Preparation Of Silica Material Derived From Rice Husks

Silica is an important functional material, which has broad application fields andgreat market demand. Many problems impeded the growth of the silica industry.(1)The quartz sand was used as the raw material to produce silica. In recent years, highgrade quartz is running short, and low grade quartz which is hard to purify is growing.High grade silica is facing to make bricks without straw.(2) Sodium silicate isprepared via reaction of quartz sand and sodium hydroxide at1300~1500°C. Hightemperature required high consumption of energy, which accounted more than70%ofthe total cost, belongs to the high energy consumption industry.(3) High temperaturereaction of quartz and sodium hydroxide is accompanied by massive carbon dioxidewhich resulted the greenhouse effect. Looking for renewable raw materials anddeveloping green synthesis process have been a crucial research topic recently.China is a large agricultural country, which produce40million tons of rice huskannually. The yield of silica from rice husk is7.2million tons. Rice husk waspyrolyzed as a source of energy, and the by-product rice husk ash which turned intothe waste of the environment was not fully leveraged. The biosilica of the rice husk asthe raw material to produce various silicas not only solved the resource of rawmaterials, but also made full use of rice husk, which shifted the waste material ofvalue.Firstly, this thesis researches the technological conditions of sodium silicate withhigh modulus which was produced by pyrolyzed rice husk. Then sodium silicate wasused to produce porous silica microspheres with high specific surface area andmonodisperse nanoporous silica.1. Preparation of sodium silicate with high modulus using alkali-solution underatmospheric conditions In this chapter, the sodium silica with high modulus was produced, which used ricehusk as raw materials via pyrolysis and the control of the concentration of alkalinesolution and the impregnation ratio.(1) The characteristics of amorphous silica was kept under the low-temperaturepyrolysis of rice husk. The sodium silicate was produced under atmospheric pressureand low temperature conditions, and the highest modulus was3.44, which could becontrolled on demand.(2) The sodium silicate was colourless and transparent by increasing the pyrolysistime at low temperature and removing the tar of the pyrolysis ash completely.(3) The advantage of this study is under atmospheric pressure and low temperaturecondition, low energy consumption, small investment, simple production process,wide application and high efficiency compared with the traditional process.2. Preparation of mesoporous SiO2microspheres using microemulsion methodThe sodium silicate derived by pyrolysis process of rice husk was used as theprecursor to prepare silica. The stable mesoporous silica microspheres were preparedusing microemulsion method by interfacial reaction.(1) The morphology of the particle is entirely spherical, and the size of the particlewas uniform and controllable. The particle size is about5μm at12000rpm rotationnumber for5min emulsification.(2) The silica spheres have three-dimensional network structure with rich pores.The mass ratio of SiO2:PEG:NH4Cl was10:1:40. The proportion of mesopore wasmore than80%and the specific surface area reached979m2·g-1by adding precipitantand pore-making agent. After washing and calcining, the purity of the particle washigh with organic reagent removed completely.(3) In this chapter, the regular morphology silica spheres were firstly prepared fromamorphous biosilica in the rice husk turned into functional material, which was asignificant breakthrough.3. The sol-gel process of preparing monodisperse nano-silicaMonodisperse nano-silica was produced by combining sol-gel method withhydrothermal reaction. The drying methods as freeze drying and the dehydration of organic solvents were used.(1) Sodium silicate solution was diluted and precipitated by3mol/L H3PO4at thespeed of0.2mL/min. The gel reacted in hydrothermal condition at120C for22h.The size of the nano-silica was about20nm and monodispersed mostly, whichovercame the character of agglomeration.(2) The specific surface area of the nano-silica is high and the pores distributedconcentrated. These characters increased with the surface-active agent added.(3) The freeze drying was used to ensure the integrity of the pore structure andprevent collapse of the pore during drying process.A variety of high value-added materials included sodium silicate with high modulus,mesoporous silica microsphere and monodispersed nano-silica were obtained byseries of experimental researches. This study provides a new solution which exploitedthe most value of renewable resources for the comprehensive utilization of silica inrice husk.