addition, papermaking is subject to the process with high consumption of energy. The overall consumption is around 0.98 ton standard coal for 1 ton dry paper production. Anaerobic digestion of pulp & paper mill sludge could develop an approach to reducing coal consumption in papermaking plants by methane generation, so as to meet the requirement of market globalization competition. On the other hand, a large amount of anaerobic digestate is produced after anaerobic digestion of sludge, and solid–liquid separation is generally operated to the digestate. Regularly, the liquid part can be recycled to fields as NPK fertilizer [18], and the solid residue of digestate is disposed by landfill or composting. Otherwise, papermaking is an industry leader from the standpoint of recycling and sustainability, the main reason being that virgin and recycled fibers can be used together, sometimes contributing with complementary characteristics. Since the solid phase of digested deinking sludge (DDS) contains most of the secondary fibers/fines which reached 41.87% based on the total solid (TS) (see Table 1), and less tannins, resins and other pigments compared to the original deinking sludge, it might be another potential material to manufacture specific papers. What’s more, abundant active hydroxyls are contained in the tiny fibers/fines which are the principal organic components of the digested deinking sludge. The possible reason for the digested sludge playing an important role in the series of esterification, etherification, graft copolymerization, cross-linking and other derivative reactions are related to the hydroxyls [19]. Besides, non-woods are used extensively in pulp and paper production in several countries due to its abundance and costeffectiveness, and the scarcity of forest resources especially in Asian countries. Many studies have been carried out on non-wood pulping for pulp and paper production [19–21]. Since China is severely short of woods for pulp and paper making, over 80% of woody papermaking materials in China depend on importation. Looking for new sources of raw materials for papermaking in domestic is challenging. Meanwhile it has become an urgent task for relevant researchers. Nowadays, China is the biggest non-wood producer in the world. Straw has become the largest source of nonwoods for the paper industry. Like the traditional non-woods of
Table 1 Secondary fiber sources and loads for the corrugating medium paper manufacture. Treatment no.
Softwood pulp (%, TS)a
Wastepaper pulp (%, TS)
DS or DDSb (%, TS)
Ctrl. 1 2 3 4 5 6
20 20 20 20 20 20 20
80 70 60 50 40 30 20
0 10 20 30 40 50 60
a b
Based on dry weight. DS and DDS were added as secondary fiber sources, respectively.
straws, reeds, bamboo and so on, deinking sludge has already been tested as a specific papermaking raw material [22]. Furthermore, it is a recommended wastewater reduction method by recycling of wastewater with simultaneous recovery of fibers [23]. The most common technique for reclaiming fiber is to recycle primary sludge back into the fiber – processing system. Some recycled paperboard mills and some manufacturers of unbleached and bleached pulp and paper have reduced sludge volume by reclaiming the fiber, fillers, or both in sludge to be reused within their pulping and papermaking processes [24,25]. Fiber recycling has played an important role in the sustainable development of pulp and paper industry with the benefits of environmental pollution decrease, leading to an extension of fiber life cycle, forest conservation and landfill requirement reduction. Otherwise, to the authors’ knowledge, the recycle of fibers from digested deinking sludge to manufacture paper has not been reported. In this study, digested deinking sludge was investigated as a raw material to manufacture corrugating medium paper, mixed with wastepaper pulp and softwood pulp. Deinking sludge was also studied for corrugating medium paper manufacture as a comparison. The ratios of feedstocks were determined based on the sheet properties. This study aimed to reclaim a valuable approach to reusing the secondary fibers in deinking sludge, which not only reduced the environmental pollution, but also developed a highly efficient reutilization of waste fibers, including methane production and corrugating medium paper manufacture.
Materials and methods Experimental material preparation The DS was collected from Guangzhou Pulp & Paper Plant (Guangdong, China). The plant uses waste newspaper as a main raw material to manufacture newsprint. The DS was generated in the process of wastepaper pulping by means of flotation deinking and dewatering (see Fig. 1). The DDS samples were collected from the dewatered digestate of deinking sludge after anaerobic digestion with nitrogen supplement of monosodium glutamate waste liquor. Raw softwood unbleached pulp with a beating degree of 208 SR was imported from the United States, which was obtained from the Kraft pulping process. Raw wastepaper pulp originated from the waste corrugating medium paper and cardboard, of which the beating degree was 208 SR, was provided by a papermaking plant in Dongguan (Guangdong, China). The raw softwood pulp and wastepaper pulp was pretreaed by the following process before used for the downstream handsheet formation. Washing and screening by a Messmer Somerville screen (71-20-00-0002, the Netherlands) were carried out to the raw softwood pulp and wastepaper pulp, respectively, and then disintegrating for each pulp (3 L, 2.5 wt%) at 3000 rpm for 200 s by using a disintegrator (PTI 95568, Australia). After that, beating the softwood pulp and wastepaper pulp by using a PFI mill (Mark V1 PFI, Hamjern Maskin 621, Norway) till the degree of 408 SR and 358 SR was observed, respectively. And then centrifugal dewatering at 1000 rpm, 5 min was implemented to the pulps. Each material was prepared prior to the experiment, and put into nylon ziplock bags storing in a refrigerator (0–4 8C). The main characteristics of the DS and DDS, such as pH values, moisture contents, organics contents and ash contents were analyzed. The chemicals including French chalk (papermaking grade, 800 meshes), cationic polyacrylamide solution (industrial grade, 0.1% (w/v)), aluminum sulfate solution (analytical pure, 10% (w/v)) and liquid rosin solution (industrial grade, 50% (w/v)) were applied for handsheet formation [26,27], and all of them were purchased from Tai’an Qineng Chemical Sci-Tech Co., Ltd (Tai’an, China).
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Fig. 1. Deinking sludge formation in the wastepaper pulping process.
