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Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

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Environmental officer needs suggestions for aluminum bright dip NOx emissions

2007

Q. Hi,

I work as an Environmental Health Officer for a local council in Lancashire, England. Part of my job involves regulating various industrial processes, including a few anodisers. My question concerns a current problem I've got with a plant that carries out chemical brightening prior to anodising.

The brightener they use is Ultralyte 170XL, a mix of phosphoric, sulfuric and nitric acids. The fumes generated when the works are lifted out of the brightening tank are extracted through two wet scrubbers and washed with a caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] solution, which they currently test the pH of using Drager tubes but is soon to go on a continuous pH monitoring unit.

My problem is that the plant has a permit to operate that includes conditions about not emitting odour and discoloured fumes outside of the site boundary. However, I am getting complaints from nearby residents about "toxic fumes". Every time a jig is lifted out of the brightening tank (about 4 times an hour), the emission from the scrubber stack turns a really horrible looking yellow/brown for about 5 minutes. The guys at the plant say they're doing everything they can to minimise it (keeping pH of liquor at correct level, maximising residence time in scrubber etc).

Really though, I would like to see the emission remain as close to white as possible, but don't know what to suggest. Any ideas would be greatly appreciated.

cheers,

Richard

Richard Walsh
Regulator - Pendle Brough Council, Lancashire, UK

A. Chemical brightening of aluminum is necessary business (we wouldn't have street light reflectors or dental lights without it), but it's nasty business. The brown color is NOx fumes, and indeed "Nox"ious. The basic problem, I believe, is that this gas is soluble in water to the extent that simple fume scrubbing probably passes too much though. I have seen 2 packed-bed wet scrubbers used in series to good effect, but I hope that a ventilation equipment manufacturer or another bright dipping shop can give us both some ideas.

Would that more of our USA regulators adopt your "working together" attitude!

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

2007

A. Spike some ferrous sulfate ⇦ this on eBay or Amazon [affil links] into the scrubber liquid?

Ferrous salts are frequently used to remove NOx from offgas.

Dave Wichern
Consultant - The Bronx, New York

2007

A. Hi Richard !

I've designed scrubbers for quite a few Bright Dip operations ... with nary, to my knowledge, any come back.

However, the design was very DIFFERENT from conventional mass transfer scrubbers.

These were 'eliminators' ...horizontal design, 10-12 micron capture, and then in '74'-ish I designed a 3 micron one which I called a LMITS, Low Micron Inertial Type Scrubber .... which INCO tested out at 86% efficiency for a 0.2 to 0.8 microns in a sulfuric spectrum.

My theory is that in plating operations, Gases and Fumes
DON'T EXIST ... just invisible to the eye droplets ... but
nitric is an S.O.B.

The LMITS had a blade type eliminator leading into a mesh pad followed by a final b.t. eliminator ... overall over
3 million cfm of these units.... and I didn't use any
caustic ... just plain ole H20.

Why, why, why two scrubbers? The main problem with sheets
was the high %age of fumes ... and we used to use a totally
enclosing hood with curtains at each end and the fumehood
ducting was at the very top.

Am now retired ....

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

A. The scrubber was not designed properly. It takes a residence time of the NOX in the scrubber to get it to come out of the gas phase into a dissolved phase. Caustic speeds up the process, but it is not the final answer. There are a number of retrofits that will work, but they are dependent on the space you have available as well as the amount of money, not to mention the down time for the retrofit.

James Watts
- Navarre, Florida
2007

2007

A. Does the Ultralyte 170XL include a fume suppressant? Commonly used bright dip fume suppressants are ammonium sulphate or monobasic ammonium phosphate. I believe urea has been used, too.

caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] solution is rather slow in scrubbing NOx and probably removes more CO2 from the air. Add a little hydrogen peroxide, perhaps 1%, to convert the NOx to nitric acid which is easily neutralized. The solution need to be slightly alkaline, which minimizes carbonate formation. Or, water + hydrogen peroxide can be used to recover nitric acid. Or, hydrogen peroxide + nitric acid solution could be used in the first scrubber, with dilute caustic in the second. Some references:

'Nitrogen Oxide (NOx) Abatement with Hydrogen Peroxide,' FMC Corp. www.envsolutions.fmc.com/Portals/fao/Content/Docs/Nitrogen%20Oxide%20(NOX)%20abatement%20with%20H2O2.pdf

'Absorption of nitrogen oxides in aqueous solutions in a structured packing pilot column,' J. L. de Paiva & G. C. Kachan, Chemical Engineering and Processing, Volume 43, pages 941-948 (1994).

'The absorption-oxidation of NOx with hydrogen peroxide for the treatment of tail gases,' D.Thomas and J.Vanderschuren, Chem. Eng. Sci, Vol.51, pages 2649-2654 (1996).

'Absorption of dilute NOx into nitric acid solutions containing hydrogen peroxide,' D.Thomas, S.Brohez and J.Vanderschuren, TransIChemE, Vol.74, Part B, pp 52-58 (1996).

'Removal of Tetravalent NOx from Flue Gases Using Solutions Containing Hydrogen Peroxide,' D. Thomas & J. Vanderschuren, Chemical Engineering & Technology, Volume 21, pages 975-981 (1998).

'Nitrogen oxides scrubbing with alkaline solutions,' D.Thomas and J.Vanderschuren, Chem. