The Indian high ash coal collected from Makum coal fields of Assam is crushed, sieved and air dried using lab scale instruments. Based on the preliminary results, the coal sample is treated in a lab scale microwave oven at 700 W of input power for 120 s. The physical and morphological properties of untreated coal show that there is an increase in the fines upon the treatment. The chemical analysis shows that the carbon and fixed carbon fractions are found to increase by 17.66% and 21.3% respectively whereas the ash and oxygen mass fractions are observed to decrease by 15.56% and 28.91% respectively. Due to this, the maximum settled concentration is observed to decrease by a maximum of 19.8%, and the time to reach maximum settling is observed to increase by a maximum of 58.5%. The rheological characteristics reveal that all the slurries behave like a Newtonian fluid at 10–30% solid concentration whereas a pseudoplastic behavior is observed at 40–60% solid concentration. The apparent viscosity is observed to reduce by a maximum of 46.11%. The optimum proportion of treated coal is observed to be 40% by mass of coal.