Study On Low-Pollution Pulping And Bleaching Of Fast-growing Eucalyptus And Pulp Fiber Properties

Eucalyptus grows fast and behaves well in pulp and paper making,it is one of the fast-growing trees which are developed emphasisly in the "Forestry-pulp-paper integration" project,and the utilization of fast-growing eucalyptus is important for the sustainable development of China's paper industry.Researches on kraft cooking,oxygen delignification and elemental chlorine free(ECF) bleaching of fast-growing eucalyptus were carried out in this thesis;the effects of fiber properties on pulp beating and sheet properties were also studied;which will provide the fundamental base and technique support for the clean pulping and bleaching of fast-growing eucalyptus.The modelling and design software(MODDE) was used for the design and evaluation of the kraft cooking experiments of fast-growing eucalyptus.Based on the MODDE mathematic model of kraft cooking,it can be learned that the variations of effective alkali charge,sulfidity, temperature and time affected the pulp properties significantly,for the pulp with a given Kappa number,pulp screened yield,viscosity and brightness could not be maximized simultaneously.Mathematic model is based on the cooking experiments,it could predict pulp properties at a given cooking conditions,at the cooking conditions of effective alkali charge 17%,sulfidity 29.5%,temperature 150℃,time 135 min,the pulp properties predicted by model were:Kappa number 18.1(upper 21.5,lower 15.3),screened yield 50.3%(upper 51.2%,lower 49.5%),brightness 34.1%ISO(upper 36.1%ISO,lower 32.2%ISO) and viscosity 1429 mL/g(upper 1627 mL/g,lower 1232 mL/g).The kraft cooking can also be optimized by model:as the target Kappa number was set up and the weight of screened yield, viscosity and brightness were mamximized with different values(from 0 to 1),the optimal cooking conditions,including effective alkali charge,sulfidity,temperature and time,could be established.Similarly,screened yield,brightness or viscosity can also be as a target for the technological optimization.During the kraft cooking process,the temperature,effective alkali charge and time affected the pulp HexA content significantly,and the variation of sulfidity did not influence the pulp HexA content.Lower temperature,lower effecitve alkali charge and shorter time resulted in a pulp with lower HexA content and higher screened yield.The effects of alkali charge,temperature,pressure and time on oxygen delignification were studied.For eucalyptus kraft pulp with Kappa number 17.1,the optimized oxygen delignification conditions were:alkali charge 2%,temperature 100℃,O₂ pressure 0.5 MPa, time 60 min;and the oxygen delignifed pulp with Kappa number 9.5,brightness 58.4%ISO, viscosity 795 mL/g was obtained.The utilization of surfactant during oxygen delignification process could improve the delignification efficiency and pulp brightness,and pulp viscosity decreased slightly.The oxygen delignification reinforced with three different surfactants(S1, S2 and S3) were also carried out,the optimized charges of the three surfactants were all 0.2%, and surfactant S3 led to better results than the others.With the charge of S3 increasing from 0 to 0.2%,pulp Kappa number decreased from 9.5 to 8.1,brightness increased from 58.4%ISO to 62.3%ISO and viscosity decreased from 795 mL/g to 744 mL/g.Changes of lignin structure during kraft cooking and oxygen delignification were analysis by means of FTIR and ³¹P-NMR.The results showed that during kraft cooking process the ester links between lignin units were cleaved,more syringyl lignin units were dissolved,the contents of aliphatic OH,phenolic OH and COOH groups in residual lignin increased apparently.The oxygen delignification process resulted in the increase on the content of COOH groups and decrease on the content of phenolic OH groups which indicated that the phenolic rings were partially opened during oxygen delignification process,COOH groups were formed at the same time.The oxidization of lignin side chains led to increase of COOH groups and decrease of aliphatic OH.Two different eucalyptus pulps with Kappa numbers of 24.1(CK1) and 17.1(CK2) were prepared by kraft cooking process,the ODQP and DQP bleaching sequences were applied to these two kraft pulps respectively.After oxygen delignification,the Kappa number of CK1 decreased from 24.1 to 16.9,which was close to that of CK2,meaning that the