Transforming coal washery rejects into carbon nanomaterials via microwave pyrolysis for waste to value conversion

This study presents a sustainable approach for converting coal washery rejects into high-value carbon nanomaterials (CNMs) using microwave-assisted catalytic pyrolysis. Initial characterization of the raw coal tailings revealed a high fixed carbon content (40.27 wt%) and calorific value (20.4 MJ/Kg). Through beneficiation techniques such as froth flotation and oil agglomeration, the carbon content was enhanced to 73.97-75.93 wt% with an increased calorific value of 27.2-30.7 MJ/Kg. The cleaned coal concentrate was subjected to microwave pyrolysis in the presence of Fe-based catalysts. Under optimized conditions (810 W microwave power, 45 µm catalyst particle size, and 2:1 coal-to-catalyst ratio), the process resulted in a 66.5 % yield of carbon nanomaterials with up to 80 % purity. Varying these parameters influenced the morphology of the synthesized nanomaterials, producing carbon nanotubes with an average length of 1.41 µm and diameter of 80.22 nm. Raman spectroscopy analysis revealed an I_D/I_G ratio ∼ 1 for the optimized samples, indicating a high degree of graphitization, while XRD confirmed the presence of crystalline graphite (002) planes at 2theta ∼ 26°. These findings demonstrate the feasibility of transforming high-ash coal waste into functional carbon nanostructures via a controlled microwave-heating route.