Experimental design In this experiment, the DS and DDS were pulped respectively, and then mixed with the wastepaper pulp in the ratios of 1/7–6/2 to manufacture corrugating medium paper (see Table 1). Because the fiber lengths of DS, DDS and waste paper pulp were all relatively short, in order to reach the quality requirement of corrugating medium paper with the lowest manufacture cost, 20% (based on TS) softwood pulp with the beating degree of 408 SR was added to the above pulp blends for corrugating medium paper production. Seven trials were set up with different ratios of the DS or DDS to the wastepaper pulp (see Table 1), including the control trial with no DS or DDS addition. The sum load rate of the DS (or DDS) and the wastepaper pulp was kept in 80% (based on TS) of the total materials. Triplicates for each trial were implemented. Handsheet formation Corrugating medium papermaking and properties testing were performed in the State Key Laboratory of Pulp and Paper Engineering in the South China University of Technology (Guangdong, China). The whole sheet forming process was shown in Fig. 2. First of all, washing and screening via a Messmer Somerville screen (71-20-00-0002, the Netherlands) were done to the raw DS and DDS, respectively. And then disintegrating at 3000 rpm for 120 s by using a disintegrator (PTI 95568, Australia) was carried out to each pulp (3 L, 2.5 wt%) in order to disperse the fibers/fines completely, before dewatering at 1000 rpm, 5 min by centrifuge was applied. Afterwards, under a designed grammage of 140 g m 2, three feedstocks (softwood pulp, wastepaper pulp and DS or DDS) were loaded into the same disintegrator (PTI 95568, Australia) according to the designed ratios in Table 1, and then 6% (w/w) French chalk, 1.5% (v/w) cationic polyacrylamide solution, 0.4% (v/w) dispersed rosin solution and 1.5% (v/w) aluminum sulfate solution were added to the system respectively, of which the percentage of each chemical was based on the blended pulp’s dry mass [26,27]. The mixture (3 L, 0.3 wt%) was blended at 3000 rpm for 120 s to mix completely and let all reactions go throughout. Then the blended pulp was transferred into a RapidKothen PTI paper-making machine (RK3AKWT, Australia) to form wet sheets with 200 mm diameter. After mechanical pressing, the wet sheets were delivered to a dryer and heated at 90 8C for at least 15 min in order to keep the final moisture content to be in the
range of 8.0 3.0%, and then the target corrugating medium paper was yielded. Besides, after the wet sheets were stored in a container at 23 0.5 8C, 50 1% of humidity for 24 h, the basic physical characteristics, like moisture content, grammage, bulk, ring crush strength, breaking length and roughness were measured and the data were compared to the Chinese standard values in GB/T 13023-2008. Characterization of the samples The main characteristics of the DS and DDS, such as pH values, moisture contents, organics contents and ash contents were analyzed according to the Standard Methods for the Examination of Water and Wastewater [28]. The fiber lengths were measured by a multimedia microscope (Olympus BX51, Japan), according to the method GB/T 22231-2008 by diluting fibers with DI water at a ratio of 1–100 (w/v). The fiber contents were determined with the method ISO 6865:2000. The beating degrees of pulps were determined by using the method ISO 5267:1999. The grammage was measured according to the method ISO 536:1995 using a grammage determinator (YQ – Z-45, China). The thickness was measured using a thickness tester (L & W 250, Sweden) based on the method ISO 534:1988. The bulk could be calculated according to the grammage and thickness with the following formulas, Bulk (g/cm3) = Grammage (g/m2)/Thickness (mm)/1000. The ring crush strength was measured and calculated according to the method ISO/DIS 12192 using a ring crush tester (L & W 248, Sweden). The tensile strength was measured using a tensile strength tester (L & W CE062, Sweden) based on the method ISO 1924-2:1985, from which the breaking length was obtained. The roughness was determined following the method ISO 8791-1:1986 using a roughness tester (L & W CE165, USA). The physical properties of the corrugating medium paper were analyzed according to the Chinese National Standard (GB/T 13023-2008). Results and discussion Characteristics of raw or digested deinking sludge Waste newspaper was used as a main raw material to manufacture newsprint in Guangzhou Pulp & Paper Plant. Therefore the residue from the recycled waste newspaper, including pigment, coating, tiny fibers and most printing ink particles peeled from fibers which originated from the flotation
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Fig. 2. The manufacture process for corrugating medium paper with secondary fibers from deinking sludge (DS) or digested deinking sludge (DDS), blended with softwood unbleached pulp and wastepaper pulp.
deinking process, formed the DS. On the other hand, the DDS was the dehydrated product of DS after about 50 – day anaerobic digestion. The organic components in the DS, such as printing ink particles, coating and part of fibers, were alternatively degraded in the anaerobic digestion process. Compared to the DS, the DDS contained less biodegradable organics but it was more stable and harmless for downstream utilization [29]. The fundamental properties of the DS and DDS were analyzed and shown in Table 2. The pH values of the DS and DDS were both higher than 7.0, which were related to their production processes, and the pH value of DDS was higher than that of DS due to the methanogenesis stage in the anaerobic digestion process. Both pH values of the sludges were in the range of 8.0–9.0, which indicated that they had little negative effect on the durability of drying and dewatering systems [19]. Furthermore, the organics contents of the DS and DDS were both higher than 70%. It was in accordance with the ash contents, of which both were lower than 30%. According to the sludge division by de Alda [19] based on the statistical distribution of ash contents, the DS and DDS both belonged to low-ash (<30%, w/w) sludge which was produced mainly in groundwood and some kraft pulp mills. In some cases, the fiber-length distribution of this type of sludge can be compared with that of some eucalyptus (0.9–1 mm) and pinus (1.9–2.2 mm) pulps. Herein, it could be deduced that secondary fibers were abundant in both above sludges owing to the known data of organics and ash contents. And this hypothesis was also proved by the fiber contents, which were higher than 40% (see Table 2). Higher fiber contents enable the DS and DDS to be reused as papermaking raw materials. Otherwise, compared to the fiber content in the DS, lower fiber content in the DDS was noted due to the fiber degradation in the anaerobic digestion process [30].
fibers were 1.4% and 0.7% respectively. It indicated that fines were the dominant fibers in the DS and DDS. This result was in line with other previous reports [31]. Moreover, the average fiber lengths in the DS and DDS were 0.189 mm and 0.167 mm respectively. It can be inferred that the fibers in the DDS were shortened by biodegradation in the anaerobic digestion process. Meanwhile, the proportion of fines in the DDS increased (Fig. 3). The similar result was reported by Hu [32] as well. The fiber structures in raw or digested deinking sludge The secondary fibers and impure particles in the DS and DDS could be seen clearly by the same multimedia microscope used for fiber length determination with 100 times amplification (Fig. 4(a) and (b)). Compared to the fibers in the DS, the fibers in the DDS were less and shorter, which was in accordance with the results of characteristics and fiber lengths in the DS and DDS. Moreover, the microphotographs of an individual fiber in the DS and DDS were observed by amplifying 1000 times with the multimedia microscope, respectively. The fuzzy surface of the single fiber in the DDS was significantly noted compared to that in the DS (Fig. 4(c) and (d)). It meant that crude fibers in the DDS were further biodegraded in the anaerobic digestion process, which was in line with other research results [30]. And it would affect the sheet strength which was manufactured by adding the DS and DDS. Fortunately, the strength properties of sheets can be improved by beating the sludge fibers. Therefore, although 100% sludge cannot be used to form sheets with as good strength as those from chemical pulps, sludge could still be used as an additive [19].
