Leachate Chemistry of Mixtures of Fly Ash and Alkaline Coal Refuse
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There is great interest in returning coal combustion products to mining sites for beneficial reuse as liming agents. A column study examined the effects of blending two coal fly ashes with an acid-forming coal refuse (4% pyritic S). Both fly ashes were net alkaline, but had relatively low neutralizing capacities. One ash with moderate alkalinity (CRF) was bulk blended with coal refuse at 0, 20, and 33% (w/w), while another lower alkalinity ash (WVF) was blended at 0, 5, 10, 20, and 33% (w/w). The columns were leached (unsaturated) weekly with 2.5 cm of simulated precipitation for >150 wk. Where high amounts of ash alkalinity (>20% w/w) were mixed with the coal refuse, pyrite oxidation was controlled and leachate pH was >7.0 with low metal levels throughout the study. At lower rates of alkalinity loading, trace metals were sequentially released from the WVF ash as the 5, 10, and 20% treatments acidified due to pyrite oxidation. Lechate metals increased in proportion to the total amounts applied in the ash. In this strongly acidic environment, metals such as Mn, Fe, and Cu were dissolved and leached from the ash matrix in large quantities. If ash is to be beneficially reused in the reclamation of acid-producing coal refuse, the alkalinity and potential acidity of the materials must be balanced through the appropriate addition of lime or other alkaline materials to the blend. Highly potentially acidic refuse material, such as that used here, may not be suitable for ash/refuse codisposal scenarios.
The exclusion of coal fly ash from regulation as a hazardous waste has led to increased interest in returning ash to the coal fields for disposal. Bulk blending alkaline fly ash with acid forming coal refuse may present a disposal option that also aids in the control of acid mine drainage (AMD). A column leaching study was initiated to examine the leachate quality from acid forming coal refuse-fly ashblends. Coal refuse alone (2.2% total S), and bulk blended coal refuse and alkaline fly ash (20 and 33% ash, w/w) were packed into 20-cm diameter leaching columns and run under unsaturated conditions for over 4 yr. Leachates were analyzed for pH, electrical conductivity, Fe, Mn, and SO4/2- content. The coal refuse columns acidified quickly and produced leachates that, at peak levels, contained high contents of acidity (pH 1.6), Fe (10 000 mg L-1), SO4/2- (30 000 mg L-1), and Mn (300 mg L-1). The high levels of metals in these leachates decreased over time. The ash- treated columns maintained leachate pH values near 8.0 with very low metal levels. The bulk mixing of alkaline fly ash and coal refuse, at high blending rates (>20%), appears to be an effective codisposal option that also provides long-term AMD control. Only B and SO4/2- appeared to leach at any significant level and the quality of leachates from the ash-treated columns was significantly improved with respect to the untreated coal refuse.
An analytical procedure involving sequential chemical extractions has been developed for the partitioning of particulate trace metals (Cd, Co, Cu, Nl, Pb, Zn, Fe, and Mn) into five fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter, and residual. Experimental results obtained on replicate samples of fluvial bottom sediments demonstrate that the relative standard deviation of the sequential extraction procedure is generally better than ± 10%. The accuracy, evaluated by comparing total trace metal concentrations with the sum of the five individual fractions, proved to be satisfactory. Complementary measurements were performed on the individual leachates, and on the residual sediments following each extraction, to evaluate the selectivity of the various reagents toward specific geochemical phases. An application of the proposed method to river sediments is described, and the resulting trace metal speciation is discussed.
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