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Cyanide plating waste water treatment

A discussion started in 2004 and continuing through 2017 . . .


Q. Problem with Cyanide Mixture treatment

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 870 mv 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, Arizona, USA


A. 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 isolate 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
consultant - Cleveland, Ohio


Q. We are working with a cyanide plating line and we have seen some issues with the water effluent from the WWTS, we are not able to destroy all the cyanide using bleach at a pH of 9 - 12 and after that we are adding hydrogen peroxide. Is there any suggestion to destroy all the cyanide.

Carlos Rodriguez
Plating Engineer - Tucker, Georgia, USA


A. Couple of things we need to know to help you. What is the ORP setting (are you using ORP to feed the bleach and peroxide). This will tell us if you are adding sufficient oxidizer to complete the reaction. What is the reaction/ retention time.

Typically with a conventional type of cyanide destruct, which you are doing, problems lie with the reaction times or the ORP settings/ pH settings.

Gordon Djani
water specialist - Greenville, South Carolina


A. One caveat; make sure your ORP probe is calibrated before you go relying on it. This may be accomplished with quinhydrone. A saturated solution of this is made in pH 4 and 7 buffers. These solutions should have ORP values of +264 and + 84 mV respectively.

Personally, I always batch treated cyanide. The only ORP indicator I ever used was a vial of KI starch paper [linked by editor to product info at Amazon]. They have the advantage that there is very little that can go wrong with them.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


A. Agree with Gordon and David; better control seems advisable. Unless monitoring free chlorine, a pH of 9-12 seems imprecise - an ORP setpoint too low at pH 9 could be too high at pH 12. Try an ORP of 200 mV* at pH 11-11.5.

Also, the effluent may contain cyanate resulting from the high pH cyanide destruct. Follow the high pH reaction with a cyanate destruct at pH 7.5-8.5 and an ORP >600 mV.*

*Determine ORP settings using the actual controller probes.

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.

O&M of Surface Finishing Wastewater Treatment Systems
Clarence Roy


A. For rapid cyanide destruction with bleach, a pH of 9 is too low. The pH should be greater than 10.5. At this pH all of the amenable cyanide is destroyed nearly as fast as the bleach is mixed. At lower pH values, the reaction is much slower.

As mentioned above, only cyanides amenable to chlorination are destroyed in this process. Potentially, there may be cyanide-metal complexes, like ferricyanides, that are not destroyed by this process and will still test as "Total Cyanide". If this is your problem, you need to eliminate any contact between iron and cyanide, or use additional treatment of some type.

Lyle Kirman
consultant - Cleveland, Ohio

Reaction between Cyanide and Hypochlorite


Q. Yes, I need to know the reaction ratio between cyanide and bleach. Can anyone provide with the equation that shows the destruction of cyanide using bleach?

Has any of you used a test kit using ampules to test free cyanide in the discharge? Is there any on site test that any of you can recommend for a limit of 0.015 ppm cyanide?

please help!

Carolina Bruce
- Texas City, Texas

Plating Waste Treatment
Kenneth F. Cherry


A. Hi Carolina. With chlorine gas the reaction is simply:
Cl2 + NaCN + 2 NaOH = NaCNO + 2NaCl +H2O

So with sodium hypochlorite it's probably essentially:
NaOCl + NaCN = NaCNO + NaCl

But the equations have provisos and disclaimers, and hazards abound, so it would be best to view them in context in, for example, the Metal Finishing Guidebook. In the metal finishing industry, where the federal standards are about 20 times less stringent than the number you quote, I found that if the treatment were properly designed, simply testing the effluent for free chlorine with potassium iodide starch paper assured compliance. But, sorry, I don't personally know of an easy test for the numbers you want to monitor to.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. The good news is; yes, you can analyze down to that level with equipment you can buy for about $500. The bad news is; you have to do a cyanide distillation on your wastewater, a process that requires a skilled operator. You can teach yourself how to do it (I did) but it's not a trivial 'add a packet and compare the tube to a color chart' thing at all.