kraft pulping was extended by the oxygen delignification.The bleaching results showed that ODQP pulp with the 86.1%ISO brightness and 783mL/g viscosity was obtained from CK1 pulp when the active chlorine charge was 2.5%,H₂O₂ charge was 2.5%;the DQP pulp with 85.4%ISO brightness and 706 mL/g viscosity was also obtained from CK2 pulp at the active chlorine charge of 4.0%,H₂O₂ charge of 2.5%.The property testing of the sheets made from the 86.1%ISO ODQP pulp and 85.4%ISO DQP pulp indicated that the ODQP pulp had better tear index and less beating energy consumption at a given beating degree but lower tensile index and burst index compared with those of the DQP pulp.Kraft cooking extended by oxygen delignification can improve pulp bleachability.The DQP and(DQ)P bleaching sequences were applied to CK2 pulp.The bleaching results showed that the DQP pulp with brightness 85.4%ISO and(DQ)P pulp with brightness 85.3%ISO were obtained when the active chlorine charge was 4.0%,H₂O₂ charge was 2.5%. The brightness of the(DQ)P pulp was not sensitive to the variation of the pH value in(DQ) stage in the pH range of 2 to 4,the pulp brightness didn't change significantly.The property testing of the sheets made from the DQP and(DQ)P bleached pulps indicated that both pulps had very good strength properties,the(DQ)P pulp had better tear index and less beating energy consumption at a given beating degree but lower tensile index and burst index compared with those of the DQP pulp.The experiment results indicate that it is feasible to combine D stage and Q stage into a(DQ) stage.O(DQ)P and O^*(DQ)P(O^*:Oxygen delignification reinforced with surfactant S3) bleachings were carried out with CK2 pulp.With the same final brightness about 84%ISO, O^*(DQ)P pulp consumed less active chlorine and peroxide compared with the O(DQ)P pulp; although O^*(DQ)P pulp needed a higher beating energy,the tensile index,tear index and burst index of the pulp were better than those of O(DQ)P pulp at the same ^oSR value.Generally, the strength properties of DQP and(DQ)P pulps were superior to those of O(DQ)P and O^*(DQ)P pulps.Wheat straw bleached pulps responded to refining more easily than eucalyptus bleached pulps and that the differences were due mainly to the higher contents of fines and xylan in the wheat straw pulps.During the beating process,the strength property of the eucalyptus pulps developed fastly because of the fiber swelling and fiber laminating;in the case of wheat straw bleached pulps,large amount of fines were formed instead of fiber swelling and fiber laminating during beating,which resulted in a very limited increase in pulp strength property. At a given beating degree,the strength properties of eucalyptus bleached pulps were much better than those of wheat straw bleached pulps.The eucalyptus bleached pulps had higher cellulose contents and moderate hemi-cellulose contents,which led to significant progress on the pulp strength properties; although wheat straw bleached pulps were easily beaten,the lower contents of cellulose and higher contents of hemi-cellulose(especially xylan) resulted in limited improvement on pulp strength properties.At a given beating degree,the eucalyptus bleached pulps had higher total fiber charges than those of wheat straw bleached pulps,and the total fiber charges were not affected by the beating;the surface fiber charges of eucalyptus bleached pulps were higher than that of wheat straw bleached pulps at a given beating degree,and were developed by beating.The initial number of dislocation was higher and that the number increased more rapidly during beating for wheat straw fibers than for eucalyptus fibers.These results may explain the lower and slower development of the strength properties of paper sheets prepared from wheat straw pulps than in that from eucalyptus pulps.The fines had a significant effect on sheet properties.Wheat straw pulps had a very high content of fines(35%～47%).Removal of fines from the wheat straw pulps improved not only the drainability of the pulps suspension but also the mechanical and optical sheet properties,this indicated that the fines in the wheat straw pulps acted mainly as filler with low bonding properties.Removing the fines from eucalyptus pulps led to a decrease on sheet tensile index and burst index,an increase on sheet tear index,and it could be learned that the fines in the eucalyptus pulps was beneficial to the pulp strength properties.