Characterization and comparison of leaching behaviors of fly ash samples from three different power plants in Turkey

• Fly ash samples were collected from three lignite coal-fired power plants.
• SoFA includes higher trace element concentrations than YFA and SFA.
• The higher leached ratios were obtained in TCLP than ASTM D3987 tests.
• Remarkable changes occurred in SFA residue during the leaching process.
• Inert waste behavior of SFA and YFA is caused by ettringite formation.

This study reports the characterization and the leaching behaviors of three fly ashes taken from different low grade-lignite-fired stations. Fly ashes were characterized in terms of their physical, chemical, mineralogical and morphological properties. Leaching behaviors of these ashes were studied by means of two standard leaching tests, namely TCLP and ASTM D3987. In this context, both the eluates and solid residues of these tests were investigated. Some selected major, minor and trace element (Ag, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Se, Sr, Ti, Zn) concentrations were analyzed in the leaching eluates. Calcium was a major cation having the highest extraction ratio in all samples regardless of the leaching test used. On the other hand, in ASTM tests, the extraction ratios are higher than 0.1% for selected elements (except Mg, Zn) whereas in TCLP tests, they are higher than 1% for the majority of selected elements. Solid residues were also analyzed to determine the mineralogical and morphological transformations during the leaching process. While lime and anhydrite were consumed in Yatagan fly ash (YFA) and Soma fly ash (SFA), secondary phases such as ettringite and/or calcium silicate hydrates precipitated in their solid residues; however, similar transformations could not be observed in Seyitomer fly ash (SoFA). Therefore, YFA and especially SFA can be accepted as inert wastes due to the ettringite formation. Since ettringite stabilizes several toxic elements such as As, Cr, Se and Sb by incorporating them to its structure.