Mineralogical components of some thermally decomposed lignite and lignite ash from the Ptolemais basin, Greece

Abstract

Two samples of Pliocene lignites from the Ptolemais basin of Greece, one from the upper and one from the lower lignite seams, were heated and dried in air at 50°C intervals from 50 to 1200°C. The two lignite samples initially contained the same minerals, namely calcite, dolomite, quartz, kaolinite, illite, pyrite and gypsum, but in different proportions. The lignite sample from the upper lignite seam is rich in Fe 2O 3, CaO and SO 3, while that from the lower lignite seam is rich in SiO 2 and Al 2O 3. Hematite, periclase, melilites, calcium ferrite and brownmillerite are constituents of the 1200°C lignite ash from both samples. The heating conditions and the chemistry of the samples lowered the formation temperatures of brownmillerite, which appeared in both samples at 950°C. In the Fe 2O 3, CaO- and SO 3-rich sample, magnesioferrite is present from 850 to 1100°C and hematite appears at 300°C. In the SiO 2- and Al 2O 3-rich sample, magnesioferrite was not detected at any temperature and hematite appeared at 600°C. Anhydrite, which normally decomposes in air at 1638°C, is the main constituent at 1150°C, on heating the lignite sample that was rich in Fe 2O 3, CaO and SO 3. Anhydrite diminishes at 1200°C. In the SiO 2- and Al 2O 3-rich lignite sample, anhydrite is main constituent at 1100°C, but diminishes considerably at 1150°C and decomposes at 1200°C.