Study On The Viscosity Reduction And Silicon Removal And Thermal Properties Of Rice And Wheat Straw Oxygen-alkali Pulping Black Liquor

Crop stalks are plentiful,low-price and short-growth bioresources,as ready raw materials for pulping and papermaking.Unfortunately,the development of straw pulping has been seriously impeded by the troublesome recovery of their pulping black liquors due to the remarkable existent silica etc.,as we known.In other words,the prospect of straw pulping industry is significantly dependent on if the technologies for clean production of straw pulp reach the expectant request.Soda-oxygen pulping can bring about the reduction of the silicon content?SiO₂?and viscosity of BL,considered as one of the most competitive methods for industrialization of straw pulping.However,it is true that the knowledge,such as the chemical components,structures,physical properties,thermochemical properties etc.,of its black liquor we have mastered is too poor to provide a fundamental technological support for the commercial recovery of the BL.And that is the problem to be revealed and even solved in this thesis.Additionally,the correlation of lignin and carbohydrates dissolved in the BL with oxygen and soda synergism in soda-oxygen pulping was investigated too.The main results and conclusions were summarized as follows:1.Soda-oxygen pulping resulted in the declination of the SiO₂ content in its BL by more than 60%,and the solids content of the resultant BL was easily concentrated up to c.a50%which can meet the minimum requirement of total solids concentration?TS?for commercial recovery system.Si O₂ in the BL can be further removed by simple membrane filtration process,and its TS could be increased to 55%or even 60%accordingly,nearing the level of concentrated BL from wood pulping.Besides,the barrier of the suspended solids in the BL through membrane filtration depended on its pH.The pH of BL was adjusted to 11,the filtered BL had the lowest silica content,while both Volumetric Isothermal Expansivity?VIE?value and pyrolysis loss rate were the highest.2.BLs of rice straw soda-oxygen pulping at different end points?pH<10,pH=11,pH=12?were obtained form the given cooking time.Their viscosity and thermodynamics properties before and after centrifugation or membrane filtration were investigated.On one hand,with soda-oxygen pulping prolonging,pH of the BL at the end point of pulping would be decreased,resulting in the precipitation of more SiO₂ on the fiber surface.Centrifugation or filtration had little effect on silica removal from BL at pH>11,but more than half of the silicon could be removed.And there was little difference between the effect of filtration and centrifugation on the silica removal was not significant.On the other hand,different pulping end points means that the BL subjected to oxidation reaction to varying degrees.With deeper oxidation reaction,the structures of lignin and carbohydrates obtained from BL were further changed.As a result,furfural and acetic acid were released from the BL after the deepest oxidation during pyrolysis at 50^oC lower temperature than BL obtained from soda-oxygen pulping.3.Physicochemical and structural features,the physicochemical characterization of the BLs from the traditional NaOH loading?TASO?18%?w/w?and high NaOH loading?HASO?36%?w/w dry straw?rice soda-oxygen pulping were comparative evaluated.Compared with TASO pulping,HASO pulping had serious damage to the the?-O-4 and G ether linkages in lignins and glycosides of carbohydrates result in the Mw of lignin and carbohydrate decreased to 1/2 and 1/3 respectively.In spite of this,the deepended oxidation had a certain selectivity to some specific lignin structure,and made the Mw distribution relatively concentrated.4.After low-temperature alkali pretreatment,75%of the lignin could be dissolved out from the wheat straw without substantial degradation of the cellulose.Dissolution of lignin can be advanced by the pretreatment,and the resultant lignin with some narrow distribution of Mw could be gained under particularly set conditons.The lignin with molecular weights concentrated in Mw 1700-2000 and 4300-4800 can be obtained at 6%and 22%NaOH loading?w/w dry straw?respectively.5.Lignins from the BL of wheat straw soda-oxygen?SOL?and soda-AQ pulping?SAL?were identified for their structural changes and pyrolysis behavior.It was indicated that oxidative groups were introduced into lignin structure,and the oxidation reaction selective broke down syringyl ether linkage while retained?-O-4 linkage.SOL has lower pyrolysis temperature that furfural,acetic acid and lignin monomers Guaiacol can be produced at 100^oC and 50^oC lower than that of SAL.