Ultrasonic and microwave irradiation-aided leaching for effective desulphurization of high sulphur North-East Indian coal

This present investigation focuses on desulphurization of high sulphur North-East Indian coal under ultrasonic and microwave irradiation-aided chemical leaching. The powdered coal was treated under four different conditions, such as alkali leaching under low-energy ultrasound energy (US), acid leaching under ultra-high frequency microwave energy (MW), ultrasonic followed by microwave treatment (US–MW) and microwave followed by ultrasonic treatment (MW–US). The ultrasonic treatment was conducted using 0.1 M NaOH solution under ultrasonic energy of 20 kHz for three different residence times. Similarly, microwave treatment was performed using a mixture of 1:1 (HCl + H₂O₂) solution for three different microwave energy levels. The raw and treated coal samples were investigated using various characterisation techniques. The ultimate analysis and X-ray photoelctron spectroscopy spectra reveal a substantial reduction in inorganic, organic as well as total sulphur content of coal. The experimental findings reveal the removal of sulphur from coal in the order of US–MW > MW–US > US > MW. However, for demineralization of coal, the sequence changes to MW–US > US–MW > MW > US. During US–MW treatment of coal, a maximum removal of 65.6% of total sulphur and 35.3% of ash has been observed. Therefore, alkaline leaching under ultrasonic irradiation followed by acid leaching under microwave irradiation is recommended as the superior technique for desulphurization of coal.

Graphical Abstract