Modes of occurrence of non-mineral inorganic elements in lignites from the Mile Basin, Yunnan Province, China

Non-mineral inorganic elements (NMIE; e.g., Al, Mg, Ca, and Fe) in lower-rank coals are released in a more reactive atomic form than the same elements in crystalline mineral particles during coal combustion and gasification, thus allowing them to participate more easily in slagging and fouling processes. Quantitative analysis of these non-mineral inorganics is therefore helpful to better understand their behavior during coal utilization. Lignite samples collected from the Mile Basin in southeast Yunnan Province, China, were studied to provide further insights into the modes of occurrence of the NMIE Al, Mg, Ca, and Fe. Geochemical and mineralogical investigations were performed using a multiple analytical approach, including a three-step sequential leaching process, oxygen plasma low-temperature ashing, X-ray powder diffraction (XRD), field emission-scanning electron microscopy in conjunction with energy-dispersive X-ray spectrometry (FE-SEM), and inductively coupled plasma mass spectrometry (ICP-MS). The Mile lignites have an elevated content of authigenic gypsum, which was deposited from dissolved SO42− and organic-bound or dissolved Ca. A large proportion of the Ca, Mg, and Fe in the Mile lignite occurs as dissolved salts, ionic species attached to carboxylates, and metallic species held in organometallic complexes, which were completely leached by water, ammonium acetate, and NH4-EDTA, respectively. In contrast, only a low proportion of Al in the lignite occurs in non-mineral inorganic form. Non-mineral Fe, Al, and Ca largely occur as organometallic complexes; only a small proportion of Ca occurs in a water-soluble form or in extractable carboxylates if present. Magnesium, however, occurs in various non-mineral inorganic forms, and could be leached not only by NH4-EDTA but also by water and ammonium acetate solutions. Iron is effectively leached only by NH4-EDTA and the leaching does not appear to affect the occurrence of pyrite in the coals.