Environmentally Friendly Pulping and Bleaching of Bamboos and Bagasse from Sudan

Abstract

Sudan is almost entirely depending on imports to satisfy its needs of pulp and paper. Out of the total paper consumption (92%) only 8% is locally produced. The pulp and paper industry is looking for unconventional fibrous raw materials for producing different kind of papers and boards. Sudan is endowed with both wood and nonwood lignocellulosic raw materials, all of them are available at the moment due to the end of the civil war in the South, especially the softwood forests. There must also be a sufficient supply of good quality raw material for running the process throughout the year. It has been shown that nonwood fibrous materials have high biomass production capacity and the pulp yields obtained have in most cases been higher than those from wood species. Nonwood fibres are an important source of raw materials, especially in developing countries. They can be by-product of agriculture such as bagasse and cereal straws, or industrial crops like bamboo, esparto, sisal, flax and hemp. Two bamboo species, Oxytenanthera abyssinica and Bambusa vulgaris together with an industrial by-product, namely bagasse, have been investigated for their potential for producing pulp and paper. The density (for bamboos), fibre dimensions, morphological indices and chemical composition of the raw materials studied were determined. The kraft, kraft-AQ, soda, soda-AQ, AS-AQ and ASAM pulping processes were applied and some of the pulps produced were bleached according to the TCF bleaching methods. The trials to improve the pulp properties included blending of bamboo and bagasse pulps. The average basic density of the two bamboo species was in the range of tropical hardwood of 150-1150 kg/m3 and in the middle for commercial pulpwoods (350-650 kg/m3). The fibre dimensions were microscopically evaluated. The average fibre length of Oxytenanthera abyssinica (1.5 mm), Bambusa vulgaris (2.1 mm) and bagasse (1.7 mm) were in the range of softwood and bamboo (1-7 mm). The fibre length of bambusa is considered as long, but that of oxytenanthera is medium-long. The fibre width of the two bamboo species was nearly the same, medium-narrow and of hardwood range (10- XI 35 μm). According to flexibility coefficients oxytenanthera have higher bonding strength (tensile, burst, double folds). The thicker wall of bambusa (4.6 μm) compared to that of oxytenanthera (3.7 μm) indicated a very different morphological indices and behaviour. The narrow fibres of bagasse (8.4 μm) had wide lumens (5.2 μm) and thin cell wall thickness (1.6 μm). Their good flexibility coefficient (62) indicate their ability to collapse easily and to form interfiber bonding important for paper strength. The chemical composition of the raw materials was typical for non-woody plants. They have high ash content, especially silica and low lignin contents. The very good cellulose content of bagasse (54.8%) predicted very good to excellent pulp yield. The higher cellulose content (56.6%) of oxytenanthera predicted better yields than from bambusa. The overall chemical composition indicates the suitability of alkaline methods for pulping with reasonable alkali charges and predicted good to excellent yields. This was demonstrated by cookings with different alkali charges. Bagasse was pulped easily with alkali charge of 10.9-12.4 % as Na2O for 155-160 minutes cooking time at maximum temperature of 170 oC to bleachable kappa number. Bamboo cooking was carried out with alkali charges of 17-18.6 % as Na2O for 165 minutes at 170 oC The bagasse alkaline sulphite-anthraquinone pulping gave a very high screened yield (57%) of a bleachable pulp with a very low kappa number, high viscosity, and high initial ISO brightness. The bagasse soda–AQ cooking gave excellent yield of nearly 58%. The kraft pulp of bamboo was superior to soda and soda-AQ pulps in all strength properties except for the tensile index of bambusa soda-AQ pulp. Addition of anthraquinone in the kraft pulping of bamboo increased the yield and improved the strength properties. The best bamboo cooking results were obtained with the ASAM process, which gave higher pulp yield and superior optical and strength properties than the kraft process. Higher yield, lower kappa numbers, superior viscosity and brightness were obtained at high pulp strength level for the both bamboo species. XII Oxytenanthera AS-AQ pulp was blended with the strongly bonding bagasse AS-AQ pulp in three ratios (10:90, 30:70, 50:50). With all blending ratios the tensile strength of oxy/bag pulp blends was improved, and the values were more or less the same as for 100% bagasse pulp. On the other hand, the tear index of the blends was improved compared to that of 100% bagasse pulp, and 50:50 blends were with the highest tear strength. TCF bleaching with Q1O/PQ2P sequence was applied for bagasse soda, soda-AQ and AS-AQ pulps. The bleached pulps reached a final ISO brightness of 71.6, 74.1 and 76.9% respectively at a kappa number of 1.6 – 3.9 and viscosity in the range of 790 -990 ml/g. The oxytenanthera pulps Kraft and ASAM were bleached with OQ1O/PP sequence. Good bleachability for the oxytenanthera pulps was comparatively difficult to attain as it required additional chemical charges due to the high lignin content of the unbleached pulps. The kappa numbers of the kraft and ASAM pulps were reduced from 24.8 to 6.6 and from 19.4 to 8.0 respectively. The ISO brightness obtained was about 82 % for the both pulps .The viscosity loss for both pulp types and especially for the ASAM pulps was small (from 1024 to 885 g/ml and 1114 to 1054 g/ml respectively). The addition of DTPA at the chelation stages (Q) improved the delignification selectivity, resulting in an increased ISO brightness in both cases. In the final peroxide stage the addition of DTPMPA (0.05%) and MgSO4 had been proved beneficial for successful peroxide bleaching. All raw materials chosen were well suited for obtaining pulp with acceptable to excellent yield with the most suitable pulping process tried, according to their individual characteristics. They could develop into predominant fibre resources for good quality pulps for use in printing and writing papers