Silicon Dioxide Purification From Coal Fly Ash By Phase Separation And Preparation Of High Silica Adsorbent

As the national economy and the urbanization progress rapidly,the demand and consumption of coal are rising.In this context,the output and storage of coal fly ash（CFA）,a by-product of coal combustion,are increasing progressively year by year,while the comprehensive utilization ratio of CFA has not been significantly improved in recent years.Meanwhile,the current value of the main utilization modes of CFA is generally low.The content of Si O₂ in CFA is approximately 30%to 60%,and the high-value application of CFA can be realized by recycling and purifying it.To maximize its application value,it is of vital significance to enhance the performance of purified Si O₂ and develop comprehensive utilization technology of high silica products.In this thesis,the thermal phase separation-acid leaching method was employed to remove metal oxide from CFA and purify Si O₂.B₂O₃was used as phase separation agent to explore the better process of CFA phase separation and metal removal,and investigate the mechanism of CFA phase separation and the process of metal migration.The experimental results demonstrated that the removal ratio of metal oxide in CFA could reach more than 95%under the condition of the B₂O₃ addition ratio being 25%,the heat treatment temperature being 1100℃and the heat preservation time being 1 h.Thus,the high silica products with purity of 97.22%were obtained.Moreover,molecular dynamics simulation was applied to simulate the phase separation of Si O₂ and B₂O₃at the atomic level.The Si and B atoms before and after the phase separation were studied using the root-mean-square deviation of coordinates and radial distribution function,which verified that the phase separation could be quickly completed in the high temperature section of the cooling process after the melting achieved the uniform state.The high silica products obtained by phase separation-acid leaching method were porous materials with a specific surface area of 364.21 m²·g^-1,an average pore diameter of 4.06 nm and a pore volume of 0.24 cm³·g^-1.To optimize the pore structure of high silica products,hydrothermal synthesis method was adopted to prepare mesoporous Si O₂ material from high silica powders after thermal treatment-acid leaching.The results showed that when Si/CTAB/Na OH ratio was 1:0.65:0.6,with ethyl acetate content of 5 m L,crystallization temperature being 80℃and crystallization time being 1.5 h,the maximum specific surface area,average pore diameter and pore volume of the obtained mesoporous Si O₂ were 774.42 m²·g^-1,7.27 nm and 1.41 cm³·g^-1 respectively.Its specific surface area was twice that of raw materials.Meanwhile,its pore volume was six times that of raw materials,and its pore size distribution was concentrated.Thus,its pore structure was effectively enhanced.To examine the adsorption performance of mesoporous Si O₂,its adsorption performance was tested with the organic pollutant methylene blue.The results indicated that the material’s adsorption capacity for methylene blue was 186.81 mg·g^-1,with the removal ratio of 98.84%when the p H was 11,with the adsorption temperature being 298.15 K,the initial concentration of methylene blue being 200 mg·L^-1,and the dosage of mesoporous Si O₂ being 1 g·L^-1,showing a good adsorption effect.The adsorption of methylene blue by mesoporous Si O₂ was monolayer adsorption,which is classified into spontaneous exothermic process.The adsorption capacity was related to the active sites on the surface of mesoporous Si O₂,and the driving force of adsorption lied in the number of active sites.The adsorption process includeed both physical adsorption and chemical adsorption.