Electroplating Wastewater Treatment Q&A's, Problems & Solutions

A. Nickel and Cyanide must be separated. Because Nickel-Cyanide complex is very difficult to break down. You can use two stage cyanide oxidation processes for cyanide waste. A conventional heavy metal precipitation using caustic soda [affil links] works fine with Nickel bearing waste .

A. Hello Rifat. Here in the U.S., such a system would include a treatment step for destruction of cyanide by alkaline chlorination, a pH adjustment system for the nickel, and then separation of the precipitated solids with a clarifier. The sludges from the clarifier would be treated in a recessed plate filter press. That's certainly not the only way, but it's the most common approach. Good luck.

A. 1 Molar NaCN = 49 grams NaCN/liter = 6.5 ounces/ US gallon. Alkaline chlorination is NOT recommended for concentrated solutions because of dangerous side reactions like poisonous gases, and eruption. The reaction in dilute solutions is the following formula (reference, Graham's Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] , 3rd Edition, VanNostrand, New York).

2NaCN + 5 NaOCl + 2NaOH = 2Na₂CO₃ + 5NaCl + N₂ + H₂O

In electroplating wastewater, where alkaline chlorination has been used for many years, the cyanide concentration may be only a few hundred mg/liter and the rest of the components of the original solution have also been diluted. Bleach [adv: bleach/sodium hypochlorite in bulk on eBay or Amazon] is added, with good mixing, to a permanent blue reaction to Potassium Iodide starch paper [affil links] as an endpoint, and/or watch the reaction with an Oxidation-Reduction Potential electrode.

Whether your plating bath is proprietary or not, it may be worthwhile to reclaim the silver, and avoid a waste treatment job. Even if you have an exemption to waste treatment due to laboratory status, it is still not a good idea to dump cyanide down the drain.

Perhaps there is someone in your area who reclaims silver from medical film who could take the solution? I don't know if they still use cyanide in the process.

A. I believe that there may be several patents and/or proprietary processes on the subject. A literature search may give you some leads.

A. Hello David. I believe that you will find this was originally called the "Kastone Process". This was a DuPont process, although I am not sure if they are still marketing it themselves. Still, it should be a good search term for you. Happy hunting.

A. Hi Marta. Back in 1982 I wrote a pamphlet entitled "20 Ways to Reduce Water Usage in Plating Shops", and it's now in our on-line library.

Luck & Regards,

A. A pH of 9.5 is a bit low for Ni.

Your best bet would be to add a bit of sulfide type precipitant, such as sodium sulfide, or dimethyldithiocarbamate. This would be a lot more forgiving than 'plain vanilla' hydroxide precipitation.

A. Good afternoon:

You didn't mention whether the nickel present in the wastewater discharge is dissolved nickel or nickel bearing suspended solids which passed through your treatment system. I'll assume that its dissolved nickel that's causing your problems.

As a preventative measure, if you are using cyanide based solutions in your finishing operations, please make sure you are not accidentally mixing cyanide bearing wastewater and nickel bearing wastewater. Nickel-cyanide complex can be tough to waste treat.

If you don't have a problem with nickel-cyanide complex, then the sulfide or carbamate additions mentioned by David Wichern are a great idea. Just remember not to exceed the recommended dosage; use only enough to precipitate out the stubborn metals in solution. Carbamate compounds are toxic to marine wildlife.

A. Not only that: if you use too much, it won't work as well. This is of particular concern with DTC, because getting rid of an excess is not so easy. You wind up with this unbreakable tea colored colloid. To break it, you need to add a bunch of ferrous or alum or something. It's a real pain in the neck.

That is why I always liked sodium sulfide. Easy to get rid of excess S2-, just blow air through it, or add some hypochlorite. Unfortunately, NaS2 has some OSHA issues associated with it that make people not want to have it in the plant.

A. Before deciding that DTC is preferable to sodium sulfide or simple lime precipitation, readers should see:

http://www.state.in.us/idem/your_environment/wrcac/whiteriver/complaint/complaint.pdf

opinion It describes a pollution catastrophe -- 187 tons of fish killed in the White River in Indiana due to a gross, excessive discharge of DTC from an electroplating plant. As far as I can determine, this was by far the worst pollution incident ever caused by an American electroplating plant ... and ironically it was not from an electroplating waste but from a treatment chemical nobody wants but which was required to meet over-the-top discharge standards. An interpretation which probably resonates with platers is that if you are relentlessly pressured to use incredibly powerful toxins like DTC in pursuit of that meaningless last fraction of a milligram of metal, you open the door for accidents that are far more injurious. Not saying that we should view the White River disaster quite exactly that way (I don't know all the facts, and neither operators nor management should have released the DTC to the river) -- but smarter people than me, like Dr. Leslie Lancy, who brought waste treatment of electroplating wastes to this country, struggled mightily to teach us that the old idiom "the cure is worse than the disease" most definitely applies to "split milligram" discharge standards. His words fell on plugged ears. 187 tons of dead fish later I hope we've started listening but I doubt it. And did any regulator lose their job for the requirement that DTC be on hand for that fraction of a milligram? Of course not! It was all on the operators, the management, the system designers -- none of who wanted anything to do with DTC :-( Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey ---- Ed. note: Sorry, but Indiana has removed that link. The White River pollution disaster was so significant, however, that you'll have no problem googling for info about it.

