Biomechanical Pulping of Loblolly Pine Chips with Selected White-Rot Fungi l

Transcription

1 36 M. Akhtar et al.: Biomechanical Pulping of Loblolly Pine Chips Holzforschung 47 (1993) Biomechanical Pulping of Loblolly Pine Chips with Selected White-Rot Fungi l By Masood Akhtar 2, Michael C. Attridge 2, Gary C. Myers 3, and Robert A. Blanchette 4 2 University of Wisconsin, Biotechnology Center, Madison, WI 53705, U.S.A. 3 USDA Forest Products Laboratory, Madison, WI 53705, U.S.A. 4 Dept. of Plant Pathology, University of Minnesota, St. Paul, MN 55108, U.S.A. Keywords Biomechanical pulping Biopulping Bioreactor Energy savings Paper strength properties Paper optical properties Loblolly pine (Pinus taeda) White-rot fungi Summary: Loblolly pine chips were treated with several white-rot fungi in two different bioreactor configurations for four weeks prior to refiner mechanical pulping. Irrespective of the bioreactor configuration, all fungal treatments caused some chip weight loss and saved electrical energy during fiberization and refining as compared to the untreated control. Some of the fungal treatments improved strength properties of the handsheets, whereas brightness and light scattering coefficient of the handsheets were decreased after all of the fungal treatments. Opacity of the handsheets after the fungal treatments remained unchanged. Based on energy savings and improvements in the strength properties, regardless of bioreactor type, the white-rot fungus Ceriporiopsis subvermispora appeared to be superior to the other white-rot fungi tested. When incubated in stationary tray bioreactors, C. subvermispora caused only 6% weight loss, saved 42% energy during fiberization and refining, improved burst index by 32% and tear index by 67%, as compared to the control. Introduction Mechanical pulping operations consume large quantities of electrical energy to produce high yield but relatively weak pulps with useful optical properties (Kano et al. 1982; Kurdin 1979; Leask and Kocurek 1987; Pulp and Paper 1989; West 1979). Such pulps are desirable for printing papers because of their optical properties. Groundwood process produces the weakest mechanical pulp with the best optical properties, and is the least energy intensive. Refiner mechanical pulping processes produce stronger pulps with reduced optical properties, but require more energy. Adding steam pressure to the refining operation (thermomechanical pulp) (TMP), and chemicals together with steam pressurization (chemithermomechanical pulp) (CTMP), retains more of the basic fiber length and improves paper strength (Beath and Mihelich 1977; Higgins et al. 1978; Kurdin 1979; Mokvist et al. 1985; Wegner 1982, 1987). However, the increased strength properties are offset by reduced optical properties, and the TMP and CTMP processes may actually increase energy consumption (Kurdin 1979; Beath and Mihelich 1977). The CTMP process also generates a troublesome dilute waste liquor stream. Mechanical pulp production is increasing (Atack 1985; Jackson 1985), although growth has re- 1 This article was written and prepared by Government employees on official time, and it is therefore in the public domain and not subject to copyright. The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. cently been slowed by an increased use of recycled fibers (Leask 1991). The disadvantages of mechanical pulping processes are the primary reasons for evaluating the potential of using fungal treatments prior to mechanical pulping (biomechanical pulping) (Akamatsu et al. 1984; Ander and Eriksson 1975; Setliff et al. 1990). We have achieved promising results with a nonoptimized bench-scale process that uses selected white-rot fungi to treat wood chips prior to refiner mechanical pulping (Leatham et al a,b; Myers et al. 1988). With the aspen chips, the process reduces refiner energy consumption, and improves strength properties of handsheets made from the biomechanical pulps. Here we report the results of bench-scale studies of biomechanical pulping of loblolly pine chips treated by selected white-rot fungi in two different bioreactor types. A preliminary report was presented at an international conference (Leatham et al b). Material and Methods Fungal selection Seven different white-rot fungi were selected for this study, based upon their ability to selectively remove lignin from small loblolly pine blocks (Leatham et al c). Fungi selected were Phlebin (Merlius) tremellosa (PRL-2845), Phanerocharete chrysosporium (BKM-F-1767), Dichomitus squalens (TON-429). Hypodontia Setulosa (FP sp), Phlebia brevispora (HHB-7099-sp). Ceriporiopsis subvermispora (FP sp), and Phlebia subserialis (RLG-6074-sp). They were maintained on potato dextrose agar (PDA) (Difco Laboratories, Detroit, MI) slants and kept refriger- Holzforschung / Vol. 47/ 1993/ No. 1 Copyright 1993 by Walter de Gruyter Berlin New York

