The Study Of Thermostable Alkali-Tolerant Xylanase Aided Bleaching In Softwood Kraft Pulp

The main effects of xylanase-aided bleaching are to improve bleachability, decrease chlorine dosage and reduce the environment pollution. Thesis has studied softwood kraft pulp aided bleaching with thermostable alkali-tolerant xylanase. The main conclusions are as follows:(1)The optimal reaction temperature of thermostable alkali-tolerant xylanase is 70 ± 1 ℃, and the pH is 9.0. According to the research of the thermostability of thermostable alkali-tolerant xylanase from 60℃ to 75℃, it can be found that the relative enzyme activity decreased with time increasing and changes of relative activity at 60℃ and 65℃ were slower than those at 70℃ and 75℃, besides, thermostable alkali-tolerant xylanase showed favourable pH stability and the relative enzyme activity of thermostable alkali-tolerant xylanase was more than 92.9% when the pH between 8.8 and 11.0.(2) Optimizing the process condition of thermostable alkali-tolerant xylanase aided bleaching in softwood kraft pulp by the response surface method. The optimal process was as follows:xylanase dosage was 12.07 U/g dry pulp, reaction time was 98.98 min and pulp concentration was 10.04%. Then, discussing the effect of thermostable alkali-tolerant xylanase pretreatment for ECF bleaching under the optimal process, the result indicated that the dosage of hlorine dioxide reduced 20%, the pulp yield and tear index fell slightly and other physical properties maintaining well while achieving the same brightness.(3) After thermostable alkali-tolerant xylanase aided bleaching, the surface of softwood kraft pulp was loose and rough, appeared tear and grooves, furthermore, it’s altitude difference increased, lignin content reduced and carbohydrate content increased based on the analysis of SEM, XPS and AFM. The results of X-RD showed that crystallinity of softwood kraft pulp rised with thermostable alkali-tolerant xylanase pretreatment. The analysis results of FT-IR and HPLC showed that thermostable alkali-tolerant xylanase could decompose xylan of pulp, which was beneficial to chemical bleach and playing an effective role on pulp bleaching.(4)To extract LCC from untreated pulp and treated pulp by thermostable alkali-tolerant xylanase, whilst analyze its structure with FT-IR and CP/MAS 13C-NMR. The results proved that the structure of LCC from treated pulp by thermostable alkali-tolerant xylanase was destroyed, which was beneficial to the dissolution of residual lignin.