Removal of heavy metals from fly ash and the impact on its quality

✍ Scribed by Henk W Nugteren; Maria Janssen-Jurkovícová; Brian Scarlett

• Publisher: Wiley (John Wiley & Sons)
• Year: 2002
• Tongue: English
• Weight: 128 KB
• Volume: 77
• Category: Article
• ISSN: 0268-2575
• DOI: 10.1002/jctb.600

✦ Synopsis

Abstract

Most coal fly ashes currently produced in The Netherlands contain leachable trace elements in amounts higher than allowed by Dutch law for its free application as a granular building material. To improve their technical and environmental quality, these ashes were subjected to forced leaching using water, citrate, oxalate, EDTA and carbonate solutions. With water alone, the free lime and substantial amounts of Mo (30%), Se (20–40%) and SO~4~^2−^ (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. Most of the extraction agents used form stable complexes or compounds with Ca^2+^, thus prewashing with water followed by treatment with extraction agents resulted in higher removal and/or lower reagent consumption. The environmental quality of the washed fly ash was evaluated using the Dutch standard leaching test (NEN7343). Leachability of oxyanions decreased by a factor of 2–4 after washing with water and by between 2 and 18 after washing with extraction agents. However, combined washing did not further decrease leaching but in some cases caused increased leaching. Selection of the washing procedure thus depends on the subsequent application of the ash, that is whether low concentrations or low mobility of trace elements are required. 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.

© 2002 Society of Chemical Industry