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Chrome reduction in dilute rinse water

2007

I have had to do chrome reduction in rinse batches that worked out well. My procedure is to adjust pH to 2.5 - 3.0, add sodium metabisulfide (here I would observe my solution changing from brown to dark green). The pH is then adjusted to 7.5 and calcium chloride and flocculant added. The resulting light green precipitate would settle out and the decanted solution or filtrate would be crystal clear meeting city limits for discharge.

My question concerns the dilute batch that I'm currently working on which is light brown in color. I carry out the procedure as above. However, it it fails to turn green at the addition of metabisulfide ( it goes dark brown instead) and the resulting solution from a light tan-beige precipitate, would be a pale yellow which contain considerable chrome.

What changes do I need for this very dilute rinse to achieve a clear filtrate?

Kenny Henry
E,H&S Coordinator/Chemist - Miramar, Florida

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It's hard to say from this distance and without concentrations, Kenny, but it sounds to me like this solution is behaving like it is predominantly iron rather than chrome. Do you have any analysis before treatment?

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

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First of two simultaneous responses --

Try retesting the pH after the metabisulfite addition. I'll bet that increases the pH to the point where you're no longer getting effective Cr6+ reduction.

And, just a litle ferrous sulfate ⇦this on eBay or Amazon [affil links] added after the metabisulfite will not hurt.

Dave Wichern
Consultant - The Bronx, New York
2007

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Second of two simultaneous responses --

Hello!

well I tell you what I did during the same process.
I'm having a tank of about 1400 ltrs. capacity. After it got filled we lowers the pH of the solution to 2--4. We normally use sulfuric Acid or HCl of commercial grade.Then we use sodium metabisulphite and then we start churning it with a electrical motor fitted to the tank for the whole night and for another one day we make it to settle down. Then after we note the pH it should be between 5--6. Then we raise it's pH to 9--10 by pouring Sodium Hydroxide in order to make the Hexvalent Chrome PPT. Within half an hour we got results i.e by taking the solution into beaker [beakers on eBay or Amazon [affil links] and let it to be settle down for half hour we see that sludge settles down and clear water is on top.U can further check that whatever treatment you have done is right or not by taking the clear solution from the beaker and add 1--2 gm. of Diphenyl Carbazide and Nitric acid, If color changes then you should again give the same treatment if not Bulls Eye. We are doing for more than 2 yrs. we have no problems. We did 4-5 ltrs. of HCl ,5--6 Kg. Sodium Metabisulphite and 5--6 kg caustic for 1200 Ltrs.

Hope this will help u.

Sean Aarchr
- Chandigarh
2007

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Ted,
The analysis prior to treatment via ICP shows: Cr 204.7 mg/l, Cu 1.357 mg/l, Ni 29.27 mg/l as metals with highest concentrations. No anlysis is made for Iron.

Thanks,

Kenny Henry
- Miramar, Florida
2007

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I wouldn't consider 204 mg/l "very dilute", but I don't know quite what's going on. I ignored what I assumed was a typo of "sulfide" for "sulfite", but obviously one thing to double check is that your reagents remain what you think they are :-)

Good luck.

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