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Effect of Magnesium-salts on Hydrogen Peroxide Bleaching of Non-wood Pulps

N. Das*
Interstate Paper Corporation 2366 Interstate Road, Riceboro, Georgia, 31323, USA

S K.Bose
Bangladesh Forest Research Institute, P .O.Box 273, Chittagong-4000, Bangladesh

D. Biswas
Bangladesh Forest Research Institute, P .O.Box 273, Chittagong-4000, Bangladesh

Abstract/ Executive Summary:

Transition metal ions present in pulp, cause wasteful decomposition of hydrogen peroxide, a pulp brightener, and thus significantly affect the delignification selectivity of hydrogen peroxide bleaching. The metal ions also affect the brightness and optical properties of pulp. The free radicals generated during the decomposition degrade carbohydrates resulting in lower viscosity and yield. It is reported in the literature that magnesium sulfate successfully adsorbs transition metal ions and thus decrease their activity. This study dealt with the effect of Mg salts in hydrogen peroxide bleaching of jute caddies pulp. It was observed that prior bleaching treatments like chelation and acid washing of pulp were efficient in removing transition metal ions as indicated by lower consumption of hydrogen peroxide. However EDTA chelation seemed better compared to acid washing. The inclusion of Mg salts improved pulp brightness. The pulp viscosity was the highest with 0.3% Mg addition for both treated kraft and soda-AQ pulps. Considering both pulp viscosity and pulp brightness, it appeared that a 0.3% Mg dose on EDTA chelated pulp, was enough in hydrogen peroxide bleaching of pulps obtained from jute caddies.

Keywords: Jute caddies fibers; Hydrogen peroxide bleaching; Kraft; Magnesium salt; Soda-AQ

Location: Interstate Paper Corporation 2366 Interstate Road, Riceboro, Georgia, 31323, USA

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Wood

Objectives:

It is, therefore, imperative to study the effect of magnesium salts on deactivation of metal ions during peroxide bleaching of non-wood pulps and to optimize the Mg doses for maximum brightness and pulp viscosity.

Methodology:

Pulping experiments The air-dried (85.5% dry moisture content) jute caddies fibers equivalent to 250 g oven dried was cooked in small stainless steel autoclaves (2L) heated in a temperature controlled air bath. Analytical grades of NaOH, Na₂S, and AQ were used. The following pulping parameters were maintained: -Liquor to fiber ratio, 7:1 -Rise of temperature from room temperature to 80ºC in 30 min and from 80 to 170ºC in 60 min; -Cooking at maximum temperature (170ºC) -Active alkali, 14 - 18% as NaOH in kraft and soda -AQ; - A total H-factor of 2000 was used for pulping. Different stages of delignification were achieved by varying the amount of effective alkali (14%, 16%, 18%) both in kraft and soda-AQ pulping. The amount of AQ used in soda-AQ cooks were 0.1% calculated on oven dry material. The cooked material was washed with fresh water and then disintegrated. Screening was done on a flat vibratory screen with 0.38 mm slot. The screened pulp was pressed to remove excess water and stored in a refrigerator in polythene bags Pulp analysis The yield and kappa number of pulp samples were determined (TAPPI Test Methods T236 cm-85). The percentage of lignin content in pulp was determined by multiplying the kappa number of pulp by a factor 0.15 (Toven et al.,2006) Preparation of pulps for hydrogen peroxide bleaching Two types of treatments likely acid treatment and chelation were given to reduce the free metal ions contents on the pulp prior hydrogen peroxide bleaching. a. Acid treatment of pulp 30 g of pulp on OD basis was taken in a stainless steel container and a sufficient amount of water was added so that the disintegrator moved freely in the slurry. The pH was then adjusted to 2.0 by adding H2SO₄, to the continuously stirred pulp slurry. The slurry was stirred for an hour. Thereafter, the water was drained out and the pulp was washed thoroughly with distilled water. b. Chelation of pulp 30 g of pulp on OD basis was taken in a stainless steel container and a certain amount of water was added so that the disintegrator moves freely in the slurry. The pH was then adjusted to 4.3 with the addition of H₂SO₄ solution. Then 0.5% EDTA, based on OD pulp was added to the slurry maintaining the temperature at 60ºC and stirred continuously for an hour. At the end of an hour the water was drained out and the pulp was washed thoroughly with fresh distilled water. Hydrogen peroxide bleaching The acid treated and the chelated pulps were taken in a poly bag. Then additional amount of water was added to have the slurry with 10% consistency. The pH of the slurry was adjusted to 11.0 with addition of NaOH solution. The different doses of MgSO₄,7H₂O followed by 2% H₂O₂ solution, (all based on OD pulp) were added to the pulp which was then mixed thoroughly. The poly bag was then sealed and placed in a water bath maintained at 80° C for 2 hours with occasionally mixing. At the end of the reaction, the poly bags were taken out. The liquor was titrated to measure the residual H₂O₂. A portion of the liquor was used to measure the end pH. Thereafter, the water was drained out and the pulp was washed thoroughly with distilled water. The viscosity and brightness of pulps were determined according to Tappi test methods T230 om-89 and T452 om-92 respectively. Estimation of residual H₂O₂ 20 mL of 4N sulfuric acid was taken in a 250 mL conical flask and 25 mL 10% KI solution was added to it. 5 mL of the spent liquor sample was added and titrated against 0.1 N Na₂S₂O₃ solution using starch as the indicator.

Source of Information: Bangladesh J. Sci. Ind. Res.51(4), 291-296, 2016

Article Link (Online Journal):

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

The selectivity ratio in kraft and soda-AQ pulp from jute caddies was highest with 14% soda-AQ pulping. The reason was that the amount of lignin removed was almost the same with all alkali doses, but the amount of carbohydrate removed was the lowest with 14% soda-AQ pulping. The result is obvious as AQ acted as a nucleophile and thus, enhanced the rate of delignification. This increased delignification and reduced the carbohydrate loss that resulted in higher delignification selectivity.

Knowledge Management: Journal