Study On The Technology And Mechanism Of Clean Pulping And Bleaching Of Agricultural Cotton Stalk

Cotton stalk is an abundant agricultural residue in north China.In this thesis,the development and the current status of pulp and paper industry in China was reviewed,and a proposal was put forward for using cotton stalk reasonably according to the situation of using non-wood fiber in China.The clean pulping and bleaching technology and its mechanism of cotton stalk were investigated in this thesis according to the existed problems in the use of this material and the development tendency of pulping technology.For solving the problem of high dirt content in cotton stalk chemical pulp,the parameters which affect the the dirt content were analyzed.The modified soda-AQ cooking and the pulp ECF bleaching were conducted. The removal of pection from cotton stalk bark was taken into the research with biological methods.The SCMP of cotton stalk and the pulp bleachability were researched systematically and some improved bleaching sequences were put forward.For improving the properties of cotton stalk refiner mechanical pulp,the white-rot fungi pretreatment technology was taken into the BMP process.Bio-mechanical pulping of cotton stalk and its mechanism were investigated in detail.Changes of fiber characteristics and lignin structure were analyzed by using advanced analysis methods,which could provide technical and theoretical support for the clean,efficient utilization of cotton stalk resource.The dirts of bleached whole cotton stalk chemical pulp came mainly from the black dust. The dirt removal ability of hydrogen peroxide bleaching was better than that of hypochlorite bleaching and chlorine dioxide bleaching.The dirt removal effectiveness of chlorine was better than that of oxygen.The impregnation-extrusion pretreatment could improve the bleachability and dirt removal of cotton stalk chemical pulp.The KP-AQ and KP pulping methods contributed to the good reduction of dirts.Modified Soda-AQ cooking of cotton stalk with pretreatment and addition of auxiliary chemicals during cooking resulted in a much lower Kappa number,much higher pulp yield and brightness and preserved pulp viscosity compared to conventional Soda-AQ pulp.ODQP sequence was used for the bleaching of cotton stalk pulp.At the dosage of 1.5%of both ClO₂ in D stage and H₂O₂ in P stage and the optimal other conditions,the brightness of bleached conventional Soda-AQ pulp reached 80%ISO,the brightness of bleached modified Soda-AQ pulp was higher than 88%ISO and the dirt content in pulp was very low,which indicated the realization of high brightness,clean bleaching of cotton stalk chemical pulp.The research of pectin removal from cotton stalk bark showed that both xylanase pretreatment and pectinase pretreatment could reduce the pectin content in cotton stalk bark. The effectiveness of pectinase pretreatment was better than that of xylanase.The pectinase pretreatment could improve pulp drainage ability and the pulp yield with lower Kappa number. For mechanical pulping of cotton stalk bark,the enzyme pretreatment could give the pulp better bleachability and lower beating degree but reduce the pulp yield.The pretreatment with pectinase and xylanase could give the bleached pulp highest brightness.The investigation of SCMP pulping of cotton stalk showed that the dosage of sodium sulfite and sodium hydroxide had strong effect on the pulp strength properties and bulk.With increasing of sodium hydroxide dosage,the pulp yield,brightness and bulk decreased,while the strength properties increased.The enhancement of pretreatment temperature could strengthen the treatment degree and improve the pulp strength properties,while the brightness, bulk and pulp yield reduced.The xylanase pretreatment could improve the brightness and strength of cotton stalk BMP pulp.The xylanase pretreatment to SCMP pulp could give the same effectiveness as that to BMP pulp.The optimum single stage H₂O₂ bleaching conditions to SCMP of cotton stalk were as follows:H₂O₂,3%;NaOH/H₂O₂.1.25;Na₂SiO₃,4%;MgSO₄,0.05%;EDTA,0.3%;60℃;150 min;pulp consistency,15%.In P₁QP₂ bleaching sequence,the bleached pulp could get the highest brightness when the H₂O₂ dosage ratio in P₁ and P₂ stage was 35:65.The chlorine pretreatment could increase the brightness of bleached pulp and improve the pulp strength. The potassium permanganate pretreatment couldn't improve the bleachability of SCMP. PaPPa and PaPP bleaching sequences could give the pulp good brightness.At a total H₂O₂ dosage of 5%,the brightness could reach 60%ISO.As total H₂O₂ dosage increased to 7%,the brightness would be higher than 66%ISO.The pretreatment of xylanase to SCMP could improve the hydrogen peroxide bleachability and the optimum pretreatment conditions were as follows:dosage of xylanase,40IU/g pulp;pH value,8;60℃;90min;pulp consistency, 8%.Both PLP and LPP bleaching sequences where L used the unpurified enzyme liquor from Lg-9 fungus and BYBF fungus respectively could improve the pulp strength.The PLP bleached pulp had lower brightness,while LPP bleached pulp had higher brightness.As crude nzyme liquor from Sdu-4 fungus and xylanase were used in the same or separate stage,the pulp bleachability could be improved.The order of pulp brightness was LXP＞XLP＞(LX)P＞XP＞LP.Trametes versicolor and Lg-9 had strong capability of producing laccase.P. chrysosporiun 25 had strong capability of producing manganese peroxidase.For these three fungi the increasing of pretreatment time could decrease the pulp yield,brightness and bulk, improve the opacity properties and strength properties.The optimal white-rot fungi pretreatment could increase the mean fiber length,reduce the fines content,improve the refining performance and pulp bleachability,enhance the bleached pulp brightness.The Lg-9 fungus was the best one in these three fungi,as the strength of the resulted pulp was highest.The lignin content in cotton stalk reduced obviously during pretreatment with Lg-9, while,with the lignin degradation,parts of carbohydrates were degraded in a certain extent. The pentosan got more degradation than cellulose.The other components got some removal in different extent.The pretreatment of Lg-9 could make the mean fiber length of cotton stalk biomechanical pulp increase and fines content decrease.Compared with cotton stalk RMP pulp,the Lg-9 pretreated BMP fiber had longer average fiber length and lower f(?)nes content. The classification analysis illustrated that the pretreatment of Lg-9 could make the resultant pulp have more content of long fiber fraction and middle fiber fraction as well as lower ccntent of fines fraction.The SEM observation showed that the mycelium of Lg-9 growed on the surface of the cotton stalk.The surface of Lg-9 treated cotton stalk had many pits,cavacities and grooves with different depth.The surface became abrasive.Some holes and cavacities existed in the fiber cell wall.The fiber pretreated by Lg-9 became soft and existed in the pulp with belt form. The enhancement of fiber softness and less damage of the fiber during refining were the reasons that Lg-9 pretreatment could improve the pulp strength properties.The UV and IR analyses illustrated that parts of lignin units were removed during the Lg-9 treatment and subsequent refining.The removal of syringyl units during Lg-9 treatment was more than that of guaiacyl lignin units,while,during refining more guaiacyl lignin units were removed than syringyl lignin units.The carbonyl content in lignin increased during the Lg-9 treatment and the following refining.Some links between lignin units were broken up. The contents of lignin side chain and phenolic hydroxyl group increased.The¹H-NMR and ¹³C-NMR analyses showed that the aromatic ring couldn't be broken up,parts ofβ-5/β-1 structue units were broken up.In addition,part of lignin molecules with carboxyl group was degraded without the structure change of the carboxyl group.