Change Hexavalent to trivalent chrome at high pH using sodium metabisulfite? 

I machine (Electro Chemical) high Nickel/Chromium components and my solutions contains metal hydroxides and chromium VI. I remove them as waste. They have a pH of > 8, (pH is high to aid machining and is essential.) is it possible to use sodium metabisulfite at this high PH to change Hexavalent chrome to trivalent chrome? If it's possible is there market for my waste product?

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Hexavalent chrome can be converted to trivalent chrome at a pH range 8.0 to 9.0 using sodium hydrosulfite.

Karl Weyermann
- Lebanon, Kentucky

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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 price than landfilling) so you are not 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.

Ted Mooney, P.E. finishing.com Brick, New Jersey

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It is important to keep hydrosulfite dry and sealed up, it can combust.

Jon Quirt
- Fridley, Minnesota

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Watch out with storing sodium hydrosulfite, though. it's water reactive.

Guy Lester
- Ontario, California

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Hi,

You can reduce Cr+6 to Cr+3 with Ferrous Sulfate; the reaction is very fast.

Moshe Yaakov
- Lod, Israel

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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 sulphurous fumes, so be warned!

Trevor Crichton R&D practical scientist The Pheasantries - Chesham, U.K.

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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

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Both sodium metabisulfite and ferrous sulphate ONLY reduce in acid conditions, pH <3, and will be of no use in this case. Sodium Hydrosulphite 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.

Geoff Whitelaw
- Port Melbourne, Victoria, Australia

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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 - maybe 1 oz./drum of wastewater.
2. Lower pH to about 3 with sulphuric 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.

Dennis Kirsch
- San Antonio, Texas

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November 17, 2010

In Dennis Kirsch response where would the ferricyanide come from in a chrome solution. We have just recently had this happen and would be interested to know the chemistry.

Phil nelson
- Victoria, BC, Canada

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November 17, 2010

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,

Ted Mooney, P.E. finishing.com Brick, New Jersey

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

Sara Michaeli     chemical process supplier Israel

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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 sulfate. 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.

Dave Wichern - The Bronx, New York

I have a question to ask, if ferric sulphate would be used as the primary precipitant and then the remaining chrome metal be reduced by sodium hydrosulfide, would this work? Has anyone tried using a combination of iron and hydrosulfide?

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Insoluble sulfide technology 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.

Larry Boldt
- Dallas, Texas

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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

Alok kumar jain
- India

Would there be a way to induce compaction of the gelatinous Cr(OH)₃ formed? I am trying to find a chemical that can induce the compaction to lower the disposal costs.

Thanks,
Shee

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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,

Ted Mooney, P.E. finishing.com Brick, New Jersey

We have tried treating hexavalent chromium with SMBS but our problem is the green color of the solution. Is there a way to convert this into a clear solution ?

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September 21, 2009

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

Regards,

Ted Mooney, P.E. finishing.com Brick, New Jersey

Dear All
I have gone through this conversation of yours, I understand that chrome trivalent sludge is problem for disposal.

I have a application which runs into approximately 1000 mt per month which requires chromium. but the metals that should not be there in the sludge are Ca, Na, K, Zn, Al,Mg.
Metals that are acceptable in the sludge are Fe, Ni, and Co in any percentage levels from 70% to 1% , but Cu, Mn less than 1%

is it possible to have this sludge prepared with ferrous sulphate and use ammonia to increase the pH?

If you all can contribute to this idea I think we can get good environmental solution to this.