UTILIZATION OF SAW MILL WASTES AS A PULPING RAW MATERIALS, KRAFT PULPING OF SAPWOOD

Authors

M. Sarwar Jahan1, M. Harun-Or-Rashid2 and S. Monilur Rahman2

Company/Organisation and addresses

1Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-E-Khuda Road, Dhaka-1205 Bangladesh 2Department of Applied Chemistry and Chemical Technology, Dhaka University, Dhaka, Bangladesh

email

sarwar2065@yahoo.co.uk

Keywords

sapwood, sulphidity, anthraquinone, delignification, pulp yield

ABSTRACT

In Bangladesh sawmill uses heartwood for timber and rejects sapwood. Presently sapwood is being used as a domestic fuel. But sapwood contain good amount of holocellulose, thus it is a good source to produce pulp. In our previous study we showed that sapwood; a sawmill waste produced comparable pulp to our to locally produced pulp.

This paper deals the effect of anthraquinone on sulphidity of kraft pulping of sapwood. The cooking conditions namely- alkali concentration, cooking time, temperature and liquor ratio were varied in low (15%) and high (30%) sulphidity. 0.1% anthraquinone was added in the low and high sulphidity pulping with varying alkali concentration and cooking time. At an optimum conditions low sulphidity kraft process produced about 44% pulp yield with kappa number of about 23. But in high sulphidity kraft process kappa number was reduced to about 20 at the same yield. An addition of anthraquinone reduced alkali requirement by 2% on oven dried raw material and cooking time by 1 hour to produced pulp yield of about 44% at kappa number 20. There is no difference between low and high sulphidity kraft process on an addition of anthraquinone. The breaking length of kraft anthraquinone process was little bit higher than that of kraft process.

INTRODUCTION

The demand of pulp and paper in Bangladesh is about 0.4 million tons/year but Bangladesh produces about 0.2 million tons/year. Therefore, it needs more pulp mills in Bangladesh. Presently pulp mills of Bangladesh use bamboo, bagasse and mixed hardwood. Unfortunately, existing conventional raw materials do not permit more pulp mills in Bangladesh. Therefore, we have to find out alternative raw materials.

Saw mills of Bangladesh use heartwood for timber and reject sapwood. This sapwood is being now used as domestic fuel. But sapwood generates carbon monoxide during burning, which is not friendly for our environment. So sapwood should be used in an alternative way, such as a pulping raw material.

The wood in young trees, and the outer wood of older trees, capable of conducting "sap" and contains living parenchyma which store carbohydrates, fats, and other food reserves, such wood tissue is commonly referred to as "sapwood" and is light in color (1). Sapwood is located between cambium and heartwood. The transition from sapwood to heartwood is accompanied by an increase in extractive content (2).

Many studies have been done on the difference between sapwood and heartwood pulping. Helena and Bruno (3) studied the presence of heartwood in the raw material used for pulping decreased pulp yield and brightness mainly as a result of a higher content of extractives in relation to sapwood. In spite of the impact of heartwood on pulping. The content of extractives differed largely between heartwood and sapwood, with respectively 19.7% and 5.8% on average. In a cross-section, the pulp yield of heartwood was always lower than the pulp yield obtained with sapwood: on average 40.0% for heartwood and 49.7% for sapwood for similar delignification degree (kappa number). Pulp yield was negatively correlated with the content of polar extractives (ethanol and water solubles). NSSC and sulfate pulping studies on Populus "Muhle-Larsen" and P. "Oxford" showed that the heartwood and sapwood differ considerably in fiber length and in chemical properties. NSSC pulps from the sapwood have higher degree of whiteness, greater tear strength and slightly lower yields than the pulp from heartwood. However, there were no significant differences between heartwood and sapwood pulps as regard to tear length, burst strength and residual lignin content. With sulfate pulps, the pulping condition significantly affected the quality of the pulp made from sapwood and heartwood; sulfate pulp from heartwood has slightly higher yield and higher ash content than those from sapwood, but, there was no significant difference as regards degree of whiteness or lignin content. Clone-dependent differences were found in the strength properties of sulfate pulps from sapwood and heartwood (4)

In the present investigation we have studied on the pulping of sapwood by kraft process with varying cooking variables. The effects of AQ on the sulphidity in kraft pulping have also been studied.

