Change Hexavalent to trivalent chrome? Chrome Reduction process

A. Hi Thyagaraj,
The blue color is the trivalent chromium ... to remove the blue color you have to remove the trivalent chromium by converting it to chromium hydroxide and settling it out.

Do some jar tests and see if the pH you are attempting to hold is too low or too high for full precipitation, or exactly what is the problem.

Readers: please do your wastewater treatment experiments with beaker [beakers on eBay or Amazon] size volumes not with hundreds or thousands of liters.
Luck & Regards,


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

A. Hello Thyagaraj,
Just a question or 2 first.
Is your effluent batch treatment or continuous flow?
If it is batch treatment i.e. in a single tank for later release then I recommend that you raise the pH to around 9 to 10. Then add a flocculant in order to draw the precipitate together.
This can than be pumped through a filter press to remove the solid content, giving you water with a much lower metal content dissolved in it.
If you have a continuous flow of water through your facility then the above procedure can be controlled with pH probes and chemical dosing pumps. With each stage, i.e. acidic chrome reduction, pH raising, flocculent addition done in separate tanks that over flow from one to the next.
Finally this water should drain into a laminar flow type settling tank to allow the solid content to fall to the bottom, with the cleaner water flowing over a weir at the top to drain.
The sludge at that forms at the bottom then gets pumped through a filter press, as above, for separate chemical waste disposal.
It is worth periodically analysing the discharge water in both scenarios to ensure that it complies with local discharge requirements.
Good Luck
Mark

A. Hexavalent chrome can be converted to trivalent chrome at a pH range 8.0 to 9.0 using sodium hydrosulfite [on eBay or Amazon].

A. Hi, Peter. There is only "a market" for the sludge in the sense that there may be companies who will recycle it (at greater or less cost than landfilling) so you reduce your chance of being stuck with the liability. But I've never heard of anyone paying for this waste product.

Sodium metabisulfite only works at pH of about 5.0 and less (and quite slowly at 5.0). Karl's suggestion sounds good to me if you are doing integrated treatment (using the treated effluent as the coolant). If you are treating end-of-pipe it may be more practical to reduce the pH and use the sodium metabisulfite.

A. Hi,

You can reduce Cr+6 to Cr+3 with ferrous sulphate [on eBay or Amazon]; the reaction is very fast.

A. I have used sodium metabisulfite to neutralise hexavalent chromium solutions from a chrome plating bath. This was done to both make the waste bath safer and easier to handle and to neutralise any chromic acid that may have crept into nooks and crannies in the plating jig or the workpiece. All I can say is that the yellow/orange hexavalent chromium solution went green, which suggests it does work. However, metabisulfite gives off sulfurous fumes, so be warned!

i. Fumes are a problem only if the pH gets too low! You need to adjust the pH of the treatment tank.

  
Tom Pullizzi
Falls Township, Pennsylvania

A. Both sodium metabisulfite and ferrous sulphate [on eBay or Amazon] ONLY reduce in acid conditions, pH <3, and will be of no use in this case. Sodium hydrosulfite [on eBay or Amazon] is the material of choice but as warned keep absolutely dry. Just dampening Hydros can cause fires. Add only amount required to take yellow colour away. Cr+3 should precipitate as hydroxide but will be of no commercial value.

A. Although Geoff is correct that metabisulphite is of no value under alkaline conditions, I agree with Tom. Very low pH in the chrome treatment tank allows instantaneous reaction, but thereby generates substantial fuming; my experience has been that if we allow for longer reaction times, chrome can be reduced at a mildly acid pH, maybe 4.5-5.0, generating no serious sulfur small.

Run jar tests to see how high you can keep the pH for sodium metabisulphite reduction of chrome if you allow extended treatment time.

Luck & Regards,

A. Sodium Metabisulfite does an excellent job of precipitating Cr+3 as the hydroxide. In general, the procedure is:
1. if there is any ferricyanide ion present, add a little laundry bleach [adv: bleach/sodium hypochlorite in bulk on eBay or Amazon] - maybe 1 oz./drum of wastewater.
2. Lower pH to about 3 with sulfuric acid.
3. Add some SMBS - use trial and error to find out how much is needed. Agitate by bubbling air.
4. Raise pH with NaOH. Be careful. I use NaOH pellets in a wire basket. When everything goes green, reaction is complete. Problem is that the precipitate is gelatinous and difficult to filter. Just let it settle and decant the supernatant liquid, leaving the precipitate behind. I can't imagine there would be anyone interested in buying the precipitate.

A. At elevated pH only hydrosulfite can reduce the hexavalent chromium to trivalent.

A. The problem with sodium hydrosulfite is that it will only bring the Cr+6 down to about 5 - 10 ppm, under alkaline conditions.

I believe the best solution to treating a hex chrome waste without bringing the pH down into the basement is ferrous sulphate ferrous sulphate [affil links]. It will work at pH 5 - 6. The problem with it is that it is only a one electron reductant, so that 3 moles of ferrous is required per mole of hex chrome. This means that it will make a great deal of sludge if your hex chrome is at all high.