If you don't have the time or personnel to devote to this, send your samples out. If you do, Hach sells a nice set-up.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


A. I have had very good success with Batch treating any and all Cyanide.

pH of 11-11.5 and add bleach until there is a clear and obvious Chlorine residual.

I typically bled my batch through my treatment system after a 3 hour reaction time. (Overkill). Better safe than sorry.

Worst case scenario, Classify it and pay a contractor to remove the waste for you..

Ryan Cook
Toccoa, Georgia

October 25, 2008

Q. I need some guidance and it seems this could be the right place... I'm a Danish manufacturing engineer student and I'm working on a project for a company that produces waste water containing free cyanide in the amount of 1 - 20 mg/L and at a 100,000 L/day.
The amount needs to be cut down to below 1 mg/L. What is the best method to do this? Can anyone suggest a company that can deliver a treatment equipment/system/solution that can take care of it?

Any help will be most welcome. Thank you in advance!

Jakob Henriksen
- Odense, Denmark

October 28, 2008

A. Hi, Jakob. The general approach is to use a reducing agent: chlorine gas for large installations and sodium hypochlorite for smaller installations. At the correct pH, the reducing agent will convert the free cyanide to cyanate rather rapidly. But one needs relevant chemical experience, and usually hands-on training, when proposing something like this because there is the possibility of releasing poisons and toxins. If the cyanide is acidified before complete conversion to cyanate, deadly HCN gas will be released. If the chlorine is excessively acidified, copious amounts of chlorine gas and other toxic gasses will be released. You may also require a secondary treatment where the pH is lowered and additional chlorine is added to convert the cyanate to carbon dioxide and nitrogen. Recognize that adding acid to the solution is a hazard unless you are positive that all the free cyanide has already been destroyed. Please see the site's Environmental Directory for some suggested system vendors.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Treatment of cyanide waste with hydrogen peroxide

September 28, 2010

Q. Hello to all, I have a question if you can help me:
We treat cyanide waters with hydrogen peroxide. Does anyone know what value of ORP I should have when the oxidizing reaction is about to finish? I worked until now only with free cyanide test kit, but because of the high quantity of CN- in wastewater, sometimes I get errors in results (even with dilution). How should I use ORP when treating CN- with peroxide in order to monitor efficiently the destruction of cyanide?
Sincerely yours,

Sabina E. [last name deleted for privacy by Editor]
- Romania

October 14, 2010

A. Doing a Google search for Cyanide Treatment using peroxide I found:

On their web site they have a link to Technical Papers that also might help.

There may also be an alternative method for cleaning up cyanide but I'm still investigating that. Will get back to you when I have completed my research.

Larry Clark
- Brunswick, Georgia

March 7, 2012

Q. I have a 0.03 discharge limit for cyanide and heard that gluconates can "tie up" the cyanide and create potential discharge problems... Is that true?

Bill Muller
- Gallatin, Tennessee, USA

March 15, 2012

A. Gluconates interfere? Never heard of that, though it's possible.

Organic aldehydes are hellacious "killer" interferences with total CN analysis. Not organic acids, though, far as I know.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

March 24, 2013

A. Hi. Maybe there was a slight misunderstanding. Gluconates are, as Dave notes, organic acid salts, and I believe that they will complex or "tie-up" metals rather than cyanide. Typical waste treatment of alkaline cleaners with high concentrations of gluconates is
1. acidification, followed by
2. adding non-regulated metal salts like magnesium, aluminum, iron, or calcium to use up the gluconate's complexing power, then
3. raising the pH to precipitate the regulated metals.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Dealing with iron cyanide and total cyanide

March 21, 2013

Q. I have cadmium and zinc cyanide bearing wastewater. Calibrated the pH sensor and verified that the ORP sensor would measure 87 mV and 264 mV buffers +- 20 mV.

