Wastewater Treatment Problem: Trouble Doing Cyanide Oxidation

A. Hi John,

If you are adding bleach, and you can't measure any excess free chlorine, then there probably isn't any free chlorine, it has all been consumed, and you need to add more.

At a pH of > 10.5, the reaction between bleach and free cyanide is extremely fast, it reacts nearly as fast as you can mix it. As the pH is lowered, the reaction speed slows down significantly. So, if you reduced the pH, you may see the ORP rise, but you would not be destroying the cyanide nearly as rapidly, and you would risk creating cyanogen chloride (a powerful kind of tear gas). One whiff and it will scare the hell out of everyone.

It sounds like you have a higher cyanide concentration than you expected, and just need to add more bleach.

Incidentally, even though all of the handbooks recommend it, adding acid in the second stage to convert the cyanate to carbon dioxide (actually bicarbonate) and nitrogen is not without risk. If the first stage fails due to an ORP probe or other failure or error, you would be adding acid to a cyanide solution. Most of the companies that I know of, have 2 stage treatment, but leave the acid pump turned off, and just let the second stage serve as a redundant first stage with added retention time. If you have to do the second stage due to aquatic toxicity or other requirements, I would recommend redundant probes (both pH and ORP) in the first stage with an alarm if they vary more that a small amount from each other.

A. This may sound too simple an answer, but have you checked whether the instrument you are using has a polarity reversing switch (generally DIP switches) inside the case?

Regards,

A. Lyle has offered a great answer to John's original question, and the only thing I would add to it is that potassium iodide starch paper [affil links] is very inexpensive and will give you an instantaneous and direct reading of whether free chlorine remains in the system as it should. If the paper turns blue (to light purple) you're in good shape, and there is unlikely to be any free cyanide left. Then you'll know if your ORP meter is informing you or misleading you.

You don't want to try to destroy cyanide at such a low pH, Carolina. As Lyle said, you risk formation of cyanogen chloride. Further, the reaction speed will slow to very nearly zero, and it is not safe to try to hold either cyanide or chlorine in solution at a pH so close to neutral. Remember that pH is a logarithmic scale, and 10.5 is about 300 times as alkaline as 8.0. Good luck.

A. It sounds like you haven't added enough sodium hypochlorite. It would help to know what your total cyanide concentration is (free + complexed cyanide). It takes a minimum of 3.1 grams of available chlorine for each gram of cyanide for first stage oxidation to be completed.

A pH of 11.5 is higher than needed. Anything > 10.5 is sufficient.

Q. Lyle, we only determine free cyanide in the copper plating bath.The usual concentration is 10 g/l. The sample I have tried in the lab is 3 g/l. The qty I have calculated via the equation is 162 lt NaOCl for the tank of 1 m3. The qty for the sample of 100 ml is 16,2 ml. But there is no result, neither ORP proper indication nor KJ starch test paper. All this I need to prepare an automatic treatment for CN oxidation for our wastewater with pH and ORP indicator. I have tried to add an excess of NaOCl with no result.
Do you think is possible to try only one stage of oxidation of CN? To add only some qty of NaOCl without trying to succeed the second stage of turning CNO to N2 and CO2? Can I dispose CNO to the environment?
Thank you for your help
Matina

Q. Is it okay if I add NaOCl and then measure the CN concentration with the photometer, even if I don't get the right measure in ORP? Is there a problem if I send away CNO to the environment? By the end of the oxidation I have less than 0.08 ppm CN, but I have no idea if I have CNO or CO2 and N2.
Thank you for your help.

A. Matina,

The concentration of complexed cyanide is much higher than the concentration of free cyanide. If you know the copper concentration then you can estimate this because most of the copper exists in solution as Cu(CN)₂^-. At high levels of free cyanide, there may also be some Cu(CN)₃^-2.

This should help to estimate the amount of hypochlorite needed.

Whether or not you can dispose of cyanates depends upon your discharge regulations. In the USA, cyanates are not usually regulated, but new treatment systems have to be designed with 2-stage oxidation.

Lyle thank you so much for your help! The concentration of Cu is around 3 g/l. I think that we don't have regulation for the disposal of cyanates. Anyway, I will keep trying to succeed with the 2nd stage.
Thank you again
Matina

A. This waste sounds like a great candidate for electrolytic treatment. That way, you get salable metal instead of sludge you must pay to haul.

The problem with stopping at the cyanate stage is that it breaks down into NH3, which solubilizes Cu. This will increase the amount of precipitant (almost certainly required) to attain the discharge limit.

A. Dave has a great answer. Heat to 180 °F, then electrolise at, say, 5 amps per gallon with steel plates as anode and cathode. In 50 hours or so, you will deposit almost all the copper as saleable metal, and reduce the cyanide to the ppm level.