Oxygen Delignification And Bleaching With Chlorine Dioxide Of Reed Kraft Pulp

Traditional bleaching with chlorine-based chemicals is being eliminated gradually, while oxygen delignification and bleaching with chlorine dioxide are currently widely used with better environmental and social benefits.The technology of oxygen delignification and bleaching with chlorine dioxide of reed kraft pulp was studied in this paper.The parameters of oxygen delignification and bleaching with chlorine dioxide were optimized. The influence of high temperature on bleaching with chlorine dioxide, the kinetics of delignification and carbohydrate degradation of oxygen delignification were studied. The mechanism of each stage of the bleaching process was also analyzed by FT-IR. The results were as follows.The kinetic study of oxygen delignification should include the kinetics of both delignification and carbohydrate degradation. The kinetics of delignification model used the power model:Delignification reaction order in lignin, concentration of NaOH and oxygen pressure was 6.72, 0.87 and 0.44 respectively, and the activation energy was found to be 80.96KJ/mol. The carbohydrate degradation during oxygen delignification could be described as the two zero order reactions:The carbohydrate degradation reaction order at rapid degradation and slow degradation in the concentration of NaOH was 0.23 and 0.20 respectively, while that in oxygen pressure was 0.21 and 0.19 correspondingly, and the activation energy was 32.40KJ/mol and 39.63 KJ/mol respectively.The optimal conditions of one-stage oxygen delignification of reed kratf pulp were as follows: time 60min, temperature between 90℃and 100℃, concentration of NaOH between 2% and 3%, oxygen pressure between 0.4MPa and 0.6MPa, MgSO4 charge 0.2% and pulp consistency 10%. Around 45% of the lignin was reduced but the carbohydrate degradation was seriously leading to a certain degree loss of physical strength, especially the selectivity of temperature and concentration of NaOH were even worse. The optimal conditions on D₀ bleaching of chlorine dioxide of reed kraft pulp after oxygen delignification were as follows: final pH value between 3.0 and 4.0, ClO₂ charge between 0.8% and 1.0%, time 60min, temperature between 55℃and 65℃, among them. Final pH value was the biggest influence factor on D₀ bleaching of chlorine dioxide.After D₀EpD₁ of reed kraft pulp after oxygen delignification, the highest brightness could be obtained when the ratio of active chlorine charge between D₀ and total active chlorine charge was in the range from 5/8 to 6/8, and there was 40.41% reduction of viscosity and 4.66% bleaching loss in the chlorine dioxide bleaching.Either D₀ or D₁ under high temperature of 85℃could make final brightness improved by around 1%ISO.The FT-IR of the reed kraft pulp after oxygen delignification showed that the absorption peak at 1643-1645cm^-1 of C=O increased, the hydroxyl which in the lignin side chain C_αwas oxidated, and generated C_αcarbonyl structure, with C_αcarbonyl structure it can easily make the C_α-C_βfracture, and generate aldehyde or carboxy group, cased the lignin degradation.The absorption peak of 1321 cm^-1 and 1245-1253 cm^-1 were decreased,which indicated the degradation of the syringe and condensation guaiacyl lignin.The absorption peak of 1163-1165 cm^-1, 1057-1058cm^-1 and 897cm^-1 were seriously decreased,it was show that the carbohydrate degradation was much more.After chlorine dioxide bleaching, absorption peak of 1643-1645cm^-1 have little decrease, which indicated the decomposition of the aldehyde or oxidation of carboxy group.The yellowing pulp absorption peak was decreased at 3379-3416cm^-1, and increased at 1643-1645cm^-1, it may because of that in hot induction hydroxyl generated to carbonyl and making the pulp yellowing.