Assessment of acid mine drainage potential of flotation slurry from a tailing dam in a copper mine, Murgul, Northeastern Turkey

Acid mine drainage potential of flotation slurry from a tailing dam in an active copper mine (Murgul) in Turkey was evaluated using an integrated approach combining static and kinetics tests, mineralogical and geochemical characterization and geochemical modeling. Acid mine drainage potential of the tailing waste was determined by acid base accounting tests and humidity cell experiment. During ABA tests, paste pH, fizz rating, sulfur speciation, acid and neutralization potentials of the slurry were measured. The rates of metal release and mechanism were determined from the humidity cell experiment. Leachates sampled weekly from the humidity cell were monitored for their pH, electrical conductivity, oxidation–reduction potential, alkalinity, sulfate and metal contents. In addition, at the end of the experiment, cell material was sampled along the flow path to determine the possible AMD products formed. Results of the ABA tests indicated that the tailing waste exhibited acidic character. Paste pH of the waste was 4.35. Acid potential (116.5 kg CaCO3/t) of the waste was about 34-folds more than its neutralizing potential (3.44 kg CaCO3/t), indicating that tailing waste had a high acid producing potential. In contrary to the ABA tests, the results of humidity cell experiment indicated that leachates of the tailing waste exhibited neutral to alkaline pH values (pH 7.4–8.2) throughout the experiment. The rates of metal (e.g., Fe: 0.04–0.001 mg/kg/week, Cu: 0.005 mg/kg/week) and sulfate release (24 mg/kg/week) were very low. Metals in leachate showed similar elution behavior. The rates of metal release were initially fast and then decreased significantly, suggesting that different processes including desorption, mineral dissolution/precipitation and oxidation played a role on metal elution behavior. Neutral to alkaline pH values observed throughout the humidity cell experiment suggesting that acidity produced by the pyrite oxidation was buffered by the low neutralizing capacity of the waste. Low permeability of the tailing waste and the presence of the unaltered pyrite grains significantly limited the kinetics of the acid mine drainage formation. Evidence of pyrite oxidation and the formation of ferrihydrite precipitates was visible especially at surface of the waste that was in contact with air and water. Dissolution of gypsum also controlled the observed sulfate release behavior from the tailing waste. Mass balance calculations performed on each metal showed that the percentage of mass eluted from humidity cell for Mn, Zn and Cu was 8.6, 4.9 and 3.6%, respectively, while it was <1% for the rest of the metals investigated. Results of this study suggest that slow metal leaching kinetics of the tailing waste reduce the risk of metal contamination in the surrounding surface water bodies that is in contact with the waste material.