Study on preparation of ultra-clean coal from bituminous coal using the ionic liquid extraction

Abstract

ABSTRACT In order to realize clean and efficient utilization of low rank coal, ultra-clean coal preparation is an important method. A clean and efficient method of producing ultra-clean coal is urgently needed, given the high environmental impact and corrosiveness of the chemicals used in the traditional chemical preparation of ultra-clean coal. In this study, ultra-clean coal was prepared by leaching using acidic imidazole ionic liquids, and the effects of temperature, time and concentration on the reaction process were investigated. The ash composition of the sample and the ash removal mechanism under the ionic liquid were revealed, and the evolution law of the microstructure and functional group characteristics was clarified. The research showed that the concentration and temperature had a large influence on the ash content of the product during the leaching process, and the reaction time had a small influence. The minimum ash content of the ultra-clean coal could reach 0.89% (dry basis). In the ionic liquid system, iron, calcium and magnesium in the coal were effectively removed, the iron oxide content was significantly reduced and calcium salts except calcium sulfate were effectively removed, while periclase was almost completely removed. However, the removal of silicon and aluminum was poor. Most of the alumina and silica remained in the ultra-clean coal. Extracted coal had more cracks and rougher surface than raw coal, the BET surface area decreased from 2.8579 to 2.4032. Under the influence of ionic liquid, the Aar/Aal ratio, which evaluates the aromaticity of coal, increased from 0.3702 to 0.8569, indicating that the aromaticity of coal was significantly improved. The CH2/CH3 ratio describing the length and branching degree of aliphatic chains in coal decreased from 4.0849 to 3.1516 which showed that ionic liquids can effectively reduce the length of aliphatic side chains, weakening the coal’s oxidizing activity. The pyrolysis curve of coal shifted to the high temperature zone and the peak temperature increased, indicating the effect of ionic liquids on the chemical stability of coal.