Behavior and mechanism of impurity removal in the process of producing ultra-low ash coal from long-flame coal by low-temperature alkali melting and acid step leaching

Coal, besides diamond and graphite, is the largest source of carbon material in nature. Due to its high chemical reactivity, long-flame coal will be an important precursor of coal-based functional carbon materials. However, coal’s deep desulphurization and ultra-pure preparation have become the main bottlenecks that affect its development. Taking Xinjiang long-flame coal as raw material, the mechanism of removing impurities in long-flame coal to produce ultra-low ash coal by low-temperature alkali melting and acid step leaching was studied. The behavior and mechanism of impurity removal were further explored. The results showed that 0.80 % ash could be extracted during alkali melting, and the yield could reach 73.26 %. After leaching and enrichment with HNO₃, the ultra-low ash coal with 0.009 % ash was obtained. The maximum demineralization rate was 99.42 %. The migration mechanism of Si/S/Fe/Al elements was analyzed. Some Si/S elements were removed as oxides, while a small amount of Si/S reacted with NaOH to produce oxides such as Na₂SiO₃ and Na₂S. Fe/Al elements generated Fe(OH)₂, NaAlO₂, and other precipitates. Further in the acid leaching process, the oxide generated by Si/S was dissolved, resulting in a small amount of undissolved quartz precipitation. Fe/Al was almost removed. The Si/S dissolution-Fe /Al leaching-step removal mechanism in the process of producing ultra-low ash coal from long-flame coal was revealed. The ultra-low ash coal prepared by this method has high application value, which can provide raw materials for the positive electrode of sodium ion batteries and high-quality activated carbon.