Bleaching Process And Kinetics Study On Ozone Bleaching Of Low Consistency Hardwood Pulp

Recently,the discharge from paper mills have been highly paid attention than before,the bleaching stage is the key stage through the process of pulp and paper making to do the clean producing.Ozone has been considered as an effective agent for the bleaching of chemical pulp,and an environmentally friendly process in pulp bleaching as well.Hardwood is a common raw material in China paper mills,so it is meaningful to study ozone bleaching on hardwood pulp.In this paper,a self-designed low consistency ozone bleaching device was used to study the ozone bleaching of hardwood sulfate pulp.Low consistency pulp bleaching was applied to discuss the influences of pulp consistency,pH,ozone consumption and reaction time on pulp properties,and the response surface method was also implemented to optimize the pulp bleaching conditions based on single factor experiment,the optimum conditions were obtained as 3%of Consistency,2.0 of pH,and 1.0%of ozone consumption.Through the above conditions,the pulp brightness was 43.5%ISO,viscosity was 700 mL·g^-1,kappa value was7.47.The empirical models on ozone bleaching of low consistency hardwood kraft pulp has been studied.To study the influence of ozone consumption on pulp properties,the variable was consumption of ozone,responses measured were kappa number,viscosity and brightness.It was found that ozone consumption was an important factor affecting pulp bleaching.The kappa number estimation function of hardwood kraft pulp by the ozone consumption was:K=7.359*(O_3CON)^-0.2318,and R² was 0.9987.The dividing point was 0.8%of ozone consumption,two fitting curves about viscosity and brightness were subsequently obtained,respectively.And the relationship models between bleaching efficiency,selectivity and ozone consumption are established.The kinetic study of ozone bleaching should include the kinetics of both delignification and carbohydrate degradation.The kinetics of delignification model used the power model:The activation energy was found to be 40.190KJ/mol.The carbohydrate degradation during oxygen delignification could be described as the two zero order reactions:The activation energy was 68.133KJ/mol and 40.132KJ/mol respectively.The mechanisms of the low consistency ozone bleaching had been analyzed by the infrared spectrum,the X-ray diffraction,fiber quality analyzed,microscope and scanning electron microscopy（SEM）.The results showed that the ozone destroyed the chromogenic groups,lignin structural units,and then the fragments dissolved from the pulp,then the purity of cellulose was increased.The addition of V additive can help improve the crystalline of ozone bleached pulp,and protected the cellulose.The fiber morphological character results showed that the surface of fiber was damaged and some grooves appeared on the fiber surface after ozone bleaching.The addition of V additive could effectively protect the surface of the fibers and reduce the damage caused by ozone.