Alumina and Silica Extraction and Byproduct Development Directly from Chemical Deashing of Coals

Abstract

Coal ashes (minerals) could be chemically removed to produce ultraclean coals for advanced utilizations such as oil substitutes and electrode materials. To eliminate secondary pollution and reduce deashing cost, chemicals should be recycled and valuable byproducts developed, in addition to ultraclean coals. In this work, an advanced alkali–acid (NaOH–HCl) chemical method featuring submolten salts was used to deash coals with high ash of 27.95%, and ultraclean coals were prepared with low ash of 0.62%. The alkali solutions after treating coals were regenerated by adding CaO, and the resulting precipitates were transformed into CaSO4 by adding dilute H2SO4, while alumina and silica were dissolved in acid solutions. The hydrochloric acid (HCl) after treating coals could be largely regenerated by evaporation. From concentrated solutions after evaporation, silica gels occurred with high purity, which were then filtered for the production of silicate fertilizer, highlighting low heavy metal content and Na2O. Concentrated H2SO4 was added into the remaining acid filtrate, and sulfates were precipitated and redissolved to remove CaSO4. By further two-step calcinations, alumina of high purity (98.6%) could be produced. Alumina and silica extraction and byproduct development from directly deashing the coals were compared with those from fly ashes.