Metal finishing Q&As since 1989

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Condensing Drag-outs

I am a recently graduated engineer working in a plating/anodising shop in the UK.

I want to condense contents of drag-out tanks to reduce the volume and allow it to be returned to plating solutions. I would ideally like to evaporate them in their tanks. Assuming I have adequate extraction, is this practice acceptable? What are the practical implications?

Jonson Brennan
Plating / Anodising - Isle of Man, UK
2005

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Evaporation is one weapon in the arsenal. Yes it can be quite practical, but is best employed when the process tank is hot rather than room temperature. It is more practical, though, to evaporate water from the process tank and feed the dragout tank to it rather than to try to concentrate the dragout tank to process concentration. You'll also need several rinse tanks after each process for this to be practical, and should use DI water throughout because any salts in the incoming water will also get concentrated.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2005

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I presume you have counter-flow rinsing - if not, go for it. I would also suggest you do an initial spray rinse with "pure" water (i.e., DI or RO or distilled) so that this rinse water goes straight back into the process tank. This will need careful monitoring so you don't flood the process tank and get it all over the floor - you can balance evaporation losses against the spray rinse top-ups. It will also reduce the amount of contamination going into the rinse tanks. Water and material consumptions reduced - problem solved.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005

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2005

Hi Jonson

Many, MANY years ago I used to collect the chrome fumes with a device I called a Dry Scrubber. This might give you some ideas.

It consisted of nothing more than 2 banks of sine curved PVC mist eliminator blades ... back in the 80's, this met and beat B.C.'s pollution standards for chrome by nearly 300 times (0.34 grains of chrome sulfuric versus allowable limits of ll grains).

As ALL plating emissions (some exceptions are some but not all of the nitric compounds) are neither FUMES no GASES, what we have are invisible droplets ... and said dry scrubber would remove them to l2 microns at full efficiency.

Have a gander on GOOGLE for LMITS and S-LMITS ... It might give you some better thoughts. But these are 3 microns and need 3" SP.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).