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Influence of Active Alkali on the Kinetics of Kraft Pulping of Muli Bamboo (melocanna Baccifera)

Mohammed Alam Shah,
Associate Professor,
Department of Chemistry, University of Chittagong, Chittagong, Bangladesh

Nimai Chandra Goswami
Research Student
Department of Chemistry, University of Chittagong, Chittagong, Bangladesh

Dr. A.F.M. Akhtaruzzaman,
Head, Pulp and Paper Division,
Bangladesh Forest Research Institute, Chittagong, Bangladesh

Abstract/ Executive Summary:

The rate of delignification increased with an increase in active alkali charge. The transition points between the initial and bulk, and between the bulk and residual phases shifted to a lower lignin content when the alkali charge was increased. The use of an active alkali charge of 16% as NaOH caused the residual phase to start at a lignin yield of 2.36 on oven dry bamboo. This lignin corresponds to a chlorine number of 5.06 or a kappa number of about 30 Normally a bleachable grade pulp from muli bamboo is made at a kappa number of 20. Consequently, an active alkali charge of 16% seems to be insufficient to confine the cook in the bulk phase. The situation improved on increasing the active alkali charge to 18% as NaOH Then, the second transition point occurred at a lignin yield of 1.22% on oven-dry bamboo, I,e. a chlorine number of 2.44 or a kappa number of about 15. Further increase of alkali charge may not be necessary because of a serious loss of carbohydrates. The loss of carbohydrates was maximum both in the initial and residual phases and minimum in the bulk phase. The alkali consumption also behaved similarly.

Keywords: Active alkali, Kinetics, Kraft pulping, Melocanna baccifera, Kappa number

Location: Pulp and Paper Division, Bangladesh Forest Research Institute, Chittagong.

Start Date: 00-00-0000

End Date: 00-00-0000

Program Area: Postharvest and Agro-processing

Commodity: Bamboo

Objectives:

To study the kinetic behavior of kraft pulping of the muli bamboo

Methodology:

Handling of chips

The study was carried out with mill-cut muli bamboo chips. The chips were air-dried for 10 days. The air-dried chips were separated from the dust and fines by screening on a 16 mesh screen. The screened chips were hand sorted to remove the oversized chips. The hand sorted chips were then collected, sampled in polythene bags and stored in a cold storage. The dry matter content of the chips was about 85%.

Cooking

The cooks were carried out in 2 liter stainless steel autoclaves in a thermostatically controlled heated air bath. The quantity of bamboo used in each cook was 250 g of chips (oven-dry basis). The cooking liquor was prepared with technical grades of NaOH and Na₂S. Three doses of active alkali at 16, 18 and 20% as NaOH were used. Other parameters of the cooks were.

Sulphidity, 25%
Liquor to bamboo ratio, 4:1
Rise of temperature from room temperature to 70°C by 15 minutes and from 70°C to 170°C by 90 minutes.
Cooking temperature, 170°C
Total cooking time (excluding 15 minutes for the rise iof temperature t0 70°C), varying

Post cooking treatments

After cooking, the autoclaves were cooled rapidly with running water. Black liquor samples were then taken and cooked chips were washed overnight under running water. Defibration was then done in a laboratory model Sprout Waldron disc refiner using a plate clearane of 0.25mm. Chips with cooking time of 20,40 and 60 minutes were defibrated twice. The rest were defibrated once. After refining, the pulp was screened on a flat vibrating screen with 0.38 mm stots. If there was any screening rejects, it was refined in the refiner and mixed with the screened pulp. The screened pulp was transferred into a thick cloth bag and dewatered by pressing in a screw press. The dewatered pulp was then well mixed mechanically. A portion of it was dried to a constant weight (SCAN-C3:78) and the yield was determined. The pulp was stored in the cold storage in a sealed polythene bag for subsequent analysis.

Analyses

The black liquor was analyzed according to the Swedish method for the determination of residual alkali. The chlorine number of the pulp was determined according to SCAN-C 29.72. It was converted to klason lignin + acid soluble lignin by multiplying it ba a factor of 0.90, because the chlorine number obtained was less than 26. The lignin yield on oven dry bamboo was obtained by multiplying the pulp lignin content by the total yield value. The crbohydrate yield was calculated by subtracting the lignin yield from the total yield.

Source of Information: IPPTA (Indian Pulp and Paper Technical Association) Vol. 3(2):62-68, 1991.

Article Link (Online Journal):

Funding Source:

1. Government

Budget: 00

Total Budget (Tk.) : 00

Achievement/Findings:

The results of this study with muli bamboo:There were three distinct phases of delignification. The rate of delignification increases with an increase in the alkali charge. The transition points between the initial and bulk, and between the bulk and residual phase change to a lower lignin content with an increase in alkali charge. An active alkali charge of 16% as NaOH seems to be insufficient to confine the cook in the bulk delignification phase. This could be improved by increasing the alkali dose to 18% as NaOH.

Knowledge Management: Journal