2 M. Akhtar et al.: Biomechanical Pulping of Loblolly Pine Chips 37 ated until used. PDA plate cultures were inoculated from these slants and incubated at 27±1 C and 65±5% relative humidity for 10 days for all the fungi, execpt for P. chrysosporium slants which were incubated at 39±1 C and 65±5% relative humidity for 5 days. Wood chips Freshly cut Loblolly pine (Pinus taeda L.) pulpwood-size logs were obtained from the Bienvillc National Forest, Mississippi. Logs were debarked and chipped to a nominal 6 mm size. Chips were placed in plastic bags and frozen to prevent the growth of contaminating microorganisms. Seed inoculum preparation Frozen chips were thawed and mixed to obtain uniform samples. Modified chemically defined medium (Leatham 1983) containing 40g glurcose kg -1 chips (oven dry weight basis) was added in sufficient quantity to 200 grams of chips (oven dry weight basis) to raise the chip moisture content to 60%, The modified chemically defined medium was used to increase the fungal biomass and suppress cellulose degmdation, These chips (200 grams dry weight basis for inoculating stationary tray bioreactor or 300 grams dry weight basis for inoculating rotating solid drum bioreactor) were put in 2800 ml Fernbach flasks, flask tops were covered with aluminum foil, and the flasks were autoclaved for 45 min. at 121 C. After cooling to room temperature, sterilized chips in each flask were inoculated with plugs from plate cultures, and the inoculated chips were thoroughly mixed. Flasks were incubated for 4 weeks at 27+1 C and 65±5% relative humidity for all the fungi, execpt for P. chrysosporium that was incubated for 4 weeks at 39±1 C and 65±5%, relative humidity. These precolonized chips were used to seed the bioreactors. Chip preparation and bioreactor inoculation Frozen wood chips were thawed and thoroughly mixed to obtain uniform samples. Modified chemically defined glucose-containing medium as mentioned above was added to the chips to raise the chip moisture content to 60%, For the fungal treatments, 1800 grams (oven dry weight basis) of chips were put in each stationary tray bioreactor, and 2700 grams (oven dry weight basis) of chips were put in each rotating solid drum bioreactor. For the control, only 2000 grams (oven dry weight basis) of chips were put in a stationary tray bioreactor.the rotating solid drum and stationary tray bioreactor are illustrated and described in previous publications (Akhtar et al. 1992; Leatham et al a,b; Myers et al. 1988). Each bioreactor was autoclaved for 90 min. at 121 C and cooled to room tempemture. A 200 gram (oven dry weight basis) seed inoculum was added to the wood chips in a stationary tray bioreactor containing 1800 grams of chips (oven dry weight basis), and a 300 gram (oven dry weight basis) seed inoculum was added to the wood chips in a rotating solid drum bioreactor containing 2700 grams of chips (oven dry weight basis) and mixed thoroughly. Bioreactors were scaled and placed in an incubator maintained at 27±1 C and 65±5% relative humidity for all the fungi, execpt for P. chrysosporium where the incubator was maintained at 39±1 C and 65±5%. relative humidity. Incubation time for each of these bioreactors was, 4 weeks. A control bioreactor containing 2000 grams of chips (oven dry weight basis) was treated identically to the inoculated bioreactors, except it was not inoculated. Chip fiberization, pulp refining, and handsheet production At harvest, the untreated (control) and the fungus-treated chips were fiberized in a Sprout-Waldron 5 model D 2202 single rotating 300-mm-diameter disk atmospheric refiner. Energy consumed dur- 5 The use of trade and company names is for the benefit of the reader and does not constitute an official endorsement or approval of any service or product by the USDA to the exclusion of others that may be suitable. ing fiberization and refining was measured using an Ohio Semitrorric 5 model WH integrating watt meter attached to the power supply side of the 44.8-kW electric motor. Energy consumption values for fiberizing and refining arc reported as kw.h kg -1 (oven dry weight basis) with the idling energy subtracted (idling energy