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Zn removal from wastewater

We need to remove Zn from our effluent to a level of 2 mg/liter. We are producing varistor ceramics so the main impurities in wastewater are suspended ZnO and dissolved Zn. The suspended particles are removed by hydroxidation, coagulation and flocculation but up till now we have not found any proper method for removal of dissolved zinc. Please advise.

Jost Razinger
electronic ceramics - Zuzemberk, Slovenia
2004

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2004

The first trial should be pH adjustment to see if that alone will precipitate the dissolved zinc. If not, the next trial would be the addition of alum or ferric chloride ⇦ this on eBay or Amazon [affil links] or calcium chloride before pH adjustment to see if that will drive the zinc out of solution. If not, try lowering the pH to about 4, then adding one of the above co-precipitants, and raising the pH again. If you still are meeting with no success, you need to investigate what complexing/chelating agent is keeping the zinc in solution (ammonia, etc.?)

Usually, a final sulfide precipitation with calcium polysulfide, or DTC, or a proprietary like DeGusa's TMT15 will very greatly reduce the solubility of zinc, but there are severe potential environmental downsides to sulfide chemistry (including some horrific fishkills from overdosing), so it should be applied only if necessary, and only after investigating local rules and conditions. This kind of project is best addressed by experienced wastewater treatment engineers, and certainly only by people with chemistry qualifications.

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

Mr Razinger,

It will help you greatly if you maintain the pH in your precipitation tank at a pH no higher than 9.45 Any higher than this results in the formation of a zincate (zinc hydroxide complex) that results in zinc going back into solution and dubious outfall results. I experienced this problem 5 years ago with acid and alkaline zinc plating but by using the above managed to maintain outfall consent of less than 2 mg/litre.

I hope this helps!

Nigel D Gill, B.Sc. MIMF AIEMA MRSC
- Glasgow, Scotland
2004

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Since you are faced with both suspended and dissolved zinc there could be significant simplification using sacrificial electrolytic methods. UNLESS there is a strong complexing agent present, as Ted suggests, then you should be able to kill both sources in one process.

Paul Morkovsky
- Shiner, Texas, USA
2004

Can you let me know the influent concentration of dissolved Zn? I might suggest a suitable media for this.

Anita U. Ang Co
R&D; Centre - Mississauga, Ontario, Canada
2007