I ran the cyanide destruction process
1st stage 11.0 pH, 670 mV for 40 min free chlorine at 120 mg/l to 200 mg/l

2nd stage
1st stage 8.13 pH, 900 mV for 90 min free chlorine at 120 mg/l to 200 mg/l

Collected total cyanide 53 ppm and amendable cyanide <20 ppm. Thought we might have complex cyanide with copper or iron.

Tested for iron 28.7 mg/l, Cu = 0.171 mg/l

Does this mean we have iron cyanide complex, if so how do we treat the wastewater and how do we get iron out of the rinses.

Bart Hamlett
Engineering and Wastewater - Placentia, California

Starch Paper

April 2014

A. Hi Bart. I concur with Dave Wichern that the easiest way to check for free cyanide is potassium iodide starch paper (the starch paper doesn't actually check for cyanide, it checks for bleach -- but if there is excess bleach and the pH is right there won't be free cyanide), so I would use that to confirm that you destroyed all the free cyanide in the first stage.

You may be able to recognize iron cyanide from the "prussian blue" color, but to my knowledge there is no cheap and practical treatment for it. That's why unlined steel tanks should not be used anymore, not steel anode baskets. Keep the cyanide from contacting steel, and don't let it mix with nickel or other wastes until the cyanide is destroyed. Better yet, switch to alkaline non-cyanide zinc, and see if you can go with a zinc alloy in place of the cadmium plating.

Luck and Regards,

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

Ed. note: For still more on this topic, please also see letter 6852, "Wastewater treatment issues: Cyanide destruction questions".

April 9, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I have a solution containing 90-110 mg/l of sodium cyanide. I plan on using the process described in the "Managing Cyanide in Metal Finishing" (EPA 625) to oxidize the cyanide into N2 and CO2. I will add sodium hypochlorite to the solution which will start at a pH of 10.5 until the ORP reads +325-400 mv after 20 minutes. The second step calls for lowering the pH to 8.5 but does not specify how. Can sodium bicarbonate be used? If not, which dilute acid should be used (I have phosphoric and citric on site)? The second stage after the pH is reduced to 8.5 is to add more sodium hypochlorite until a ORP of +600-800 mv is read after 60 minutes. Are any problems with this process visible to anyone??

Cameron kivela
Process engineer - Boulder, Colorado, usa

April 17, 2014

A. A saturated solution of bicarb has a pH of 8.3, if memory serves. I guess it would work but it would take a lot. I used to simply use sulfuric acid. Phosphoric would work, but might generate excess solids if certain anions are present. Just make sure all the "free" cyanide is gone before acidifying. Sometimes, sparingly soluble CN compounds will precipitate and settle to the bottom. Make sure that that's not the case.

The cyanate oxidation step is not strictly required. It will spontaneously break down into ammonia and carbonate. NH3 might cause problems if you have a lot of copper and or nickel.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

March 18, 2016

A. If you treat the cyanides which are amenable with NaClO, you can then proceed with metals precipitation which will lower your total CN measurement to less than .5 ppm; any sludge generated will contain metals and cyanide

Mike Shoup
- Pennsylvania usa

CN increases instead of decreasing after chlorination

August 30, 2016

Q. I'm having cyanide concentrations in wastewater samples as 1.5 g/l in form of KCN tested by auto titrator. But when I treat it with hypochlorite, the concentration gets increased rather than getting decreased, although I maintain the ORP levels to 250 for first stage and pH to 10.5.

