Geochemical Analysis of Leachates from South African Coal Fly Ash: Up-flow Percolation Test

Abstract

Fly ash is a waste product generated from combustion of coal in coal-fired power stations, and is composed of major and minor species. Fly ashes from South Africa power stations are mostly alkaline and on contact with water release major and minor species in significant quantities. However, there is a need to assess the geochemical factor influencing the mobility of these species in fly ash in order to understand the effect of ash disposal on the environment. The up-flow percolation (column) test was employed on the fly ashes from two South African coal-fired power stations to understand how major and minor species are mobilized from the fly ashes. PHREEQC geochemical modeling software was employed to determine whether the concentrations of the species leached out of the fly ashes after contacting water were controlled by the precipitation of secondary mineral phases. The results of the leaching test showed that the initial leachates from the fly ashes contain high concentrations of species such as Ca, Mg, Na, K, SO4, Sr, Ba, which decreased as the leaching continued until steady states were reached. Fe, Mn, Se, As, Cu, Pb, Mo and Cr concentrations were also high initially before decreasing to lower levels overtime. Precipitation of secondary mineral phases controlling the leaching of most of these species was predicted by geochemical modeling. Ca and SO4 were predicted to be controlled by portlandite, anhydrite and gypsum precipitation; Barite and celestite were predicted by PHREEQC as the mineral phases controlling Ba and Sr in the leachates; Mg was controlled by sepiolite. PHREEQC did not predict the formation of secondary mineral phases controlling Na and K. The calculated saturation indices (SI) showed that the leachates were slightly supersaturated with respect to brucite. Pyrochroite, ferrihydrite, Ba3(AsO4)2, SrSeO4, Cu(OH)2, Pb(OH)2, CaMoO4 and BaCrO4 were predicted as the secondary mineral phases controlling the release of Mn, Fe, As, Se, Cu, Pb, Mo and Cr respectively. This study gives insight into the possible secondary mineral formation in the ash/water system that could control the release of toxic species into the environment.

Department(s)

Mining Engineering

Keywords and Phrases

Fly ash; Mineral phases; PHREEQC; Saturation indices; Up-flow percolation test

International Standard Book Number (ISBN)

978-153610533-9;978-153610516-2

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2024 Elsevier, All rights reserved.

Publication Date

01 Jan 2017

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