Cross-Layer Leaching of Coal Fly Ash and Mine Tailings to Control Acid Generation from Mine Wastes

Abstract

Coal fly ash can be used in in various configurations (e.g. as cap, bottom liner, or blending) at a mine site, but comparative studies investigating their capacity to control acid mine drainage are limited. Batch and column leaching experiments were conducted to investigate the effects of fly ash-mine tailings mix ratios and system configurations on leachate chemistry. Acidic mine tailings (pH 2.72) were obtained from waste piles at a former gold and pyrite mine. Mixing the fly ash with the tailings in a 1:1 (w/w) ratio decreased Zn, Ni, Fe, Mn, Pb, and Cu leaching by 90 ± 6%, increased the pH from extremely acidic (2.9) to alkaline (8.0), and decreased electrical conductivity from 4 to 2.5 mS cm−1 due to solute precipitation. Using the fly ash as a ‘chemical liner’ beneath the tailings, applying the fly ash as both a cap and bottom liner, or blending the fly ash with tailings produced significantly less acidity, salinity, and metal leaching than using the fly ash as a cap. The capacity of fly ash to control acid generation is attributed to its acid neutralizing capacity and high pH (11.1).