Effect of leaching high sulphur subbituminous coal by potassium hydroxide and acid on removal of mineral matter and sulphur☆

The effect of leaching coal samples from Boragolai and Ledo collieries of Makum coal fields, Assam, situated in north eastern region of India with potassium hydroxide solution alone at 95 and 150 °C as well as followed by mild acid on demineralization and desulphurization was investigated. Potassium hydroxide alone leads to 2–19% demineralization and 16–30% desulphurization of the coal samples at 95 °C. Demineralization of the coals decreases to 1–11 and desulphurization increases to 26–43% on increasing the temperature to 150 °C. The decrease in demineralization is due to increased precipitation of potassium aluminosilicates. Demineralization of the coal may be enhanced to 28–45 and 39–68% and desulphurization to 22–35 and 34–53% at 95 and 150 °C, respectively, by leaching the potassium hydroxide treated coal with 10% hydrochloric acid which decomposes the potassium aluminosilicates to certain extent. The treatment almost completely remove the inorganic and up to 37% organic sulphur from the coal samples.