Leaching of Sulfidic Alum Shale Waste at Different Temperatures

摘要

Four different alum shale waste products (unprocessed shale, weathered fines, processed shale and shale ash) from Kvarntorp, Sweden, were leached at different temperatures (-18°C, +22°C and +70°C) in order to elucidate the influence from freezing (frost wedging) and high temperatures (heat generated during sulfide oxidation). Unprocessed shale and the weathered fine fraction have an acidic pH while the processed shale and the shale ash have circum neutral pH. Leaching was performed at liquid solid ratio of 10:1 at room temperature followed by treatment at different temperatures for 24 hours (cycle repeated 10 times). pH, electrical conductivity, redox, alkalinity, acidity, sulfate, major cat ions and trace elements were measured. pH was almost one pH unit lower in the heat treated shale ash samples compared to the samples kept in the freezer. No significant pH differences were observed for the other samples. Iron and sulfate concentrations were found to be higher in the heat treated samples still containing pyrite (unprocessed shale and weathered fines) indicating a higher rate of oxidation. When it comes to trace elements molybdenum, for instance, significantly higher concentrations were leached from the processed shale compared to the unprocessed shale indicating increased leachability due to transformation of the primary minerals.