It was about the time when the US WFGD industry began experiencing very aggressive duplex alloy vessel material failure via corrosion, that WFGD ORP entered the vernacular. It wasn’t too long before WFGD ORP control seemed to almost take center stage. It lingers still.
One of the offshoots of this new-found ORP talisman was the belief that by controlling oxidation air flows, we could not only minimize persulfate (S2O82-) formation, which was implicated in this dastardly corrosion business, but minimize conversion of selenium (IV), referred to as selenite, to selenium (VI), referred to as selenate.
The graph directly below gives a sense of what was widely adopted as the Se/ORP Relationship.
Selenate is a difficult ion to remove in phys/chem wastewater treatment systems, while selenite is seen as not nearly as troublesome. It’s simple, really. The new ORP guru’s said you need to lower the ORP to lower the selenate level, and possibly eliminate your bioreactor cost. Certainly, minimize the impact.
The myth of such indirect selenium control slowly became entrenched, with patents eagerly sought, even leading to the development of an on-line “sulfite analyzer”. We’re making gypsum, right?
Now, with the sulfite analyzer, did we suddenly find a tool that could help the customer save money? Could they “control” the ORP? If so, please explain to me how it helped the customer save money?
Another little minor issue is lack of an approved EPA analytical method to determine selenite and selenate concentrations in WFGD slurries. So, why did we go down this road? ORP meters…sulfite analyzers…oxidation air flow adjustments. This isn’t pH that we’re measuring.
From the MercSelAs (MSA) perspective, selenium precipitation within the WFGD doesn’t rely on an ORP meter, OR a sulfite analyzer. The essential nature of the gypsum-laden slurry, the precipitation of the incoming sulfur dioxide into gypsum, and its subsequent crystallization – this is our field of invention.
Our patented technology partitions selenium to the slurry solid phase due to the robust crystal stability between Ba-SeO3-SeO4-SO4. It works because of the absorption, dissolution, and co-precipitation with the incoming sulfur dioxide and subsequent crystallizing nature of the solids-burdened, sulfate laden slurry solution. A solids-free, sulfate-free solution is quite the opposite of what is needed to make this technology work. It is counter-intuitive.
Why not consider such a test at your WFGD system, and see what selenium reductions you can achieve to lower your bioreactor cost? Since selenium is not a metal, phys/chem systems using sulfide precipitation are ineffective.
At least a partial solution to this selenium issue lies in the WFGD. We own equipment that allows MSA testing at full-scale to be arranged. December 2025 is still four years away. Why not see how MSA could lower your bioreactor cost? How much could you save?