Extraction of Silica and Alumina from Coal Fly Ash using Box-Behnken Design for the Synthesis of Na-A Zeolites.

The extraction of soluble silicates and aluminates from coal fly ash using microwave irradiation from a domestic microwave oven is reported. Fifteen experimental conditions for the extraction were obtained using the Box-Behnken design through manipulation of three variables namely NaOH concentration, power level of microwave irradiation and irradiation exposure time. The variables with three values each were; NaOH concentration (2, 4 and 6 M), exposure time to microwave irradiation (1, 3.5 and 6 min) and power level of the irradiation (10, 50 and 100 Watts). Analysis of silicon and aluminium ions in the extracted liquids from the microwave assisted irradiation reactions between coal fly ash and NaOH solution was performed using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and the concentrations obtained compared with that of the thermal fusion reaction. The extracted silicates and aluminates (after the molar ratio of SiO2 to Al2O3 was adjusted to unity by adding aluminium foil) were incubated at 95°C for 72 hours to afford products which were characterized by their XRD patterns. These were found to contain Na-A zeolites, gibbsite and sodalite octahydrate. A correlation between the concentrations of Si4 and Al3 in the extracts and the types of zeolites produced after hydrothermal is attempted.