IS FUNGAL TREATMENT OF ASPEN CHIPS BENEFICIAL FOR CTMP?

Authors

Bruce Sitholé, Marie-Josée Rocheleau, Robin Berlyn, Cyril Heitner, and Larry Allen

Company and address

Paprican, Pointe Claire, Quebec, Canada

email

bsithole@paprican.ca

Keywords

populus, chips, chemithermomechanical pulping (CTMP), fungi, mechanical properties, physical properties, brightness, extractives, refining, toxicity

ABSTRACT

Is fungal treatment of aspen chips beneficial for chemi-thermomechanical pulping? To answer this question, aspen chips were treated with fungus, seasoned in insulated boxes and then pulped in a pilot-plant refiner. Chemi-thermomechanical pulp (CTMP) from aspen chips treated with a commercially-available fungus Cartapip™ and aged for three weeks, results in CTMP with higher strength properties than CTMP from untreated chips. Such treatment also results in a 13 to 15% savings in refining energy, as has been previously reported in the literature. However, seven weeks of aging after the fungal treatment is detrimental to the strength properties, and requires higher refining energies. On the other hand, pulps made from fungal-treated chips and aged for seven weeks have brightness values that are 7% higher than those of pulps from untreated chips. Although, fungal treatment is known to degrade wood extractives, it nevertheless does not reduce the toxicity of aspen pulping effluents, nor does it decrease the acetone extractives content of the CTMP.

TITLE

Aspen (Populus tremuloides Michx.) is a hardwood that is now widely used for pulp production in North America. In kraft pulping, the alkaline conditions do not seem to be able to completely break down some of the wood resin components of aspen1,2. Seasoning, good barking, addition of tall oil in the digester, and addition of surfactants during oxygen delignification are some of the measures that can be used to improve deresination3–5. Another aspect of the use of aspen is the toxicity of its wood leachate6. A recent comprehensive Canadian study undertaken to elucidate the nature, strength, and persistence of aspen leachate toxicity and the chemical composition of the leachate showed that the leachate was rich in phenols, organic carbon and organic nitrogen, but much of its toxic effect was attributed to other unidentified constituents7.

In previous work we have demonstrated that treatment of aspen chips with Cartapip 97®, a fungus that thrives on wood resin components, accelerates deresination of the chips without adversely affecting their kraft pulping properties8,9. We have now expanded the study to ascertain if the fungal treatment can also be beneficial for other pulping processes that use aspen wood, namely chemithermomechanical pulping (CTMP).

Mechanical pulping studies of fungal treated chips, termed biomechanical pulping, have shown that certain fungi do have beneficial effects. For example fungal treatment of loblolly pine chips with a white-rot fungus, Ceriporiopsis subvermispora, for four weeks followed by refining on a single-disc mechanical refiner resulted in extensively fibrillated fibres that appeared to be associated with improved paper strength properties and decreased specific energy requirements during refining10–12. Studies using another fungus, Phanerochaete chrysosporium, led to a suggestion that fungal treatment likely involves enzymatic softening and swelling of the wood cell-wall thus facilitating mechanical fibre development during refining13. Fungal treatment of softwood chips with Cartapip 97®, Ophiostoma piliferum, has shown that the treatment improves pulp properties14,15 but no similar studies have been done with aspen chips.

Our main objective in this study was to assess whether or not fungal treatment of aspen chips with Cartapip 97® leads to savings in refining energy and improved pulp properties. Another objective was to determine if the fungus can reduce COD, BOD and toxicity of CTMP pulping effluents. The last objective was based on the postulate that, if wood resin components are the cause of effluent toxicity, their removal by the fungus should result in reduced toxicity of aspen effluents.

MATERIALS AND METHODS

Fungal inoculum:

This study was conducted with a commercially available fungus, Cartapip 97®, which thrives on wood resin but does not stain chips16-17.  Prior to its use, the Cartapip inoculum was assayed on Potato Dextrose Agar plates to determine the number of colony forming units in the product. Five grams of Cartapip were hydrated in 100 ml of tap water at 25°±5°C, pH 5-6, for 30 minutes and then 1 ml of the suspension was diluted with 300 ml of water to prepare the inoculum.  Its viability was 5x106 colony forming units / ml.