The distribution of fiber lengths In this study, 150 pieces of fiber in the DS and DDS were measured by a multimedia microscope, respectively. The distribution of fiber lengths was shown in Fig. 3, which was classified as long fibers (more than 1.0 mm), short fibers (between 0.2 and 1.0 mm) and fines (less than 0.2 mm). As shown in Fig. 3, the proportion of fines in the DS and DDS were 76.1% and 87.6% respectively, while short fibers were 21.1% and 15.9%, and long
Table 2 The characteristics of deinking sludge and digested deinking sludge.
pH value Moisture content (%) Organics (%, TS) Ash (%, TS) Fiber (%, TS)
Deinking sludge
Digested deinking sludge
7.99 0.06 67.18 2.86 77.56 2.00 22.44 2.00 43.26 0.11
8.59 0.01 59.27 1.70 70.44 1.48 29.56 1.48 41.87 0.66
Fig. 3. The distribution of fiber lengths in deinking sludge and digested deinking sludge, respectively.
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fibers and the great porosity in sheets. Similar results were also reported [33,34]. Moreover, in the Chinese standard of corrugating medium paper (GB/T 13023-2008), the bulk of qualified product is required to be more than 0.45 g cm 3. In this study, the bulk of the corrugating medium paper manufactured with all loads of secondary fibers from the DS and DDS could meet the standard requirement. Even though the adding proportion of fibers from the DS or DDS reached 60%, the bulk of formed corrugating medium paper was about 112.2% higher than that of the qualified product based on the Chinese standard. It indicated that replacing wastepaper with DS or DDS as specific papermaking materials could assure the bulk requirement for qualified sheets with a lower cost. Regarding the corrugating medium paper made with the same secondary fiber loads from the DS or DDS, no significant variation of bulk was noted. It could be further predicted that anaerobic digestion proceeded a higher recycling of deinking sludge by biogas and papermaking material production.
Fig. 4. Microphotographs of fibers in deinking sludge and digested deinking sludge respectively, recorded by the multimedia microscope.
The properties of corrugating medium paper The bulk It could be observed in Fig. 5(a) that as the dosage of DS or DDS was increased, the bulk of corrugating medium paper tended to decrease. When the load of secondary fibers from DS or DDS was enhanced to 60%, the bulk of corrugating medium paper had both declined by around 13%, compared to the control treatment without secondary fibers addition. The main reason was that the disintegration effect of secondary fibers in the DS and DDS was worse than that in the wastepaper due to the different fiber characteristics, which resulted in the weak physical bonding of
The ring crush strength As shown in Fig. 5(b), with the increased secondary fiber loads from the DS or DDS, the ring crush strength of corrugating medium paper tended to drop down. Compared to the control treatment, when the load of secondary fibers from the DS or DDS was increased to 60%, the ring crush strength of corrugating medium paper decreased by 31.88% and 30.63%, respectively. It is for the reason that the ring crush strength is determined by the strength, length and cohesion of interfibers [33,35]. Compared to the fibers in the wastepaper, the fiber physical properties of the raw or digested deinking sludge were poorer. It resulted in the decline of the ring crush strength of the corrugating medium paper manufactured with the sludge addition. Besides, the average ring crush strength of the corrugating medium paper made with the DDS application was 8.51 0.31, which was 3.3% in average lower than that with the DS addition. It was in line with the result that fibers
Fig. 5. The properties of corrugating medium paper manufactured with different sources and loads of secondary fibers (a-Bulk, b-Ring crush strength, c-Breaking length, dRoughness).
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(e.g. cellulose, hemicellulose and lignin) in feedstocks were partly biodegraded by anaerobic digestion and their strengths were also weakened [36]. Nonetheless, in the Chinese standard of corrugating medium paper (GB/T 13023-2008), the ring crush strength of qualified sheets is required to be over 4.4 N m g 1, if the grammage of corrugating medium paper is equal or more than 140 g m 2. As shown in Fig. 5(b), the ring crush strength of the corrugating medium paper manufactured with the secondary fibers from the DS or DDS could all reach the standard requirement. Even if the load of secondary fibers from the DS or DDS was 60%, the ring crush strength was about 60.2% higher than that of the qualified product. It declared that the DS or DDS playing a role as supplemented papermaking materials could guarantee the ring crush strength for qualified sheets. The breaking length The breaking length of corrugating medium paper dropped off with the incremental load of secondary fibers from the DS or DDS (Fig. 5(c)). Compared to the control treatment, the breaking length of the corrugating medium paper manufactured with adding the DS or DDS had decreased by 30.63% and 35.00%, respectively. It might be owing to the weak connection of fibers in the DS and DDS, since higher breaking strength depends on the greater bonding capability for the interfibers [35]. Herein, as the dosage of the DS or DDS increased, the breaking length of corrugating medium paper reduced. As mentioned in the Chinese standard (GB/T 13023-2008), the breaking length of qualified sheets is required to be over 2.5 km. In Fig. 5(c), the breaking length of the corrugating medium paper manufactured by adding 30% or less secondary fibers from the DDS could meet the requirement for qualified products. Otherwise, 40% or less secondary fibers from the DS could reach the standard. It declared that anaerobic digestion further weakened the combination of fibers in deinking sludge. Nevertheless, in the pulp and paper making process, the shortage of interfiber connection could be fixed alternatively by increasing pulp beating degrees which could improve the fiber binding for higher breaking lengths [37]. The roughness Being opposite to the above indexes, the roughness of corrugating medium paper enhanced slowly with the increased DS dosage (Fig. 5(d)). If 60% of secondary fibers from the DS was added to the papermaking system, the roughness of corrugating medium paper would increase by 6.38% compared to the control treatment. It might be due to the different fiber surface structure and length distribution in the DS and the wastepaper. Otherwise, the roughness of the corrugating medium paper manufactured with the DDS addition firstly decreased by 1.71% at the secondary fiber load of 10% and then gradually increased till 4.36% increment of roughness was observed when the secondary fiber load was 60%. Compared to the DS and wastepaper, more tiny particles and shorter fines were contained in the DDS. Fatehi et al. [38] reported that the increased fine content in pulp would result in a lower average fiber length and leading to the decrease of sheet coarseness. Herein, the roughness of corrugating medium paper dropped down with the DDS addition at the secondary fiber loads of 10–20%. Nevertheless, the roughness of the corrugating medium paper manufactured by adding over 30% secondary fibers from the DDS increased with the enhanced DDS dosage. It could be predicted that the rougher fibers in DDS defined the sheet courseness rather than the fines, when the secondary fiber loads were higher than 30%. Besides, the roughness of corrugating medium paper for all trials wasn’t taken into comparison to that in the Chinese national standard because it is not requested in the standard.
