Alteration of Coal Fly Ash Induced by Aging Treatment: Insights from Mineral Quantification Analysis

The mineral composition of coal fly ash (FA) subjected to an aging treatment was analyzed using mineral liberation analysis (MLA) to elucidate the alteration behavior of FA, including changes in the leaching behavior of toxic elements such as B, As, and Se. Three types of FA generated from a coal-fired power plant in Japan, with initial pH values of 4, 9, and 12, were subjected to leaching experiments. These FA samples exhibited different alteration behaviors depending on their initial pH. FA1, with a pH of 12, exhibited a significant decrease in the leaching of B and Se after aging, and the MLA results revealed a slight change in the major mineral contents. However, the amount of ettringite, Ca₆Al₂(SO₄)₃(OH)₁₂•26H₂O, slightly increased during the aging period, which was consistent with the leaching behavior of Ca, SO₄^2–, and Al. In contrast, FA2, with an initial pH of 4, did not exhibit decreases in the leaching of B, As, or Se during aging, and the MLA results revealed a significant increase in quartz owing to the dissolution of aluminosilicates during aging. For FA3, with an initial pH of 9, the leaching of B and Se decreased slightly following aging, and the MLA results revealed a significant increase in quartz and mullite and a significant decrease in muscovite-like K-aluminosilicate. Furthermore, the amount of ettringite increased slightly, indicating the potential formation of monosulfate (kuzelite), Ca₄Al₂(OH)₁₂(SO₄)•6H₂O, considering the stability of ettringite under the given pH conditions. These findings suggest that the immobilization of toxic elements during aging is influenced by the initial pH and secondary mineral phase formation. Moreover, the results offer valuable insights into the environmental alteration behavior of FA and demonstrate the applicability of MLA for characterizing heterogeneous environmental materials with complex compositions, such as FA.