Disposal, uses and treatments of combustion ashes: a review

Ashes resulting from the combustion of fuels can contain mobile compounds that may give rise to significant pollution on disposal or during use. Source reduction has limited potential in reducing such pollution. Thus for polluting elements immobilization and forced extraction may be considered and regarding organic pollutants: destruction. Thermal destruction of hazardous organics in the preset of a catalyst is practised. In case of immobilization permanent sequestration of once mobile elements in quartz-type silica lattices is an attractive option to prevent future pollution by such compounds. So far full sequestration of hazardous compounds in quartz-type silica lattices has not been demonstrated. Partial sequestration has been shown and is widely applied. Extraction of vanadium from ash generated by the combustion of heave oil is practiced commercially. Extraction of germanium from coal ash is pursued in Russia and China. The extraction of small pieces of metal from bottom ash of municipal waste incinerators by magnetic and eddy currant separation can be profitable. Washing of ashes from municipal waste incineration is also practised. Among the more generally applicable forced extraction processes, biohydrometallurgy seems relatively promising for the extraction of heavy metals because necessary inputs generate a relatively low environmental burden. As processes for high efficiency immobilization and forced extraction of metals and other elements cannot be evaluated well as to their long-term actual performance in the field by current leachability tests, it is presently not possible to settle the question of which press is preferable to reduce leaching. Moreover, the overall evaluation of destruction, immobilization and forced extraction is dependent on the weighting factors regarding respective environmental impacts. (C) 2004 Elsevier B.V. All rights reserved.