EXPERIMENTAL

Material
Sapwood was collected from the sawmill in Dhaka City. It was cut to about 2-1-1 cm in size.

Pulping
Pulping was carried out in a 20-l capacity batch cylindrical reactor heated by means of electrical resistance and was rotated by a motor. The normal charge was 1 kg of moisture free sapwood. The following parameters were maintained:

- The active alkali concentration was varied from 16 to 22% as Na2O on oven dried (o.d.) sapwood for kraft process and kraft-AQ processes.

- Sulphidity was 15% and 30%

- The cooking temperature was varied from 160-1800C

- The cooking time was varied from 1 to 4 hours at the maximum temperature.

- The liquor to material ratios was 4, 5 and 6.

- 0.1% AQ was in both low and high sulphidity kraft-AQ processes.

After digestion the pulp was washed free of black liquor. The pulp yield was determined as percentage of oven-dry raw materials. The kappa number of the pulps was determined according to Tappi Test Methods (T 236).

Evaluation of pulps
Sapwood pulp was beaten in a Valley beater and hand sheets of about 60gm/m2 were made in a Rapid Kothen Sheet Making Machine according to German Standard Methods number 106. The sheets were tested for tensile (T 404os 61), burst (T 403m 53) and tear strength (T 414m-49), double fold (T 423m-50) according to TAPPI Standard Test Methods.

RESULTS AND DISCUSSION

Sapwood pulping in low and high sulphidity kraft process with and without anthraquinone was performed with varying cooking variables namely- active alkali, cooking time, temperature and liquor ratio and shown in Tables 1-8.

Low sulphidity (15%) kraft pulping
Table 1 represents the pulp yield and pulp properties with the variation of active alkali. The cooking time was held constant for 3 hours at 1700C. It is clearly seen from the Table 1 seen that there was a large acceleration of delignification rate when active alkali was increase from 16 to 18%, after that the increase of delignification rate was reduced with an increase of active alkali. Pulp yield was decreased with increasing active alkali. The physical properties of sapwood pulp were increased with increasing alkali up to 20% then decreased. This is in good agreement with the studies of alkaline pulping of wood and nonwood (5, 6).

Table 1: Effect of alkali concentration on the pulp yield and pulp properties in kraft pulping of sapwood (Cooking time 3 hour at 1700C, material to liquor ratio 1:5 and sulphidity 15%).

Alkali concentration (% on o.d. sapwood)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

16
18
20
22

45.8
44.9
43.9
39.6

27.3
23.2
22.6
19.2

7.4
7.5
8.0
7.7

3.6
4.0
4.1
3.8

5.6
5.7
5.9
5.8

401
443
441
395

Table 2 shows the effect of cooking time on the pulp yield and pulp properties of low sulphidity kraft pulps from sapwood. It is clearly seen from Table 2 that pulp yield and kappa numbers were decreased rapidly with increasing cooking time up to 3 h. The increase of cooking time from 3 to 4 hour kappa number reduced only 1.2 units. This was possibly due to pulping process reached to the residual stage.

The physical properties of kraft pulp at low sulphidity in different cooking time are also shown in Table 2. All physical properties were increased with increasing cooking time up to 3 hour then decreased with increasing cooking time.

Table 2: Effect of cooking time on the pulp yield and pulp properties in kraft pulping of sapwood (AA 20%, temperature 1700C. material to liquor ratio 1:5, Sulphidity 15%).

Cooking time (hr)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

1
2
3
4

49.6
47.4
43.9
40.4

30.2
24.3
22.6
21.4

7.7
7.9
8.0
7.3

3.3
3.8
4.1
3.7

5.2
5.7
5.9
5.4

297
391
441
414

Table 3: Effect of temperature on the pulp yield and pulp properties in soda pulping of sapwood (Cooking time 3 hour AA 20%, material to liquor ratio 1:5 and sulphidity 15%).

Cooking temperature 0C

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

160
170
180

43.0
43.9
40.1

29.8
22.6
23.6

7.3
8.0
6.9

3.7
4.1
3.8

5.2
5.9
5.4

369
441
400

The effect of temperature on pulp yield and pulp properties is shown in Table 3. Over a given period, kappa number decreased as temperature increased. The yield decreased with an increase in temperature because the velocity constant increased for both delignification and carbohydrate degradation reaction.