The proportionate DS or DDS addition for qualified corrugating medium paper manufacture Overall, the addition of sludge (DS or DDS) as a raw papermaking material negatively affected the properties of laboratory sheets. Based on the physical properties of the corrugating medium paper (see Fig. 5(a)–(d)) and the cost minimization with the sludge recycling as much as it could be, the best load of secondary fibers from the DS to manufacture corrugating medium paper was 40%, mixed with 40% wastepaper and 20% softwood pulp. With this raw material recipe, the properties of qualified corrugating medium paper were as follows, grammage 137.64 (1.79) g m 2, moisture content 7.45 (0.42)%, bulk 0.52 (0.01) g cm 3, ring crush strength 8.73 (0.32) N m g 1, breaking length 2.60 (0.11) km and roughness 10.70 (0.09) mm. As regards to the DDS addition for corrugating medium papermaking, the best load of secondary fibers from the DDS was 30%, blended with 50% wastepaper and 20% softwood pulp. With this raw material recipe, the properties of qualified corrugating medium paper were as follows, grammage 142.44 (1.09) g m 2, moisture content 8.35 (0.12)%, bulk 0.53 (0.01) g cm 3, ring crush strength 8.99 (0.31) N m g 1, breaking length 2.60 (0.12) km and roughness 10.32 (0.16) mm. According to the physical properties of the corrugating medium paper made from the DS or DDS addition, higher ring crush strength and breaking length were observed by adding the DS, but lower roughness was noted with the DDS addition at the same secondary fiber load. Overall, reusing DDS to manufacture corrugating medium paper provides a new valuable approach to recycling DS digestate. Conclusion This research concentrated on the feasibility of reusing DDS to make corrugating medium paper, compared to the sheets obtained from DS addition. It claimed that the qualified corrugating medium paper could be obtained when the load of secondary fibers from the DDS was equal or less than 30%. A new valuable approach was provided to conversing DS into high-value by-products, like biogas and a specific papermaking material, as well as reducing the secondary environmental pollution caused by DS landfill, incineration, and so on. It can also promote the energy-saving and emission-reduction in the pulp and paper industry, which deserves further pilot-scale experiments for the future application. Acknowledgements The authors would like to thank the Fund for Pear River Talents of Science and Technology in Guangzhou, China (No: 2012J2200082) and the Specialized Research Fund for the Doctoral Program of Higher Education in China (No: 20114404120021) for financially supporting this research. References [1] Q.W. Chen, Introduction to Modern pulping technology of wastepaper, Light Industry Press, Beijing (CN), 2005p. 1. [2] B. Liu, Y. Chao, Recycled Fibers and Deinking Technology of Wastepaper, Chemical Industry Press of China, Beijing (CN), 2010p. 404. [3] A. Blanco, C. Negro, E. Fuente, L.M. Sanchez, Cell. Chem. Technol. 42 (2008) 89. [4] S.P. Raut, R. Sedmake, S. Dhunde, R.V. Ralegaonkar, S.A. Mandavgane, Constr. Build. Mater. 27 (2011) 247. [5] M. Likon, J. Saarela, Macromol. Symp. 320 (2012) 50. [6] S. Tandy, J.C. Williamson, M.A. Nason, J.R. Healey, D.L. Jones, Soil Use Manage. 24 (2008) 217. [7] M.C. Monte, E. Fuente, A. Blanco, C. Negro, Waste Manage. 29 (2009) 293. [8] N. Voc´a, T. Kricˇka, T. C´osic´, V. Rupic´, Zˇ. Jukic´, S. Kalambura, Plant Soil Environ. 51 (2005) 262. [9] J.T. Novak, M.E. Sadler, S.N. Murthy, Water Res. 37 (2003) 3136.
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JIEC-2873; No. of Pages 7 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Production of corrugating medium paper with secondary fibers from digested deinking sludge Dexing Yin a,b, Yunqin Lin a,*, Zhihuan Chen a, Jialiang Qiao c, Min Xiao a, Dehan Wang a a
College of Natural Resources and Environment, South China Agricultural University, Guangzhou 510642, China Beijing CIIT Environmental Co., Ltd., Beijing 100016, China c College of Food Science, South China Agricultural University, Guangzhou 510642, China b
A R T I C L E I N F O
Article history: Received 27 August 2015 Received in revised form 30 January 2016 Accepted 14 March 2016 Available online xxx Keywords: Corrugating medium paper Secondary fibers Deinking sludge Anaerobic digestion Fiber loads
A B S T R A C T
This work focused on developing an approach to reusing digested deinking sludge for corrugating medium paper manufacture by recycling secondary fibers. The fiber content in the digested deinking sludge (DDS) was around 41.87%, and the main proportion of fibers (87.6%) was subject to secondary fines with lengths less than 0.2 mm. The qualified corrugating medium paper was obtained when the secondary fiber load from DDS was equal or less than 30%. Corrugating medium paper manufactured by adding DDS could maximize reuse and recycling of deinking sludge via methane and papermaking material production, and reduce the environmental pollution as well. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction The pulp and paper industry, which is in rapid development nowadays, has become one of the main industries in China. Simultaneously, waste water and sludge are produced in large quantities in the pulping and papermaking processes. Deinking sludge (DS), which is formed in the process of wastepaper pulping by means of flotation deinking, dominates over 50% of the total sludge yield in the pulp and paper industry and it will reach 70% in the following years because more and more wastepaper is reused as a papermaking raw material [1]. Generally, 80–150 kg dry deinking sludge or 160–500 kg wet deinking sludge is generated when one ton dry pulp is produced [2]. How to appropriately treat the huge amounts of deinking sludge has become a vital issue to be addressed for pulping and papermaking plants. In China, randomly discarding sludge is banned owing to the serious secondary pollution. The representatives of traditional conversion technologies for pulp & paper mill sludge (including deinking sludge and sewage sludge) are incineration and composting, even though some researchers have been trying to look for some different ways for pulp & paper mill sludge reusage, such as cement and brick manufacture, specific sorbent production
* Corresponding author. Tel.: +86 020 85280296; fax: +86 020 85287672. E-mail address: [email protected] (Y. Lin).
