Sulfate Removal - Puzzling Gypsum Addition

[wardens355]

A plant is considering adding lime, sodium aluminate, and gypsum to precipitate sulfate (CaSO4 - Gypsum!!!). They call the gypsum a seed source or precipitation enhancement agent, and seem to have pilot tests to show it works. I have no idea how adding the product that precipitates can aide in its formation... anyone ever heard of doing this?

 

[MortenA]

Lots of percipitation processes benefit from seeding (especially any kind of crystallisation process) (i could mention sugar production and ice formation but there are numerous examples). However, these rate processes are often difficult to control and to consitently demonstrate an effect. Since gypsum been around for a long time i guess there should be litterature data to support this.

Best regards

Morten

 

[micalbrch]

Coal fired power plants add lime and water to the flue gas to bond the sulfur dioxice which is produced during burning. The result is gypsum which the sold.

 

[wardens355]

I understand that the plant is producing a lot of gypsum during the SOx control, but the lab testing indicated that pure, reactant grade gypsum was required to get better removal of sulfate in the wastewater stream. The lab tests used both recycled gypsum sludge and lab grade gypsum, and only saw good results with the lab grade material. Unfortunately, they did not test the other gypsum stream produced in the scrubbing process. Hopefully they can use that product, rather than shipping in pure gypsum, which will be expensive and probably upset the owners (since they produce a lot of gypsum).

 

[bimr]

Calcium sulfate (gypsum) is being added to reduce the carbonate (when excess alkalinity is present)to required levels. The reaction is as follows:


Na2CO3 + CaSO4 = CaCO3 ¯ + Na2SO4

sodium carbonate + calcium sulfate


= calcium carbonate + sodiumsulfate

Calcium sulfate is being added to precipitate the carbonate, nothing more. This is the same reaction that is involved in the reduction of noncarbonate calcium hardness.