Evaluation of coal fly ash for use in mine waste management

This study investigates the utilization of alkaline coal fly ash as an alternative mine waste management strategy to control acid mine drainage (AMD). A series of laboratory experiments were carried out to identify suitable geotechnical and geoenvironmental properties of fresh and landfilled coal fly ash from Atikokan Thermal Generating Station (Atikokan TGS), Ontario that are required for use in mine waste management. Two possible schemes of coal ash utilization for mine waste management were also investigated: (i) as a barrier material to retard the migration of heavy metals and acidity into the surrounding environment, and (ii) as an additive to mine tailings to prevent AMD formation and metal release.;The feasibility of using coal fly ash and bentonite-fly ash mixtures as a barrier material for mine waste was investigated using a constant flow fixed-ring permeameter. The measured hydraulic conductivity of the coal fly ash was of the order of 2 x 10-9 m/s when the fly ash was permeated with deionized water, and significantly decreased when the permeant was switched to AMD. Addition of bentonite to the coal fly ash lowered its hydraulic conductivity during water permeation but no further significant change was observed upon switching the permeant to AMD. Chemical analysis of the effluent from the hydraulic conductivity tests indicated that heavy metals present in AMD were attenuated and were well below the leachate criteria set by the Ontario Government.;This study also evaluated the efficiency of coal fly ash addition to reactive mine tailings to control the AMD formation and subsequent heavy metal mobilization. Atikokan coal fly ash and Musselwhite mine tailings were mixed at different proportions and the drainage chemistry and the optimal mix was investigated using static (acid base accounting (ABA)) and kinetic (column) testings. The ABA results indicated that fly ash possessed strong alkaline (neutralization) potential and could be used in the management of reactive mine tailings, thus ensuring prevention of AMD in the long term. Column tests conducted in the laboratory to further investigate the long-term performance of fly ash in the neutralization and mitigation of AMD from tailings similarly showed that mixing fly ash with mine tailings reduces dissolution of heavy metals by providing alkalinity to the system.;Keywords. coal fly ash; mine waste; acid mine drainage; acid base accounting; acid neutralization capacity, sorption capacity; hydraulic conductivity; saturation index.