and agriculture land application [3–6]. Nevertheless, evident shortcomings of applying both above main technologies have been found. Regarding incineration, the difficulty to dehydrate, the low efficiency of energy generation, the trouble to cleanse the toxic exhausted gas and the overall high investment hinder its application [7]. As regards to composting, it is a widely used bio-conversion technology for sludge. Currently, some plants have been set up and they focused on the pulp & paper mill sludge reutilization by composting in China [8]. However, low nitrogen contents, long time for lignocellulose decomposing, lower contents of humic acid in compost and a high risk of heavy metal pollution existing in compost land application have become the main problems to stagnate the development of sludge composting [9– 11]. Thus, investigating new technologies to reuse pulp & paper mill sludge, especially the deinking sludge which yield is growing with the increasing recycle of wastepaper, is significantly important. As mentioned by many authors, the deinking sludge is rich in polysaccharides, cellulose, hemicellulose, lignin and other inorganic components [12]. On the other hand, anaerobic digestion has been highly concerned in the world as a renewable energy production technology for solid waste from various sources, like agricultural residues, municipal solid waste, green trimmings and sludge. Anaerobic digestion of pulp & paper mill sludge (including deinking sludge and sewage sludge) for methane production was also reported currently. Lin et al. [13,14] studied the possibility of
http://dx.doi.org/10.1016/j.jiec.2016.03.026 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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anaerobic co-digestion of pulp & paper sewage sludge and monosodium glutamate waste liquor and found the accumulative methane yield could reach 88.82 mL g 1 VS (volatile solid). Some other authors found that by controlling pH, hydraulic retention time and temperature, it was possible to optimize the acidogenic and acetogenic phases in the sludge anaerobic digestion process and took good advantages of the soluble organics produced in the hydrolysis stage [15,16]. Besides, Ratnieks and Gaylarde [17] investigated the effect of systemic pH on anaerobic digestion of paper mill sludge, and found that when the systemic pH was 7, the sludge degradation efficiency could be increased to 60%. Domestic and foreign research results indicate that it is feasible to use pulp & paper mill sludge to generate methane for heat or electricity recovery. In addition, papermaking is subject to the process with high consumption of energy. The overall consumption is around 0.98 ton standard coal for 1 ton dry paper production. Anaerobic digestion of pulp & paper mill sludge could develop an approach to reducing coal consumption in papermaking plants by methane generation, so as to meet the requirement of market globalization competition. On the other hand, a large amount of anaerobic digestate is produced after anaerobic digestion of sludge, and solid–liquid separation is generally operated to the digestate. Regularly, the liquid part can be recycled to fields as NPK fertilizer [18], and the solid residue of digestate is disposed by landfill or composting. Otherwise, papermaking is an industry leader from the standpoint of recycling and sustainability, the main reason being that virgin and recycled fibers can be used together, sometimes contributing with complementary characteristics. Since the solid phase of digested deinking sludge (DDS) contains most of the secondary fibers/fines which reached 41.87% based on the total solid (TS) (see Table 1), and less tannins, resins and other pigments compared to the original deinking sludge, it might be another potential material to manufacture specific papers. What’s more, abundant active hydroxyls are contained in the tiny fibers/fines which are the principal organic components of the digested deinking sludge. The possible reason for the digested sludge playing an important role in the series of esterification, etherification, graft copolymerization, cross-linking and other derivative reactions are related to the hydroxyls [19]. Besides, non-woods are used extensively in pulp and paper production in several countries due to its abundance and costeffectiveness, and the scarcity of forest resources especially in Asian countries. Many studies have been carried out on non-wood pulping for pulp and paper production [19–21]. Since China is severely short of woods for pulp and paper making, over 80% of woody papermaking materials in China depend on importation. Looking for new sources of raw materials for papermaking in domestic is challenging. Meanwhile it has become an urgent task for relevant researchers. Nowadays, China is the biggest non-wood producer in the world. Straw has become the largest source of nonwoods for the paper industry. Like the traditional non-woods of
Table 1 Secondary fiber sources and loads for the corrugating medium paper manufacture. Treatment no.
Softwood pulp (%, TS)a
Wastepaper pulp (%, TS)
DS or DDSb (%, TS)
Ctrl. 1 2 3 4 5 6
20 20 20 20 20 20 20
80 70 60 50 40 30 20
0 10 20 30 40 50 60
a b
Based on dry weight. DS and DDS were added as secondary fiber sources, respectively.
straws, reeds, bamboo and so on, deinking sludge has already been tested as a specific papermaking raw material [22]. Furthermore, it is a recommended wastewater reduction method by recycling of wastewater with simultaneous recovery of fibers [23]. The most common technique for reclaiming fiber is to recycle primary sludge back into the fiber – processing system. Some recycled paperboard mills and some manufacturers of unbleached and bleached pulp and paper have reduced sludge volume by reclaiming the fiber, fillers, or both in sludge to be reused within their pulping and papermaking processes [24,25]. Fiber recycling has played an important role in the sustainable development of pulp and paper industry with the benefits of environmental pollution decrease, leading to an extension of fiber life cycle, forest conservation and landfill requirement reduction. Otherwise, to the authors’ knowledge, the recycle of fibers from digested deinking sludge to manufacture paper has not been reported. In this study, digested deinking sludge was investigated as a raw material to manufacture corrugating medium paper, mixed with wastepaper pulp and softwood pulp. Deinking sludge was also studied for corrugating medium paper manufacture as a comparison. The ratios of feedstocks were determined based on the sheet properties. This study aimed to reclaim a valuable approach to reusing the secondary fibers in deinking sludge, which not only reduced the environmental pollution, but also developed a highly efficient reutilization of waste fibers, including methane production and corrugating medium paper manufacture.
