Basic Research Of The Utilization Of Amorphous Silica In High Alumina Fly Ash

High alumina fly ash (HAFA) is a typical solid waste whose emission amount is about 25 million tons per year, and the emission leads to serious pollutions. HAFA presents about 50 wt% alumina content, which is the same with the low grade bauxite. Therefore, it is seen as potential alternative to bauxite. Its utilization can resolve the environment pollution, and can effectively relieve the strain on China’s aluminum resources. The extraction of alumina with alkali is widely studied. In this paper, in view of the utilization of the desilication solution generated in the utilization process of HAFA, the routes to prepare calcium silicate and silica-calcium carbonate were developed. In the recycle process of alkali, phase and morphology change of the silicon-based materials were investigated. The structure and morphology of the crystals in the reaction were well controlled. Through the researches, the preparation of high-value silicon materials can be prepared, silicon treatment in HAFA can also contribute to environment-friendly utilization of low-grade ore and other kind of solid waste containing high silicon contents. The main contents and results are as follows:(1) The preparation of calcium silicate nanofibers in the alkaline system was studied, and the effects of the conditions were investigated. The optimum conditions are as follows:Liquid/solid ratio is 7, initial concentration of alkali is 2 mol/L, Ca/Si ratio is 1, temperaute is 240℃, and reaction time is 5 h. Under these conditions, L/D ratio of the nanofiber can reach 100. Meanwhile, the effects of the reaction temperature, CA/SI ratio, and initial concentration of alkali were systematically investigated, and the correlation between the conditions and the phases is built. Accoding to the relation between the phase and morphology, calcium silicate in wollastonite group possess the morphology of nanofiber. Based on the transmission electron microscope (TEM), selected-area electron diffraction (SAED), and surface energy calculation of foshagite, the formation of nanofiber was explained through the crystal growth.(2) A two-step route to prepare calcium solicate hydrate (C-S-H) and xonotlite was reported, and the effects of Ca/Si ratio, concentration of sodium ions, and temperature were investigated. The single-chain tetrahedron structure in C-S-H obtained in the alkaline system with short reaction time was determined through 29Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR). The X-ray photoelectron spectroscopy (XPS) analysis results confirmedthe existence of sodium ions as Na-OSi and Na-OH in the synthesized C-S-H.Moreover, Na+ combinations in the interlayer can be removed through ion exchange with 3-order countercurrent washing. The content of sodium ions can be declined to 0.35% with 5 times washing water, and xonotlite can be prepared under hydrothermal conditions. In addition, the phase can be affected by alumimun, and tobermorite can be obtained with the Al/Si ratio of over 2.69%. The synthesis of tobermorite can be prohibited through the addition of silicon or EDTA.(3) A two-step route to prepare silica with high surface area and superfine calcium carbonate was reported. Focusing on the preparation process of silica through carbonation, the effects of the conditions on the surface area of silica were investigated. Under the optimal conditions, the surface area of silica can get to 420.82m2/g. The carbonation route for silica areogel synthesis with high surface area was studied, and the effects of the silicon concentration, aging temperature, surface modification, solvent exchange et al on silicaareogel properties were also concerned. Under the optimum conditions, the bulk density of silica can be 0.25 g/mL, and the surface area can be 700.72 m2/g. Through potentiometric titration and in-situ infared spectroscopy, the change in the system was investigated. In order to prevent the separation of the mixed solvent, pre-exchange was adopted to make sure the reaction between trimethylchlorosilane (TMCS) and silica. Then, the hydrophobic structure on the suface can be formed to reduce the density of silica areogel.(4) In the highly alkaline causticization system, the coupling control on the CO32-conversion and the CaCO3 crystallization depended on the complicated reaction conditions. The causticization conditions in HAFA utilization process were determined as the reaction temperature of 50℃, stirring rate of 200 rpm, feeding time of 30 min, Ca2+/CC>32- ratio of 1.1, and Ca(OH)2 concentration of 2 mol/L, respectively. With these conditions, the conversion of carbonate can reach 95.67%. The alkali can be well recycled and superfine CaCO3 with the single crystal size of about 100 nm is obtained. In addition, the influences of alkali and Ca(OH)2 on crystallization process was investigated by the study of morphology change with TEM image and SAED patterns.(5) In causticization, the crystallization kinetics of CaCO3 was investigated in a continuousMixed-suspension-mixed-product removal (MSMPR) crystallizer. The volume growth rate, nucleation rate, and agglomeration kernel of CaCO3 were each obtained. Crystal growth is surface-integration-limited, nucleation is size-limited, and relatively low growth and nucleation rate coefficients are obtained due to the combination effect of excess OH- with Ca2+. The positive order of mean residence time for CaCO3 agglomeration in the reaction system indicates that agglomeration increases with increasing mean residence time. Meanwhile, the increase in magma density induces greater agglomeration at 0.011 mol/L, but increasingly frequent and energetic collisions break down the agglomerates at a high solid concentration of CaCO3.