Perhaps a combined approach? Use hydrosulfite to reduce the bulk of the chrome, then polish with ferrous.

Hope this is of some help.

A. WHILE PRECIPITATING, IF INSTEAD OF NaOH, MgO IS USED, IT FORMS STABLE PRECIPITATE. SIMILARLY, BRING pH TO AROUND 2 TO GET BETTER RESULTS AT pH 3 IT TAKES LONGER TIME FOR REDUCTION
AKJAIN

A. We have developed an insoluble sulfide technology that reduces Hexavalent chrome to the trivalent state at a near neutral pH. This technology produces less sludge than the traditional SMBS and ferrous treatment process and is less costly and safer than the Hydrosulfite method. It also doesn't generate the corrosive SO2 odors that come from the traditional SMBS treatment methods. We have also found that the TDS of the treated supernate is much lower making the treated water more susceptible for water reuse. Larry B Dallas, Texas [last name deleted by Editor] 2003 A. My company has developed a proprietary reducing agent which is effective at a pH range of 3-4, and does not give off any sulfur dioxide fumes when converting CR6 to CR3. If is a completely non-hazardous product, with no combustibility problems. If your interested contact us at (xxx) xxx-xxxx. A.J. R East Rutherford, New Jersey "Internet advertising price/demand curve: Infinite demand at zero cost, zero demand at infinitesimal cost" -- T. J. Pullizzi, 1995 Larry's product and/or A.J.'s may indeed by an improvement over conventional treatments. Readers are encouraged to google the subject! And Larry & A.J. are very welcome to advertise here or elsewhere. But hopefully readers who saw the once terrific sci.chem newsgroups blown away by allowing 'comment spam', and who saw so many websites lost to comment spam, and so many sites lose their credibility due to thinly disguised 'sponsored posts' (which we have never allowed) will understand why we delete contact info so as to refuse to let comment spam destroy this one :-) Ted Mooney, P.E. RET Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. No one has mentioned using calcium polysulfide for Cr(VI) reduction. We use that to treat hex chrome in groundwater in situ. No pH adjustment is required. Sometimes a ferrous iron salt is added to facilitate immobilization of the Cr3+ as the sulfide or with iron oxyhydroxide.

A. Hi, Shee.

No chemical can do that to my knowledge. But "difficult to filter" doesn't mean impossible to filter. A filter press preloaded with filter aid is one way to filter it. Good luck.

Regards,

A. Hi, Teddy. Hexavalent chromium reduction with SMBS is only the first step in the treatment. After that, the pH must be brought back up and the green trivalent chrome converted to chromium hydroxide and precipitated out.

Regards,

A. Hi, Phil. I think Dennis was referring to the treatment of a mixed waste which contained chrome but was not exclusively chrome; rather, it also included some cyanide waste.
There is no cyanide in chromic acid, but some older hexavalent chromate conversion coating chemistries included cyanide as an accelerator.

Regards,

A. Hi Muhammad. This entire thread is on that subject, but you get a more specific answer without a more specific question. I don't understand what makes your situation different from everyone else's because you haven't told us yet :-)

Regards,

A. Hi Muhammad. Sorry, but we may be having a language difficulty. It's already been explained that:
Step 1 is to reduce hexavalent chrome to trivalent with a reducing compound; usually this is sodium metabisulfite (SMBS) [on eBay or Amazon] at a pH of below 5.
Step 2 is to raise the pH to precipitate the trivalent chrome as chromium hydroxide so it will settle or can be filtered.

But you must explain your situation before anyone can help you further.

Are you trying to convert a beaker of dilute wastewater, which contains water and hexavalent chromium into trivalent chromium for some reason? That can be done by lowering the pH to below 5.0 and slowly adding a small amount of sodium metabisulfite, under a lab hood, until the faintly amber color of the water turns to faintly blue-green, indicating the conversion is complete.

Are you trying to convert fasteners that already have a hexavalent chromium conversion coating on them to RoHS-compatible trivalent chromium? It is not possible; the old coating must be removed.

Are you trying to convert a hexavalent chromium electroplating bath to a trivalent chromium plating bath? It is not possible; you must discard the old bath and start new.

Are you trying to design an industrial wastewater treatment plant for treating the effluent from a hexavalent chromate conversion coating line or a hexavalent chrome electroplating line? That may involve additions of sulfur dioxide or sodium metabisulfite after lowering the pH, or employing the previously discussed sodium hydrosulfite. But both hexavalent chromium and the sulfur-bearing chemicals like sulfur dioxide, sodium metabisulfite, and sodium hydrosulfite are quite hazardous chemicals. You can't add them to an unknown waste stream without severe hazards. If you over-acidify sodium metabisulfite, for example, it releases hazardous quantities of poisonous gasses.