Materials and methods Experimental material preparation The DS was collected from Guangzhou Pulp & Paper Plant (Guangdong, China). The plant uses waste newspaper as a main raw material to manufacture newsprint. The DS was generated in the process of wastepaper pulping by means of flotation deinking and dewatering (see Fig. 1). The DDS samples were collected from the dewatered digestate of deinking sludge after anaerobic digestion with nitrogen supplement of monosodium glutamate waste liquor. Raw softwood unbleached pulp with a beating degree of 208 SR was imported from the United States, which was obtained from the Kraft pulping process. Raw wastepaper pulp originated from the waste corrugating medium paper and cardboard, of which the beating degree was 208 SR, was provided by a papermaking plant in Dongguan (Guangdong, China). The raw softwood pulp and wastepaper pulp was pretreaed by the following process before used for the downstream handsheet formation. Washing and screening by a Messmer Somerville screen (71-20-00-0002, the Netherlands) were carried out to the raw softwood pulp and wastepaper pulp, respectively, and then disintegrating for each pulp (3 L, 2.5 wt%) at 3000 rpm for 200 s by using a disintegrator (PTI 95568, Australia). After that, beating the softwood pulp and wastepaper pulp by using a PFI mill (Mark V1 PFI, Hamjern Maskin 621, Norway) till the degree of 408 SR and 358 SR was observed, respectively. And then centrifugal dewatering at 1000 rpm, 5 min was implemented to the pulps. Each material was prepared prior to the experiment, and put into nylon ziplock bags storing in a refrigerator (0–4 8C). The main characteristics of the DS and DDS, such as pH values, moisture contents, organics contents and ash contents were analyzed. The chemicals including French chalk (papermaking grade, 800 meshes), cationic polyacrylamide solution (industrial grade, 0.1% (w/v)), aluminum sulfate solution (analytical pure, 10% (w/v)) and liquid rosin solution (industrial grade, 50% (w/v)) were applied for handsheet formation [26,27], and all of them were purchased from Tai’an Qineng Chemical Sci-Tech Co., Ltd (Tai’an, China).
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Fig. 1. Deinking sludge formation in the wastepaper pulping process.
Experimental design In this experiment, the DS and DDS were pulped respectively, and then mixed with the wastepaper pulp in the ratios of 1/7–6/2 to manufacture corrugating medium paper (see Table 1). Because the fiber lengths of DS, DDS and waste paper pulp were all relatively short, in order to reach the quality requirement of corrugating medium paper with the lowest manufacture cost, 20% (based on TS) softwood pulp with the beating degree of 408 SR was added to the above pulp blends for corrugating medium paper production. Seven trials were set up with different ratios of the DS or DDS to the wastepaper pulp (see Table 1), including the control trial with no DS or DDS addition. The sum load rate of the DS (or DDS) and the wastepaper pulp was kept in 80% (based on TS) of the total materials. Triplicates for each trial were implemented. Handsheet formation Corrugating medium papermaking and properties testing were performed in the State Key Laboratory of Pulp and Paper Engineering in the South China University of Technology (Guangdong, China). The whole sheet forming process was shown in Fig. 2. First of all, washing and screening via a Messmer Somerville screen (71-20-00-0002, the Netherlands) were done to the raw DS and DDS, respectively. And then disintegrating at 3000 rpm for 120 s by using a disintegrator (PTI 95568, Australia) was carried out to each pulp (3 L, 2.5 wt%) in order to disperse the fibers/fines completely, before dewatering at 1000 rpm, 5 min by centrifuge was applied. Afterwards, under a designed grammage of 140 g m 2, three feedstocks (softwood pulp, wastepaper pulp and DS or DDS) were loaded into the same disintegrator (PTI 95568, Australia) according to the designed ratios in Table 1, and then 6% (w/w) French chalk, 1.5% (v/w) cationic polyacrylamide solution, 0.4% (v/w) dispersed rosin solution and 1.5% (v/w) aluminum sulfate solution were added to the system respectively, of which the percentage of each chemical was based on the blended pulp’s dry mass [26,27]. The mixture (3 L, 0.3 wt%) was blended at 3000 rpm for 120 s to mix completely and let all reactions go throughout. Then the blended pulp was transferred into a RapidKothen PTI paper-making machine (RK3AKWT, Australia) to form wet sheets with 200 mm diameter. After mechanical pressing, the wet sheets were delivered to a dryer and heated at 90 8C for at least 15 min in order to keep the final moisture content to be in the
range of 8.0 3.0%, and then the target corrugating medium paper was yielded. Besides, after the wet sheets were stored in a container at 23 0.5 8C, 50 1% of humidity for 24 h, the basic physical characteristics, like moisture content, grammage, bulk, ring crush strength, breaking length and roughness were measured and the data were compared to the Chinese standard values in GB/T 13023-2008. Characterization of the samples The main characteristics of the DS and DDS, such as pH values, moisture contents, organics contents and ash contents were analyzed according to the Standard Methods for the Examination of Water and Wastewater [28]. The fiber lengths were measured by a multimedia microscope (Olympus BX51, Japan), according to the method GB/T 22231-2008 by diluting fibers with DI water at a ratio of 1–100 (w/v). The fiber contents were determined with the method ISO 6865:2000. The beating degrees of pulps were determined by using the method ISO 5267:1999. The grammage was measured according to the method ISO 536:1995 using a grammage determinator (YQ – Z-45, China). The thickness was measured using a thickness tester (L & W 250, Sweden) based on the method ISO 534:1988. The bulk could be calculated according to the grammage and thickness with the following formulas, Bulk (g/cm3) = Grammage (g/m2)/Thickness (mm)/1000. The ring crush strength was measured and calculated according to the method ISO/DIS 12192 using a ring crush tester (L & W 248, Sweden). The tensile strength was measured using a tensile strength tester (L & W CE062, Sweden) based on the method ISO 1924-2:1985, from which the breaking length was obtained. The roughness was determined following the method ISO 8791-1:1986 using a roughness tester (L & W CE165, USA). The physical properties of the corrugating medium paper were analyzed according to the Chinese National Standard (GB/T 13023-2008). Results and discussion Characteristics of raw or digested deinking sludge Waste newspaper was used as a main raw material to manufacture newsprint in Guangzhou Pulp & Paper Plant. Therefore the residue from the recycled waste newspaper, including pigment, coating, tiny fibers and most printing ink particles peeled from fibers which originated from the flotation
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Fig. 2. The manufacture process for corrugating medium paper with secondary fibers from deinking sludge (DS) or digested deinking sludge (DDS), blended with softwood unbleached pulp and wastepaper pulp.
