Mineralogy and chemical mobility in a weathered ash dump site, South Africa
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Date
2010
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Publisher
University of the Western Cape
Abstract
Coal fly ash generated from coal fired plants poses potential health risks to humans and plants in the environment due to the surface enrichment of the ash with various toxic trace elements during combustion. Only a small portion of the fly ash produced every year as a result of the increase in the demand for electricity, is being utilized. The bulk of the ash is disposed of in ash dams and landfills. Rain water as well as waste water from the ash slurry serves as leaching medium for the toxic elements into the environment,
especially into the groundwater. This study aims at understanding the chemical and mineralogical omposition of the weathered fly ash, the distributive pattern of species down the ash dump, the various mineral phases with which the elements are associated and the change in mineralogy as a result of weathering over time. Methods employed in this study included the use of XRF and total acid digestion of the samples followed by ICP AES/MS analysis of the leachates to identify and quantify the major, minor and trace elements in the ash samples, pore water chemistry of the samples to determine the species soluble in water at various horizons in the drilled core, XRD and SEM/EDS to determine the mineralogy and morphology of the fly ash samples and a 5 step sequential extraction procedure to understand the various mineral phases with which the elements in the fly ash are associated. Fly ash samples were obtained from a core drilled to 32 m at Kragbron in the Free State Province of South Africa. Geology of the area falls under the Karoo Supergroup and the study site is underlain with Jurassic dolerite rock of the Karoo Supergroup. The ash was disposed at the dump as slurry in a layered form. The bulk chemical composition as determined by XRF showed AhO3, SiO2, Fe2O3 and CaO as the major oxide constituents in the fly ash samples. Kragbron ash belongs to Class F according to ASTM C618, as the sum of the percentage composition of SiO2, AhO3 and Fe2O3 revealed by XRF was greater than 70 % and the lime content was less than 10 %. Loss on ignition values in most of the samples (between 15 m and 22 m) was higher than specified by ASTM C618. This was as a result of the different coals used in the combustion units at the power stations. Comparison between the results obtained from XRF and the ICP analysis of the fly ash digestates was good in some of the elements. Al, Si and Na concentration was higher with XRF than with ICP. This was expected because the elements are present in reasonable amounts in fly ash. The concentration of Ca, Fe, Mg, K and Ti was higher with ICP than with XRF. This ought not to be as these elements are also present in appreciable quantities in fly ash. Results obtained with XRF showed more accuracy because the technique gives total composition of the solid sample and chances of contamination are minimal. Results obtained from XRD showed mullite, quartz and calcite to be the major crystalline mineral phases identified in most of the ash samples. Hematite and calcite were observed at 22 m depth in the ash dump. The inclusion of hematite at 22 m could be due to mixing with parts of the bedrock during the initial construction of the ash dump as hematite was not identified in the previous depths. Mullite, quartz, anorthite and diopside were the minerals identified at 23 m. The presence of mullite also indicates a mixture of ash and bedrock components. Literature revealed the bedrock to be dolerite. Major minerals revealed by XRD from 24 m to 32 m were quartz, diopside and anorthite commonly
found in dolerites. The pH pattern observed in the profile showed strong weathering at the surface of the dump between 1 m and 5 m and it was alkaline for all the samples. Electrical conductivity (EC) was ery high at 16 m depth and the trend coincided with that which was observed in Ca, Na, K, Ba, so/- and are indicating the presence of these elements in highly soluble forms at that depth. Al was observed in high concentrations at 15 m and 18 m due to its presence in soluble hydroxide form. Se, As, Cr, Zn, Cu and V showed a considerable release in the fly ash pore water leachates. Ti, Pb, Co, Fe, Mn and Mg were present in water insoluble phases in the fly ash samples.
Description
>Magister Scientiae - MSc
Keywords
Mineralogy, South Africa, Ash disposal, Weathered, Coal