Abstract The formation and accumulation of carbonate minerals during the hydrogenation of low rank coals can lead to operational difficulties. For Victorian brown coals, carbonate formation increases with increasing hydrogenation temperature (380–440°C) and with increasing reaction time. The amount of carbonate formed is proportional to the amount of ion-exchangeable calcium although under certain reaction conditions magnesium is also involved. Vaterite is the predominant carbonate mineral formed at low temperatures (380°C), but as the temperature increases, calcite, the more stable form, is observed. After further increase in temperature and particularly at long reaction times, dolomite begins to form. Small rod-shaped particles (approximately 2 μm long) of calcium carbonate were identified in one hydrogenation residue, and these appeared to be intimately associated with the cellular structure of the coal, indicating that they were formed during the early stages of coal decomposition.
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