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Modification of Chlorine Dioxide Bleaching of Gmelina Arborea (gamar) Pulp

M. Sarwar Jahan*
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh

M. M. Uddin
Department of Chemistry, Dhaka College, National University, Dhaka-1205, Bangladesh

M. A. Kashem
Physical Instrumentation Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh

Abstract/ Executive Summary:

Elemental Chlorine Free (ECF) pulp bleaching is now the dominant pulp bleaching process in globally. In most bleachery, chlorine dioxide is over-consumed. About two thirds of the chlorine dioxide is wasted in useless side reactions. In the study, kraft pulp from Gmelina arborea (gamar wood) was bleached by ECF bleaching in modified sequences. Oxygen prebleaching was carried out to decrease ClO₂ requirement, which reduced kappa number of kraft pulp by 47.6% and increased pulp brightness by 21.7 percent points. Several sequences were tested based on the application of limited charges of ClO₂ during successive ClO₂ and extraction stage. Application of this concept allowed a 33% reduction of ClO₂ to reach target brightness. The kraft pulp could not reach target brightness of 80% in DED sequences using even 30 kg ClO₂ /ton of pulp, while splitting of same amount of ClO₂ charge into DEDED sequences reached the pulp brightness to 81.1%. But oxygen delignified kaft pulp reached 79.6% brightness using 25 kg ClO₂/ton of pulp in DED sequences. In the splitting of ClO₂ charge into DEDED sequences, Oxygen pulp reached to 85% brightness by using only 20 kg ClO₂/ton pulp.

Keywords: Pulp bleaching; Gmelina arborea (gamar); Chlorine dioxide; Chlorine dioxide Splitting and Brightness

Location: Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Pulp

Objectives:

In this study, gamar wood chips was cooked by kraft process in mill conditions and the produced pulp was oxygen delignified prior to ECF bleaching. Kraft and oxygen delignified pulps were ClO₂ bleached by splitting total ClO₂ charge. The delignification and brightness after D0E in varying ClO₂ charge were determined. The final brightness and effluent load after splitted ClO₂charge were also studied.

Methodology:

Raw materials Gamar wood chips were collected from Karnaphuli Paper Mills (KPM) and cooked by kraft process. The cooking was carried out in thermostatically control digester, rotating at 1 rpm. The cooking conditions were 170 °C for 120 min, liquor to material ratio 4, active alkali charge 18% (as Na₂O) on oven dried (o.d.) wood basis. After digestion, pulp was washed until free from residual chemicals, and screened in a flat vibratory screener (Yasuda, Japan). The screened pulp yield, total pulp yield and screened reject were determined gravimetrically as percentage of o.d. raw material. The kappa number (T 236 om-99) of the resulting pulp was determined in accordance with Tappi Test Methods. Oxygen delignification Oxygen delignification (OD) was carried out in thermostatically control digester, rotating at 1 rpm. OD conditions were 110 °C, retention time 60 min, pulp consistency 10 %, NaOH 2 %, MgSO₄ 0.3 % and O_2-pressure 3.5 kg cm^-2. The kappa number, viscosity and brightness of the resulting pulp was determined in accordance with Tappi Test Methods (T 236 om-99), (T 230 om-99) and (T 452 om-92), respectively. Bleaching Kraft and oxygen delignified pulps were bleached by D0EpD1 and D0EpD1ED2 bleaching sequence. The ClO₂ charge was varied from 0.5% to 2.5% and temperature was fixed at 70 °C for 45 min in the D0 stage. The pH was adjusted to get end pH 2.5 by adding dilute H₂SO₄. In alkaline extraction stage, temperature was 70 ^oC for 120 min and NaOH and H₂O₂charges were 2% and 0.5%, respectively. Using same the ClO₂ charge, DED sequences were splitted into DEDED sequences. In the final D1 and D2 stage, the pH was adjusted to get end pH 4.5 and 6.5, respectively, by adding dilute NaOH and temperature was 70^oC for 120 min. The pulp consistency was 10% in all the stages. The brightness (T 452 om-92) and viscosity (T 230 om-99) of the bleached pulp were determined in accordance with Tappi Test Methods.

Source of Information: Bangladesh J. Sci. Ind. Res. 52(4), 247-252, 2017

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Kraft pulp from Gmelina arborea was bleached by ECF bleaching in modified sequences. Oxygen delignification reduced kappa number by 47.6%. D0E kappa number decreased rapidly at low ClO₂ charge followed by slower decrease, which indicated over consumption of ClO₂ by non-useful reactions. Splitting of DED stages into DEDED stages increased pulp brightness from 51% to 80% using 25 kg ClO₂/ton of pulp for kraft pulp. Splitting of ClO₂ charges, pulp brightness reached to 85% by using only 20 kg /ton of pulp only for oxygen delignified pulp. Splitting of ClO₂ charge, reduced over consumption of ClO₂ through removing solubilized lignin by-products (like mucomic acid) resulted better delignification and higher brightness of the bleached pulp.

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