Ultrasonic assisted cleaning of high sulphur Indian coals in water and mixed alkali

• Semi-ultra clean coal (≤ 1.0% ash yield) are produced.
• The maximum mineral matter removal (87.52%) is obtained.
• The ultrasonic clean coals show good thermal properties.

The present investigation reports the application of ultrasonic energy (20 kHz) to few high sulphur Indian coal samples in aqueous and mixed alkali media (1:1 KOH and NaOH) for removal of sulphur and ash for industrial applications. The study shows maximum removal of ash, pyritic sulphur, sulphate sulphur and total sulphur with 87.52, 83.92, 12.50 and 18.80% respectively. The XRD and FTIR spectral analyses reveal reduction in ash and different forms of sulphur contents in coals by use of ultrasonic energy. The changes in SEM morphology of the ultrasonicated coal samples are attributed to the occurrence of the cavitations in the coal structure. The ash fusion temperatures (AFT) and the calorific values of the ultrasonicated coal samples indicate the enhanced quality of these coals for utilization in power sector. The coal leachates after ultrasonication are found to be predominant in K, Ca and Mg, which can be discharged sustainably. The possible mechanism of coal cleaning by ultrasonic energy is also discussed in the paper.