How does MercSelAs MSA remove selenium from a Wet Flue Gas Desulfurization System?
We can use SO2 as a proxy for SeO2.
Perry and Chilton’s Chemical Engineers’ Handbook places the WFGD Absorber within the broad category of Diffusional Operations, and more specifically, Gas Absorption. Additionally, within Gas Absorption, this fundamental mass-transfer of the SO2 in the flue gas to the liquid, and the subsequent outcome, is further broken out as diffusion with reaction. When “a chemical reaction takes place in the liquid with the absorbed molecules, the concentration profiles are altered.” The WFGD Absorber, designed for SO2 removal, alters the slurry to otherwise avoid a quick descent into exceedingly acidic conditions. It does this via the controlled feeding of finely ground limestone, CaCO3. It is the continuous dissolution of the Ca2+ into the liquid that precipitates the dissolved SO2 into the solid CaSO4. This is the chemistry, mass-transfer, balancing act in which gypsum byproduct is being made.
Once the SO2 diffuses into the slurry liquid, it assumes ionic form, which allows it to react with the Ca2+ from the dissolving limestone. This mass is transferred into CaSO4 via precipitation as a solid.
So, it isn’t simply the SO2 mass being transferred from the gas to the liquid, but also Ca2+ being dissolved within the slightly acidic absorber solution, and the precipitation of the CaSO4 which keeps the absorber clock ticking.
The WFGD absorber can only be properly understood when realizing that all forms of earthly matter; solid, liquid and gas; are part of the harmony that allows the system to work as required. While it is the flue gas that enters the absorber and exits with a different compositional structure, no less does the compositional structure of the liquid and solid material within the absorber change.
And this is also the way it works for SeO2. But in the case of SeO2, it is the addition of the Ba2+ from the sulfide of barium solution from which the barium selenate and barium selenate are precipitated, and subsequently removed from the WFGD. Yes. There is 5000 – 10000+ ppm of sulfate in the WFGD absorber slurry, but it is co-precipitation of the co-crystal, which binds selenium and sulfur into the heterogeneous mixture. Once selenium is incorporated into the crystal lattice it is unlikely to be leached out in the WFGD absorber, and subsequent downstream environments.
Once the gaseous constituents are dissolved into the liquid, the heavy lifting begins.