Letter 40024

Using Oxygen gas for Cyanide oxidation  

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We are currently looking for ways to lower our chemical costs. We currently use chlorine bleach for oxidation and sodium bisulfite for dechlorination. Is it possible to use oxygen gas, injected into a carrier stream, to oxidize the cyanide in the first stage of the process? The first stage is accomplished in a seperate tank from the second stage, where several first stage tanks are combined to second stage.

David M. Shook
helicopters - Grand Prairie, Texas, USA

Do not think that O2 will work well at all. It probably will take ozone or peroxide (diluted for safety)

James Watts
- Navarre, Florida

Oxygen may work some if you also use a UV light.

Paul Morkovsky
- Shiner, Texas, USA

Yes, you can oxidize cyanide with oxygen gas. In fact, you can hydrolyze it with just water. At 50 atm pressure, and 200 degrees C, that is. There are some proprietary cyanide destruct processes that operate this way, mainly used by centralized waste disposal facilities that treat concentrated cyanide wastes that other people generate.

This is a good example of the "thermodynamics vs kinetics" problem. Whether a reaction *can* occur, and, how fast it *will* occur, are two separate issues.

Another example are the two cyanide treatment chemicals that Mr. Watts mentions - hypochlorite and peroxide. Hypochlorite, or chlorine gas, reacts with cyanide very rapidly at pH 10 - 11.5. It will also oxidize cyanate - with enough chlorine you can take cyanide all the way down to nitrogen gas and carbon dioxide. Peroxide, on the other hand, is slow to react even in the presence of catalysts with cyanide, and will not oxidize cyanate at all. And, it is very tough, under normal conditions, to get the cyanide below about 25 ppm with peroxide alone.

My approach to concentrated cyanide wastes (2 - 5 g/l) is to take the pH down to about 11 with sodium bicarbonate, add copper ions as a catalyst, then peroxide over a period of hours to bring the cyanide down to about 50 ppm. The endpoint of this treatment stage is signaled by a color change to green, as copper is liberated from its colorless cyanide complex. Then, I use hypochlorite to bring it down the rest of the way. The advantages of this are: 1) little cyanogen chloride is generated and 2) the volume of the waste is not increased as much as it would be if hypochlorite alone was used. Peroxide is "stronger" in the sense that it contains more equivalents of oxidizer per gallon.

Hope this is of some help.

Dave Wichern - The Bronx, New York

I have seen a process described in which you add sodium chloride and then pass a current through two electrodes. Chlorine is produced at the anode and destroys cyanide. To be effective there are several practical details you will need:- Presumably some agitation will be necessary. The anode current density. The anode material, presumably carbon. The pH.

Nick Clatworthy
- Whitstable, Kent, Great Britain