Preparation Of Natural Mediator From Bagasse Pulp Spent Liquor And Its Application In LMS Bleaching

Lignin is one of the main byproducts of pulp mills, which is one of the most widelyproduced resources in nature. Preparing phenols and aldedhydes which have a high addedvalue from lignin, can provide natural mediator for LMS bleaching.This not only make abetter use of lignin,but also provide a new direction for bio-bleaching(LMS).In this study, bagasse was used as raw material for soda-AQ pulping,and the bagassealkaline lignin was participated from the black liquor. Bagasse alkaline lignin was degradedby catalytic wet air oxidation (CWAO) with five different catalysts, and the orthogonalexperiment was carried out to find out the optimal conditions and the key influencing factorsduring bagasse alkaline lignin degradation by CWAO with Co₂O₃as catalyst.At the same time,LQP bleaching process with three kind of simulated natural mediators and one kind ofsynthetic mediator in L stage were carried out to find the effect on the brightness and strengthproperties of the pulp and COD value of the waste water.Then, lignin were isolated andpurified from black liquor,unbleached bagasse pulp,pulp with laccase treatment and pulp withlaccase/vanillin system treatment (here vanillin was used as the natural mediator), someanalyses about the structure change of lignin through FT-IR,¹H-NMR and¹³C-NMR werealso carried out.In the study on the degradation of bagasse alkaline lignin by CWAO, bagasse alkalinelignin was degraded by CWAO with five different catalysts. The results showed that the mainproducts were: p-hydroxybenzaldehyde,vanillin,syringaldehyde,syringic acid,vanillic acidand acetosyringone.The concentration of aromatic substances increased when the timeincreased.Catalysts had a significant effect on the concentration of aromatic substances. UsingCo₂O₃as catalyst, the concentration of vanillin,syringaldehyde and acetosyringone were400.2mg/L,351.4mg/L,and248.7mg/L respectively, and the concentration of vanillin wasraised82.60%comparing to that without catalyst.Using AQ as catalyst, the concentration ofvanillin was344.9mg/L which was1.5times of that without catalyst. Using Co₂O₃,CuSO₄,MnO₂,AQ,Fe₃O₃as catalysts,the total concentrations of aromatic aldehydes were1696mg/L,1658mg/L,1584mg/L,1347mg/L and1250mg/L respectively,which explainedthat using Co₂O₃as the catalyst was the best, and CuSO₄and MnO₂were next, the catalysts of AQ and Fe₃O₃contributed only for several kinds of aromatic aldehydes with smallmolecules.Orthogonal experiment was carried out to find out the optimal conditions and the keyinfluencing factors during bagasse alkaline lignin degradation by CWAO with Co₂O₃ascatalyst. The experimental results show that vanillin, syringaldehyde andp-hydroxybenzaldehyde are the main products, and these3major products concentrationaccounts for nearly50%of total concentration of9products; the optimal level combination isalkali concentration of1mol/L, lignin concentration of40g/L, top temperature of170℃,total pressure of2.0MPa and reaction time of50min. The most important influencing factoron the concentration of the3main products is the total pressure, the next are ligninconcentration, alkali concentration, top temperature, and the least important one is reactiontime. At the same time,the concentration of alkali lignin degradation products extracted byethyl acetate is lower than that extracted by the mixture of butyl alcohol and propyl alcohol.Soda-AQ bagasse pulp was used as raw materials, and LQP bleaching process with threekind of simulated natural mediators and a kind of synthetic mediator in L stage were carriedout to find the effect on the brightness, strength properties and COD value of the waste water.The result showed that after processing of LQP with natural mediator in L stage, thebrightness, the brightness stability and strength properties were all improved; at the same time,COD values of wastewater from pereoxide stage with laccase/natural mediator pretreatmentwere much lower than that with laccase/synthetic mediator HBT as well as that without anymediator.Thus, simulated natural mediators had more advantages than the synthetic mediatorin improving the properties of the pulp and reducing pollution of bleaching waste water.The FT-IR,¹H-NMR and¹³C-NMR analysis showed that:(1) the number of conjugatearomatic carbonyl for the residual lignin with LMS treatment is more than that with laccasetreatment,because of the oxidation of the C on the lignin side chain by LMS treatment,andgenerated-carbonyl;(2) this kind of bagasse lignin type was GSH,both S and G units weremore than H unit;(3) LMS treatment could cause the oxidation of C on the side chain of Gstructure and the partial cleavage of β-ether bond.