was measured without a chips or plup load). Chips and crumbed pulp were hand fed into the preheated refiner, adjusting feed rate to keep the load between 6 and 15 kw. The initial plate setting was 0.46 mm, and the refining process was repeated with a decrease in the plate setting after each successive pass until the Canadian Standard Freeness (CSF) of the pulp was dropped below 100 ml. Care was taken to prevent fines loss by collecting the pulp slurry in closed plastic containers, transferring the pulp slurry to a canvas bag, and pressing to dewater the pulp. After each refiner pass, the pulp was dewatered to about 25% solids content in a wine press and the pulp mat was crumbed before the next pass. Details about the weight loss determination, handsheet preparation and testing methods have been described previously (Leatham et al. 1990a,b). Energy values, strength properties, and optical propertics were regressed to 100 ml CSF to facilitate comparison. Results and Discussion The objectives of this study were to evaluate biomechanical pulping of loblolly pine chips with selected white-rot fungi as a means of reducing electrical energy consumption and improving paper strength properties. Energy savings All of the fungal treatments applied to the loblolly pine chips saved electrical energy, regardless of bioreactor configuration, when comparisons were made with an untreated control (Figs. 1 and 2). P subserialis and C. subvermispora incubated in the rotating solid drum bioreactor (Fig. 1) were almost equal, and gave the highest amount of energy savings (32-35%). Energy savings due to C. subvermispora were even better (42%) in a stationary tray bioreactor (Fig. 2). These results show that C. subvermispora, irrespective of the bioreactor configuration, can give substantial refiner energy savings. The specific cell wall changes that are responsible for the reductions in energy consumption during fiberization and refining of fungal pretreated chips are not known. Gross Fig. 1. Energy savings during biomechanical pulping of loblolly pine chips with seven white-rot fungi incubated in rotating solid drum bioreactors. The control chips required 2.0 kw.h kg -1 (oven dry weight basis) electrical energy to produce 100 ml-canadian Standard Freeness pulp. Energy savings due to fungal pretreatments were calculated as compared to control.

3 38 M. Akhtar et al.: Biomechanical Pulping of Loblolly Pine Chips Fig. 2 Energy swings during biomechanical pulping of loblolly pine chips with five white-rot fungi incubated in stationary tray bioreactors. The control chips required 2.0 kw.h kg -1 (oven dry weight basis) electrical energy to produce 100 ml-canadian Standard Freeness pulp. Energy savings due to fungal pretreatments were calculated as compared to control. Strength properties Pretreatment by some of the fungi led to increased burst and tear strength in comparison to the untreated control (Tables 1 and 2). When fungi were incubated in rotating drum bioreactors, C. subvermispora, R brevispora and H. setulosa increased burst indices by 29, 31 and sq~., respectively and tear indices by 71, 56 and 74%, respectively (Table 1). The other fungi did not improve these properties. When fungi were incubated in stationary tray bioreactors, C. subvermispora gave the greatest improvement in strength properties, improving burst index by qz~. and tear index by 67% (Table 2). The other fungi tested either slightly improved or slightly decreased these properties. When the treatments were in rotating drum Table 1. Weight loss and physical properties for 60 g m -2 handsheets made from pulps obtained from untreated loblolly pine chips (control) and from loblolly pine chips treated with seven white-rot fungi incubated in rotating solid drum bioreactors. Table 2. Weight loss and physical properties for 60 g m -2 handsheets made from pulps obtained from untreated loblolly pine chips (control) and from loblolly pine chips treated with five white-rot fungi incubated in stationary tray bioreactors. changes in the amouts of cell wall polymers are not correlated with efficacy (Leatham et al a,b). Loss of wood substance during fungal treatment Weight losses from the loblolly pine chips treated with the fungi ranged from 1 to 7% when incubated in a rotating solid drum bioreactor (Table 1), and from 4 to 6% when incubated in a stationary tray bioreactor (Table 2). These weight losses for the biomechanical pulps are well within the range of high-yield pulping processes. bioreactors, tensile indices were slightly increased, whereas densities of the handsheets were either not affected or slightly decreased (Table 1). And in stationary tray bioreactors, the fungal treatments led to no change or slight decreases in tensile indices and densities (Table 2). Reduced handsheet densities imply and screen fractionation indicate that the fungustreated wood chips were separated into relatively intact wood fibers on refining, with a reduction in fine material generated during the fiberization and refining. These results are in accord with our previous results on aspen and loblolly pine using several fungi