The physical properties were increased with an increase of temperature from 1600 to1700C then decreased with another increase of temperature to 1800C.

To ensure the bulk penetration of all chips, it is important that sufficient liquor be charged to the digester to immerse the chips completely. To find out an optimum liquor ratio required in sapwood pulping it was varied and shown in Table 4. It is seen that 1:5 liquor ratio was the most suitable amount for proper penetration of sapwood chips in kraft pulping. There were no significant effects on the physical properties of sapwood kraft pulp with the variation of liquor ratio.

Table 4 Effect of material to liquor ratio on the pulp yield and pulp properties in soda pulping of sapwood (Cooking time 3 hour at 1700C, active alkali 20% and sulphidity 15%).

Liquor ratio

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

1:4
1:5
1:6

46.2
43.9
47.8

24.3
22.6
27.2

7.8
8.0
7.7

3.9
4.1
4.2

5.8
5.9
5.4

425
441
435

High sulphidity (30%) kraft pulping
The effect of active alkali charge at higher sulphidity on the pulp yield and pulp properties are shown in Table 5. The experiments were carried out 1700C under a constant reaction time of 3 hours. The yield decreased from 46.1% at 16% active alkali to a low value of 40.2% at 22% active alkali, while the corresponding drop in kappa number was from 21.6 to 17.1. The physical properties were increased with an increase of alkali from 16 to 18% then again an increase of alkali decreased properties.

A comparative drop in yield was observed for longer duration of pulping (Table 6). As expected initially delignification rate was considerably faster than the later stage of pulping. Breaking length and burst index were increased with increasing cooking time from 1 to 3 hours of cooking after this period of cooking these properties were decreased. But tear index and double fold number were increased up to 2 hours then decreased.

Table 5: Effect of alkali concentration on the pulp yield and pulp properties in high sulphidity kraft pulping of sapwood (Cooking time 3 hour at 1700C, material to liquor ratio 1:5 and sulphidity 30%).

Alkali concentration (% on o.d. sapwood)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

16
18
20
22

46.1
43.9
43.4
40.2

21.6
20.9
19.8
17.1

8.0
7.9
7.8
7.5

3.6
4.4
4.2
3.9

5.8
6.0
6.1
5.9

455
496
461
432

Table 6: Effect of cooking time on the pulp yield and pulp properties in high sulphidity kraft pulping of sapwood (Active alkali concentration 20%, temperature 1700C. material to liquor ratio 1:5).

Cooking time (hr)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

1
2
3
4

48.1
44.4
43.4
40.0

32.9
22.4
19.8
18.0

7.7
7.9
7.8
7.5

3.8
4.0
4.2
4.0

5.8
6.0
6.1
5.8

439
467
461
431

Kraft-anthraquinone (AQ) pulping
Table 7 represents the effect of active alkali in the low and high sulphidity kraft-AQ pulping of sapwood. The nature of delignification of sapwood in low and high sulphidity was almost similar in entire range of active alkali. Pulp yield was deceased to about 43% at 18% active alkali from 46% at 16% alkali in both sulphidity. All physical properties of sapwood kraft-AQ pulps were increased with increasing alkali concentration from 16 to 18% then decreased.

Table 7: Effect of alkali concentration on the pulp yield and pulp properties in low and high sulphidity kraft-AQ pulping of sapwood (Cooking time 2 hour at 1700C and material to liquor ratio 1:5).

Alkali concentration (% on o.d. sapwood)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g) 400SR

Burst index (kPa.m2/g) 400SR

Breaking length (Km) 400SR

Double fold number 400SR

Low sulphidity

16
18
20
22

46.3
43.6
43.1
40.2

21.5
20.8
19.4
16.9

7.6
7.9
7.6
7.3

3.9
4.2
4.2
4.0

5.8
6.1
6.1
5.9

465
452
419
412

High sulphidity

16
18
20
22

46.1
43.8
43.1
40.2

20.3
20.3
19.4
17.1

7.7
7.8
7.6
7.1

4.0
4.1
4.1
3.9

6.1
6.2
6.1
5.8

387
393
391
364

Table 8: Effect of cooking time on the pulp yield and pulp properties in low and high sulphidity kraft-AQ pulping of sapwood (Active alkali concentration 18%, temperature 1700C. material to liquor ratio 1:5).

