Preparation And Carbon Footprint Assessment Of High-performance Nanosilica From Rice Husk

Biomass energy is regarded as an ideal and clean renewable energy for its carbon-dioxide free and eco-friendly properties. The development and utilization of biomass energy has important significance in mitigating the greenhouse effect, reducing the air pollution and easing the energy pressure. The high-valued utilization of rice husk can solve the pollution problems caused by these agricultural wastes, and it is vital to ease the energy crisis and environmental problems. Rice husk is thought to be an ideal precursor to prepared nanosilica, which is the most widely used nano material with the most industrial output.In recent years, the preparation of silica and nanosilica from rice husk has been reported repeatedly. However, there are many problems during these technologies, such as the products are single properties, low purities, gross particle sizes and easy to agglomerate. Especially the agglomeration and dispersion of nanosilica particles are the aporia and hot spots of the preparation of nanosilica. For the above reasons, a series of studies have been carried out in this paper, to realize the scale production of biobased nanosilica and the high value-added utilization of rice husk.Firstly, based on the activity problems of rice husk during the usage of its siliceous composition, the effects of several pretreatment methods (acid leaching, grinding and a new water soaking process included) on the removal of metallic impurities and thermal decomposition behavior of organic compositions of rice husk is studied. The pyrolysis kinetics of various pretreatment processes, including water soaking, acid leaching and grinding, were investigated using thermogravimetric analysis. These results provide useful information for the following manufacturing methods of biobased nanosilica, as well as the rational design and scaling up of pretreatment reactors.Secondly, aiming at the low purity problem of traditional pyrolysis method, an optimized process is described, and nanosilica products with a purity of 99.9% are obtained. The characteristics of the nanosilica prepared from this method are superior to those of fumed silica. The mechanism of pyrolyzation at high temperature, water soaking and grinding is clarified to fill up the gap of the related theories. Results show that, pyrolysis temperature is a key factor to the purities of nanosilica products, and pyrolysis at about 610℃ is most benefit to the recovery of nanosilica from rice husk. Lower pyrolysis temperature results in a higher carbon content of nanosilica. Excessive pyrolysis temperature leads to the partial replacement of O atoms by free carbon atoms of pyrolysis intermediates when bonding with Si atoms, which forming a structure of [Si(O,C)4], destroying the original [SiO4] structure, and reducing the silicon content of nanosilica. The adoption of water soaking pretreatment can remove most of the metallic impurities from the rice husk, destroy the carbon skeleton of rice husk, and improve the quality of the nanosilica products indirectly. The optimal preparation conditions are determined, the dynamic analysis results of the last chapter are verified.On the basis of improved pyrolysis method, in view of the shielding dispersion principle of pyrolyzed black pigment on the surface hydroxyl layer of SiO2, a novel two-staged thermal synthesis method of generating nanosilica from rice husk via pre-pyrolysis in CO2/N2 atmosphere combined with calcination in O2 atmosphere is performed. This method is performed to realize the directional adjustment to control properties like pore characteristic, purity and particle size of nanosilica products, as well as to solve the problem of nanosilica products preformed in a single property. Properties of nanosilica products are adjusted by controlling the pre-pyrolysis atmosphere and temperature, which dominating the volume shrinkage rate of black pigment and gas pressure of rice husk pores. Nanosilica particles of different levels of purity and textural properties are produced. The materials synthesized at the designated pyrolysis atmosphere and temperature are characterized to have a purity of 95.8-99.6%, a specific surface area of 204.3-352.6 m2/g, a total pore volume of 0.3513-0.5228 cm3/g, and a particle size of 5-20 nm.Nanosilica products obtained from pyrolysis method are hydrophilic. The huge hydrophilic hydroxyl groups on the outside surface make the silica particles easy to aggregate, difficult to be dispersed, leading to the defects of the interface between the materials, and limiting its wide application. Therefore, to obtain hydrophobic and highly-dispersed nanosilica, a novel method of generating nanosilica from rice husk via high pressure homogenization and surfactants combined with vacuum freeze-drying is proposed on the basis of cavity expansion and doubly charged ions theory. Nanosilica powder of high dispersion, high purity and high transparency are prepared. The formation of secondary particles and the hard agglomeration of SiO2 particles during drying process in the traditional precipitation method are well settled. The dispersion mechanism of high pressure homogenization, surfactants and vacuum freeze-drying process are clarified. The materials synthesized by this method are characterized to have DBP absorption factor of 3.67 ml/g, a purity of 98.9%(after calcinated at 500℃), a whiteness of 98.8% and an average particle diameter of about 35-40 nm. The optimal alkali dissolving conditions to prepare sodium silicate and the optimal precipitation conditions to prepare nanosilica are determined.Finally, the carbon footprints of rice husk-based nanosilica products are conducted by using life cycle assessment method. The environment compatibility of these products is compared to that of nanosilica products prepared by an industrial precipitation method, aiming at optimization process and seeking a system process with highly environmental benefits. The CO2 emissions, global warming potential value and dust potential value of rice husk-based nanosilica products are greatly significantly less than that of nanosilica products prepared by an industrial precipitation method. Therefore, the recovery of nanosilica from rice husk has a beneficial effect on the reduction of greenhouse gas emissions, the mitigation of climate warming, the decrease of PM2.5 and PM10 content in the air, and the remission of mist.