What improvements am I required to do, or what other treatment options would be available instead of this treatment?

vanashri limaye
- Ahmedabad,Gujarat ,India

August 30, 2016

A. 1.5 g/l is pretty high for chlorination. I imagine you get a lot of cyanogen chloride offgassing.

You might try 50% hydrogen peroxide. Lower the pH with sodium bicarbonate to less than 11, add a 1/2 pound of copper sulfate, then add peroxide until you see a color change to light green. It's good to use a tank with plenty of freeboard, because at the end, all the excess peroxide autolyzes and if the tank is too full it will go all over the place.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

August 30, 2016

Q. Thanks for the prompt reply; it will surely be of great help. But I would like to ask will it create HCN at low pH ... as according to literature I came across that at lower pH HCN may vaporize? So is it possible practically too? And what shall be the retention time kept for H202 process? And here we have 30% H202 available so will it be ok if I use 30% instead of 50?
And is ozonation or any other such treatment possible in such high concentrations along with H202?
Thanx again.

Vanashri Limaye [returning]
- Ahmedabad,Gujarat,India

August 31, 2016

A. Sure, it's fine to use 30% H2O2. It will just take more.

I did this treatment many times and never had a problem with HCN evolution. That's why you use bicarbonate, no matter how much is added the pH will not go below about 8. What might be a problem is the mist generated by O2 bubbles breaking the surface. I'd wear a cartridge respirator, never had a problem.

The residence time will vary depending on the metals present. I used to treat a 2,000 gallon batch of zinc cyanide waste with 1 - 1.5 g/l CN in about two hours.

Ozone is a great help, though to have it do the whole job would take a heck of a lot. Where it would be useful is in the finish, because I was never able to get the CN under about 50 mg/l with peroxide alone. I'd have to chlorinate, which has its disadvantages. It generates trihalomethanes by reacting with the brighteners and what not in the waste.

Colorimetric Analysis
by Snell & Snell

There's a good test for excess peroxide. Dissolve as much TiO2 in conc. H2SO4 as will go. Dilute this 1:4 with H2O. Combine 2 ml of this with 10 ml of the waste you're processing. A yellow color will develop, proportional to the H2O2 concentration. This is the pertitanic acid method, for details see Snell and Snell's "Colorimetric Methods of Analysis." It's from 1952, it's an oldie but a goodie.

Good luck! Be careful.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

September 1, 2016

Q. Thank you very much sir for your great help indeed it worked but I faced some problems though. I experimented according to your H2O2 guidelines ,and that did decrease the cyanide content to 'not detectable' on initial trial and on second trial 0.31, 0.13 g/l were obtained.

But I would like to ask that when CuSO4 was added precipitates were formed (with light green color pre H2O2 addition), and then I added H2O2, but those ppts were not soluble and on taking the samples after half an hour or so, as it contained some ppts might be I obtained higher concentration -- can this be the case for higher concentration of cyanides? If yes, then ppt must not contain free cyanides, a lot of confusion has started in my mind, please do help me out. And is it possible that H2O2 may get evolved (due to its half life) not completing the reaction? Is it needed to increase its solubility by adding anything?

Vanashri Limaye [returning]
- Ahmedabad, Gujarat, India

May 30, 2017

A. Dear Friends.
First we should attend to the pH value of the cyanide waste water. It should be maintained at 9-11 (preferably 10 ) so that the ORP electrode shows the correct value of desired ORP. The wrong pH will void the electrode function. So first adjust pH by adding NaOH (dilute) or mix the alkaline degreasing with your wastewater.

Then add calcium hypochlorite or use ozonation instruments to bring the negative/low ORP to +500. Please note that if you use Chlorine compounds, you should allow delays in injection periods. This way the Chlorine has enough time to react with complexes; otherwise the Cl level will be in excess and the output water look yellowish with bad chlorine smell. After delays and injections you will have the carbonate of the metals (Cu, Zn) and some sludge.

Then you can use milk of lime to raise the pH to 10-12 to precipitate all metal salts. Use some aeration instrument such as blowers to remove excess chlorine and also mix the lime milk with wastewater. After secondary pH control and some water polishing you can reuse it.
Please note: Safety first.

Alireza Reyhan
- Tehran, Iran, Persia

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