The mineralogy of fly ash refers to both amorphous and crystalline phases and other mineral fractions in the fly ash. Fly ash is a complex mixture of different minerals and high amounts of toxic elements are associated with these minerals (Saikia et al., 2006). The major mineral phases that have been commonly identified in fly ash include: quartz (SiO2); mullite, (Al6Si2O13);
tricalcium aluminate, Ca3Al2O6; alite, (Ca3SiO5); belite (Ca2SiO3); magnetite, (Fe3O4); hematite,
(Fe2O3); lime, (CaO); anhydrite, (CaSO4); periclase, (MgO); melilite, (Ca2(Mg,Al)(Al,Si)2O7);
merwinite, (Ca3Mg(SiO4)2,calcite (CaCO3), pyrite (FeS2) and thenardite, (Na2SO4) and a small
portion of unburned carbon (Tishmack and Burns, 2004; Adriano et al., 1980; Steenari et al., 1999). Most of the quartz in fly ash originates from coal as silt and sand sized particles and it remains in the ash because it survived thermal transformation during combustion (Helmuth, 1987). Although bituminous coal ash may contain more than 50 wt% of SiO2, only about 5-10
wt% is present as quartz (McCarthy et al., 1990). Some Si is present in mullite but a greater portion of the Si is found in the amorphous glass phase (Tishmack and Burns, 2004). Mullite is the principal Al-bearing mineral in low-Ca bituminous coal. It originates from direct crystallization of clay minerals or by devitrification of glass on cooling (Hubbard et al., 1984). South African coal ashes have high quantities of mullite because low-Ca bituminous coal which is rich in kaolinite is utilized during combustion. Fly ash with high Ca content contains lower
18
amounts of mullite because of the presence of mica-illite or smectitic clays in their parent coal and also because some of the Al combines with Ca to form Ca3Al2O6 (Tishmack and Burns,
2004). The principal Fe-oxides minerals in coal ash are magnetite (Fe3O4) and hematite (Fe2O3).
Magnetite results mainly from the volatilization and oxidation of iron bearing minerals in coal, mainly pyrite (FeS2). Hematite crystals survive the brief exposure to high temperature like quartz
while pyrite is oxidized to magnetite with the release of SO2. About a third to half of the Fe
present in fly ash is in the form of magnetite and hematite, which are largely inert (McCarthy et
al., 1990). The rest is contained in the glass phase and becomes available when the glass
dissolves (Helmuth, 1987). In addition to quartz, mullite and hematite, moderate to high Ca fly ashes contain other calcium bearing minerals which are reactive in the presence of water. The most common is Ca3Al2O6 which occurs with Ca-aluminate-rich glass (Tishmack et al., 1999).
The principal Mg phase, periclase (MgO) is about half of the Mg present in sub-bituminous and lignite fly ashes. Magnesium is also found in melilite and merwinite, neither of which is known to significantly react with water (Tishmack and Burns, 2004). Most of the S in fly ash is in the form of anhydrite, which forms as a solid on the surface of the sub-bituminous and lignite fly ash particles (McCarthy et al., 1989). Sulphur accumulates on the surface of fly ash particles from reactions between Ca, Na, O and SO2 as the fly ash leave the hot part of the furnace and enter
low temperature zones (Fishman et al., 1999). Small amounts of anhydrite have been observed in fly ash with CaO content as low as 12 % (McCarthy et al., 1989).
The most abundant phase in a class F ash is the glass that results from the melting of clays and subsequent exsolution of mullite from the melt. Major minerals in class F fly ash are quartz, the ferrite phase and mullite. Because of the higher calcium content of the class C ash, the mineral assemblage is quite different. Quartz, ferrite phase and mullite are present as in class F but the alteration of the clay content of the coal in the presence of calcium results in a suite of silicates, aluminosilicates and oxide phases instead of large amounts of glass. Only a minor amount of glass is formed which contains a rather high concentration of alumina. It is chemically, extremely reactive. Most notable among the mineral phases are lime, di-calcium silicate and tri- calcium silicate, periclase, gypsum/anhydrite can also be found (Scheetz, 2004). Studies have revealed that the mineralogical analysis of fly ash showed 70-90 % of the particles to be consisting of amorphous ferro-aluminosilicate glassy spheres (Solem and McCarthy, 1992;
19
Spears, 2000). The glass is generated from clay minerals such as feldspar, mica, chlorite and any other easily melted minerals (Hulett et al., 1980; Spears, 2000). The crystalline phase present in fly ash consist of quartz (SiO2), mica, chlorite, feldspars, mullite (3Al2O3.2SiO2), spinel
(FeAl2O4), haematite (Fe2O3) and magnetite (Fe3O4) depending on the mineralogy of the feed
coal (Norton et al., 1986; Vassilev and Vassileva, 1997; Vassileva et al., 2005; Kutchko and Kim, 2006). Sakorafa et al, 1996 in the study carried out on the fly ash from Megalopolis lignite field, Peloponnese (Southern Greece) reported the main minerals present in the fly ash to be quartz, anhydrite, plagioclase, haematite, gehlenite and calcite. The presence of lime, alkali feldspars, bassanite, gypsum, mica and unburnt lignite are found in minor and trace amounts. Erol et al., 2000, in their study on the fly ash obtained from çayirhan thermal power plant, Turkey, reported the presence of quartz, mullite, enstatite, anorthite and haematite as the major mineral phases present in the ash. Volatile elements released from the coal matrix during combustion enter into the vapour phase. As they cool, these gaseous compounds may condense into very small spherical particles and on the surface of other particles leading to surface enrichment of the species (Kim and Kazonich, 2004). In a previous study (Fatoba, 2008) carried out on some South African fly ashes (Secunda and Tutuka fly ashes), the major crystalline mineral phases in both Secunda and Tutuka fly ashes were found to be quartz (SiO2) and mullite (3Al2O3.2SiO2). Lime
and calcite phases were also identified but these phases were present in low amounts. The mineralogical compositions of the ashes from the two power stations are not different and showed a similar composition.