Procurement, preparation and fungal treatment of aspen chips:

Fifty 8-foot fresh aspen logs, ranging in diameter from 6 to 10 inches, were obtained from a farm in southern Québec and stored outdoors at an ambient temperature close to freezing for 9 days before processing.  The logs were debarked with a Morbark ring debarker and then chipped with a horizontal feed (spout-under-shaft) overhead-discharge "Eeger Beever" Morbark chipper. The unit is diesel powered, belt driven, and equipped with a single clamp mounted knife (wedge angle 30.5o) on a 103 cm diameter disc. Chipping was carried out at 650 rpm with nominal chip length set at 19 mm.  The chips were screened on a 3 x 6-foot Overstrom vibratory screen to remove oversized and undersized chips. The debarking, chipping, and screening were done over three days in four successive batches; one batch for each of four storage boxes.

The screened chips were stored in insulated boxes, Figure 1, to simulate seasoning in full-size piles. Similar boxes have been successfully used to simulate seasoning in mill chip piles18.  The boxes were made either from plywood or oriented strand board. The sides of the boxes were lined with a 11-cm thick layer of polystyrene foam insulation.  The lids, made of 11-cm thick polystyrene foam, were fitted with nine 10-cm diameter holes, arranged around the circumference of a 60-cm diameter circle.  The positioning of a plywood cover over the holes in the lid controlled the degree of venting from the top of the box. The floor panel was also fitted with nine 10-cm diameter holes, as was the 5-cm overlying layer of polystyrene foam.  Expanded metal mesh on the upper side of the floor prevented chips from falling out. Sliding Masonite panels on the underside of the box allowed the holes to be covered over albeit not tightly sealed.  The coverage of the holes was adjusted as needed to stabilize the temperature of the chips inside the boxes. The interior dimensions of the box were 84 cm X 84 cm X 150 cm high.  The volume of each box, lid in place, was 1 m3.  Each box was mounted on skids, elevating it 9 cm above the floor.

Inoculation of chips with Cartapip was performed while filling two of the boxes with the chips; the fungal suspension was sprayed onto the chips via a hand-pressurised sprayer. The fungal dosage on the chips was 0.1AU (activity units) per tonne of chips. The other two boxes were filled with untreated chips onto which an equivalent amount of water to that used in spraying the Cartapip, was applied.  It was determined that the moisture content of the chips in all four boxes at the beginning of the ageing experiment was the same. Each box contained 50 kg OD chips.

Once each box was filled, a temperature probe (thermocouple) was located in the centre of the mass of chips. The temperature inside each box was scanned at hourly intervals throughout the study and the data were logged on a computer.  The boxes were stored at room temperature prior to being reclaimed for pulping trials.  Two boxes, one containing untreated chips and the other with Cartapip treated chips, were reclaimed after about 3 weeks (26 days) whereas the last two boxes were reclaimed after 7 weeks (actually 54 days).  The growth of Cartapip on treated chips was evaluated by ELISA using procedures previously described19.

Chemithermomechanical Pulping of Aspen Chips

The chemithermomechanical pulping of aspen chips involved a modest chemical impregnation of steamed chips prior to the refining stage. For each of the four CTMP experiments, 50 kg of O.D. chips were needed. Steaming of the chips was performed under pressure, which was followed by impregnation of the steamed chips with 1.5% of sodium hydroxide and 2% of sodium sulphite (based on dry-wood weight). The one-stage refining was performed with a disc refiner, Andritz Sprout Bauer model 400H, in a single stage, at five different levels of energy ranging from 2 to 6 GJoules / tonne. The five levels of energy were selected based on past experience with aspen refining. The pulps generated at the different levels of energy were collected separately.

The refined pulps were disintegrated in a 30-L polyethylene tank at a consistency of 1%, for 60 minutes at 50°C. The disintegration in hot water was necessary to remove the latency of the refined fibres. The physical properties of the pulps were measured using standard PAPTAC procedures.

Determination of Extractives Contents of Chips and Pulps

Treated chips, untreated chips and their corresponding pulps were freeze-dried and then finely ground (2-mm mesh size) on a Wiley mill. The extractives contents of the samples were determined by solvent extraction with acetone using a Soxtec extractor (System HT6, Tecator, Fisher Scientific). Double extractions were performed20.

Analysis of Wood Extractives by Gas Chromatography

A gas chromatograph (HP5890 Series II Plus, Hewlett Packard) with a flame ionization detector was used for the analysis of the wood extractives. A short capillary column (2-metre non-polar HP-5 column from Hewlett Packard) was used. The use of short capillary columns enables the elution of both the high boiling point lipid fractions and the lower boiling point fatty acids. Conversion of fatty acids into methyl esters that are amenable to analysis by gas chromatography was achieved by in situ methylation with trimethylanilium hydroxide (MethElute Reagent, Pierce, Rockford, Il). The in situ methylation was performed by injection of a mixture of extractives and MethElute into the injection port of the gas chromatograph. Injection of individual standards ranging from fatty acids to triglycerides enabled us to assign retention times for the compounds. From the retention times, compounds of similar chemistry were grouped into six retention time zones as previously described1.

