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Precipitation and treatment of chelates in waste streams


I am a waste water treatment operator supporting a chemical finish shop. We use electroless nickel which contain the chelant EDTA. We have a batch treatment system which uses Reduction and precipitation in the conical bottom batch tanks. We treat and let bed settle and decant the clear liquid into a slant plate clarifier that uses a polyelectrolyte polymer to facilitate metal fallout. We have to use inordinate amounts of ferrous sulfate in batch tank precipitation and this creates a lot of sludge and still sometimes doesn't work. How can we solve this problem and how can chelates be detected using an Atomic Absorption Flame Spectroscopy?

Robert Lester
- Dallas, Texas


It is indeed difficult to waste treat electroless nickel for the reasons you present. Many electroless nickel vendors take back the spent solution for reprocessing, and your time might be better spent arranging for that than trying to develop a treatment process that most plating jobshops long ago despaired of and demanded an alternative to.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey


Robert, Ted is right. The manufacturers of electroless nickel solutions will arrange to have the spent concentrates hauled for you.

There are more efficient methods of reduction treating (if that's the way you want to go, however) than ferrous sulfate. A few years back, I had great success treating nickel batches with sodium sulfide (yuck!), which generated very small sludge volumes. Nasty and dangerous stuff, though. Other companies have used 0000 steel wool [affil. link to info/product at Rockler] to reduce nickel ion, then disposed of the ammoniated residual liquid according to state and fed laws.

If you're trying to detect NHx chelators using AA, you're barking up the wrong tree. If you don't have an HPLC, just dilute a target sample with DI water and run it through the NH3-N program on your PC lab spectrophotometer. You'll have to set up known standards of the waste you're analyzing, however, to draw straight lines to EDTA.

randy fowler
Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA

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