Morphological and Chemical Characteristics of Different Non-wood Species and Their Effect on Pulping

Abstract

Bangladesh is a densely populated agricultural country. As the allocated forestland for pulpwood production is very limited and a substantial quantity of crops residues is generated each year, the later can substitute pulpwood. But it is hard to use agriculture residues as pulping raw materials in conventional pulping process due to its high content of silica and fines. In this context, twenty-two residues of crops produced in Bangladesh were assessed in terms of their anatomical, chemical and morphological characteristics. The conventional soda-anthraquinone (AQ) pulping and alternate formic acid/peroxyformic acid (FA/PFA) pulping processes were evaluated for the suitability of the selected agricultural residues for pulp and paper production. The non-wood samples showed a lot of variation in terms of anatomical, chemical and morphological characteristics. These samples showed a low to high level of α-cellulose (27.6- 54.3%) and lignin content (20%). Alkaline oxidation showed that the lignin of many of these non-wood residues was composed of high proportion of p-hydroxyphenyl propane unit. The ash content varied from 0.64% in dhaincha stalks to 15.1% in rice straws. Anatomically, most of the agricultural wastes consisted of parenchymatic cells in different proportions which generated fines in the pulping process. The fiber length of the samples varied from 0.62-1.91 mm and the range falls within the fiber length range of hardwood and softwood. The fiber wall thickness of most of these non-wood materials was thinner than wood. The pulp yields in soda-AQ pulps of the samples were 21.01-64.39% with kappa number 4.92- 42.43, while pulp yields in FA/PFA process were 34.5-60.7% with kappa number 10.9-24.1 depending on crops residues. A low pulp yield was obtained in soda-AQ process and the formic acid process produced better pulp yield from the non-wood raw materials because of the retention of silica. Bleaching of unbleached and oxygen delignified soda-AQ treated pulps were evaluated in elemental chlorine free bleaching (D 0 (ClO 2 /D HT (E P )D 1 ) with varying chlorine dioxide ) charge (kappa factor 0.15, 0.20 and 0.25) and temperature (70 and 85 and temperature exhibited lower kappa number and higher brightness after alkaline extraction (EP) stage and high temperature ClO 2 delignification (D HT o C). ClO 2 charge ) exhibited higher final pulp brightness. Residual hexeneuronic acid content in final pulp from most of the non-wood plants were lower and exhibited 1-2% higher pulp brightness in D HT delignified pulp and D HT process than D 0 process. Oxygen process discharged lower COD load in the effluent. The paper sheets prepared from the unbleached, bleached and refined unbleached pulps from both processes showed good physical properties. Mathematical model between chemical and morphological characteristics with pulp yield and physical properties of the prepared paper sheets were developed. The pulp yield and kappa number were positively correlated with holocellulose content and lignin content, respectively. The papermaking properties of unbeaten pulps were depended on pulp fines, external fibrillation. Positive relation was observed for the fiber length with tear index of unbeaten pulp.