Study On The Ozone Bleaching Of Oxygen Delignified Hardwood Pulp And The Preparation Of Carbon Quantum Dot Photocatalyst (C/TiO₂) From Lignin

In recent years,the fiber resource shortage and environmental protection pressure are the major factors that restrict the sustainable development of pulp and paper industry in China.The high efficient,green separation technology of fiber components and low carbon,environmental friendly bleaching technology are effective ways to alleviate fiber resources shortage and to reduce pollution load,and ozone is one of the green bleaching agents with good development prospects.In this paper,ozone bleaching technology was used to bleach broad-leaved wood pulp after oxygen delignification treatment,the bleaching process parameters were optimized,and their effects on pulp and paper properities were studied.Besides,the carbon quantum dot photocatalyst was prepared by using the pre-hydrolyzed liquid lignin as material from pulping process.This research results could provide theoretical basis and technical support for the pulp clean preparation and the high value utilization of lignin.The main research contents and results are as follows:Ozone bleaching technology was used to bleach eucalyptus kraft pulp after oxygen delignification treatment.The effects of pulp concentration,bleaching time,p H and ozone dosage on the properties of bleached pulp and paper were studied,respectively.Based on the evaluation of viscosity and brightness,the influence degree of bleaching process parameters on pulp bleaching effect follows the following sequence:pulp concentration>bleaching time>ozone dosage>p H.The results show that when 0.9%dosage of ozone was used in bleaching process to bleach 35%concentration of pulp with 2.0 p H for 3 min,the best bleaching effect was achieved.At this point,bleached pulp with 76.9%ISO brightness was produced,and the viscosity and Kappa number of it were 535 m L/g and 2.71.Besides,physical properties of paper made from bleached pulp were tested,the tensile index of paper was up to 57.44 N·m·g^-1,and the burst index,tear index and folding times were 4.77 KPa·m²·g^-1,7.95 m N·m²·g^-1,89 times,respectively.Ozone bleaching technology was used to bleach eucalyptus caustic soda and anthraquinone pulp after oxygen delignification treatment.The effects of ozone concentration,temperature,type and dosage of protectant on the bleached pulp and paper properities were studied.Based on the evaluation of viscosity and brightness,the results show that when 80 g/Nm³ concentration of ozone was used in bleaching process to bleach pulp with 1.0%usage of oxalic acid as protective agent at 10oC,the best bleaching effect was achieved.At this point,bleached pulp with 72.8%ISO brightness was produced,and the viscosity and Kappa number of it were 678 m L/g and 4.18.Besides,physical properties of paper made from bleached pulp were tested,the tensile index of paper was up to 78.91 N·m·g^-1,and the burst index,tear index and folding times were 4.99 KPa·m²·g^-1,5.96 m N·m²·g^-1,38 times,respectively.Poplar wood was pretreated with hot water,cooked by the kraft method,treated with oxygen delignification(poplar H/KP/O pulp),then poplar pulp was bleached by ozone.The effects of pulp concentration,bleaching time and ozone dosage on the bleached pulp and paper properities were studied.The results show that when 0.9%dosage of ozone was used in bleaching process to bleach 35%concentration of pulp for1.5 min,the best bleaching effect was achieved.At this point,bleached pulp with84.5%ISO brightness was produced,and the viscosity and Kappa number of it were424 m L/g and 1.91.Besides,physical properties of paper made from bleached pulp were tested,the tensile index of paper was up to 47.85 N·m·g^-1,and the burst index,tear index and folding times were 2.09 KPa·m²·g^-1,5.35 m N·m²·g^-1,4 times,respectively.The fiber surface of eucalyptus KP/O pulp was damaged,fibrillation phenomenon was appeared,and some fiber fines and fragments were produced.The crystallinity of pulp decreased from 55.26%to 48.32%indicating that carbohydrates degraded severely and ethylene glycol as protective agent had no positive effect.The fiber surface of eucalyptus soda-AQ/O pulp was damaged obviously and small fragments were produced,the phenomenon of fluffiness and fibrillation was appeared at the same time.The crystallinity of pulp slightly decreased from 56.39%to 53.26%indicating that carbohydrates degraded little and compound acid had a significant protective positive effect.Lignin in pre-hydrolysis liquor coming from pulping process was used as raw material.The functional utilization of lignin was discussed preliminarily,and the lignin-based carbon quantum dots(C/TiO₂-X)were prepared by adsorbing lignin on the surface of TiO₂ nanosheets after carbonization.The CO₂ reduction properties of lignin-based carbon quantum dots were analyzed.The results showed that the C/TiO₂-3sample had the best CO₂ reduction performance,and about 19.50%of lignin and 2.80%of dissolved sugars were adsorbed on the surface of the sample.The photocatalytic of CO₂ reduction efficiency increased by 300%compared with untreated TiO₂ nanosheets.The introduction of lignin carbon quantum dots not only improved the separation efficiency of photogenerated carriers,but also improved the adsorption capacity of TiO₂nanosheets for CO₂.