deinking process, formed the DS. On the other hand, the DDS was the dehydrated product of DS after about 50 – day anaerobic digestion. The organic components in the DS, such as printing ink particles, coating and part of fibers, were alternatively degraded in the anaerobic digestion process. Compared to the DS, the DDS contained less biodegradable organics but it was more stable and harmless for downstream utilization [29]. The fundamental properties of the DS and DDS were analyzed and shown in Table 2. The pH values of the DS and DDS were both higher than 7.0, which were related to their production processes, and the pH value of DDS was higher than that of DS due to the methanogenesis stage in the anaerobic digestion process. Both pH values of the sludges were in the range of 8.0–9.0, which indicated that they had little negative effect on the durability of drying and dewatering systems [19]. Furthermore, the organics contents of the DS and DDS were both higher than 70%. It was in accordance with the ash contents, of which both were lower than 30%. According to the sludge division by de Alda [19] based on the statistical distribution of ash contents, the DS and DDS both belonged to low-ash (<30%, w/w) sludge which was produced mainly in groundwood and some kraft pulp mills. In some cases, the fiber-length distribution of this type of sludge can be compared with that of some eucalyptus (0.9–1 mm) and pinus (1.9–2.2 mm) pulps. Herein, it could be deduced that secondary fibers were abundant in both above sludges owing to the known data of organics and ash contents. And this hypothesis was also proved by the fiber contents, which were higher than 40% (see Table 2). Higher fiber contents enable the DS and DDS to be reused as papermaking raw materials. Otherwise, compared to the fiber content in the DS, lower fiber content in the DDS was noted due to the fiber degradation in the anaerobic digestion process [30].
fibers were 1.4% and 0.7% respectively. It indicated that fines were the dominant fibers in the DS and DDS. This result was in line with other previous reports [31]. Moreover, the average fiber lengths in the DS and DDS were 0.189 mm and 0.167 mm respectively. It can be inferred that the fibers in the DDS were shortened by biodegradation in the anaerobic digestion process. Meanwhile, the proportion of fines in the DDS increased (Fig. 3). The similar result was reported by Hu [32] as well. The fiber structures in raw or digested deinking sludge The secondary fibers and impure particles in the DS and DDS could be seen clearly by the same multimedia microscope used for fiber length determination with 100 times amplification (Fig. 4(a) and (b)). Compared to the fibers in the DS, the fibers in the DDS were less and shorter, which was in accordance with the results of characteristics and fiber lengths in the DS and DDS. Moreover, the microphotographs of an individual fiber in the DS and DDS were observed by amplifying 1000 times with the multimedia microscope, respectively. The fuzzy surface of the single fiber in the DDS was significantly noted compared to that in the DS (Fig. 4(c) and (d)). It meant that crude fibers in the DDS were further biodegraded in the anaerobic digestion process, which was in line with other research results [30]. And it would affect the sheet strength which was manufactured by adding the DS and DDS. Fortunately, the strength properties of sheets can be improved by beating the sludge fibers. Therefore, although 100% sludge cannot be used to form sheets with as good strength as those from chemical pulps, sludge could still be used as an additive [19].
The distribution of fiber lengths In this study, 150 pieces of fiber in the DS and DDS were measured by a multimedia microscope, respectively. The distribution of fiber lengths was shown in Fig. 3, which was classified as long fibers (more than 1.0 mm), short fibers (between 0.2 and 1.0 mm) and fines (less than 0.2 mm). As shown in Fig. 3, the proportion of fines in the DS and DDS were 76.1% and 87.6% respectively, while short fibers were 21.1% and 15.9%, and long
Table 2 The characteristics of deinking sludge and digested deinking sludge.
pH value Moisture content (%) Organics (%, TS) Ash (%, TS) Fiber (%, TS)
Deinking sludge
Digested deinking sludge
7.99 0.06 67.18 2.86 77.56 2.00 22.44 2.00 43.26 0.11
8.59 0.01 59.27 1.70 70.44 1.48 29.56 1.48 41.87 0.66
Fig. 3. The distribution of fiber lengths in deinking sludge and digested deinking sludge, respectively.
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fibers and the great porosity in sheets. Similar results were also reported [33,34]. Moreover, in the Chinese standard of corrugating medium paper (GB/T 13023-2008), the bulk of qualified product is required to be more than 0.45 g cm 3. In this study, the bulk of the corrugating medium paper manufactured with all loads of secondary fibers from the DS and DDS could meet the standard requirement. Even though the adding proportion of fibers from the DS or DDS reached 60%, the bulk of formed corrugating medium paper was about 112.2% higher than that of the qualified product based on the Chinese standard. It indicated that replacing wastepaper with DS or DDS as specific papermaking materials could assure the bulk requirement for qualified sheets with a lower cost. Regarding the corrugating medium paper made with the same secondary fiber loads from the DS or DDS, no significant variation of bulk was noted. It could be further predicted that anaerobic digestion proceeded a higher recycling of deinking sludge by biogas and papermaking material production.
Fig. 4. Microphotographs of fibers in deinking sludge and digested deinking sludge respectively, recorded by the multimedia microscope.
The properties of corrugating medium paper The bulk It could be observed in Fig. 5(a) that as the dosage of DS or DDS was increased, the bulk of corrugating medium paper tended to decrease. When the load of secondary fibers from DS or DDS was enhanced to 60%, the bulk of corrugating medium paper had both declined by around 13%, compared to the control treatment without secondary fibers addition. The main reason was that the disintegration effect of secondary fibers in the DS and DDS was worse than that in the wastepaper due to the different fiber characteristics, which resulted in the weak physical bonding of
The ring crush strength As shown in Fig. 5(b), with the increased secondary fiber loads from the DS or DDS, the ring crush strength of corrugating medium paper tended to drop down. Compared to the control treatment, when the load of secondary fibers from the DS or DDS was increased to 60%, the ring crush strength of corrugating medium paper decreased by 31.88% and 30.63%, respectively. It is for the reason that the ring crush strength is determined by the strength, length and cohesion of interfibers [33,35]. Compared to the fibers in the wastepaper, the fiber physical properties of the raw or digested deinking sludge were poorer. It resulted in the decline of the ring crush strength of the corrugating medium paper manufactured with the sludge addition. Besides, the average ring crush strength of the corrugating medium paper made with the DDS application was 8.51 0.31, which was 3.3% in average lower than that with the DS addition. It was in line with the result that fibers
Fig. 5. The properties of corrugating medium paper manufactured with different sources and loads of secondary fibers (a-Bulk, b-Ring crush strength, c-Breaking length, dRoughness).
