Limited availability and large industrial demands of high-grade coals have forced many coal-dependent industries to shift their preference towards low-grade coals to meet the feedstock requirements. The low-grade coals due to their poor washability nature do not respond efficiently toward existing coal beneficiation techniques, making the cleaning process challenging. Inefficient cleaning of such coals could potentially lead to environmental problems such as solid waste generation and gaseous emissions during combustion. Therefore, it is important to upgrade the existing coal beneficiation techniques for improving the clean coal quality, and simultaneously enhance the efficiency of the process. In the past few decades, many techniques have been developed to improve existing coal beneficiation techniques. Among them, ultrasound technology has gained significant attention due to its ability to enhance the process performance. The incorporation of ultrasound can significantly increase the clean coal yield under the substantial effect of cavitation and streaming. In this paper, an overview on the recent development in ultrasonic-based coal beneficiation techniques and the role of ultrasound in improving the efficiency of various coal beneficiation techniques are discussed. This includes a critical review of the ultrasound mechanism in enhancing coal demineralization, desulphurization, grindability, slurryability and dewatering.
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.
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