ADVANCES IN ACID AND ALKALI LEACHING FOR COAL DESULFURIZATION: A REVIEW

Abstract

Coal is a key global energy source. However, using it creates considerable environmental challenges. This is due to the mineral matter and sulfur compounds in coal, which contribute to ash production and the release of pollutants like sulfur dioxide when the coal is burned. Chemical leaching has proven to be a successful method for coal beneficiation, significantly decreasing ash and sulfur content while maintaining calorific value. This article offers an extensive examination of diverse leaching methodologies, encompassing acid leaching, alkali leaching, acid–alkali combination treatments, and oxidative leaching. Acid leaching with hydrochloric acid (HCl), hydrofluoric acid (HF), nitric acid (HNO₃), and hydroiodic acid (HI) effectively facilitates the selective extraction of mineral matter and sulfur compounds, with HNO₃ and HI attaining a sulfur reduction of up to 90% under optimal conditions. Alkali leaching efficiently solubilizes silicates and alumina, whereas integrated acid–alkali treatments improve demineralization by treating remaining constituents. Oxidative leaching, utilizing agents like hydrogen peroxide and ozone, enhances the removal of pyritic and organic sulfur. The combined use of these processes can yield ultra-clean coal with negligible environmental impact. This article examines process characteristics, constraints, and recent improvements, emphasizing avenues for sustainable coal consumption and cleaner energy production.