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(e.g. cellulose, hemicellulose and lignin) in feedstocks were partly biodegraded by anaerobic digestion and their strengths were also weakened [36]. Nonetheless, in the Chinese standard of corrugating medium paper (GB/T 13023-2008), the ring crush strength of qualified sheets is required to be over 4.4 N m g 1, if the grammage of corrugating medium paper is equal or more than 140 g m 2. As shown in Fig. 5(b), the ring crush strength of the corrugating medium paper manufactured with the secondary fibers from the DS or DDS could all reach the standard requirement. Even if the load of secondary fibers from the DS or DDS was 60%, the ring crush strength was about 60.2% higher than that of the qualified product. It declared that the DS or DDS playing a role as supplemented papermaking materials could guarantee the ring crush strength for qualified sheets. The breaking length The breaking length of corrugating medium paper dropped off with the incremental load of secondary fibers from the DS or DDS (Fig. 5(c)). Compared to the control treatment, the breaking length of the corrugating medium paper manufactured with adding the DS or DDS had decreased by 30.63% and 35.00%, respectively. It might be owing to the weak connection of fibers in the DS and DDS, since higher breaking strength depends on the greater bonding capability for the interfibers [35]. Herein, as the dosage of the DS or DDS increased, the breaking length of corrugating medium paper reduced. As mentioned in the Chinese standard (GB/T 13023-2008), the breaking length of qualified sheets is required to be over 2.5 km. In Fig. 5(c), the breaking length of the corrugating medium paper manufactured by adding 30% or less secondary fibers from the DDS could meet the requirement for qualified products. Otherwise, 40% or less secondary fibers from the DS could reach the standard. It declared that anaerobic digestion further weakened the combination of fibers in deinking sludge. Nevertheless, in the pulp and paper making process, the shortage of interfiber connection could be fixed alternatively by increasing pulp beating degrees which could improve the fiber binding for higher breaking lengths [37]. The roughness Being opposite to the above indexes, the roughness of corrugating medium paper enhanced slowly with the increased DS dosage (Fig. 5(d)). If 60% of secondary fibers from the DS was added to the papermaking system, the roughness of corrugating medium paper would increase by 6.38% compared to the control treatment. It might be due to the different fiber surface structure and length distribution in the DS and the wastepaper. Otherwise, the roughness of the corrugating medium paper manufactured with the DDS addition firstly decreased by 1.71% at the secondary fiber load of 10% and then gradually increased till 4.36% increment of roughness was observed when the secondary fiber load was 60%. Compared to the DS and wastepaper, more tiny particles and shorter fines were contained in the DDS. Fatehi et al. [38] reported that the increased fine content in pulp would result in a lower average fiber length and leading to the decrease of sheet coarseness. Herein, the roughness of corrugating medium paper dropped down with the DDS addition at the secondary fiber loads of 10–20%. Nevertheless, the roughness of the corrugating medium paper manufactured by adding over 30% secondary fibers from the DDS increased with the enhanced DDS dosage. It could be predicted that the rougher fibers in DDS defined the sheet courseness rather than the fines, when the secondary fiber loads were higher than 30%. Besides, the roughness of corrugating medium paper for all trials wasn’t taken into comparison to that in the Chinese national standard because it is not requested in the standard.
The proportionate DS or DDS addition for qualified corrugating medium paper manufacture Overall, the addition of sludge (DS or DDS) as a raw papermaking material negatively affected the properties of laboratory sheets. Based on the physical properties of the corrugating medium paper (see Fig. 5(a)–(d)) and the cost minimization with the sludge recycling as much as it could be, the best load of secondary fibers from the DS to manufacture corrugating medium paper was 40%, mixed with 40% wastepaper and 20% softwood pulp. With this raw material recipe, the properties of qualified corrugating medium paper were as follows, grammage 137.64 (1.79) g m 2, moisture content 7.45 (0.42)%, bulk 0.52 (0.01) g cm 3, ring crush strength 8.73 (0.32) N m g 1, breaking length 2.60 (0.11) km and roughness 10.70 (0.09) mm. As regards to the DDS addition for corrugating medium papermaking, the best load of secondary fibers from the DDS was 30%, blended with 50% wastepaper and 20% softwood pulp. With this raw material recipe, the properties of qualified corrugating medium paper were as follows, grammage 142.44 (1.09) g m 2, moisture content 8.35 (0.12)%, bulk 0.53 (0.01) g cm 3, ring crush strength 8.99 (0.31) N m g 1, breaking length 2.60 (0.12) km and roughness 10.32 (0.16) mm. According to the physical properties of the corrugating medium paper made from the DS or DDS addition, higher ring crush strength and breaking length were observed by adding the DS, but lower roughness was noted with the DDS addition at the same secondary fiber load. Overall, reusing DDS to manufacture corrugating medium paper provides a new valuable approach to recycling DS digestate. Conclusion This research concentrated on the feasibility of reusing DDS to make corrugating medium paper, compared to the sheets obtained from DS addition. It claimed that the qualified corrugating medium paper could be obtained when the load of secondary fibers from the DDS was equal or less than 30%. A new valuable approach was provided to conversing DS into high-value by-products, like biogas and a specific papermaking material, as well as reducing the secondary environmental pollution caused by DS landfill, incineration, and so on. It can also promote the energy-saving and emission-reduction in the pulp and paper industry, which deserves further pilot-scale experiments for the future application. Acknowledgements The authors would like to thank the Fund for Pear River Talents of Science and Technology in Guangzhou, China (No: 2012J2200082) and the Specialized Research Fund for the Doctoral Program of Higher Education in China (No: 20114404120021) for financially supporting this research. References [1] Q.W. Chen, Introduction to Modern pulping technology of wastepaper, Light Industry Press, Beijing (CN), 2005p. 1. [2] B. Liu, Y. Chao, Recycled Fibers and Deinking Technology of Wastepaper, Chemical Industry Press of China, Beijing (CN), 2010p. 404. [3] A. Blanco, C. Negro, E. Fuente, L.M. Sanchez, Cell. Chem. Technol. 42 (2008) 89. [4] S.P. Raut, R. Sedmake, S. Dhunde, R.V. Ralegaonkar, S.A. Mandavgane, Constr. Build. Mater. 27 (2011) 247. [5] M. Likon, J. Saarela, Macromol. Symp. 320 (2012) 50. [6] S. Tandy, J.C. Williamson, M.A. Nason, J.R. Healey, D.L. Jones, Soil Use Manage. 24 (2008) 217. [7] M.C. Monte, E. Fuente, A. Blanco, C. Negro, Waste Manage. 29 (2009) 293. [8] N. Voc´a, T. Kricˇka, T. C´osic´, V. Rupic´, Zˇ. Jukic´, S. Kalambura, Plant Soil Environ. 51 (2005) 262. [9] J.T. Novak, M.E. Sadler, S.N. Murthy, Water Res. 37 (2003) 3136.
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Please cite this article in press as: D. Yin, et al., J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.03.026