NON-MINERAL INORGANIC CONSTITUENTS OF BUKIT ASAM COAL NATURE AND MODE OF OCCURRENCE

The nature and mode of occurrence of the non-mineral inorganic fractions in coal from the Bukit Asam coalfield South Sumatra Basin Indonesia were analysed using a combination of selective leaching, low temperature ashing, XRD, XRF and electron micropobe techniques. Geologically the rank of coal at Bukit Asam was influenced by igneous intrusion. Fourteen coal samples were used in this study. Samples are chosen to represent different rank levels as well as seam horizons. The inorganic elements, in the form of dissolved salts, exchangeable ions and organometallic complexes, although present in small proportions, contribute significantly to the mineral matter components of Bukit Asam low rank coal. Electron microprobe analysis showed small but significant concentrations of inorganic elements (Al, Ca and Fe) occurring in the individual macerals in two sub bituminous and twosemi anthracite samples. There is a tendency for some of the elements, particularly Ca and Al to be lost from the organic matter with rank advance. The concentrations of elements indicated by the microprobe are relatively in good agreement with the levels of element mobility indicated by selective leaching of the same whole-coal samples. The results of this study also suggest that non-mineral inorganic was likely act as a precursor for the reactions resulting in the formation of new minerals in the high rank coals at Bukit Asam.

Formation of Tetra-Coordinated Aluminum in the Low Temperature Ashes

Formation of tetra-coordinated aluminum in low temperature coal ashes was investigated. Coal samples were intensively air-oxidized at 350oC for 160 hours, then 27Al MAS NMR spectra of obtained ashes were acquired and compared against those for initial coals, a reference ash and Al standards. It was found that chemical shift at 4 ppm with coals corresponds to that for triaquohydroxo- diphenoxido-aluminum(III) complex and the shift at 72 ppm with resulting coal ashes agrees with that for Al tetra-coordinated with oxygen. So, in the presence of coal organics the tetracoordinated Al is formed under low temperature conditions. Formation of this form of Al can be elucidated as follows. Probably, Al in coal originates from kaolinite, as this mineral is very frequent in coal. Mentioned Al-complex was formed through the long-term reactions of kaolinitic hexacoordinated Al with phenols/polyphenols as prevailing oxygen-contaning components in coal substance. During this process the octahedral structure of hexa-coordinated Al was retained, but ligands coordinated around the central Al atom changed. At first, delamination of a kaolinitic structure through the effect of coal organics proceeded and separation of the Al gibbsite octahedral and silica tetrahedral layers occurred. Then, two oxygens in the Al octahedron were substituted with two phenoxide groups while OH groups were preserved. As result, Al-tetrahydroxo-diphenoxide ion was formed, further stabilized in a water environment as Al(H2O)3(OH)(C6H5O)2. During low temperature ashing this complex was transformed to Al tetra-coordinated with oxygen only at 350oC.