Demineralization and desulfurization of high-sulfur Assam coal with alkali treatment

Demineralization and desulfurization of high-sulfur coal from Assam (Makum coalfield), India, was investigated using aqueous solutions of sodium hydroxide, potassium hydroxide, and their mixtures (1:1) alone as well as followed by mild hydrochloric acid treatment. Compared to the alkali and acid alone, successive treatments with the alkali and acid resulted in significant removal of mineral matter and sulfur from the coal. Demineralization and desulfurization was found to increase with the increase in alkali concentration. Alkali treatment resulted in formation and precipitation of sodium/potassium aluminosilicates, which subsequently undergo decomposition and solubilization in the presence of acid-forming soluble salts. It is possible to remove 50-54% of the ash, total inorganic sulfur, and around 25% organic sulfur from the coal by treatment with mixtures (1:1) of 16% sodium hydroxide and potassium hydroxide solution followed by 10% hydrochloric acid.