The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Most Effective Method to Precipitate Zinc

What is the best and most effective method to precipitate zinc out of water? We are discharging water with 3 ppm of zinc to a sewer line but need to bring that down to 1 ppm-1.5 ppm. Please take into account that cost is a big factor.

Phet Sinthavong
Environmental Firm - Olympia, WA, United States
2004

---

There wouldn't be a need for environmental firms like yours, Mr. Sinthavong, if the answers to such questions did not depend upon flow rates, source of the contaminant, interfering complexers, etc. So it would be a good idea to hear the full list of such facts before making a suggestion. But sulfide post-treatment, either with sodium polysulfide (cheapest), DTC (sodium diethyl dithio carbamate) or a proprietary such as Degussa's TMT 15 (most expensive) has become common in such applications.

You usually will need an organic coagulant to go with the sulfide because the concentration of zinc is so low that you will otherwise end up with such a fine floc that it will be impractical to filter or settle.

Also we wary of the dangers of overly casual application of sulfide or DTC. Look up "White River Fish Kill": Sadly and ironically, the biggest environmental disaster in electroplating industry history wasn't a discharge of untreated plating waste, but a discharge of those treatment chemicals no plating shop ever wanted, but which are now needed to comply with untenable treatment standards unachievable with conventional precipitation technology :-(

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004