RESULTS AND DISCUSSION

Extractives Contents of Chips

The acetone extractives content of the fresh wood was 2.1%. The reproducibility of the extractives was such that triplicate measurements of a sample agreed within 2%. Analysis of the extractives of fresh aspen by gas chromatography showed that fatty acids, steryl esters, and triglycerides were the main classes of compounds present. Fatty acids, steryl esters, and triglycerides constituted 1.7% (by weight) of the wood, whereas sterols, waxes, and diglycerides, represented less than 0.2% of the wood.

Figure 1 shows the effect of aging on the acetone extractives of untreated and fungal treated aspen chips. After aging for three weeks, 29% of the extractives were removed in untreated chips, while only 16% of the extractives of chips treated with Cartapip were removed. The expected improvement in the deresination of aspen chips treated with Cartapip 97® was not obtained after aging for three weeks. After seven weeks, 30% of the extractives were removed in untreated chips, while 35% of the extractives of treated chips were removed. This represents a slight, but probably not significant, improvement of 5% over natural aging of untreated chips.

Figure 1

These results are contrary to previous observations where the reduction in wood resin content was greater in fungal treated chips than in the untreated chips8. Sampling error probably accounts for the discrepancy: inoculation of the chips done via a hand-held sprayer most likely did not result in uniform coating of the 50 kg chip samples. Taking 250 g of chips from each box for extractives content analysis may result in non-representative sampling of the chips in the boxes. Thus the chip samples obtained for solvent extraction may or may not have had the full benefit of fungal treatment. In our previous study, 250 g samples were inoculated in plastic bags and this enabled uniform coverage of the chips by the fungal inoculum. In future work, it would be more prudent to take subsamples from different areas of the box and mix them to obtain a more representative sample.

Chemithermomechanical Pulping of Aspen Chips

The results for the CTMP yield of aspen chips after three and seven weeks of seasoning are summarized in Table I. The pulping yields for untreated and Cartapip treated chips are above 97%. It is evident that there was minimal fibre weight loss induced by fungal treatment, aging the chips or their chemical treatment.

TABLE I. Chemithermomechanical pulping yield.

Aging Period

(weeks)

Control

Untreated Chips

Cartapip 97®

Treated Chips

0

98.2±2.2%

 

3

98.8±2.8%

97.4±2.0%

7

97.3±6.0%

100±6.0%

Figure 2 compares the energy consumed on refining treated and untreated chips. Refining at specific energies above 4.2 GJ/tonne was not possible for treated chips aged for three weeks. Fresh aspen chips required 4.5 GJ/t to produce 450-mL CSF pulp. After aging for three weeks, the energy required to produce the same CSF pulp decreased to 4.3 GJ/t and 3.7 GJ/t for untreated chips and Cartapip-treated chips, respectively. This represents an improvement of 13 % in energy savings for the Cartapip-treated chips. The energy savings were further enhanced by 25% for untreated chips aged for seven weeks, whereas the energy savings for the chips treated with Cartapip improved to 18%. The pattern of energy consumption for pulping after three weeks shows that the growth of fungi on aspen chips appears to contribute in softening the chips considerably so that the wood fibres separate more easily. This confirms observations made on CTMP refining of aspen chips treated with other fungal species21.

Figure 2

Handsheets of pulps from treated chips aged for three weeks exhibited increased tensile strength (Figure 3) over the pulps from untreated chips. After aging for seven weeks, however, the tensile strength for pulps from untreated chips far exceeded that of pulps from treated chips. A similar pattern was observed for the Scott bond (Figure 4). The relationship between tear and tensile index is shown in Figure 5. Tear at a given tensile strength increased after three weeks whereas the opposite was observed after aging for seven weeks.

Figure 3

Figure 4

Figure 5

The observed increase in the strength properties of handsheets for pulps from aged chips cannot be attributed to an increase in fibre length. The average lengths of fibres from untreated and treated chips were not distinctly different (data not shown). Instead, strength development in chemithermomechanical pulping may be attributed in part to a larger number of acid groups on the fibre surface. This is corroborated by the increased total ion content (151 to 166 mmol/kg) after fungal treatment for three weeks (Table II). However, seven weeks of aging with Cartapip appeared to produce pulp with a total ion content of 150 mmol/kg suggesting that fungal treatment was ineffective or that, during the last four weeks of the seven-week treatment, acid groups were lost. More extensive data are needed to confirm these observations.

