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Air pollution control from anodising/anodising tanks

Which is the best & economical system to control air pollution problems from the anodising tank of H2SO4 (18'X6'X4'H)fumes & etching tank of caustic solution (18'x3'x4'H) fumes with a central air pollution control system?

Dhiren Parikh
environmental engineer - AHMEDABAD, GUJARAT, INDIA
2004

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You need lip-mounted exhaust systems on each tank, with the exhausted air then passing through a packed-bed fume scrubber.

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

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First of three simultaneous responses --

I would add a pH controller to feed a mild caustic solution to keep the pH above what your sewer requirements are. You do need a minimum of flow thru water to avoid sulphate buildup on your packing.

James Watts
- Navarre, Florida
2004
2004

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Second of three simultaneous responses -- 2004

Dhiren,

Re your anodizing sulfuric, yes, most definitely you need a lip exhaust.

Re the scrubber, ..... NO .... a mass transfer packed bed scrubber for the sulfuric is NOT necessary. However, those emissions need to be removed. You'll note I didn't say that they were fumes, they're NOT ... the emissions in this case consist of invisible low micron droplets of acid.

The answer in this case is to use a low micron (horizontal blade type) mist eliminator.

These designs are now being in Australia ... but they started in Canada.

For a general purpose scrubber, yes, a packed bed design used to be de rigeur ... but now there's a very low micron eliminator scrubber (below 3 micron capture), also being made in Australia ... which is far less expensive to buy.

I have over 3 million cfm of annotated end users which proves a point or two, doesn't it?

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

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Third of three simultaneous responses --

Mr Parikh,

Use Stable fume suppressants in the Sulfuric acid anodising bath. Use Alkaline stable fume suppressants in the Caustic etch.

Look for proprietary makers of these products. These work very effectively and stop fumes from occurring. You can achieve zero fumes. That's abatement.

Regards,

Asif Nurie [deceased]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016
2004

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Dhiren,

I'd like to make a couple of observations on this discussion which will hopefully be of help.

The fumes or mists from these baths are mostly generated when the product is entering or exiting the acid/alkali, the rest of the time the generation is minimal. Using lip extraction is probably the best way of capturing the fume, they work really well when the product is in the bath, but you have to regulate the speed of the product entering or exiting the bath so you effectively regulate the generation of the fume during the peak time. Fume suppressants work wonderfully well until you break the surface of the liquor in the tank, and that happens when you move the product into or out of the bath,and thats when maximum fume generation occurs, they help but don't rely entirely on them.

My calculations based on the size of the tanks, suggest you would need double lip extractors on the wide tank pulling 17750 CFM and a single unit on the narrow tank pulling 6550 CFM. Combined a total extraction rate of approximately 24500 CFM. Not knowing what length of ducting you would need I am guessing by saying you should budget on a fan at that volume with a SP of about 6"WG. Scrubbers. A packed tower or mass transfer scrubber would have to be nearly 9' in diameter to cope with the estimated extraction rate. I would suggest you would need a 'plastic fan' in case of carryover from the scrubber, and this type of scrubber is notorious for carryover. You would need pH control as James suggests and it would have to be both acid and alkali as at any time your scrubber could be scrubbing acid or alkali or both depending on what is happening in the tanks. For sure you would not like acid or alkali carryover from the scrubber falling to the ground close by. The spray medium would have to be changed quite regularly as the chemical content builds up from the scrubbing action and the neutralising action.

I go along with Freeman for the horizontal inertial type scrubber. They are physically at least 60% less than the packed tower units as they operate at much higher velocities, and this reflects in their price as well. They don't have a carryover problem as they are in fact very efficient mist eliminators so you can use a 'steel fan' after them. Steel fans are about 30 to 40% the cost of a 'plastic fan' and are comparatively smaller in size for their duty. You don't have to neutralise the spray medium, in fact you can use process effluent from the site effluent treatment facility, and that equates to cost savings in water and chemicals. Maintenance is easy, an occasional hosing of the interior should do.

There you are Dhiren, make your choice.

Tony Johnston
pollution control - Melbourne, Australia
2004