Experimental Study On The Preparation Of Nano-silica From The Pretreated Rice Husk Combustion In Fluidized Bed

Active rice husk ash can be widely used in various industrial fields as an economic raw material,due to the abundance of amorphous nano-silicon.The annual output of rice husk is huge in China.Rice husk combustion to generate industrial steam/hot water and simultaneously to prepare active rice husk ash(nano-silica)is an efficient resource utilization method.However,the process of rice husk combustion to prepare nano-silica have some key problems,namely,the melting reaction between metal impurities(alkali metals)and silica will occur during the combustion process of rice husk,which reduces the combustion efficiency of rice husk and seriously effects the quality of rice husk ash.Another key issue is that the disadvantages of unstable combustion and discontinuous operation of traditional combustion equipment in the laboratory seriously restrict the industrial application of nano-silica preparation process by rice husk combustion.In this thesis,a method for preparing nano-silica by combining rice husk pretreatment with circulating fluidized bed combustion is proposed.The leaching behavior and leaching mechanism of metal impurities in rice husk during acid pretreatment is then systematically studied and the pretreatment param eters is also optimized.Finally,based on the optimized pretreatment conditions,rice husk combustion tests are conducted in a bench-scale fluidized bed and a specially designed megawatt double-circulating fluidized bed system to prepare active rice husk ash,and the technological feasibility of process is analyzed by characterization of rice husk ash.Based on the above studies,this thesis provides experimental basis for the resource utilization of rice husk and the industrial preparation technology of nano-silica,which has important practical significance.The complex acid pretreatment process was firstly simplified to approach the industrial applications.On this basis,the changes of rice husk and rice husk ash after acid pretreatment were compared in terms of chemical composition,micro-structure and activity,and the changes of influence of combustion temperature on rice husk ash was also analyzed.Thereby,modification effect of acid on rice husk and rice husk ash was preliminarily verified.The results show that the removal effect of metal impurities in rice husk is obvious after pretreatment;the activity and purity of rice husk ash obtained from the rice husk combustion in the muffle furnace is greatly improved;the restriction of combustion temperature on the performance of rice husk ash is obviously weakened.It is proved that the simplified acid pretreatment has significant effect on the modification of rice husk ash.Consequently,the effects of the main pretreatment parameters on the leaching of metal impurities in rice husk using orthogonal test and statistical regression analysis were studied,and the optimal pretreatment conditions for industrial production were found.The orthogonal test results show that factors such as acid type,concentration and leaching time have different influence degree and significance on the leaching of metal impurities K,Na and Ca.By response regression analysis,the dual-factor leaching regression equation is obtained;the comprehensive optimal pretreatment conditions are finally screened out,in coordination with the contradiction of leaching conditions of the three metal impurities.Based on the optimization analysis of metals leaching in rice husk,the leaching behavior of rice husk during acid pretreatment was further studied and the leaching mechanism of metal impurities was discussed by establishing the kinetic equation.The results show that the leaching of inorganic components and trace organic components occur simultaneously during the acid pretreatment process of rice husk;the leaching process of metal impurities is a pseudo-second leaching reaction,which is divided into two stages: rapid leaching and slow leaching,and is mainly leached by ion-exchange.Combined with the above studies,the proportion of occurrence forms of the main metal impurities in rice husk was discussed.It is concluded that the content proportion and location distribution of the three occurrence forms of metal impurities in rice husk are different.The difference in the occurrence forms and distribution determine the leaching behavior of the metal impurities.Aiming at the proposed process of preparing nano-silica from the pretreated rice husk combustion in circulating fluidized bed,the combustion test of pretreated rice husk in a bench-scale fluidized bed was first carried out based on the optimal pretreatment conditions.The improvements in the combustion condition of rice husk in fluidized bed and the performance of rice husk ash by physicochemical characterization after pretreatment were studied,and the weakening of effect of fluidized bed temperature on the performance of rice husk ash was discussed.The results indicate that rice husk can be stably burned in the bench-scale fluidized bed at 600-900 oC after acid pretreatment,and no agglomeration problem occurs.The obtained rice husk ash is composed of nanoparticle,which is rich in mesoporous pores,high purity and ultra-low residual carbon content.The quality of rice husk ash also decreases very slowly with the increasing in temperature.Therefore,the feasibility of preparing nano-silica by acid-pretreated rice husk combustion in fluidized bed was verified under laboratory condition.Focusing on the industrial application of this process,a 0.7 MW double-circulating water-cooled fluidized bed boiler system was specially designed and constructed,and pilot-scale tests of rice husk combustion were carried out in this system.The actual combustion condition of pretreated rice husk in the double-circulating fluidized bed and the performance of the obtained rice husk ash were discussed.The quality of products was finally analyzed by comparison with the rice husk ash obtained under other conditions and the relevant national standards.The results indicate that the pretreated rice husk can be also stably burned in the industrial-scale circulating fluidized bed,with high combustion efficiency and low CO emission,and the pollutant emissions in flue gas meet the national standard.The yield of rice husk ash(nano-silica)is up to 14%,with a small number of gray particles in white,high purity and activity,low residual carbon content and developed porosity.As a nano-silica product,its overall performance is at an excellent level and the market potential is huge,which is worthy of promotion and application.