Improvement of environmental quality of coal fly ash by applying forced leaching

A coal fly ash containing leachable trace elements in amounts higher than allowed by Dutch law for its free application as granular building material was subjected to forced leaching using water, citrate-, oxalate-, EDTA- and carbonate-solutions. With water only, the free lime and substantial amounts of Mo (30%), Se (20–40%) and SO42− (40–50%) are removed. Using the extraction agents, these percentages increase to 60–90% and in addition 15–20% Cr, V and Sb are removed. The environmental quality of the washed fly ash was checked with the Dutch standard leaching test (NEN7343). Leachability of Mo, Se and S decreased by a factor of 2–4 after washing with water and a factor 2–18 (including Cr) after washing with the extracting agents. Removing extractants and metals remaining in the moisture after filtration will further decrease leaching. For a feasible process reduction of reagent consumption and simple methods for process-water recycling are needed.