Alkaline-oxygen Pulping Of Wheat Straw At Low Temperature And Tcf Bleaching Of Akaline-oxygen Pulps

Wheat straw is one of the most abudant non-wood fiber raw materials forChina's papermaking industry. However, the difficulties of alkali recovery due to thelarger viscosity of wheat straw pulping waste liquid which restraint the developmentof traditonal pulping technology. NaOH-AQ/O2two-stage cooking process is asuitable method for wheat straw pulping and environmental friendly. This papermainly study on the wheat straw alkali-oxygen alkali two stage low temperaturecooking and TCF bleaching technology.Wheat straw after alkaline-AQ pulping was disposed with oxygen directlywithout washing.An L9(34)orthogonal test was designed to study the effects of alkali charge,oxygen pressure, cooking temperature and holding time on wheat straw alkaline-oxygenpulping. The result showed alkali charge had the maximum influence on yield, while holdingtime had the minimum influence but the declined of viscosity was not obvious. Alkali chargehad the maximum influence on kappa number, while oxygen pressure had the minimuminfluence. The optimal conditions were determined as follows: alkali charge on oven dry pulp(as NaOH)1%, oxygen pressure0.6MPa, the maximum temperature85℃and holding time atthe maximum temperature60min. At this optimal condition, kappa number decreased11.8%.The wheat straw alkaline-oxygen pulp was obtained with yield52.6%, viscosity1042mL/g,the kappa number17.9, black liquor viscosity3.4mPa.s. Compared with the first stage alkalinepulping, viscosity decreased5.2%, kappa number reduced11.8%, and black liquor viscositydecreased33.3%.Alkaline-oxygen cooking of wheat straw at low temperature (85℃).Akaline-oxygen pulps were bleached through pressure hydrogen peroxide at lowtemperature(80℃), Kappa number was reduced from17.9to9.4¹1.9; when theH₂O₂dosage range between0%and5%, brightness increased from37.1%ISO to45.6%⁶0.6%ISO, and have certain linear relationship with Kappa number:︱brightness increment︱=︱decreased Kappa number︱х1.57; the yield between86.6%⁸9.5%,meanwhile there is a standard linear relationship between decreasedyield and reduced Kappa number:︱lessened yield︱=︱reduced Kappa number︱х0.19.Wheat-straw soda/soda-oxygen pulps were bleached through pressurized H₂O₂(Ppt). Kappa number of pulp decreased with increasing of H₂O₂and Na2SiO3charge.Kappa number was reduced from17.9to6.7, the removal of remaining lignin up to 62.6%, Brightnesses increase from36.2%ISO to73.6%ISO. That is to say that Pptbleaching had highefficiency of delignification and bleaching properties.Brightnesses of the bleached wheat straw soda-oxygen pulps by oxygenpretreatment-pressurized-H₂O₂bleaching (Ppt-Q-P) and pretreatment-pressurizedH₂O₂-H₂O₂bleaching(Ppt-P) both reached to about80%ISO, while brightness of thepulp bleached by sequence of PptQP was4.0%(ISO)higher than that by the PptPsequence, the viscosity of bleached pulp reached about929.6mL/g¹025mL/g.