Study On New Environment-Sound Wheat Straw Alkaline-Oxygen Pulping Technology

Wheat Straw fiber is rich in source, convenient in reproduction and good in pulpability. Through two stages Alkaline-Oxygen pulping (alkaline soaking and alkaline-oxygen cooking), wheat straw pulp with high screen yield, high brightness and excellent physical properties is obtained. In this paper the studies are focused on the puling technology, pulping mechanism, black liquor recycling and alkaline-oxygen pulp bleaching.To some extent, alkaline soaking condition dominates the pulping result. Soaking liquor-to-straw ratio 4: 1, soaking NaOH concentration 1% and soaking temperature 90癈 are the premises to achieve ideal pulping result. The optimum Alkaline-Oxygen cooking conditions for the soaked wheat straw are as the followings: dosage of NaOH 8%, max temperature 120癈, time at max temperature 30 mins, straw to liquor ratio 1: 4, initial oxygen pressure 5 kg/cm2. Under similar Kappa number, compared to Soda-AQ pulp, Alkaline-Oxygen pulp is 5% higher in screen yield and 19%ISO higher in brightness. Under the same beating degree, the burst index and tear index of Alkaline-Oxygen pulp are similar to those of Soda-AQ pulp, but the breaking length of Alkaline-Oxygen pulp is much higher than that of Soda-AQ pulp.The SiOa and total pollution contents of the black liquor of Alkaline-Oxygen pulping are much lower than those of the Soda-AQ pulping black liquor. However, compared to Soda-AQ pulping, the total liquor to straw ratio of Alkaline-Oxygen pulping is much higher. Thus the recycle of the black liquor is indispensable. Investigation shows that satisfied pulping result can be achieved with 5 times recycle of the black liquor.Degradation of carbohydrate mainly takes place during the increasing temperature stage and no obvious decreasing of carbohydrate content is founded in keeping max temperature stage. But gas chromatographic (GC) analysis indicates that the relative contents of the constituents of the carbohydrate are greatly changed. There are sharply increasing of mannose and galactose contents. Different from carbohydrate degradation, there are two turning points of residual lignin removing. 7.23% residual lignin is removed in the increasing temperature stage and 1.87% residual lignin is removed in the keeping max temperature time from 0 to 30 mins. Compared to Soda-AQ pulping the cleavage of aromatic ring and formation of HOO~ in Alkaline-Oxygen pulping are particular.Alkaline-Oxygen pulp has better bleachability for its high virgin pulpbrightness. Compared to Soda-AQ pulp, to bleach the Alkaline-Oxygen pulp to about 70%ISO with NaClO single bleaching stage, 60%-80% effective chlorine is saved. Through single step hydrogen peroxide bleaching with 2%dosage of hydrogen peroxide the Alkaline-Oxygen pulp can be bleached to the brightness above 70%ISO. Brightness above 80%ISO can be achieved through the pretreatment with KMnO4 or Na2C2O6.