Geochemical fractionation of hazardous elements in fresh and drilled weathered South African coal fly ashes.

The chemical reactions of dry-disposed ash dump, ingressed oxygen, carbon dioxide, and infiltrating rainwater affect mineralogical transformation, redistribution, and migration of chemical species. Composite samples of weathered coal fly ash taken at various depths and fresh coal fly ash were examined using organic petrographic, X-ray diffraction, X-ray fluorescence techniques, and successive extraction procedures. Results obtained show relative enrichment of glass, Al-Fe-oxides, calcite, and tridymite in the weathered CFA, but the fresh CFA is enriched in mullite, inertinite, maghemite, and ettringite. The enrichment of the weathered CFA in amorphous glass suggests higher reactivity when compared to fresh CFA. The evident depletion of soluble oxides in the weathered CFA is attributed to flushing of the soluble salts by percolating rainwater. Comparative enrichment of examined elements in water-soluble, exchangeable, reducible, and residual fractions of the weathered CFA is partly due to the slow release of adsorbed chemical species from the alumina-silicate matrix and diffusion from the deeper sections of the particles of coal fly ash. Sodium and potassium show enrichment in the oxidisable fraction of fresh CFA. The estimated mobility factor indicates mobility for Ca, Mg, Na, Se, Mo, and Sb and K, Sr, V, Cu, Cr, Se, and B in fresh and weathered CFAs, respectively.

Natural weathering in dry disposed ash dump: Insight from chemical, mineralogical and geochemical analysis of fresh and unsaturated drilled cores.

Some existing alternative applications of coal fly ash such as cement manufacturing; road construction; landfill; and concrete and waste stabilisation use fresh ash directly collected from coal-fired power generating stations. Thus, if the rate of usage continues, the demand for fresh ash for various applications will exceed supply and use of weathered dry disposed ash will become necessary alternative. As a result it's imperative to understand the chemistry and pH behaviour of some metals inherent in dry disposed fly ash. The bulk chemical composition as determined by XRF analysis showed that SiO2, Al2O3 and Fe2O3 were the major oxides in fresh ash and unsaturated weathered ashes. The unsaturated weathered ashes are relatively depleted in CaO, Fe2O3, TiO2, SiO2, Na2O and P2O5 due to dissolution and hydrolysis caused by chemical interaction with ingressing CO2 from the atmosphere and infiltrating rain water. Observed accumulations of Fe2O3, TiO2, CaO, K2O, Na2O and SO3 and Zn, Zr, Sr, Pb, Ni, Cr and Co in the lower layers indicate progressive downward movement through the ash dump though at a slow rate. The bulk mineralogy of unsaturated weathered dry disposed ash, as determined by XRD analysis, revealed quartz and mullite as the major crystalline phases; while anorthite, hematite, enstatite, lime, calcite, and mica were present as minor mineral phases. Pore water chemistry revealed a low concentration of readily soluble metals in unsaturated weathered ashes in comparison with fresh ash, which shows high leachability. This suggests that over time the precipitation of transient minor secondary mineral phases; such as calcite and mica might retard residual metal release from unsaturated weathered ash. Chloride and sulphate species of the water soluble extracts of weathered ash are at equilibrium with Na+ and K+; these demonstrate progressive leaching over time and become supersaturated at the base of unsaturated weathered ash. This suggests that the ash dump does not encapsulate the salt or act as a sustainable salt sink due to over time reduction in pore water pH. The leaching behaviours of Ca, Mg, Na+, K+, Se, Cr and Sr are controlled by the pH of the leachant in both fresh and unsaturated weathered ash. Other trace metals like As, Mo and Pb showed amphoteric behaviour with respect to the pH of the leachant. The precipitation of minor quantities of secondary mineral phases in the unsaturated weathered ash has significant effects on the acid susceptibility and leaching patterns of chemical species in comparison with fresh ash. The unsaturated weathered ash had lower buffering capacity at neutral pH (7.94-8.00) compared to fresh (unweathered) ash. This may be due to the initial high leaching/flushing of soluble basic buffering constituents from fly ash after disposal. The overall results of the acid susceptibility tests suggest that both fresh ash and unsaturated weathered ash would release a large percentage of their chemical species when in contact with slightly acidified rain. Proper management of ash dumps is therefore essential to safeguard the environmental risks of water percolation in different fly ashes behaviour.

