48062

Molybdate  

March 5, 2008

For the first time in three years our WWT department had a violation. Oddly enough it was for Molybdate. Doing some research on this really doesn't show any other companies with that problem at the moment. Moly being a normal replacement in steel I just found it odd that no other area had any trouble with it.

Justin Ater
Lead Chemical Technician - Dallas, Texas, USA

This is indeed unusual, so I'm a bit out of my depth also. But I think the molybdenum may or may not be coming from stainless steel, but you also need a chelator like ethylene diamine in your process chemistry in order to form a molybdate.

You haven't told us what your shop does, or what the molybdate limit is, and I think that information would be helpful.

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

Justin,

You do not mention the types of processes that create the effluent for your WWT. This may be important. As an example - if you were doing zinc plating and black chromate and switched to a trivalent black or changed vendors of trivalent black, you may now be using a product which could contain a molybdate (ammonium, potassium, sodium, etc) in addition to the trivalent chrome salts. This may not be on MSDS depending on the requirements of RCRA (Sections 302/303, 308) and OSHA since I believe most moly compounds are not DOT regulated.

To make what is becoming a long story shorter (sorry) find out what if anything has changed that would be feeding the effluent stream. If you seek help here, the more information you can provide, the better the quality of help (I hope).

Gene

Gene Packman
- Great Neck, NY

Most plants aren't regulated for molybenum, so you won't find much about discharges with it on the internet. Cooling tower corrosion inhibitors and blackening baths are other possible source. It's pretty hard to remove molybdates from water, because they are very soluble.

Three facilities that I know of had to convert to zero discharge to meet their limits. Two of these were in the lightbulb business where they use molybdenum wire as a mandrel to wind the tungsten filiment around, followed by dissolving away the mandrel. One was a plant that made the wire, the other plant made light bulbs.

Lyle Kirman
water treatment systems - Cleveland, Ohio

I have had success treating molybdate by lowering the pH to 4 (using sulfuric acid), and adding ferric chloride. Keep an eye on the pH as the ferric will lower the pH (be very diligent with the pH control as it is extremely important). After aboy 20 minutes of mixing, raise the pH to 7-8 and flock

Gordon Djani
- Greenville, South Carolina