Deep Deashing of Long−Flame Coal by a Synergistic Alkali−Acid Process: Optimization and Mechanism

Long−flame coal, with its abundant reserves and favorable structural properties, serves as an excellent raw material for preparing coal−based carbon materials such as hard carbon. However, the high ash content in natural and conventionally processed long−flame coal significantly limits its high−value applications. To achieve deep deashing and prepare ultra−low−ash coal, a synergistic alkali–acid deashing process was investigated, and the underlying mechanism was preliminarily explored. Results showed that the alkali−leaching followed by acid−washing process enables effective deep deashing. Under optimal alkali−leaching conditions—7 mol/L NaOH, liquid−to−solid ratio of 7∶1, 190 ℃, 90 min—combined with acid washing, ultra−low−ash coal with an ash content as low as 0.17% was successfully produced. Alkali leaching effectively removes Al− and Si−bearing minerals, while acid washing targets Fe−bearing minerals, with the two steps exhibiting synergistic effects during deep deashing. This study provides valuable insights into the deep deashing of long−flame coal for ultra−low−ash coal production and enhancing its added value.