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Cyanide destruction with peroxide (Cu catalysed)

March 24, 2009

I am working to re-start an old cyanide detoxification facility at GSBPL and want to conduct some laboratory test work before plant trials.

Question: What dosage rates of peroxide and Cu will be considered enough(base case) for detoxification of cyanide by oxidation. Also, in previous operations, evaporation and even boiling of peroxide was experienced during hot weather (in rural Ghana,West Africa). Any ideas on how to overcome this?

Ideas very much appreciated.

Ahmed-Salim Adam
metallurgist - Bogoso,Ghana

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March 28, 2009

Treatment of concentrated (1 - 3 g/l CN(a)) cyanide wastes with peroxide is an old specialty of mine. I did it successfully for years.

Bring the pH of the cyanide waste down to about 11 with sodium bicarbonate. Some people try to do this with acids but that is kind of suicidal. The proper Cu catalyst concentration is something like 30 - 40 mg/l.

Add the peroxide slowly and gradually with lots of stirring. Depending on the cyanide concentration, this step will take 2 - 6 hours. The sign of the completion of the treatment is the appearance of a lime green color owing to the liberation of Cu from its cyanide complex (colorless) and its transformation into the ammonia [on eBay or Amazon] complex (dark blue).
There will also be some foaming as any excess peroxide in the batch autolyzes and oxygen is liberated. If you have been careless and added a large excess of peroxide, the tank of waste will foam over like a stein of beer and make an ungodly mess.

This treatment will leave 40 - 50 mg/l of "free" cyanide in the waste. I used to have to do a polish treatment with bleach to attain my discharge limits. It has no effect at all on iron cyanide complexes. Also, unlike chlorination, peroxide will not oxidize cyanate or ammonia. This may make the final step - metals removal - a bit harder.

Good luck! Do plenty of bench tests first. Also, make sure to wear a respirator. There is little chance of releasing hydrogen cyanide if you do as I suggest, but peroxidation always makes the waste foam up, generating a mist that is very unhealthy to breathe.

Dave Wichern
Consultant - The Bronx, New York