Study On Environmentally Friendly Bleaching Technology And Mechanism Of Non-Wood Pulp

Although the structure of raw materials has being adjusted through increasing the proportion of wood pulp and by effectively utilizing the waste paper source, non-wood species will still remain an important fiber resource. Wheat straw, bamboo and bagasse are the main non-wood species for papermaking in China. In this paper, ECF and TCF bleaching technologies and the mechanisms of bamboo, wheat straw and bagasse pulps were studied, which will provide theoretical understanding and technique support for realizing non or less pollution bleaching in our country.Through studying OQP bleaching of non-wood pulps, it was found that stronger oxygen delignification conditions, namely pulp consistency 10%, NaOH charge 2.5 %, MgSO₄ 0.5%, O₂ pressure 0.7Mpa, 105°C and 75min, resulted in better effect of delignification, and small loss of the viscosity of pulps. The optimal pH value of chelation was 3. The optimal process conditions of H₂O₂ bleaching were: pulp consistency 10%, H₂O₂ charge 3.5%, NaOH charge1.5% (wheat straw), 1.6% (bagasse), MgSO₄ 0.05%, 90°C and 4h. It was beneficial for the improvement of final brightness if pressurized H₂O₂ bleaching was applied, but the viscosity decreased slightly.Low chlorine dioxide charge DQP bleaching was carried out. The results indicated that the wheat straw, bamboo and bagasse pulps can be bleached to the brightness of above 83 % ISO by DQP sequence under low chlorine dioxide charge (wheat straw and bamboo1.5%, bagasse 1.0%, calculated as active chlorine) and low H₂O₂ charge (1.5% for the three pulps), and the DQP bleached pulps had higher viscosity and strength and better brightness stability. The optimal pH value in D stage was 4. The suitable EDTA charge in Q stage of DQP sequence for the three pulps was 0.1%-0.2%. The charge of MgSO₄(calculated as Mg²⁺) was: 0.02% (bagasse), or 0.04% (bamboo), while wheat straw pulp did not need to add Mg²⁺ due to the high Mg²⁺ content in pulp.Under the similar brightness level, compared with CEH bleached pulps, OQP and DQP bleached pulps had better brightness stability, much higher pulp viscosity, better strength properties and lower pollution loads. The OQP and DQP bleached pulps needed less PFI revolution or less beating energy to reach the same strength properties as the CEH bleached pulps. The OQP and DQP bleached pulps had better strength at the same beating degree. The total pollution loads (CODcr, BOD₅ and SS) of QP and DQP effluent were much lower than those of CEH effluent.