The untreated chips were colonized by wild-type fungi that stained the chips, consequently, there was a 7% loss in pulp brightness after aging the chips for seven weeks (Table III). In a mill the loss in brightness would create additional costs in bleaching chemicals. Pulps from treated chips did not lose brightness and opacity; the Cartapip 97® fungus is an albino strain of O. piliferum that does not stain the wood chips. Its growth on wood chips appears to compete successfully with colonization by the wild-type fungi.

The data in Table II and in Figure 6 show that, in general, the extractives contents of the pulps increased with increase in refining energy applied on the aspen chips. Aging for three weeks did not significantly alter the quantity of wood resin in pulps from chips treated with the fungus. Aging for seven weeks, on the other hand, resulted in lower extractives content in pulps made from untreated chips than in pulps made from treated chips.

Sampling errors that appeared in the analysis of extractives content did not materialize in the CTMP studies because whole sample lots of each box were processed.

TABLE II. Extractives content of CTMP samples.

Chips

Refining Energy (GJ/t)

Extractives Content (%)

Acid Groups

-COOH (mmoles/kg)

-HSO3 (mmoles/kg)

 

 

 

124.1

134.1

133.9

26.9

22.6

41.1

Untreated

aged for 3 weeks

2.06

4.22

5.87

0.76

1.12

1.01

122.5

126.9

128.2

31.3

36.0

37.5

Treated:

aged for 3 weeks

2.08

4.16

NA

0.80

1.12

NA

133.3

135.2

NA

33.4

45.3

NA

Untreated

aged for 7 weeks

2.08

4.24

5.81

0.57

0.74

0.66

133.6

123.2

140.9

45.4

45.6

52.9

Treated:

aged for 7 weeks

2.08

4.22

6.18

0.67

1.00

0.86

126.7

114.7

118.2

24.4

44.1

29.0

Figure 6

TABLE III. Brightness and opacity of chemithermomechanical pulps from treated and untreated chips.

Chips

Specific Energy

(GJ/t)

Brightness

(%)

Opacity ISO

(%)

Fresh

4.3

58.7

91.1

Untreated: aged for 3 weeks

4.2

55.6

93.1

Treated: aged for 3 weeks

4.2

56.3

92.1

Untreated: aged for 7 weeks

4.2

52.3

93.7

Treated: aged for 7 weeks

4.2

56.8

92.6

SUMMARY AND CONCLUSIONS

The effects of treating aspen chips with Cartapip observed in this work are summarized in Table IV:

TABLE IV. Changes induced in some properties of aspen chips and CTMP pulp after fungal treatment with Cartapip.

Property

Percent Change in Parameter After Fungal Treatment

3 weeks

7 weeks

Chip extractives

none

none

CTMP extractives

none

none

Specific refining energy (450 ml CSF)

–14

+30

Tensile index

+32

–21

Tear index

+32

–12

Scott Bond

+31

–22

Brightness

+1

+8

Opacity

–1

–1

Cartapip treatment of aspen chips for three weeks is beneficial for energy savings and strength properties. However, it is evident from our work that seven weeks of treatment can cause serious degradation of the chips and lead to large negative effects on specific energy of refining, pulp properties and effluent toxicity.

It must be noted, however, that conclusions may only be applicable under the conditions used in this work. In mill chip piles, the temperature, moisture and consequently fungal activity, may be significantly different from the ones in the present study. Further work is required, therefore, to assess the effect of the Cartapip treatment over a broader range of conditions.

Although threes studies were performed on aspen wood, they are nonetheless applicable to other hardwood species found in South Africa such as eucalyptus.  Fungal treatment targets wood extractives, hence any wood species would be amenable to fungal treatment. Indeed, previous studies have shown that pine wood species can also be treated by fungal treatment22.

ACKNOWLEDGEMENTS

We wish to recognize the expertise and assistance provided by the following: Derek Dranfield, Christine Lapointe, Keith Miles, Steve Morris, and Mike Stacey. Thanks to Daniel Ouellet for reviewing the manuscript.

REFERENCES

1. Sitholé, B.B., Sullivan, J.L., and Allen, L.H. (1992). Identification and quantitation of acetone extractives of wood by ion exchange and capillary GC with a spreadsheet program, Holzforschung, 46(5):409–416.

