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

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• Preparation of ultra-low ash coal by low-temperature melting and acid leaching. • The migration law of Si/S/Fe/Al reveals the demineralization mechanism. • The minerals in long-flame coal have been deeply removed. • The ash content of ultra-low ash is 0.009%, and the demineralization rate is 99.42% 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 HNO 3 , 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 Na 2 SiO 3 and Na 2 S. Fe/Al elements generated Fe(OH) 2 , NaAlO 2 , 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.