Waste treatment in zinc plating

A. 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?

A. 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 wastewater. 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 ⇦liquid caustic soda in bulk on Amazon [affil link] , though. Some ferrous sulfate ⇦this on eBay or Amazon [affil links] 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.

A. I would suggest that you try some beaker [beakers on eBay or Amazon [affil links] 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.

A. 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.

A. 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.

A. 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.

A. Hi Zulkarrnaen. Sorry, but I don't think you can receive even a wild estimate for that question.

To my knowledge chemical precipitation would not be used for BOD removal and carbon filtration would not be used for zinc removal. Perhaps you have transposed something and you are planning on chemical precipitation for zinc, and carbon filtration for BOD?

For a first cut, you can probably use the effluent limitations as a guideline for zinc removal by precipitation because in much of the world they are supposedly based on what the regulators feel is possible. In other words, they think chemical precipitation will get the zinc down to 2 ppm or 4 ppm or whatever the regs in your area say.

Apologies but I could not begin to hazard a guess about BOD removal by carbon filtration.

Regards,