Cooking time (hr)

Pulp yield (%)

Kappa number

Tear Index (mN.m2/g)

Burst index (kPa.m2/g)

Breaking length (Km)

Double fold number

Low sulphidity

1
2
3
4

46.9
43.6
41.4
38.3

26.7
20.8
18.1
18.0

7.7
7.9
7.7
7.3

4.0
4.2
4.3
4.0

5.9
6.1
6.0
5.9

369
452
434
409

High sulphidity

1
2
3
4

47.4
43.8
42.1
40.0

27.4
20.3
18.4
18.0

7.7
7.8
7.6
7.4

4.1
4.1
4.2
3.8

5.8
6.2
6.3
6.0

335
393
357
307

As is seen in Table 8 that pulp yield reduced from 47 to 38% with kappa number 27 to 18 in increasing cooking time from 1 to 4 hours in low sulphidity kraft-AQ process. High sulphidity kraft-AQ process showed pulp yield 46 to 40% with almost similar kappa number in increasing cooking time from 1 to 4 hours. Physical properties of both kraft-AQ pulps showed similar nature.

The effect of AQ in low and high sulphidity on the delignification of sapwood pulp in respect to increasing active alkali is given in Figure 1. A desired kappa number could be attained in the kraft15 cook with 18% active alkali. An addition of 0.1% AQ in the liquor reduced the active alkali by 16% to reached the kappa number about <25. Kraft 30, kraft30-AQ and kraft 15-AQ showed almost similar nature. Similar results were obtained in wood pulping (7-9).

Figure 1

Figure 2 shows the effect of AQ in low and high sulphidity kraft delignification of sapwood in respect to cooking time.

Figure 2

It is seen from Figure 2 that the addition of AQ reduced the cooking time to reach a particular level of delignification. The desired kappa number <25 could be attained in low sulphidity kraft with cooking time of 3 hours and in high sulphidity kraft below 3 hours. The kappa number than <25 and was reached in kraft 15-AQ cook in 2 hours. It is thus observed that on addition of AQ in the low sulphidity white liquor reduced the cooking time by 33%. An addition of AQ in the high sulphidity white liquor, same delignification was observed as in the low sulphidity. Therefore, AQ is more favorable in low sulphidity kraft pulping.

Compared with normal kraft pulping, kraft-AQ showed marked lower kappa number at the same yield (Figure 3).

Figure 3

At pulp yield of about 44%, high sulphidity kraft, kraft-AQ pulps showed almost similar kappa number that was about 3 points lower than the low sulphidity kraft. Van Allen et al. (10) changed the sulphidity from 15 to 29%, the total pulp yield at a given permanganate number increased by 0.8% on o.d. wood, which was not statistically significant, it was indicated to that observed by Aurell (11). Tasman (12) observed an increase of 2% for Jack pine and a decrease of 0.3% for Black spruce for a change in sulphidity from 20 to 35%. Therefore, it appears that the magnitude of pulp yield change with sulphidity in kraft cooking is species-dependent. In the alkaline-AQ process, a nucleophilic addition of AHQ-2 to a lignin quinone methide to give an adduct, followed by an elimination reaction that regenerated AQ and lead to - aryl ether cleavage (13, 14). The predominant productive delignification event appeared to be cleavage of the abundant -aryl ether linkage (15).

The influence of AQ on the breaking length of sapwood pulp at 400SR is presented in Figure 4.

Figure 4

The figure shows that an addition of AQ in low sulphidity kraft liquor produced pulp of about 4% higher breaking length at kappa number 25. Breaking length of kraft 30 and kraft30-AQ coincided at kappa number 25. Therefore, it seems that AQ is more effective for low sulphidity kraft process.

The effect of AQ on the burst index of kraft pulp at 400SR is shown in Figure 5. Kraft-AQ was superior to the normal kraft pulp.

Figure 5

The Figure 5 shows that normal kraft-15 exhibited highest burst index value at kappa number 22 whereas kraft 15-AQ at kappa number 18. This value was about 14% higher than the normal kraft-15. This means that an addition of AQ, the delignification can be continued to a lower kappa number without an adverse effect. Kraft-30-AQ gave about 5% higher burst index than the normal kraft-30 at kappa number 22.

