Study On Environmentally Friendly Wheat Straw Soda-AQ Pulping Technology And Mechanism

This study focus on the environmentally friendly pulping technology of wheat straw. Thesuitable end point of wheat straw was determined, the obtained conditions were applied in thestudy of black liquor replacement cooking and extended delignification process. The mechanismand potential application properties of a high pure thermostable and alkaline tolerant xylanasepretreatment on bleaching properties and drainage of wheat straw pulp were studied byDesign-Expert software, XRD, FQA, FT-IR, Alkaline Nitrobenzene Oxidation, Ozonation, SEMand HPLC. The results could provide sufficient informations for the establishment ofenvironmentally friendly pulping technology. The results showed that:1．The optimized of the cooking conditions were alkali dosage17.0%, cooking temperature150℃, liquor ratio4.5:1, time at maximum temperature60min, AQ0.05%. The obtioned pulpproperties were yield48.9%, kappa number13.3, viscosity27.3mPa·s, whiteness37.8%ISO.The pulp properties were affected by the end point of cooking. It was found that at the optimumcooking conditions, time at maximum temperature45～60min (Kappa number13～15) was theright end point of cooking. ODQP,OQ(PO)1(PO)2and OD0EpD1D2were the high selectivitybleaching process;2. The effects of black liquor replacement cooking (BLRC) on wheat straw Soda-AQ pulpswere investigated. The residual AQ and dissolved carbohydrates in the black liquor are beneficialto the delignification selectivity while the dissolved and degraded lignin can slow down thedelignification rate. When the black liquor replacement rate was50%⁶0%, the pulp yield couldreach51%. With fibre fractionation, FQA, SEM and lignin structure properties analysis, therewere no remarkable difference on the fiber morphology, physical and drainage properties, lignincondensation and β-O-4structure degradation, between treated by the BLRC compared with thecontrol Soda-AQ cooking;3. Single stage oxygen delignification of wheat straw pulp was about50%. Both H₂O₂reinforced oxygen delignification and two stage oxygen delignification was more than60%. Thechain of lignin and carbohydrates maybe were cut off during oxygen delignification process. ByFT-IR, SEM, XRD and lignin structure properties analysis, the increase of lignin phenoichydroxyl group enhanced the dissolution and subsequent reactivity of lignin. Meanwhile,the lignin condensation was limited, and the side chain β-O-4structure and methoxyl contentwere decreased. Non-crystalline area of cellulose was effect by oxygen delignification, however,there were no remarkable influence on the fiber morphology. In addition, oxygen delignificationprocess was enhanced after H₂O₂added. Compared with single stage and H₂O₂enhanced oxygendelignification, two stage oxygen delignification has a high deliginification and selectivity.4. The high pure thermostable and alkaline tolerant xylanase has strong absorption and specific hydrolysis on xylan. A high temperature (60℃～80℃) and pH (7.0～9.5) were proper inthe pretreatment process. The center of the response surface experiment indicated that thebleached pulp accumulation yield, viscosity deceased rate and finally whiteness was about90%,5%and88%ISO respectively after xylanase treatment at conditions pH7.0～8.0, temperature75℃～85℃, time1.0～1.5h, pulp consistency10%～13%, xylanase dosage2.0～4.0IU/g andinitial pulp kappa number13～15.5. The better bleaching efficiency and low pollution was achieved after the xylanasetreatment. The final bleached pulp brightness was close to90%ISO. The ClO₂dosage in the D0stage could be saved by23.5%(14%of total ClO₂dosage) to achieve the same brightness (87%ISO) of the unpretreatment bleached pulp. By HPLC, FT-IR, SEM, XRD and lignin structureproperties analysis, while the xylanase pretreatment decrease hemicellulose exclusively, HexAand lignin decreased without causing an obvious change in lignin structure characteristics. Thesewere the mainly reason for improving bleaching performance. The bleaching chemicals caneasily penetrate inside the pulp fibers and residual lignin can be easily dissolved in the bleachingbecause of fiber porosity increase after xylanase treatment.6. Xylanase treatment could improve drainability of pulp significantly and decrease cationiccharge demand and fiber wetting properties at suitable conditions (pH8.0,temperture80℃,time1.0h,pulp consistency10%,xylanase dosage2.0～4.0IU/g). Compared with untreated pulp, theFreeness increased11%～14%and WRV decreased3%. The enzymatic treatment resulted inenergy savings of28.6%highly during refining. Loose structure and cracks were observed on thefiber surfaces and cross section after xylanase treatment. The material in fiber wall was degradedor removed during refining, resulting in weak points in the fiber wall structure at high xylanasedosage.7. The environmental friendly straw pulping process was founded through systematic studyon wheat straw pulp and Soda-AQ key technology and mechanism: In the optimization ofSoda-AQ cooking, the appropriate cooking end point with extended delignification of pulp wasselected (kappa number was13～15), and then the high selectivity of two stage oxygendelignification was be used, delignification rate could achieve60%; The pulp was treated byhigh-purity and heat-stable xylanase after oxygen delignification, pulp yield was about95%,kappa value dropped to a certain extent and viscosity constant after xylanase treatment; Pulpdrainability and bleaching performance were all improved after xylanase treatment. After ECF orTCF bleaching, pulp cumulative rate was around90%, viscosity reduction rate was about5%,whiteness was about90%ISO, while bleaching chemicals and bleaching effluent pollution loadcould reduce than the traditional bleaching process; The black liquor could be appropriate forreuse cooking. Chemical consumption and alkali recovery pressure could be reduced, and pulpyield increase by4%.