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

Letter 12215

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?

Peter G
- Middlesbrough, England, UK

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

Karl Weyermann
- Lebanon, Kentucky

There is only "a market" 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 is good.

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

  

Thanks,

Can anyone put me in touch with any such companies in the UK?

Peter G
- Middlesbrough, ENGLAND, UK

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

Jon Quirt
- Fridley, Minnesota

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

Guy Lester
- Ontario, California

Hi,

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

Moshe Yaakov
- Lod, Israel

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 - UK

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

Tom Pullizzi Platronica.com Falls Township, PA

 

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, Vic., Australia

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 supernatent liquid, leaving the precipitate behind. I can't imagine there would be anyone interested in buying the precipitate.

Dennis Kirsch
- San Antonio, Texas

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

Sara Michaeli     chemical process supplier Israel

  

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?

Rick Fudalewski
- Mississauga, Ontario, Canada

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

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 Shee Pagsuyoin - Quezon City, Philippines August 8, 2009 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 ? Teddy Fabic color coating line - Manila, Philippines 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 finishing.com Brick, New Jersey