4 M. Akhtar et al.: Biomechanical Pulping of Loblolly Pine Chips 39 (Leatham et al c; Akhtar et al. 1992). The increased burst and tear indices of the handsheets suggest that the increased handsheet strength may be caused by increased interfiber bonding and that the individual wood fibers were not weakened by fungal treatment applied to the loblolly pine chips. Similar results on aspen using several fungi have been reported previously (Leatham et al c). It is interesting to note that regardless of the bioreactor configuration, C. subvermispora showed an increase both in burst and tear indices over the control. Optical properties There is no apparent distinction between bioreactor types with regard to handsheet optical properties (Tables 1 and 2). All of the fungi reduced brightness and light scattering coefficients compared to the untreated control. Opacity of the handsheets remained unchanged after the fungal pretreatments. The phenomenon of reduced optical properties accompanying improved strength properties in the biomechanical pulps also occurs with other mechanical pulping processes (Kurdin 1979). Our previous studies also demonstrated that aspen wood chips treated with several white-rot fungi under similar experimental conditions substantially decreased brightness and light scattering coefficients (Leatham et al b). However, the pulps obtained from the fungus-treated aspen wood chips responded well to peroxide bleaching (Myers et al. 1988). Biomechanical pulping of loblolly pine chips with several white-rot fungi in two different types of bioreactor appears to be a high yield process. All fungal pretreatment saved electrical energy during fiberization and refining. Some of these fungi improved burst index and tear index of the handsheets. There was no apparent correlation between weight loss, energy savings, and improvements in the strength properties. Based on energy savings and improvements in the strength properties, C. subvermispora was identified as the best fungus. A U.S. patent describing this entire process has been granted (Blanchette et al. 1991). Acknowledgements The authors wish to thank T. Kent Kirk, USDA Forest Products Laboratory. Madison, Wisconsin for his review of the manuscript and John Koning, Jr, and Richard Burgess, University of Wisconsin Biotechnology Center (UWBC), Madison, Wisconsin for valuable discussions. We thank Ian Reid, Pulp and Paper Research lnstitute, Quebec, Canada for isolate PRL-2845 of Phlebria tremellosa and the Center for Forest Mycology Research, Forest Products Laboratory, Madison, Wisconsin for all other isolates used in this study. We also thank Louis C. Lunte, Marguerite S. Sykes, Sanya Reyes-Chapman, Leatha Damron, Kerry Katovich and Freya Tan for technical assistance. This work was supported by a Biopulping Consortium involving 20 pulp and paper, and related companies, the UWBC, and the Forest Products Laboratory. References Relationship between weight loss, energy savings, strength properties, and optical properties The time and effort required for each biomechanical pulping run did not permit replication and statistical treatment. It is assumed that the values for energy savings are significantly different for the fungi tested. However, variation within the handsheet strength and optical properties was found to be <10%. There appears to be no apparent relationship between weight loss of the chips, energy savings during pulp preparation and strength or optical properties. These results are in agreement with our previous results on aspen using several species of fungi (Leatham et al b,c). Selecting the best fungus Based on energy savings and improvements in the strength properties using both stationary tray and rotating solid drum bioreactors, C. subvermispora appeared to be superior to the other white-rot fungi tested. Conclusions

5 40 M. Akhtar et al.: Biomechanical Pulping of Loblolly Pine Chips Printed on recycled paper