Pulping Technology And Bleaching Mechanism Of Neosinocalamus Affins Chemimechanical Pulps

The effects of different chemicals application in the chemcial impregnation stage in chemimechanical pulping from a bamboo species Neosinocalamus affinis on pulping properties were investigated. The suitable conditions in the chemcial impregnation stage were suggested in term of technical and economical points. The bleachability of the bambaoo chemimechanical pulps were investigated in details,and the reasons for the difficulties of bleaching the pulps to a high brightness were discussed.The chemical constituents and fiber morphology of the bamboo species were tested. The average length of the bamboo fibers is 1.68mm, and the width 17.5um, and the ratio of fiber length to width is about 71.3. The extractives contents of the bamboo species are high (1%NaOH extractive accounts for 25.95%, hot water extractive takes up 5.42%, benzene-ethanol extractive is 2.12%). The ash content is 1.60%, which is lower than most gramineous fiber materials.After being screened, the accepted bamboo chips imperged in water at romm temperature overnight. After being peeled off free water and pre-steamed at atmospheric presure for 10 minutes, the bamboo chips were compressed by a MSD screw press at a compression ratio of 1:4. The compressed stocks were impregnated in the five kinds of chemical liqours, namely, alkali, neutral sodium sulfite, alkali sodium sulfite, alkali hydrogen peroxide and sodium hydrogensulfite, to obtain different impregntation effect. The impregnated bamboo stocks were then refined and bleached further. By compared in term of specific refining energy, bleachability, pulp strength properties and pollution loads of the effluents, the results demonstrated that the chemcial impregnation process by using alkali hydrogen peroxide and neutral sodium sulfite have more obvious advantages and can be expected to used as the accepted chemical impregnation methods available for bleached pulps. The pretreatment conditions of the alkali hydrogen peroxide impregnation process are: NaOH 1.5%,H₂O₂ 2%,DTPA 0.2%,tempreture 75oC, reaction time 30min, pulp consistency 25%; and the conditions for the neutral sodium sulfite impregnation process are: Na₂CO₃ 2%,Na₂SO₃ 2%,DTPA 0.2%,tempreture 75oC, reaction time 30min, pulp consistency 25%.The effects of alkali hydydrogen peroxide pretreatment on bleaching properties of bamboo CTMP pulps were evaluated. The results show that the bleachability of the bamboo pulps is not good. When the hydrogen peroxide dose exceeds 8%, the bleaching efficiency drops rapidly with only 1.5 brightness units gain at per unit chemical usage. Even bleached at a high usage level of 30% H₂O₂, the pulps was just bleached to a brightness of 63%ISO. With increasing bleaching agents, bleaching efficiency drops and the bleach yield of the pulps decreases rapidly (at 2% H₂O₂ usage, the bleach yield is 95.7 %, and at 30% H₂O₂ usage, the yield 85.5%). Meanwhile, the pollution loads of the bleach effluents increase rapidly companying with lignin being dissoved more. The pollution load reached 209.1 kg.t^-1 when the hydrogen peroxide usage was 30%.The lignin structure of the bamboo chips, the bleached and the unbleached CTMP pulps were analyzed by modern spectruom analytical instruments.. The results show that the lignin structure of the bamboo species belong to the G-S-H type, among of which G takes up 32.6%, S accounts for 38.4%, H contributs to 29.0%. Interpreted by the UV-Vis differences spectrums for the pulps before and after bleaching, the chromophore structures of carbonyl groups and aldehyde groups might be destroyed by hydrogen peroxide bleaching. Meanwhile chromophore structures of quinoidstructure which has a great influence on the absorption of visible light might be destroyed partly. With the increasing of the hydrogen peroxide usage, the changing of the quinoidstructures to colorless structures seems to be harder. It concluded that there exists some quinoid structures in the bamboo pulps which are immune to hydrogen peroxide bealching , which showed that quinoid structures of lingin may be one of the obstacles for the bamboo CTMP bleaching.