Letter 27083

Problem with Cyanide Mixture waste treatment  

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Subject: Problem with Cyanide Mixture treatment
Importance: High

I have several waste streams of cyanide bearing waste water coming to a common treatment tank. I have the following waste streams:

Copper cyanide rinse water = 300 gallons per day
Bright Alloy Rinse = (Copper Cyanide + Zinc Cyanide mixture) = 300 gallons per week
Acid Gold Rinse = 300 gallons per day.

My problem is I batch treat this waste in a 4500 gallon tank. I use Sodium Hypochlorite solution (Bleach 12%).

I add bleach to an ORP reading of 750 mv. In the past, a lower ORP level would result in incomplete cyanide destruction. Our goal is to reduce the cyanide to cyanate, adjust the pH and discharge to the POTW.

Normally we have no problem with the cyanide rinse water above treatment to cyanate. The problem arises when a Bright alloy bath is dumped into this mix. The bright Alloy bath is:

NaCN = 3.2 oz/gal
Zn = 0.13 oz/gal
Cu = 0.28 oz/gal
Sn = 0.08 oz/gal
pH = 10.5

This is a complex cyanide bath. When it mixes with the gold rinse we get a tank(4500 gals) of the following

AU - 4.77 mg/l
Cu - 29.3 mg/l
Zn - 2.57 mg/l
CN - 9.13 mg/l after 870mv contact for 5 days.

When no Gold is in the mix the batch treats in 24 hours with Total cyanide = non-detect. This is verified by outside Lab. We use the EPA method of cook down to test total cyanide. Any suggestion on how to proceed to break this cyanide complex?

Daniel M. Reeves
Waste Water Treatment Supervisor - Glendale, AZ, USA

Gold, nickel, and iron cyanide complexes are a lot more difficult to treat, and may not be destroyed by simple chlorination.

By my calculations, there are over 2.5 troy ounces of gold in your 4500 gallon batch. Perhaps it is possible to isloate the gold cyanide flow and treat it with ion exchange before it gets to the batch. Anion resins remove gold cyanide complexes very efficiently, and can hold up to several pounds of gold per cubic foot of resin. They are burned to recover the gold, which destroys the cyanide.

Alternatively, you could add some zinc dust to the batch before the chlorination step. The gold will immersion plate on the zinc and be replaced in solution with the more easily treated zinc cyanide.

Good luck, and I hope you can recover the gold!

Lyle Kirman
water treatment systems - Cleveland, Ohio

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