basal conglomerates, passing upwards to marl, sandy marl, sand, clay and ..... Bull. 1973, 52, 459. Pavlides, S. B. PhD Thesis University of Thessaloniki,. 1985.
Mineralogical and chemical investigation of fly ash from the Main and Northern lignite fields in Ptolemais, Greece Anestis
Filippidis
and Andreas
Georgakopoulos
Aristotle University of Thessaloniki, Department of Mineralogy~Petrology-Economic Geology, GR-540 06 Thessaloniki, Greece (Received 19 March 1991; revised 13 May 1991)
fly ash from the Main and Northern lignite fields of Ptolemais, Greece, is mainly composed of anhydrite, lime, calcite, gehlenite and quartz. Also present, in minor and trace amounts, are portlandite, hatrurite, gkermanite. ettringite, plagioclase, alkali feldspar, mica, gypsum, bassanite, brownmillerite, tobermorite and unburnt lignite. Calcite, quartz, feldspars and mica were initially contained in the mined lignite. Anhydrite, lime, portlandite, gehlenite, ikermanite, hatrurite and brownmillerite were formed due to the ashing process at different temperatures ( > h4002C), while gypsum, bassanite, ettringite and tobermorite were formed due to the subsequent soaking of the fly ash, at temperatures around 42-200°C. The chemical composition of the fly ash corresponds approximately to that of coal combustion slags, lignite fly ash of Gardanne-Provence, France, and Portland clinker. The chemical and mineralogical composition of the fly ash reveal properties of concern to the construction industry and even to health and the environment. Lignite
(Keywords : fly ash; lignite; mineralogical
and chemical characterization)
Lignite beds of the Ptolemais basin, Greece, are under intensive exploitation by opencast mining. The annual production of lignite in this basin exceeded 35 x lo6 tons in 1990. The lignite reserves of the Main and Northern fields are estimated to be around 217 x lo6 tons’, and are used mainly for electric power generation. The ash content of Ptolemais lignites is around 30% (db), while their calorific value2T3 (daf) is around 4400 kcal kg-‘. During lignite combustion the major, minor and trace elements may concentrate in the fly ash, while the mineral matter undergoes a series of physical and chemical changes4,5. Lignite combustion processes result in emission of particulate matter to the environment. The different industrial uses of the lignite ash depend mainly on its mineral and chemical composition. This paper reports on the mineralogical and chemical composition of fly ash from the Main and Northern lignite fields in the Ptolemais basin. Such information is vital for the construction industry and for identifying and reducing environmental hazards6-8.
GEOLOGICAL
SETTING
The sediments of the Ptolemais basin overlay both Paleozoic metamorphic rocks and Mesozoic crystalline limestones9*10 (Figure I). The Neogene-Quaternary sediments of the basin are divided into three lithostratigraphic formations. The lowest (Upper Miocene to Lower Pliocene) consists of basal conglomerates, passing upwards to marl, sandy marl, sand, clay and lignite (partly xylitic) layers. The Pliocene middle formation contains intensively exploited
lignite beds alternating with clays, marls and sands. The Quaternary upper formation consists of terrestrial and fluvioterrestrial conglomerates, lateral fans and alluvial deposits”.“. In the opencast mines of the Ptolemais basin the average thickness of the overburden is 20 m for the Main field and 41 m for the Northern field, while for the intermediate sterile bands it is 20 m and 31 m and the average thickness of mineable lignite is 15 m and 16 m, respective1y3.
METHODS
OF INVESTIGATION
AND RESULTS
The size distribution of the fly ash samples produced by dry sieving is presented in Table 1. Each fraction was examined by X-ray diffraction to give the results in Tables 2 and 3. The fly ash samples were analysed by inductively coupled plasma analysis (Table 4). Examination under the stereomicroscope revealed unburnt lignite to be present in the fly ash. The unburnt lignite grains ( > - 200 pm) were separated carefully from small fly ash samples under a stereomicroscope. Separation of the finer lignite grains was not possible. This separation showed that the unburnt lignite in the fly ash is at least 4 wt% for the Main field and at least 3 wt% for the Northern field. These values were not taken into account in Tables 2 and 3.
DISCUSSION The finer fractions are the dominant size fractions in the Ry ash. Around 34 wt% of the fly ash has a grain size
OOl6-2361/92/040373X14
(
1992 Butterworth-Heinemann
Ltd
FUEL, 1992, Vol 71, April
373
Mineralogy of lignite fly ash: A. Filippidis and A. Georgakopoulos Table 1 Grain size distribution in the fly ash samples from Main (KP) and Northern (BP) Ptolemais lignite fields Fraction
Sample
(pm)
(wt % )
(wt%)
0.1 0.2 1.1 4.0 20.1 46.9 21.6
0.2 0.2
>2000 2ooo~1000 1000~500 500-250 250- 125 125-63
