Investigation of High-Frequency, High-Intensity Ultrasonics for Size Reduction and Washing of Coal in Aqueous Medium

While ultrasonic coal-wash is not entirely new in many countries, it has not yet been practiced in India, though it would appear that the relatively high-ash content of Indian coals would render them particularly suitable for such a washing procedure. In this study, state-of-the-art ultrasonic equipment, spanning the frequency range from highly cavitational (<100 kHz), to mostly acoustic-streaming dominated (>100 kHz), were used. The coal to be studied was received from Giral mine, Rajasthan, India. Four stages for ultrasound assisted coal particle breakage have been proposed and characterized with SEM images of sonicated coal. Sono-treatment, followed by the decanting process, has been found to remove detached ash effectively. Decanted coal samples were analyzed by SEM/EDX to determine the mechanism of ash removal by sonication. The present investigation also encompasses aqueous-based ultrasonic coal desulfurization. Ultrasound in aqueous medium produces OH, H2O2, HO2, O-2, and ozone, which are strong oxidizing agents. The oxidation occurring in the presence of ultrasound is called "advanced oxidation process", and this converts the sulfur present in the coal to water-soluble sulfates, thereby enhancing sulfur removal. Larger-scale trials with high-ash and sulfur coals are strongly recommended.