Integrated Utilization Of Waste Resources In Rice Straw Pulping

There are plentiful rice straw resources in our country. But most of them aredealt with firing directly which has caused serious environmental pollution problem.The best way to achieve a reasonable conversion of biomass resources is to makepaper by straw fiber. However, the ash content of rice straw is much higher than woodfiber. The major component is silica, which is more than10%in rice straw. Duringsoda-anthraquinone pulping, the silica in rice straw can dissolve in black liquor assodium silicate. High silicon content in black liquor inhibits the recovery of alkali,including the scaling formation in the evaporator and pipes, the reduction of heattransfer efficiency, the increase of boiling point of the fused material and the influenceon causticizing treatment which is caused by calcium silicate sediment on the surfaceof lime particles. All the above factors decrease the efficiency of alkali recoverysystem. The alkali recovery system of rice straw pulping faces technological problemswhich are hard to overcome. It is important to make full use of waste resources duringrice straw pulping, solve the environmental pollution of black liquor and realize theintegrated utilization of straw resources in our country.In this dissertation, due to the problems in rice straw pulping, the recycle of silicaand lignin in black liquor as well as hemicelluloses pre-extraction onsoda-anthraquinone cooking pulp properties were studied scientifically and madeindustrial improvement.1Sodium silicate in black liquor was used as silicon resource to prepare silicananoparticles.Sodium silicate in black liquor was used as silicon resource, lignin was used as astructure directing reagent and CO2obtained from the waste gas of pulp mills wasused as a precipitating reagent to prepare lignin-modified silica nanoparticles. Theeffects of lignin concentration, pH and temperature on the properties of the materialswere investigated in detail.2Two steps method was used to remove silicon in black liquor The greatest difference between rice straw pulping and wood pulping is siliconcontent in black liquor. If silicon is precipitated, wood pulping factories can use ricestraw to make paper, and rice straw used in pulping can expand the material sourceand lower production cost. Two steps method of flue gas and lime was used to removesilicon in black liquor. In the first step, CO2simulated from flue gas was reacted withsodium silicate to form silica hydrate sediment. When pH value reached9.0, thepercentage of silicon removal could reach65.21%with little lignin coprecipitation.Lignin remained in black liquor is beneficial for its combustion to gain heat in alkalirecovery system. In the second step, lime was used to react with sodium silicate toform calcium silicate sediment, and the total silicon removal percentage reached83.51%. Two steps silicon removal can improve the efficiency of silicon removal,decrease the cost of making alkaline in black liquor. It can achieve the recycle ofcarbon resource in carbon dioxide and efficient silicon removal. After silicon removalin black liquor, the interference of alkali recovery due to high silicon content waseliminated.3The lignin/silica hybrid was prepared from black liquorTo extract lignin/silica hybrid from black liquor is an efficient way to decreasethe discharge pollution of black liquor. A discuss was presented on the conditions ofpreparing lignin/silica hybrid using in situ synthesis of sulfuric acid sedimentation,including influences of pH, reaction temperature and time on the yield andcomponential content of the hybrid. The optimum condition was pH3, reactiontemperature50℃and reaction time1h. At this time, the yield of lignin/silica hybridwas the greatest, and the mass percentage of silica was25.92%. The size of theparticles was uniform.4The activated carbon was prepared from the lignin/silica hybrid as precursorThe activated carbon was prepared from the lignin/silica hybrid as precursorusing KOH activated method. A discuss was presented on the conditions of preparingactivated carbon, including influences of the amount of activating agent KOH,reaction temperature and time on iodine adsorption capacity of the activated carbon.The optimum condition was impregnation ratio (mass ratio of KOH/hybrid)3:1, activating temperature750℃and activating time1h. At this time, the iodineadsorption capacity of the activated carbon is the greatest, and the iodine adsorptioncapacity was816.26mg/g. The BET surface area was532.6m2/g.5Hemicellulose pre-extractionDuring traditional pulping process, hemicelluloses was combusted and degradedthe main organic compound of black liquor with lignin in alkali recovery system. Asa result, the viscosity of the system was increased and the dehydration of black liquoris hard to operate. Hemicellulose extracted before pulping can be used to makeproducts of high added value such as xylose, xylitol or furfural. Conditions ofhemicellulose pre-extraction of rice straw with hot water were discussed as well asimpact on soda-anthraquinone cooking pulp properties. The results shows temperatureand extraction time influence the concentration of reducing sugar in hydrolyzate andthe yield of rice straw. Considering two points of the concentration of reducing sugarand the yield of rice straw, the optimum condition of pre-extraction was1h of heatingtime and1.5h holding time at150℃.The concentration of reducing sugar inhydrolyzate was4.65g/L. Under the same condition (alkali charge15%, solid-liquidratio1:15, dosage of anthraquinone0.09%, heating time2h), Kappa number of ricestraw pulping after pre-extraction was2.56higher than that of non-extraction. Afterhemicellulose pre-extraction, the tensile index of paper decreases obviously, and thetear index of paper increases.In this dissertation, due to the problems in rice straw pulping, integratedutilization of waste resources can prepare silica nanoparticles using sodium silicate inblack liquor. Two steps method of flue gas and lime was used to remove silicon inblack liquor. The lignin/silica hybrid was extracted from black liquor and prepared theactivated carbon. Integrated utilization of waste resources during rice straw pulpingwill not only provide experimental reference, decrease the pollution of black liquor,but also make great economical benefits and ecological value.