Civil Engineering Reference
In-Depth Information
confirmed by Gong and Kirk (1994) who reported that these heavy metals
exist largely as chlorides, primarily on the surfaces of larger particles (mostly
silicates) a view supported by Greenberg and Zoller (1978). Importantly,
they consider the relative solubility of these phases, using both leach testing
methods and equilibrium thermodynamic calculations. Their findings that
much of the fly ash (they estimate around 60%) is soluble and that heavy
metals are mobilized from the ash following dissolution are confirmed
and expanded by Johnson et al. (1996) and van der Sloot et al. (2001).
The implication of this is far reaching; not only do the fly ashes contain
heavy metal species, they are in a form which is soluble and therefore
available for environmental dispersion, increasing the impact of their
toxicity.
12.3.3 APC residues
This class of combustion product contains, to a greater or lesser extent, a
portion of the fly ash which carries over from the previous stage, where
treatment and collection of the two fractions are independent. APC residues
are distinct from fly ash in that they are formed during the final stage of flue
gas cleaning by injection of solvents and sorbents into the gas stream. To
neutralize acid gases such as HCl, SO
2
and NO
x
, alkaline solutions (typically
calcium and/or sodium carbonate and/or hydroxides - finely ground limestone
slurry is the most common) are sprayed into the gas in a 'scrubber'. The
HCl and SO
2
are rapidly converted to halite and gypsum whilst some CO
2
can be captured as sodium carbonate where sodium hydroxide is present.
Lundtorp et al. (2002) describe the use of FeSO
4
as a scrubbing reactant and
show that it results in reaction products with significantly lower solubilities
than do conventional methods. Organic compounds (especially dioxins and
furans) are captured by sorption onto the surface of activated carbon particles
also present in the scrubber fluids. Further oxidation of the gas stream may
take place, through the use of regenerative oxidizers of either the thermal or
catalytic types, but as these act solely in the gas phase (producing no solid
waste) they need not concern us further. Alba et al. (1997) and Bethea (1978)
review air pollution control technology and options for the treatment of its
residues. More recent work by Quina et al. (2008) and rani et al. (2008)
completes the picture, discussing the production, properties, composition and
treatment of MSW incinerator fly ashes. Both groups of authors advocate
the thermal conversion of APC residues to glassy ceramics, feeling that the
high solubility of some of the metal-bearing phases they contain precludes
their direct use in concrete.
It should be noted that the term 'air pollution control residues' is used
to describe more than one waste stream, which reflects the wide variation
of technology and practice in the industry. APC residues may be the solids
