Arsenic stabilization in coal fly ash through the employment of waste materials

Coal fly ash can be considered either as an industrial waste material or as a valuable raw material. This is due to the fact that the main problem in recycling this ash is the possible presence of heavy toxic metals and the necessary reliable treatment to avoid their leaching into the environment. In this paper Fluidized bed combustion (FBC) fly ash containing leachable arsenic (As) and vanadium (V) is stabilized with a new proposed technology, based on waste or by-product materials. In particular, silica fume, that is a byproduct of the smelting process in the silicon and ferrosilicon industry, is the main stabilizing agent used in this work. It is employed here, for the first time, for stabilization of arsenic. Also other ash, containing calcium hydroxide, is employed in the stabilization process.The starting fly ashes and stabilized materials have been characterized. The results of leaching tests show that, despite the lower leachability of vanadium in respect to arsenic, the proposed method results effective in the entrapment of both elements. Based on the results of characterization a different stabilization mechanism is proposed for the two elements: Vanadium seems to be stabilized by silica; concerning arsenic, it appears that calcium hydroxide acts as stabilizing agent for this element, probably due to formation of more stable phases.