Removal of AAEMs from high alkali coal under supercritical CO ₂ fluid-citric acid extraction system

In this paper, a method for dealkalization and upgrading of high alkali coal by supercritical CO2 fluid-citric acid extraction system is proposed. The leaching mechanism was explored by microstructural changes of elements and functional groups in coal samples, and single-factor optimization experiment was conducted. The result shows that supercritical CO2 fluid-citric acid extraction can effectively remove alkali and alkaline earth metals. The extraction mechanism is the reaction of H+ with inorganic minerals and the ion exchange of H+ with the form of phenolic hydroxyl and carboxyl groups. Under the optimum conditions, the removal rates of Ca and Mg are 44.95% and 65.75% respectively. The ash content of coal is 2.42%, and the content of Na2O (calculated by ash) is 1.13%, which meet the standards of power coal in China. Supercritical CO2 fluid-citric acid extraction is an effective and promising method for the cleaning of high alkali coal.