Chemical Benefication of High-Ash Indian Noncoking Coal by Alkali Leaching under Low-Frequency Ultrasonication

摘要

Coal mined from the Talcher region of Odisha, India is known to be high-ash, surface-oxidized, and noncoking in nature. It is quite challenging to beneficiate such low-grade coal by physical or physicochemical processes due to its oxidized nature and the presence of complex ash forming mineral matter in its matrix. Chemical beneficiation is one of the alternative process to such problems. However, this chemical process consumes more chemicals, treatment time, and energy, which limits its application further. Therefore, an attempt has been made to chemically beneficiate this coal cost-effectively with optimum chemicals, treatment time, and energy. In the present study, an application of ultrasound at low frequency on alkali-acid leaching is employed to investigate on the demineralization of high-ash Indian noncoking coal. The raw coal properties such as fixed carbon content, CHNS content, Hardgrove grindability index, ash fusion temperature, and gross calorific value (GCV) were investigated before the experiments. The coal samples were leached with three different types of alkali namely, NaOH, KOH, and Na2CO3 followed by H2SO4 and HCl treatment, respectively. The quality of the treated coal was examined by proximate analysis and GCV measurement. The maximum ash removal was achieved on NaOH-leached coal followed by 30% H2SO4 treatment with 73.91% demineralization and 57.21% fixed carbon. The raw and treated coal samples were characterized by Fourier transform infrared, scanning electron microscopy, and X-ray diffraction to confirm the presence of oxygenated functional groups causing surface oxidation, surface modification by ultrasonication, and the formation of alkali aluminosilicates on the coal surface, respectively. The presence of trace elements in the alkali leachates released during ultrasonication was also determined by the inductively coupled plasma atomic emission spectroscopy technique.