Design and reaction engineering aspects of fly ash processing to fertilizers
A radically new class of slow-release N-fertilizers is proposed in this dissertation. This kind of slow-release fertilizer is expected to improve both the energy efficiency of production and the plant-usage efficiency in the field. The proposed N-fertilizer is nitrided fly ash mainly in the form of SiAlON (silicon aluminum oxynitride) compounds. The compounds can be synthesized from a common industrial waste material, fly ash. The resultant products may contain up to 30 wt % N, which can be released through hydrolysis reactions under moist conditions. Therefore, nitrided fly ash has a significant potential for use as an efficient slow-release N-fertilizers. Such use would have potential environmental and economical benefits at a global scale including reductions in energy consumption, greenhouse gas production (CO 2 and N 2 O), and non-point-source pollution of streams and groundwater. This dissertation examines the effect of reaction parameters on preparation of nitrided fly ash and subsequent hydrolysis behavior. A tubular laboratory high-temperature furnace was used for carbothermal reduction and nitridation reactions. Nitrogen in the SiAlON products was measured by a nitrogen/oxygen analyzer. Field Emission Scanning Electron Microscope/Energy-Dispersive X-ray spectrometer, X-Ray Diffraction, Fourier transform infrared spectroscopy, and Brunauer-Emett-Teller surface area analysis have been used to further characterize the products. An ammonium electrode and a pH electrode were used to measure the extent of the hydrolysis. Conditions that maximize the concentration of nitrogen are also examined and discussed. The products have been prepared from fly ash of different sources. The ammonia released slowly from the hydrolysis of nitrided fly ash has the potential to be a very good N-fertilizer. Certain salts, for example calcium carbonate, can slow down the hydrolysis of the nitrided fly ash and a slow ammonia release can be achieved. The procedures to produce nitrided fly ash can be also valuable for ceramic production.