Cyanide plating waste water treatment

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 ⇦this on eBay or Amazon [affil links] 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!

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.

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 [affil links]. They have the advantage that there is very little that can go wrong with them.

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.

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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.

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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 [affil links] assured compliance. But, sorry, I don't personally know of an easy test for the numbers you want to monitor to.

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.

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..

A. Hi, Jakob. The general approach is to use an oxidizing agent: chlorine gas for large installations and sodium hypochlorite for smaller installations. At the correct pH, the oxidizing 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.

Regards,

A. Doing a Google search for Cyanide Treatment using peroxide I found:
www.h2o2.com/industrial/applications.aspx?pid=106&name=Cyanide-Treatment

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.

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.

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.

Regards,

A. Hi Bart. I concur with Dave Wichern that the easiest way to check for free cyanide is potassium iodide starch paper [affil links] (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, nor 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,

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.

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

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 sulphate, 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.

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.

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.

A. Dear Friends.
Hi
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.

A. Hi James. We appended your inquiry to one of many previous threads on the subject. The formula you need is probably:
NaOCl + NaCN = NaCNO + NaCl
…but counting on stoichiometric relationships is tricky because that one remaining molecule of NaOCl in the upper right far corner of the tank may never contact the one remaining molecule of NaCN in the lower left near corner, or it may be lost to reacting with some stray organic. So you always need some excess :-)

Hypochlorite will oxidize cyanate but it is usually done as a second stage and requires several times as much hypochlorite and a lower pH. The Metal Finishing Guidebook offers that info.

Chlorination will remove cyanides "amenable to chlorination" and that generally includes WADs but generally not SADs.

Regards,

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A. Hi Jagdish. Sodium hypochlorite destroys cyanide but the copper must be physically removed. Take a bit of your treated wastewater, simply pour it through a Whatman filter, and tell us what the copper concentration of the filtrate is vs. what you need to achieve. Then we can continue ...

Regards,

A. Hi Jagdish. There is no way to destroy copper, all you can do is separate it from your discharge water and deal with it. Your filtration test demonstrated that your chemical treatment is sufficient. But now you must remove the copper precipitate from the wastewater. This is typically done by adding a polyelectrolyte flocculation agent and passing the wastewater thru a large lamella or tube-settler clarifier so the copper has time to settle -- no matter how you do it, it takes times for the settling. Then you must concentrate the copper-bearing sludge with a filter and have it legally disposed of. Alternately you can directly filter the waste, but you probably have so much water to deal with that this is problematic. Good luck.

Regards,