Occurrence and transformation behavior of AAEMs in the flotation fraction of a typical Xinjiang coal

The occurrence and thermal transformation behaviors of AAEMs (alkali and alkaline earth metal species) in a typical Xinjiang coal (Wucaiwan) were systematically investigated.The raw coal sample was primarily separated to four fractions based on different densities by float-sink experiments using organic heavy solutions.Subsequently, the BSE-EDX (backscattered electron-energy dispersive X-ray detector) and XRD were employed to analyze the mineral matters in coal fractions.Moreover, the thermal transformation behaviors of minerals in coal were simulated using FactSage.The results show that the active minerals (i.e., AAEMs) in coal vary with the flotation density and exhibit quite different transformation characterizations.Samples with density of 1.40-1.50 g/cm³ are the major fraction (70%) for Wucaiwan coal.The Na and K exist principally in the lower density (-1.40 g/cm³) fraction;in contrast, the AAEMs are combined with the alumina-silicate in the high density (+1.50 g/cm³) fraction.Additionally, Ca is mainly in the form of calcite;and kaolinite and quartz are enriched in the high density fraction.Less Na could release at lower heat treatment temperature (500℃);thus the release amounts of Na in four fractions present less difference.The majority of Na is volatilized from the low density coal sample at 815℃;but for the high density coal samples the clay (mainly kaolinite) and quartz could capture Na and restrain its release.According to the FactSage calculation, the NaCl is found to be still stable in the coal ash at 550℃;it would start evaporate over 550℃ and generate NaAlSiO₄.The NaCl would vanish at 620℃ and the NaAlSiO₄ gradually becomes constant.Moreover, at the temperature range of 500-815℃, Ca is mainly in the form of CaSO₄ and Ca-Si-Al, while Mg varies in several forms.