Se Capture in WFGD
We know that gypsum has a tendency to adsorb different species onto the gypsum particles.? These adsorbed species, at least to some degree, are carried away by the solid gypsum particles when the slurry is dewatered.? Yet, the sulfide of barium compounds provide a mechanism allowing the precipitation and crystallization of the selenium within the WFGD slurry.? Can there be a reasonable theory as to how this occurs?
T&T held that once Se is incorporated into the Barite lattice, it would be unlikely to leach out, since they saw strong crystal stability of the Ba-SeO3-SeO4-SO4.
In co-precipitation, crystal formation can generally be seen as a competition between the aggregation velocity and the orientation velocity.? If the solubility limit is exceeded, the aggregates of molecules tend to lay themselves together and accumulate to forge larger aggregates.? The aggregates formed are mixed in a somewhat arbitrary manner and are not stable.? As this unstable aggregate loses energy, the mass becomes a more ordered lattice.? The speed at which this takes place, represents the orientation velocity.
If the supersaturation is extremely large, as in the WFGD sulfate-laden slurry liquid, the aggregation velocity will dominate and the amorphous particles which form are slowly transformed into a crystalline structure.??
Barium selenate is three orders of magnitude less soluble than calcium selenate.? Likewise, barium sulfate is three orders of magnitude less soluble than calcium sulfate.??
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What seems to be widely misunderstood, is that selenium is not a metal.? It is not a metalloid.? Sulfur and selenium are both chalcogens, with selenium sitting directly below sulfur on the periodic chart.? Which is why sulfide precipitation can not remove selenium.
Sulfate and selenate ions are very similar in size, with a difference of less than 4%.??
The WFGD absorber seems to provide an environment conducive to selenium precipitation and crystallization.???
Tomorrow, we’ll review results from operating WFGD systems.