If fly ash is collected separately from bottom ash and flue gas cleaning residues, two different methods may be applied to remove or immobilise fly ash contaminants:
Acid washing of fly ash using acidic scrubber blowdown removes heavy metals in a recyclable form and produces a fly ash which can be disposed with the bottom ash
Neutral washing followed by solidification by adding binders to condition the fly ash into a leach resistant matrix
This process is an ideal combination of two flue gas cleaning residues: Acidic fly ash washing uses acidic scrubber blowdown to wash out the heavy metals from fly ash. Heavy metals can be recovered in the effluent treatment stage and recycled.
The aim of this process is to transform fly ash into a stable, leach-proof matrix. Neutral, easily soluble salts are extracted in a cascade of stirred vessels. By controlling pH in a slightly alkaline range, soluble heavy metals are precipitated. Washing liquid is separated from the solids by filtration and cleaned in the effluent treatment stage. Dewatered solids are mixed with hydraulic binders to form a solid, stable and leach-resistant matrix when landfilled.
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Example of an acidic fly ash washing