Straw Alkaline Sulfite Depth Delignification Cooking And Bleaching Characteristics

Wheat straw has a great value of development and a good prospect as a biomass. It is important to expore the alkaline sulfite pulping (ASP) and bleaching which is benefit for the application of it. The biorefinery concept of non-wood lignocellulosic fiber biomass ASP was proposed in this study. The influences of total alkali charge, sulfurous acid degree, cooking temperature and time on the characteristics of wheat straw extended delignification and lignin sulfonation are investigated. Meanwhile, combining with the process of thermostable and alkaline tolerant xylanase (XTA) pretreatment and oxygen delignification, the research of environmently friendly bleaching (ECF and TCF) were discussed.The main results were as follows:The wheat straw ASP process has a high selectivity of the extended delignification, and its lignin sulfonation degree increased with the extended delignification. The total alkaline charge, sulfurous acid degree, maximum cooking temperature and the time at maximum temperature had significant impacts on both extended delignification selectivity and lignin sulfonation degree. A good extended delignification pulp with yield 56.8%, kappa number 8.8, and viscosity 33.3 mPa.s was obtained under the cooking conditions of the total alkali charge 18.0%, the sulfurous acid degree 85.0%, liquor ratio 3.5, maximum cooking temperature 168℃and 150 minutes at 168℃. The lignin sulfonation group content in the black liquor was up to 2.16 mmol/g. Based on the extended delignification selectivity, the lignin sulfonation and the basic pulp properties, wheat straw ASP process have good perspective for pulping biorefinery.Both XTA pretreatment and oxygen delignification have the selectivity of extended delignification. A pulp with a high yield, low kappa number and high viscosity can be obtained by XTA pretreatment. A good extended delignification pulp with yield 93.5%, kappa number 6.4, viscosity 41.1 mPa.s and the rate of lignin removal 33.1% was obtained under the enzyme dosage was 5.0 IU/g. Followed with the alkaline dosage increasing, a pulp with a high yield, low kappa number and high viscosity could be obtained by oxygen delignification. A good extended delignification pulp with yield 88.9%, kappa number 4.7, viscosity 30.0mPa.s and the O2 delignification 52.5% was obtained under the alkali dosage was 2.0%.Under the same conditions of ECF and TCF bleaching, both XTA xylanase pretreatment and oxygen delignification show efficiency of the bleaching, which enhance the brightness of each section.The use of XTA can increase the final viscosity of pulp.In addition, the final brightness of the bleached pulp with pretreatment were 1.0%ISO higher than that of unpretreatment pulp, and the viscosity was almost the same though the oxygen delignification, which also had an increment of 1.0%ISO in the final birghtness of pulp.We did not observed that the lignin condensation, lignin side chain structural change, and the crystallinity of cellulose by the xylanase pretreatment and oxygen delignification. Compared with the residual lignin condensation in wheat straw Soda-AQ pulp, the residual lignin condensation in the ASP pulp was lower, which was indicated by highβ-O-4 structure content from ozonation analysis.