Chemical Weathering in a Dry Ash Dump: An Insight from Physicochemical and Mineralogical Analysis of Drilled Cores
Abstract
A typical power station in South Africa that utilizes a dry ash disposal system produces 1.765 Mt of fly ash per annum. Fly ash is conditioned for dust suppression before being conveyed to the ash dumps with high saline effluent resulting from the pre-treatment of raw water for boiler feed. In the ash dumps it is further conditioned by irrigation with the high saline effluents, therefore the ash dumps may act as salt sinks. This study is an attempt to understand the chemical weathering of the effluent conditioned fly ash in a dry disposal scenario. A combination of leaching tests was performed for fresh ash and drilled cores obtained from various sites on the dump to estimate the highly leachable species. Results from DIN-S4 tests of the fresh ash reveal that Ca, K, Na, Mg, Ba, SO 42- , Se, Mo and Cr are highly leached from fresh fly ash. Leaching tests also revealed that major soluble components in the solution at equilibrium are Ca, Na, SO 42- and K. Weathering profiles of the ash dump cores were observed to follow a similar trend. The greatest weathering was observed to take place at the top layer (0.55-3 m depth) in the cores obtained from areas of the dump that were older (15 years and older), showing that infiltration of rain water over time has a profound effect on the decrease of the pore water pH. Analysis of the extracted pore water in each of the different drilled cores per depth profile indicated the mobility of several elements through the ash. Increased cation exchange capacity at 4-5 m depth suggests a transient mineralization zone. Darcy tests reveal that high concentrations of salts are mobilised from the initial flush of water through the ash cores during leaching. However a significant amount of the soluble salts may remain immobile as long as the ash dump does not come into contact with infiltrating water or lateral flows of groundwater. Contact of the ash with water is observed to be a crucial factor in the mobilization of constituents of the ash or of the brine used for irrigation with time. A combination of leach and hydraulic tests revealed that a dry ash dump does not act as a sustainable salt sink and evaluation of the hydrogeology of the disposal site is thus essential before dumping the ash.
Recommended Citation
M. W. Gitari and O. O. Fatoba and A. Nyamihingura and L. F. Petrik and V. R. Vadapalli and J. Nel and A. October and L. Dlamini and G. Gericke and J. S. Mahlaba, "Chemical Weathering in a Dry Ash Dump: An Insight from Physicochemical and Mineralogical Analysis of Drilled Cores," 3rd World of Coal Ash, WOCA Conference - Proceedings, UK CAER, Dec 2009.
Department(s)
Mining Engineering
Keywords and Phrases
Cation exchange capacity; DIN-S4 test; Pore water; TCLP; Weathered fly ash; X-ray diffraction analysis
Document Type
Article - Conference proceedings
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 2024 World of Coal Ash, All rights reserved.
Publication Date
01 Dec 2009