A. In addition to environmental considerations, a practical problem often occurs in lowering [Ni] using DTC or soluble sulfide: As the DTC lowers [Ni] and other metal concentrations, previously precipitated hydroxides such as Ni(OH)2 dissolve toward maintaining their equilibrium Ni, Cu, Zn, etc. solubilities. One may end up using very large amounts of DTC precipitating all metals. Hence, precipitation with DTC or soluble sulfide should only be used as a 'polishing' treatment on filter press effluent water. In which case, it's easier to do a 2-stage pH precipitation, removing most metals at pH 8.5 and residual Ni, Zn, etc. at pH 10.5.

Possibly, Ni-containing waste waters can be segregated and treated separately.

A. Dave: Good point about sulfide, and thanks for it. sulfide is a much stronger precipitant than hydroxide. But I've done a lot of jar testing and found it not as effective as DTC at approaching zero concentration of metals.

Ken: I totally agree that we shouldn't use DTC in the first stage of a wastewater treatment system, and should reserve its use (if used at all) for segregated polishing of what's left after hydroxide precipitation. But I don't think much precipitation happens at pH 8.5; about the only thing that comes out in many shops at that pH is iron.

I think the right approach was developed by Dr. Lancy before the EPA even existed: hold the pH low or make it low; add ferric chloride [on eBay or Amazon] or aluminum sulphate as a co-precipitant/chelate breaker; raise the pH to 10.5; filter or clarify; reduce the pH to acceptable discharge point. If this doesn't reduce the contaminants to a low enough level to comply with the law, change the process chemistry or change the law because the treatment regimen was right -- the use of DTC is a dangerous step in the wrong direction :-)

The above assumes, of course, that we consider end-of-pipe treatment of "mixed metals" satisfactory; and that premise should be continually reexamined as times change. Just because we used to mix them doesn't necessarily mean we should continue to.

A. Malaysia in particular has a tough go from a discharge standard, as they are usually put into categorical standards known as Malaysia a or Malaysia b, both of which exist based on WHO water standards. Usually, they do not have the luxury of a secondary or tertiary POTW, so the use of DTC is out - they don't even set limits on it (but there are limits!) Sulfide works, however you also have sulfide standards for discharge, as well as chlorine (for those of you that want to use a little hypo to destroy the sulfide). Lastly, with all this treatment, there is (or at least was) a TDS limit, so adding a raft of chemicals to treat this is probably not the panacea that it may be here.

You didn't mention what type of nickel it was - the originating bath, so the final treatment technique could be hard to find. If its chelated at all - you cannot treat it - even with Doc's method, unless you get rid of the chelator, or tie it up. Iron does that, but there are sulphate and chloride discharge standards to consider as well. Let's not forget, while KL is a BIG city, its treatment works are not what they are here -- and unlike people here that depend on well or tap water to live. There are still many many people that live by the rivers there -- washing, drinking, existing etc. That's why the WHO standards are there. They don't have the benefit of chlorination at every point.

I might suggest ion exchange as an alternate, depending on the origination of the nickel and the form, either on a point of use basis, or end of pipe if necessary, but controlling it POU is a heck of a lot easier and more efficient -- and cost effective. It can usually easily get well below the discharge point before the stream is commingled with the effluent, and once mixed, it could be virtually undetectable in the final outfall.

 Thanks, Tom, for reminding us that the question was about a discharge situation in Malaysia, not here, and that others don't live the same way we do :-)

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

A. It is true that there are discharge standards for sulfide - I operated under a pretty tough one (0.5 mg/l, Santa Clara, CA) for some years. Either because I used sodium sulfide in conjunction with ferrous sulphate ferrous sulphate [affil links], because I was treating fairly small batches (2,000 - 3,000 gallons) or because the sulfide ion is so easily oxidized, I never had a problem with it. I used to batch treat, and I'd add many pounds of sodium sulfide to batches. I never had to treat for residual sulfide with hypo or peroxide or anything else. Just the O2 diffusing into the treated water through the surface took care of any excess. The treated decant ... well, I wouldn't put it up against Chanel #5 in a Sweet Smelling Derby, but it didn't reek of H2S, either.

Tom is, of course, correct in saying that the best solution to the problem is POU control - don't make the problem in the first place! That is perhaps the wisest word yet.

A. Dear Sirs,

I was browsing the Internet in search for information for a presentation on electroplating effluent treatment when I happened to find your discussion on nickel problems.

I was a little surprised to see that the Degussa trimercapto-s-triazine, marketed as TMT 15, was not mentioned at all. TMT 15 should solve the problem easily and without toxicity.

Best regards,

---

"Plating Waste Treatment"
by Kenneth F. Cherry

on AbeBooks

or eBay or

Amazon

(affil links)

Thanks, Esko, you may be right! It's supposedly non-toxic and environmentally safe. But I'm no biochemist and don't pretend to know enough about the potential toxicity or lack of potential toxicity of that compound to hold an opinion on it.

Luck & Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. I routinely get under 1 ppm nickel but my method may not be applicable to your waste plant. I precipitate at pH 10.5 or 11 first, allow to settle, correct the pH and then discharge. I can do this because I have reduced the quantity of water used so much that I have days between discharges.