Metal finishing Q&As since 1989

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Wastewater treatment question (chromium and cyanide)

2002

I'm new to the plating industry and I have a few questions.

Where I'm working we have a wastewater treatment system who treat chromium and cyanide. This must be very common. We use ORP probe in each of these treatment tank. We reduce Cr6+ to Cr3+ with sodium metabisulfite and I want to know what is the exact reaction and what is the limit potential(mV) associated with it, so I can be sure that we are not realizing hex. chromium in water?

Same question for cyanide, here we use sodium hypochlorite and I want to make sure that the probe and the alarm are set correctly.

Thank you for your help.

Stéphane Daneault
- Québec, Canada

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From previous experience, when you are treating both chrome (VI) and cyanide, the pH of your effluent stream is as important as your mv readings. We would always get the pH in range first (10.5 - 11.0 for cyanide in our case) and then set up the mv to suit. It's probably better to set your own parameters that give you best results. You need to be able to measure the chrome (VI) content and cyanide content of your treated waste to ensure your process is effective. We also tested for free chlorine (i.e., an excess) in the cyanide stream which is also a good indicator of successful treatment (at the correct pH!). Simple test kits are readily available for all of these tests. Adjust your mv to give you the best results.

Paul Beel
- Durham, UK
2002

I have a small system that treats waste water of chromium. I need to know more about the chromium conductivity in mv.

Haitham Mohamelenour
- Khartoum
2004

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Sorry, Mr. Mohamelenour, but I'm not aware of that number being printed anywhere. It's probably somewhere in the vicinity of 300 mV, but I believe it depends on a number of things. Fortunately, even in very low concentrations of a few ppm, there is a marked color change when chromium goes from hexavalent (amber) to trivalent (blue-green). The color change is so obvious that you should be able to determine the correct mV in just a few minutes with no need for repeated chemical analyses to do it.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

There will be a sharp change in the potential in both the cases when treatment is completed.Depending on the conditions i.e., pH,presence of other heavy metals and the concentrations of the reagent the Mv may vary.Also the conditions of the ORP electrodes itself is an important factor.It is always better to do confirmatory chemical tests to ensure completion of the treatment.Also it may be noted that orp will be effective only if you have two separate streams of cyanide and chromium.If you have mixed effluent the system wiill not work at all.There one has to adopt the old conventional type of treatment.

Sasikumar P K
- Bangalore, Karnataka, India
March 20, 2008