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

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• 作者：S.A. Akinyemi^a ; ^{sakinyemi@uwc.ac.za} ; ^{akinyemi70@gmail.com} ; A. Akinlua^a ; W.M. Gitari^c ; N. Khuse^b ; P. Eze^b ; R.O. Akinyeye^b ; L.F. Petrik^b
• 关键词：Dry disposed ash ; Unsaturated ash ; Secondary mineral phases ; Leaching ; Acid susceptibility
• 刊名：Journal of Environmental Management
• 出版年：2012
• 期刊代码：82_03014797
• 类别：env
• 出版时间：15 July, 2012
• 卷：102
• 期：Complete
• 页码：96-107
• 文件大小：839 K

摘要

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鈥檚 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.