Research On The Low Kappa Number Oxygen Pulping System Of Rice Stra

Rice straw is abundant in natural resources.It seems,however,that the application of it in paper industry is conditioned by its well-known handicaps such as high silica content and more non-fibrous cells,so as hardly to reach the level of clean production of straw pulp.In this dissertation,rice straw is firstly pulped with alkali-oxygen,then highly selectively bleached with ozone.It is hoped that the two-stage pulping gives rise to high quality of bleached rice straw pulp with brightness of ca.80%ISO or more,where the selectivity of ozone bleaching is above 15.Finally,the waste liquors from ozone bleaching will be recycled to a great extent in order to eliminate the effluent ultimately,which helps to establish an ozone bleaching process with high-efficient,energy-saving and clean,for rice straw alkali-oxygen pulp,and also accelerate the utilization of rice straw pulp in pulp and paper industry.To a large extent,alkaline precooking plays a positive role in improvement of the alkali-oxygen pulping result.The alkali charge and treatment time were evaluated during alkaline precooking,and the optimum two-stage alkali-oxygen pulping conditions for rice straw were obtained as follows:NaOH dosage 19.5%,MgSO4 charge of 0.5%,maximum temperature 110℃ for 9 min,straw to liquor ratio 1:5 in precooking stage;and initial oxygen pressure 5 kg/cm2,time to max temperature 60min,maximum temperature 105℃ for 420 min in alkali-oxygen pulping stage.The resultant pulp with non-fibrous cells(Pf)had screened yield of ca.54.0%,brightness 58.5%ISO,kappa number in the rang from 4.5 to 6.7,viscosity 857 mL/g;and the pulp screened-out non-fibrous cells(Pnof)had screened yield of 45.4%,brightness 59.1%ISO,kappa number 6.1,viscosity 936 mL/g.In addition,the non-fibrous cells obtained had yield of 16.6%,brightness 53.0%ISO,kappa number 9.0,viscosity 657 mL/g.Based on the low-kappa-number pulp with non-fibrous cells,the parameters of ozone bleaching process were systematacially studied in this paper.The optimized ozone bleaching were carried out under pulp consistency 50%with ozone flow of 0.5 kg/min·odt for 10 min at pH=1.5,with addition of 1.5%oxalate and 0.5%EDTA,which brought about the good selectivity of ozone bleaching near 19.The bleached pulp presented desired properties:kappa number 2.4,brightness 80.3%ISO,viscosity 829 mL/g.It was concluded that the two-stage pulping gave rise to high quality of the bleached rice straw pulp with brightness above 80%ISO,where the selectivity of ozone bleaching was more ideal.Scanning electron microscopy(SEM)was used for observation of the appearance of the pulp fibers before and after ozonation under the optimum condition of ozone bleaching.The results showed that ozone bleaching had a moderate effect on fiber morphology,analogous to beating,like improving the fibers swelling and softening,generating a lot of fines and debris,enhancing the fiber-fiber bonds etc.The folding number,burst index,tear index and tensile index of the hand-sheets made from ozone bleached pulp increased by 42.8%,20.3%,35.4%and 16.0%,respectively,compared with unbleached pulp.However,the bleaching effect and the physical properties of the paper-sheet decreased dramatically during recycling of the bleached waste liquor in ozone bleaching.Also,the specific surface area of the fibers was determined by BET method.It was found that the fiber pore size of rice straw alkali-oxygen pulp was widely distributed in the range from 2 nm to 170 nm.While it was shrinkaged within 130 nm after bleaching.And a great number of pores with diameter 2-3 nm were distributed in the pulp fibers regardless bleaching or not,but their pore volume accounted for the few proportion of the total pore volume.The total pore volume was greatly dominated by the pores with diameter 2-50 nm.Besides,along with the waste liquors recycled in ozone bleaching,the distribution of the pore volume and pore size was changed,but it had little impact on the specific surface area.The result originated from X-ray diffraction(XRD)analysis showed that the Xray diffraction peaks from non-fibrous cells were similar to the pulp fibers,but the diffraction intensity was lower.The crystalline region of the fibrous cellulose was destroyed to some extent during ozonation,which resulted in the decrease of the crystallinity,being 65%and 60%,respectively,before and after ozone treatment.On the other hand,the crystallinity remained fluctuant kindly at about 60%on account of the diffraction intensity,being unchanged during recycling of the waste liquors.Finally,as indicated by ozone bleaching of the non-fibrous cells,carbohydrate protective agents had less positive influence on improvement of their viscosity,so that the bleaching effect was much less than that of the pulps.Consequently,it was necessary to reduce appropriately the proportion of the non-fibrous cells in the pulps,which could contribute to improve the bleaching selectivity and enhance the pulp performance,and also to provide some reference for extensive application of rice straw pulp.