Treatment of acid mine drainage with fly ash: removal of major contaminants and trace elements.

Acid mine drainage (AMD) has been reacted with two South African fly ashes in a batch setup in an attempt to evaluate their neutralization and major, trace elements removal capacity. Different fly ash: acid mine drainage ratios (FA: AMD) were stirred in a beaker for a set time and the process water analyzed for major, trace elements and sulphate content. The three factors that finally dictated the nature of the final solution in these neutralization reactions were the FA: AMD ratio, the contact time of the reaction and the chemistry of the AMD. Efficiency of the elements removal was directly linked to the amount of FA in the reaction mixture and to the final pH attained. Most elements attained approximately 100% removal only when the pH of minimum solubility of their hydroxides was achieved (i.e., Mg = 10.49-11.0, Cu(2+) = 6, Pb(2+) = 6-7). Dissolution of CaO and subsequent precipitation of gypsum and formation of Al, Fe oxyhydroxysulphates, Fe oxyhydroxides with subsequent adsorption of sulphate contributed to the sulphate attenuation. Significant leaching of B, Sr, Ba and Mo was observed as the reaction progressed and was observed to increase with quantity of fly ash in the reaction mixture. However B was observed to decrease at high FA: AMD ratios probably as result of co-precipitation with CaCO(3(s)).

The use of X-ray fluorescence (XRF) analysis in predicting the alkaline hydrothermal conversion of fly ash precipitates into zeolites.

The use and application of synthetic zeolites for ion exchange, adsorption and catalysis has shown enormous potential in industry. In this study, X-ray fluorescence (XRF) analysis was used to determine Si and Al in fly ash (FA) precipitates. The Si and Al contents of the fly ash precipitates were used as indices for the alkaline hydrothermal conversion of the fly ash compounds into zeolites. Precipitates were collected by using a co-disposal reaction wherein fly ash is reacted with acid mine drainage (AMD). These co-disposal precipitates were then analysed by XRF spectrometry for quantitative determination of SiO(2) and Al(2)O(3). The [SiO(2)]/[Al(2)O(3)] ratio obtained in the precipitates range from 1.4 to 2.5. The [SiO(2)]/[Al(2)O(3)] ratio was used to predict whether the fly ash precipitates could successfully be converted to faujasite zeolitic material by the synthetic method of [J. Haz. Mat. B 77 (2000) 123]. If the [SiO(2)]/[Al(2)O(3)] ratio is higher than 1.5 in the fly ash precipitates, it favours the formation of faujasite. The zeolite synthesis included an alkaline hydrothermal conversion of the co-disposal precipitates, followed by aging for 8h and crystallization at 100 degrees C. Different factors were investigated during the synthesis of zeolite to ascertain their influence on the end product. The factors included the amount of water in the starting material, composition of fly ash related starting material and the FA:NaOH ratio used for fusing the starting material. The mineralogical and physical analysis of the zeolitic material produced was performed by X-ray diffraction (XRD) and nitrogen Brunauer-Emmett-Teller (N(2) BET) surface analysis. Scanning electron microscopy (SEM) was used to determine the morphology of the zeolites, while inductively coupled mass spectrometry (ICP-MS), Fourier transformed infrared spectrometry (FT-IR) and Cation exchange capacity (CEC) [Report to Water Research Commission, RSA (2003) 15] techniques were used for chemical characterisation. The heavy and trace metal concentrations of the zeolite products were compared to that of the post-synthesis filtrate and of the precipitate materials used as Si and Al feed stock for zeolite formation, in order to determine the trends (increase or decrease) and ultimate fate of any toxic metals incorporated in the co-disposed precipitated residues.