Varma, et al., reported in 2019 that the peculiar characters and industrial applications of carbon nanotubes (CNTs) and carbon nanoballs (CNBs) have attracted the interests of technologists, scientists, and policy-makers over the past years. Here, the authors have synthesized CNTs and CNBs through MCL (molten caustic leaching) method from ten coal samples belonging to Ib valley Basin, Odisha, India. HRTEM (High-Resolution Transmitted Electron Microscopy), FTIR (Fourier Transform Infrared) spectroscopy and XRD (X-ray Diffraction) analyses appraised the presence of carbon nano-particles with varied sizes in the MCL products. The nanoballs are in the range of 52-64 nm and the nanotubes are within the range of 7-15 nm in diameter.
The petrographic parameters viz. various macerals and rank, as well as a measure of aliphatic bonds, have a significant impact on the amount of yield, purity, and quality of CNTs/CNBs. Carbon nano-products (CNPs) yield is increased with the fixed carbon content and decreased with volatile matter yield and mineral matter present in coals. Oxygen and hydrogen content in coal have decreased the carbon nanoproducts yield. Optical microscopic study of coals revealed that the carbon nano-products (CNPs) formation is favored with an increase in vitrinite and liptinite and decrease with inertinite macerals. Inertinite macerals, characterized by high aromaticity with aromatic rings of large size, do not take part (little participation) in reaction with alkalies and in acid treatment. The aliphatic bonds and chains in liptinites and vitrinites help in carbon nano-products synthesis. The proposed empirical equation PEC (petrographic estimate of CNPs) seems to indicate the amount of CNPs yield through the MCL process. The MCL may be a proficient technique for the synthesis of nano‑carbons as well as the removal of mineral matter from coals.
The authors proposed an empirical equation, PEC, to predict the amount of nano-products yield through the MCL process. Carbon nano-products (CNPs) including carbon nanotubes and carbon nanoballs can be synthesized from sub-bituminous to bituminous coals. MCL process appears to be an effective method for the synthesis of CNPs. Vitrinites and liptinites favor the formation of CNPs, whereas inertinites hinder the synthesis of CNPs. Generally, mineral matters do not favor the CNPs formation, but the presence of some iron contents in the mineral matter may increase the formation of CNPs. Authors recommend for systematic investigations with various types of coals with varying composition, belonging to different ranks for CNPs synthesis through the MCL process.
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