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

## TL;DR

This study examines how aging affects the mineral composition and leaching behavior of coal fly ash at different initial pH levels.

## Contribution

The study provides new insights into how initial pH and secondary mineral formation influence the immobilization of toxic elements in aged coal fly ash.

## Key findings

- FA1 (pH 12) showed reduced leaching of B and Se with slight mineral changes and increased ettringite.
- FA2 (pH 4) showed no reduction in B, As, or Se leaching and increased quartz due to aluminosilicate dissolution.
- FA3 (pH 9) showed slight decreases in B and Se leaching with changes in quartz, mullite, and K-aluminosilicate.

## Abstract

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.