From Residue Treatment
If fly ash is collected separately (e.g. upstream of flue gas scrubbing systems), two different methods may be applied: Neutral washing followed by solidification with added binders to condition residues into a leaching resistant material. Acidic scrubbing using acid scrubber blow down removes heavy metals into a recyclable form and conditions remaining ash for disposal.
Acid fly ash washing
This process is an ideal combination of two processes: Acid fly ash washing uses effluent from wet flue gas scrubbing to wash out the pollutants and remove heavy metals from fly ash. Heavy metals can be recovered in the waste water treatment stage and reused.
Acid scrubber blow down is first filtrated (candle filter) to separate solid pollutants from the liquid. Mercury is adsorbed on an activated carbon specific ion exchanger. Separation efficiency is > 99% and Hg can be recovered from the carbon and/or exchanger resin.
In a series of agitating tanks the acid dissolves heavy metals out of ash. The pH is controlled in a slightly acidic range by the addition of lime slurry; due to the alkaline character of ash, consumption of a neutralizing agent is reduced compared to plants without acidic fly ash treatment. The remaining insoluble components of the fly ash are rinsed and dewatered on a vacuum belt filter.
The dissolved heavy metals in the liquid phase are precipitated and separated from the water by flocculation and flotation; the sludge containing the metals (main component: zinc) is dewatered and transported to metallurgical zinc recovery plants.
Neutral fly ash washing
The aim of this process is to transform fly ash into a stable, leachate-proof residue. Neutral, easily soluble salts are extracted in a cascade of stirring 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 further steps. Dewatered solids are mixed with hydraulic binders to form a stable and solid body when landfilled.