The influence of fly ash additions on acid mine drainage production from coarse coal refuse

The exclusion of coal fly ash from regulation as a hazardous waste has led to increased interest in returning ash to the coalfields for disposal. Bulk-blending alkaline fly ash with acid forming coal refuse may present a disposal option that aids in the control of acid mine drainage. A preliminary column leaching study examined the leachate quality from acid-forming coal refuse:fly ash blends. Coal refuse (2.2 % total-S), and two refuse:fly ash blends (20% and 33% ash w/w) were packed into leaching columns and leached under unsaturated conditions for over four years. The coal refuse columns acidified quickly, producing peak leachate levels of acidity (pH 1.7), Fe (10,000 mg 1^-1), SO₄<sup-2(30,000 mg 1^-1), and Mn (300 mg 1^-1). Both ash blended treatments provided alkaline leachate with low metal levels. A second column study used a refuse with higher potential acidity (4% total-S), and two alkaline ash sources; one blended at two rates (20% and 33%) and one at four rates (5%, 10%, 20%, and 33%). Bulk-blended ground agricultural limestone, rock phosphate ore, and topsoil treatments were also studied. The unamended refuse treatment acidified rapidly and produced acidic (pH 1.7) leachates with high peak dissolved metal levels (>15,000 mg 1^-1 Fe; 200 mg 1^-1 Mn). Treatments blended with 33% ash produced alkaline (pH >8.4) leachates with low metal levels (Al <1.0 mg 1^-1; Cu-0.10 mg 1^-1; Fe < 2.0 mg 1^-1; Mn <3.0 mg 1^-1). Treatments containing less ash eventually acidified and ash bound metals (Mn and Cu) were stripped from the ash and eluted in quantities proportionate to the amount of ash in the blend. A field experiment with water sampling lysimeters examined bulk-blended refuse (0.8% S) with two acidic and one alkaline ash. The ash treatments did not produce consistent negative impacts on leachate water quality, and produced positive effects on plant biomass production compared to unamended controls. Overall, the inhibition of pyrite oxidation by high ash blends was attributed to alkalinity effects, chemisorption, decreased hydraulic conductivity, and decreased oxygen diffusion to pyritic surfaces. Ash alkalinity and refuse potential acidity must be balanced for environmental safety in this co-disposal environment.