Research On Reinforced Hydrogen Peroxide Bleaching Of Poplar CMP And Bleaching Mechanism

Taking fast growing wood as raw material to develop high yield pulping process is an important way to resolve the fiber materials shortage of pulping industry.But the pulp brightness may be a restrict factor as it is not easily bleached to high target.Hydrogen peroxide has been widely used in mechanical pulp bleaching.But conventional H₂O₂ bleaching has some shortcomings,such as the brightness gain and bleaching efficiency are low and the invalid decomposing reactions are violent.To improve H₂O₂ bleaching efficiency,ultrasonic radiation,TAED activator and H₂O₂/TAED displacement bleaching were carried out in this research.The H₂O₂/TAED bleaching process of poplar CMP and its pretreatment methods were fully studied.The results indicated that second adding NaOH,acetic anhydride activating,and ClO₂ activating can improve H₂O₂ bleaching efficiency and pulp brightness effectively.The H₂O₂ and H₂O₂/TAED displacement bleaching processes of poplar CMP were confirmed.The modified displacement bleaching apparatus for laboratory use was initially made to decrese channelling effect of conventional apparatus.The FTIR,element analysis,gel permeation chromatography(GPC) and proton nuclear magnetic resonance(¹H-NMR) were employed in this research in the analysis of components changes in poplar lignins before and after bleaching treatments.The optium pretreatment conditions of probe-type ultrasonic radiation for Triploid Populus tomentosa CMP were confirmed.The brightness gain was 20.7%ISO with the optium ultrasonic radiation pretreatment and 2%H₂O₂ bleaching.For the ultrasonic-H₂O₂ bleaching of Populus nigra L.CTMP,the pulp brightness gain with PT bleaching system was 19.9%ISO,which higher 7.3%ISO than conventional H₂O₂ bleached pulp.The brightness gain gets the highest value(21.2%ISO) with activator and ultrasonic radiation coexistence.Study on H₂O₂/TAED bleaching process of Populus nigra L.CTMP showed that the suitable bleaching condition were:pulp consistency 10%,H₂O₂ 4%,NaOH 2%,the molar ratio of TAED and H₂O₂ is 0.3,temperature 70℃,bleaching time 60min and the brightness gain can get 23.6%ISO.The results indicated that the P_T2P₄ procedure has better bleaching result(brightness gain was 33.5%ISO) than P_T3P₃ and P_T4P₂ procedure at the same H₂O₂ usage.The results showed that second adding NaOH,acetic anhydride activating,and ClO₂ activating can improve H₂O₂ bleaching efficiency and pulp brightness effectively.Among of them,the method of second adding NaOH was better.The brightness gain of P_Ts1P₃ was higher 4.5%ISO than that of P_T1P₃. P_TY bleaching procedure can bleach the pulp to 83.0%ISO brightness.The optimum H₂O₂ displacement bleaching process for Populus nigra L.CTMP were confirmed. Comparing H₂O₂ displacement bleaching process,the H₂O₂/TAED displacement bleaching process has shorter dynamic bleaching time(7min) and lower bleaching temperature(70℃),and the ratio of TAED and H₂O₂ was 0.05.The pulp brightness gains were 30.6%ISO and 31.6%ISO respectively with two bleaching systems.The brightness of pulp bleached with modified displacement apparatus better than conventional apparatus'.The pulp brightness of H₂O₂/TAED five-stage counterflow displacement bleaching increased 2.4%ISO and H₂O₂ consumption decreased 12.9%than those of single stage bleaching.This indicated that multi-stage counterflow displacement bleaching can decrease H₂O₂ consumption and increase bleaching efficiency.From FTIR absorption,it can be found that the conjungated carbonyl contents were reduced with P, PD and PT treatments.The results from the reduction differential absorption also proof the conclusion. The reduced degree of conjungated carbonyl is PT＞PD＞P.This showed that PT bleaching system has stronger effects on conjungated carbonyl of lignin.The o-quinone contents of bleached lignins which tested by NaBH₄ reduction method were reduced by P,PD and PT bleaching.Among three bleaching processes,PT bleaching made the o-quinone content lowest.Comparing with Populus nigra L lignins,conjungated carbonyl and o-quinone contents in Triploid Populus tomentosa lignins reduced relatively larger at the same bleaching condition.According to the ¹H-NMR results,it can be found that guaiacyl structure reduced with P,PD and PT bleaching.The methoxy proton peak increased after P and PD bleaching but reduced after PT bleaching.It indicated that PT system has more effects on lignin methoxyl than the other two systems. Phenolic hydroxyl contents increase showed that aryl-ether bond is sensitive to P,PD and PT bleaching systems.