Mesoporous Silica From Coal Combustion Wastes

Coal fly ash and coal gasification slag are two major solid wastes discharged during the energy utilization (combustion) of coal. In China, there are a huge amount of coal fly ash accumlated during the past decades, and the annually discharge amount of coal gasification slag continue to grow. It is imminent to find a solution to such large amount of solid wastes. Since the price of constructrial and agricultural raw materials are already low enough, the application of coal fly ash and coal gasification slag in these tranditional utilization fields will not show any advantages. Therefore, the handling of these solid wastes requires the development of high add-value applications. In this study, the high silica propertey of coal combustion wastes was recongized and they were used as silica source in preparing mesoporous silica. Through the wide applications of mesoporous silica, the utilization of coal combustion wastes is brought to a whole new level, which is a big step towards the consistent disposal of coal combustion wastes.This study includes two parts. One focused on the sythesis of mesoporous silica from coal fly ash and coal gasification ash, and the other focused on the evaluation of thermal stability and application feasibity of synthesized mesoporous silica. To sythesize mesoporous silica, coal gasification slag was pretreated with acid leaching and alkaline fusing to obtain high silica solids with high chemical reactivity. Ordered mesoporous silica MCM-41 with high specific surface area (1347 m2/g) and pore volume (0.83 cm3/g) was then sythesized through sol-gel method. The major problem of sol-gel method is its infeasibility in industrial manufacturing. Therefore, an industrial feasible acid leaching technology was used for the first time as a novel pore fabrication method to sythesize mesoporous silica from coal fly ash. A mathmetic model was developed to describe the pore forming process through acid leaching. The pore structure and shape varies with a increase of acid leaching reaction intensity in the following order:nonporous. wedge shape mesopores, slit shape mesopores, cylinder shape mesopores, micropores and wedge shape mesopores. Mesoporous silica sythesized at the optimal reaction intensity has a large specific surface area of 585.02 m2/g and a large pore volume of 0.54 cm3/g. These pore characteristics are competitive with those of sol-gel synthesized. Simplicity in reaction condition control has made acid leaching an exceptional choice in industrialized manufacturing of mesoporous silica.Mesoporous silica synthesized through acid leaching show better thermal stability than that synthesized through sol-gel method. After 1 h treatment at 900?, only 15% pore volume remained for sol-gel synthesized MCM-41, while the residue pore volume of mesoporous silica synthesized through acid leaching was as high as 55%. The application of mesoporous silica in phase change material and heavy metal adsorption was evaluated with a focus on the effects of pore shape. The results shows that cylinder shape mesoporous silica presents the highest effective loading (54.85%) of phase change material. The corresponding form-stable phase change material show heat capacity of 50.65 J/g and phase transition temperature of 30.9?. Its high thermal- cycling stability and resistant to leakage is favorable in building applications. Mesoporous silica with different pore shape show relatively low adsorption capacity of Pb and Cr. After surface modification with EDTA and DTPA, the adsorption performance of cylinder shape mesoporous was highly improved and show higher adsorption capacity (EDTA functionlized: 17.30 mg/g and DTPA functionlized:16.90 mg/g) than mesoporous silica with other pore shape. The EDTA or DTPA functionlized mesoporous silica show great potential in heavy metal adsorption.