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topic 2846

Waste treatment in zinc plating


We're having trouble producing a true solid from our waste treatment of electrozinc rinsewaters. We have a typical chemical precipitation process. Our sludge is too wet, more of a slurry at times. We have tried alum and lime. No major improvements. Has anyone used potassium permangante with any success? Someone has suggested this, but I've not heard of it? What about hydrogen peroxide? I'm leery of both of these, but would welcome comments.

Karen Walters
- Lexington, Kentucky

Please don't mess around with unconventional, perhaps dangerous, treatment technologies unless there is a demonstrated need. You do have a filter press, I assume -- you're not expecting the material to precipitate as a solid without dewatering? Second, what zinc process do you use: cyanide, alkaline, chloride zinc, ammoniated zinc?

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


I'm not sure what kind of bath your waste is from. The general procedure for zinc waters is adjusting the pH to 9.0 - 9.3, and then using a flocculant to make settleable solids. Personally, I have not had a lot of luck with lime treatment of zinc plating wastewaters. If you are treating the rinses from an acid chloride zinc bath, adding a little at the start of the treatment may help. I would do most of the pH adjustment with 50% caustic soda, though. Some ferrous sulfate might help, too. The fact that you mention permanganate and peroxide makes me think that you are treating a cyanide waste. Both of these chemicals have their uses in cyanide treatment, but only in special circumstances. And, neither will have any effect on cyanide that is not amenable to chlorination. This type of cyanide needs to either be precipitated, or destroyed with special techniques, such as UV irradiation.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


I would suggest that you try some beaker tests using magnesium hydroxide. It is moderately insoluble and there is a great tendency to add too much, causing the pH to rise as time goes on, possibly causing a discharge problem as well as wasting product. It gives a considerably more granular precipitate. Consider using it for the bulk of the neutralization and then finish with sodium hydroxide or lime. Beaker testing and then 5 gal bucket testing is a wise way to proceed before committing bigger dollars.

James Watts
- Navarre, Florida


There is no problem precipitating zinc hydroxide, even from the cyanide solution (once the cyanide is treated). If the sludge is too slimy using sodium hydroxide, switch to hydrated lime. Leave to settle and run through the filter press. The cake will be in a dry " biscuit" form, especially if your filter press has the facility to blow air through once the cake is formed and ready to be taken out. Unfortunately our problem only starts there. Since the new environmental laws have come into operation we can no longer dump zinc hydroxide or zinc dust from vibropolishing onto hazard dumps. We need help! Anybody with ideas as to what we can do with it is welcome. Regards Trudy

Trudy Hattingh
- Durban, South Afica


Dear Ms. Hattingh:

Here in the U.S. we have some "recycling" of zinc sludges. I write "recycling" in quotes because when a hauler has to charge more to recycle something than to dispose of it in a secure landfill, one has to question whether this is true recycling or just a "feel good" exercise in semantics and obfuscation that consumes more natural resources than it saves. If you are not allowed to dispose of zinc wastes in a secure landfill, and cannot establish recycling of zinc sludges in South Africa, I guess you have to contract to landfill it in another country--an environmental disgrace that countries like the U.S. and Canada are already involved in.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

June 9 -

Are you also treating your cleaner rinses and dragging a lot of oil and cleaner into the water? I recently started up a Micro Filtration system for Zinc sludge rejection and they occasionally had upsets when they dragged to much cleaner and oils into the water and as the previous replies recommended Ca or Mg ties it all and forms a nice pin floc. John Ring

John Ring
- Wheaton IL


Hello again, I am still looking for help in stabilising zinc hydroxide effluent sludge to be able to dispose of it on a low hazard dump. Can you help? Regards Trudy

Trudy Hattingh
- Durban, South Africa


First I had a solution of zinc chloride and a small amount of MnCl2, could you suggest me how to separate in a form hydroxide metals between Mn and Zn.

I would be very grateful if you could give me some explanation.

Tri Budiono
College Student of Indonesian University - Jakarta, Indonesia

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