The tear index of kraft-15-AQ pulp was almost similar to the normal kraft-30 pulp at kappa number about 20 as shown in Figure 6.

Figure 6

Kraft-30 pulp showed lower tear index than the kraft pulp. Previously the loss in tear index in kraft-AQ pulping was observed with black spruce (16).

Figure 7 shows the relationship between kappa number and double fold number at 400SR.

Figure 7

Kraft 15 produced highest double fold number at kappa number about 23 whereas kraft 15-AQ and kraft 30 at about kappa number 20. Kraft 15-AQ and kraft 30 showed almost similar value in the kappa number range 16-20. Unfortunately, kraft30-AQ gave inferior results.

It is known that the properties of the pulp are interdependent. Hence it is more common practice to look upon tensile tear relationships. They are considered to be the most important strength properties and at the same time they are inversely related in the majority of the cases. Such a comparison for the pulp at almost similar degree of cooking (kappa number about 22) is drawn in Figure 8.

Figure 8

The Figure shows that the tear strength of kraft-AQ and kraft 30 pulps at any breaking length was higher than the kraft 15 pulp. At higher breaking length, tear index of kraft30, kraft-AQ pulp was much higher than the kraft 15. In the initial stage of beating, no significant different was observed between normal kraft and kraft-AQ pulp in respect to breaking length tear index relationship.

CONCLUSIONS

The following conclusions may be drawn from this investigation:

- Pulp yield and kappa number of sapwood in kraft process were decreased with increasing active alkali, cooking time or temperature.

- High sulphidity kraft process showed high pulp yield and lower kappa number than that of low sulphidity kraft process.

- High sulphidity kraft process produced pulp better strength properties as compared to low sulphidity kraft pulp.

- An addition of AQ in the kraft liquor decreased cooking time or active alkali to produce of desired kappa number with higher pulp yield.

- Low and high sulphidity kraft-AQ pulps exhibited almost similar pulp yield and kappa number. The strength properties were better in low sulphidity kraft-AQ pulp. Therefore, AQ is more effective in low sulphidity kraft process.

REFERENCES

1. Kocurek, M.J. and Stevens, C.F.B. (1997). Pulp and Paper Manufacture. Volume 1: Properties of Fibrous Raw Materials and their Preparation for Pulping. CPPA P. 55.

2. Miller, B.R. 1999. Wood handbook-Wood as an engineering materials Gen. Tech Rep. PL -GTR-113. Madison, WI: US Department of Agriculture, Forest Service, Forest Product Laboratory. P.463.

3. Helena P. and Bruno E. Kraft pulping and heartwood development in Maritime pine. www .pierroton.inra.fr/WBB/Abstracts/S2o/Helena_pereira.1.htm

4. Dix,-B; Roffael,-E. Holz-als-Rohund Werkstoff. 50:1, 5-10; 38(1992).

5. Kleppe, P. J. Tappi 53(1): 35 (1970).

6. Aurell, R. Sevensk. Papperstid. 67(3): 89 (1964).

7. MacLeod, J.M., Fleming, B.I., Kubes, G.J., and Bolker, H.I., Tappi J. 63(1): 57(1980).

8. Rajan, P.S., Griffin, C.W., Jameel, H., and Gratzl, J.S. Extending the digester delignification with anthraquinone. TAPPI 1992 pulping conference Proceedinggs, TAPPI PREESS, Atlanta, p. 985.

9. Sarwar Jahan, M. Ph.D. Thesis. Studies on the pulping of jute based raw materials using various additives and their characteristics. Rajshahi University, Rajshahi, Bangladesh, 2000.

10. Van Allen N.J. Hatton J.V. and Gee, W.Y. Tappi J. 64(6):64(1981).

11. Aurell R. Svensk Papperstid 66(23): 978 (1963).

12. Tasman J.E. Trans Tech. Sect 6(1): TR 19 (1980).

13. Obst, J. Landucci L. Sanyer N Tappi J. 62 (1):55 (1979).

14. Landucci L. Tappi J. 63 (7):55 (1980).

15. Gierer J. Wood Sci Technol. 19 289(1985).

16. Blain, T.J. Tappi J. 63 (5): 125(1980).

N.B. 'Because the climate similarities between Bangladesh and Southern Africa, the results of this investigation will be applicable to some Southern African hardwood species'.

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