2. Chen T., Wang Z., Gao Y., Breuil C., and Hatton, J. V. (1994). Wood extractives and pitch problems: analysis and partial removal by biological treatment, Proceedings, 48th Appita Annual General Conference, Appita, Melbourne, pp. 473–478.

3. Allen, L.H., Sitholé, B.B., MacLeod, M., Lapointe, C.L., and McPhee, J. (1991). The importance of barking and seasoning in kraft pulping of aspen, J. Pulp Pap. Sci., 17(3):J85–J90.

4. Allen, L.H. (2000). Pitch control in pulp mills, in "Pitch Control, Wood Resin and Deresination", Back, E. and Allen, L.H. (editors), TAPPI Press, Atlanta, Ch. 13.

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7. Taylor, B.R., Goudey, J.S., and Carmichael, N.B. (1996). Toxicity of aspen wood leachate to aquatic life: laboratory studies, Environ. Toxicol. Chem., 15(2):150–159.

8. Rocheleau, M.J., Sitholé, B.B., Allen, L.H., Farrell, R., Iverson, S., and Noël, Y. (1998). Fungal treatment of chips for wood resin reduction: a laboratory evaluation, J. Pulp Pap. Sci., 24(2): 37–42.

9. Rocheleau, M.J., Sitholé, B.B., Montgomery, J., and Allen, L.H. (1999a). Fungal treatment of chips for pitch control: a field evaluation, 10th International symposium on wood and pulping chemistry — 10th Biennial ISWPC — Main symposium, Yokohama, Japan, Japan TAPPI, vol. 3, Poster presentations, pp 208–213.

10. Akhtar, M., Attridge, M.C., Myers, G.C., Kirk, T.K., and Blanchette, R.A. (1992). Biomechanical pulping of loblolly pine with different strains of the white-rot fungus, Ceriporiopsis subvermispora, Tappi J., 75(2):105–109.

11. Akhtar, M., Attridge, M.C., Myers, G.C., and Blanchette, R.A. (1993). Biomechanical pulping of loblolly pine chips with selected white-rot fungi, Holzforschung, 47(1):36–40.

12. Blanchette, R.A., Akhtar, M., and Attridge, M.C. (1992). Using Simons stain to evaluate fibre characteristics of biomechanical pulps, Tappi J., 75(11):121–124.

13. Sachs, I.B., Leatham, G.F., and Myers, G.C. (1989). Biomechanical pulping of aspen chips by Phanerochaete chrysosporium: fungal growth pattern and effects on wood cell walls, Wood Fibre Sci., 21(4):331–342.

14. Kohler, L.F., Dinus, R.J., Malcolm, E.W., Rudie, A.W., Farrell, R.L., and Brush, T.S. (1995). Improving softwood mechanical pulp properties with Ophiostoma piliferum, Proceedings, TAPPI Pulping Conference, TAPPI PRESS, Atlanta, pp. 303–308.

15. Lanouette, R., Valade, J.L., Thibault, J., Noël, Y., and Iverson, S. (1996). Mise en pâte biomécanique de pin gris (Pinus banksiana lamb.), Les Papetières du Québec, 7(4):34–44.

16. Blanchette, R.A., Akhtar, M., and Attridge, M.C. (1992) Using Simons stain to evaluate fibre characteristics of biomechanical pulps, Tappi J., 75(11):121-124.

17. Brush, T.S., Farrell, R.L., and Ho, C. (1994) Biodegradation of wood extractives from Southern yellow pine by Ophiostoma piliferum, Tappi J., 77(1):155-159.

18. Springer, E.L., and Zoch, L.L. (1970) A simulator of an outside chip pile, Tappi J., 53(1): 116-117.

19. Rocheleau, M.J., Sitholé, B.B., Allen, L.H., Farrell, R., Iverson, S., and Noël, Y. (1998) Fungal treatment of chips for wood resin reduction: a laboratory evaluation, J. Pulp Pap. Sci., 24(2): 37-42.

20. Sitholé, B.B., Vollstaedt, P., and Allen, L.H. (1991). Comparison of Soxtec and Soxhlet systems for determining extractives content, Tappi J., 74(11):187–191.

21. Leatham, G.F., Myers, G.C., and Wegner, T.H. (1990). Biomechanical pulping of aspen chips: energy savings resulting from different fungal treatments, Tappi J., 73(5):197–200.

22. Gerischer, G., and Dommisse, E., (2000). Fungal pretreatment of woodchips for alkaline pulping, Proceedings, 10th International Conference and Exhibition, TAPPSA, Durban.

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