Mineral matter transition in lignite during ashing process: A case study of Early Cretaceous lignite from the Hailar Basin, Inner Mongolia, China

Abstract

• NME are involved in the mineral transformation with the ashing temperature increasing. • The Ca for calcite formation in ash (500 °C) was sourced from the non-mineral Ca in coal. • The increase of ashing temperature causes the change of mineral crystallinity in coal. • NME are altered into crystalline minerals during the ashing process affected by temperature. Investigation of mode of occurrence and chemical transformation of non-mineral elements (NME) is of benefit to solve sintering and slagging during utilization of coal. To better understand the mineral phase transition and thermal behavior of NME during ashing process, lignite in Hailar basin and its ash obtained at different ashing temperatures are analyzed by the sequential selective leaching (SSL), X-ray fluorescence (XRF), X-ray diffraction (XRD) and field emission-scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM-EDS). The results show that the phases of minerals in coals changed and the NME are involved in the mineral phase transformation with the increase of ashing temperature. In particular, non-mineral calcium presented as sulfate minerals (gypsum, bassanite or anhydrite) in the ashes of 120 °C, 350 °C and 815 °C, while changed into calcite in the 500 °C ashes. It may be due to the carbon dioxide generated by the oxidation of organic matter reacted with the non-mineral calcium that gradually released in coal under the high temperature (≥500 °C) and pressure. The transformation mechanism of non-mineral calcium implies an interaction between organic matter and mineral matter in the ashing process.