Computer-Controlled Scanning Electron Microscopy (CCSEM) Investigation on the Heterogeneous Nature of Mineral Matter in Six Typical Chinese Coals

Abstract

The heterogeneous nature of mineral matter in six typical Chinese pulverized coals has been quantitatively characterized using computer-controlled scanning electron microscopy (CCSEM). The results show that clay minerals, quartz, pyrite, and calcite form the bulk of the mineral matter. Minor minerals, such as dolomite, ankerite, rutile, nonstoichiometric iron sulfides, and gypsum, are also detected in some samples. The particle-size distribution of the included minerals is generally finer than that of the excluded ones in a certain coal. As a consequence, the coal rich in included minerals has more small mineral particles. Regarding the association of individual mineral species, the proportion of included to excluded kaolinite varies with coal greatly. Other minerals are, however, principally excluded in nature. With regard to the modes of occurrence of major inorganic elements, it is found that Si mostly occurs as quartz and clay minerals, while Al mostly occurs as silicate minerals. Fe is primarily present as iron sulfides, iron oxide, and Fe-Al-silicate. S is partitioned into iron sulfides and gypsum. Most Ca occurs as carbonates and gypsum, with a minor fraction associated with clay minerals. Mg is mainly present as dolomite and clay minerals, with a minor fraction present as ankerite. The majority of alkali elements are associated with aluminosilicates. P is mostly associated with kaolinite and/or present as more complex compounds containing Al, Si, and other elements. Apatite is the major form of P in only one of the coals studied. Ti is mainly present as rutile and kaolinite.