Removal behavior and mechanism of AAEMs and Fe in Zhundong coal under acidic ionic liquid system

Abstract

China's Zhundong region is rich in coal resources. However, the high content of AAEMs and Fe (Na, Mg, Ca and Fe) in the coal leads to serious equipment fouling and slagging problems during combustion. In this paper, eco-friendly acidic ionic liquid [Bmim]HSO4 was used as a pretreatment solvent for the removal of AAEMs and Fe from Zhundong coal. The effects of [Bmim]HSO4 concentration on the removal of Na, Mg, Ca and Fe were comprehensively investigated. The changes of occurrence modes of four metal elements were systematically analyzed. The removal mechanism of AAEMs and Fe from Zhundong coal was expounded. The results showed that the ash content and B/A of coal treated by [Bmim]HSO4 were significantly lower than those of water leached coal and raw coal. The content of Na2O in coal ash treated by [Bmim]HSO4 was as low as 0.939%, which met the standard of power coal. The removal rate of AAEMs and Fe increased with the increment of [Bmim]HSO4 solution concentration. At the optimum concentration of 5%, the removal rate of Na, Mg, Ca and Fe were 92.89%, 81.11%, 79.31% and 48.27%, respectively. Compared with deionized water, [Bmim]HSO4 solution showed an excellent removal effect on NH4Ac-soluble and HCl-soluble metal elements. It was mainly attributed to the following two points: Firstly, [Bmim]HSO4 destroyed the ionic force and coordination interaction between metal ions and organic oxygen-containing functional groups in coal, carboxylates and phenates were protonated and H+ replaced metal ions. Secondly, the swelling effect of [Bmim]HSO4 on coal was beneficial to the removal of tiny acid soluble minerals embedded in the coal matrix, which further enhanced the removal of AAEMs and Fe in Zhongdong coal by [Bmim]HSO4.