Mineralogy and Mobility Patterns of Chemical Species in Weathered Coal Fly Ash

Abstract

Chemical interactions of disposed coal fly ash with O₂, CO₂, and infiltrating rain-water lead to chemical alteration, flushing/leaching of soluble chemical species locked in different physico-chemical forms as the coal fly ash is aging. This study was carried out to gain insight into the chemical alterations and its effects on the mobility patterns of chemical species in weathered coal fly ash. Weathered coal fly ash samples of ages 1 year and 20 years were sampled at a coal burning power station in the Mpumalanga Province, South Africa. The chemical and mineralogical compositions of the weathered coal fly ash were investigated using X-ray fluorescence spectrometry, inductive coupled plasma-optical emission spectrometry, ion chromatography, X-ray diffraction, and Fourier transform infrared spectroscopy. X-ray fluorescence results showed the presence of major oxides, such as SiO₂, Al₂ O ₃, Fe₂ O ₃, while CaO, K₂O, TiO₂, MnO, MgO, P₂ O ₅, and SO₃ occur in minor concentrations. The major elements, such as Fe, Si, Mg, K, and Mn, showed increasing trends down the depth of the core, while Ti and Al show decreasing trends down the depth of the core (20-year-old coal fly ash). Trace metals, such as P and Zn, show increasing trends down the depth, while Ba, Ni, Pb, Sr, V, and Cr increase up the depth of the core (20-year-old ash). The trace metals distribution patterns in 1-year-old ash showed increasing trends down the depth of the core for Cr, Ni, Pb, Y, S, and P, while Ba and Sr decrease down the depth of the core. Major mineral phases as revealed by X-ray diffraction include quartz and mullite. Other minerals identified are hematite, calcite, lime, anorthite, mica, and enstatite. The pH of interstitial pore water for 1-year-old and 20-year-old coal fly ash ranged from 9.0–10.6 and 7.2–9.9, respectively, while in the 2-week-old coal fly ash ranged from 9.9–10.9. Analysis of extracted pore water shows that several toxic metals, such as B, Cr, As, Mo, and Se, are leaching from the weathered ash. The core ash samples in contact with the atmosphere and those at the saturation point record the highest availability of chemical species, such as Mg, Ca, Fe, K, Na, B, Cr, As, Mo, and Se in pore water. The 1-year-old and 20-year-old coal fly ash cores showed a lower pH and greater leaching/flushing of the soluble buffering constituents than the 2-week-old placed ash.

Keywords:

• coal fly ash
• leaching
• major elements
• mineralogy
• mobility patterns
• trace metals
• weathering
• X-ray diffraction

Notes

^a L.O.I.: loss on ignition.

^b FA: fresh coal fly ash.

^c WA: weathered coal fly ash.