Why, and how, did our February 2021 testing at AEP’s John E. Amos Plant result in a 60% reduction in aqueous selenium concentration at the discharge point (ELG measurement point), if there was no phase partitioning of Se to the slurry solid phase? How were we able to continuously lower Secondary Clarifier Overflow selenium concentrations, once MSA injection began, if there was no precipitation, with subsequent phase-partitioning?
While the MercSelAs (MSA) is added to the WFGD absorbers for the reduction of aqueous selenium concentrations in the bleed slurry via phase partitioning to the solid phase, this graph of the data shows a persistent and lingering effect of continued crystallization of the selenium into the mixed crystal as the particle makes its way from the WFGD absorber to the wastewater treatment system. How else is the aqueous selenium concentration in the Secondary Clarifier Overflow dropping? The work of the T&T brothers (my term of endearment for Japanese Geo-chemists Takahashi and Tokunaga) would suggest this outcome should be expected.
There are those who have said this chemical injection approach to ELG compliance, adding MSA into the WFGD Absorber, where sulfate levels of 5,000 – 10,000 ppm are common, can’t work. Can’t work for reducing selenium in a WFGD since the sulfate will overwhelm the chemical being injected, rendering the chemical ineffective in removing selenium from the aqueous phase. They have neglected to adequately understand co-precipitation theory, and may not have reviewed the recent work (2017) of T&T.
In a sulfate supersaturated slurry solution, the concentration of the selenium ions will co-precipitate prior to the sulfate ions due to the favorable aggregation velocity. Numerous beaker tests on real WFGD bleed slurry taken from the absorber confirm this again and again. In the real world of operation, when you add precipitation and crystallization, things should be expected to improve for selenium phase partitioning, and that is what we have seen.
In their technical paper “Effective Removal of Selenite and Selenate Ions from Aqueous Solution by Barite”, T&T said “once Se is incorporated into the Barite lattice, it is unlikely to be leached out regardless of the surrounding environment because of the strong crystal stability of the Ba-SeO3-SeO4-SO4 for Se immobilization.”
As the bleed slurry leaves the WFGD absorber, the pH immediately begins to rise, and the temperature begins to fall. As the pH of the bleed slurry rises along the route from the absorber to the wastewater treatment system, T&T’s distribution coefficient, a measurement of selenium uptake in barite compared to water, shows selenite uptake is effectively tripled between 5.8 < pH < 8.0 in Artificial Seawater. In water alone, the distribution was much less robust.
So, our MSA is provides chemically-coupled control – since both mercury in the stack and selenium in the bleed can be effectively reduced. And , as a chemical intermediary, the cost is reasonable. It’s delivered as a solid, and slurried on site.
Stack Hg control. Wastewater Se control. Test equipment is ready to be shipped and tested at your site. What are you waiting for? Four years is not that far away.