If you are designing an industrial wastewater treatment plant for plating wastes, please start with a good book on the subject, such as "Operation and Maintenance of Surface Finishing Wastewater Treatment Systems" [adv: this book on eBay, Amazon, or AbeBooks]. Best of luck!

Regards,

A. "There's more than one way to skin a cat" -- keeping the barnyard metaphors going in our digital age.

It's simplest to mix the chrome wastewater with other acidic rinse waters, but it's most effective to treat wastes before mixing them with anything else.

Luck & Regards,

A. Hi Umesh. Wastewater treatment and sludge disposal is expensive. It will almost surely prove impractical to make a product from this sludge. The better approach is to focus on retaining/recycling the chromium before it enters the waste stream. Good luck.

Regards,

A. Hi Hans. There is no need to separate the more concentrated treatment chemical from the rinse water; you can collect and treat it all together. Reduce the pH of the collected waste water to about 4.5-5.0 with dilute sulfuric acid, slowly add small amounts of sodium metabisulfite (SMBS) [on eBay or Amazon] powder until all trace of amber/yellow/orange/honey color is gone and you have only a blue/green color. Then raise the pH to about 8.5-9.0 with hydrated lime and filter out / settle out any sediment/precipitate before discharging the water.

There may be local or national regulations prohibiting you from doing this, but neutralizing hex chrome with metabisulphite seems far better than discharging hex chrome. There are certainly hazards if you're not familiar with the chemistry, so keep studying. Wear rubber gloves [on eBay or Amazon] and goggles [on eBay or Amazon] at a minimum. The reaction will be slow, but safer at pH 4.5-5; lower pH is used in industry, but can release a good amount of sulfur dioxide gas -- better to keep the pH just low enough to reduce the chrome and just take your time. Good luck.

Readers: the internet is a giant one-room schoolhouse where it is impossible to keep people from reading about stuff which they may not be trained & qualified to do. So just because readers may read some steps here does not mean they have the background and skillset to safely and properly follow them :-)

Regards,

A. A small amount of Thiol Red will reduce any Chromium VI and precipitate the metals so that they can be settled or filtered.

A. Do what Mr. Mooney says. That will work.

Or, since you have so little, you could reduce the Cr6+ to Cr3+ with ferrous sulphate ferrous sulphate [affil links]. pH in the neutral range. Then get the pH up to 9 or so, let the solids settle, and dump the free liquid (should be legal) down the drain.

Dewater the settled solids, the filtrate ought to be legal to sewer. Then, put the solids in the trash.

Hi Satish. I can't tell you the limits because every country has their own, many states and zones within states have their own, and it may also depend on whether you are discharging to surface waters vs. a public treatment works.

But trivalent chrome can be precipitated. That is, at neutral to alkaline pH the chrome will not stay dissolved but will become a floc which you can settle or filter depending on your situation. Good luck.

Regards,

A. Hi shahrukh. You are only at about 1% of the solubility of sodium sulphate, so if you use sulfuric acid for pickling and sodium hydroxide for treatment, the problem may not be solvable. But if you use lime for treatment instead of sodium hydroxide, this number might sharply decline. Jar testing is definitely needed. Good luck.

Regards,

A. Rather good read on the subject of using lime successfully in Chromium treatment (tannery effluent) done in 2010 by an Egyptian research team:

https://www.researchgate.net/publication/307137491

I love this paper in that it is quite well written in such a manner as to be understandable to non-academics, and found it quite useful some years back while doing a bunch of jar testing on spent trivalent conversion coating bath solution. It's a fantastic resource!

A. Hi Jerry. I have never heard of that situation and had never even contemplated the possibility, so thanks for this warning! I have seen dried chromate powders and they can certainly be yellow and resemble powdered sulfur. So if it looks like hex chrome and analyzes as hex chrome, I guess it is.

I don't think it will prove practical to treat these dusts and powders in-situ. Dissolving them into water will give you the opportunity for quantitative analysis, reduction & precipitation, and confirmation of what you have accomplished. Further, you will have a widely accepted treatment protocol to follow.

Regards,

A. Hi Chris. Sulfuric acid is the usual choice and surely a better choice than nitric acid. Good luck.

Regards,

Ted is correct in that Sulfuric is better than Nitric; this is because Nitric acid is an oxidizer and will absolutely have the opposite effect of what you are trying to achieve in a reduction.

A. Hi Anand. Air powered diaphragm pumps are usually employed for such applications. They are well suited because they will do no shearing damage to the floc and nothing is liable to happen in the process that can damage the pump.

I think a bag filter would 'work', but poorly, because with no pressure to compact the sludge you won't get beyond a very loose mud. I have seen small installations distribute thin chrome sludges in their boiler room in a thin layer to evaporate the moisture, but thermal drying of filter bags doesn't sound practical for a 30 GPM system. Usually a clarifier (settling tank) followed by a polypropylene filter press, often with some amount of filter aid, is the only practical approach.

Luck & Regards,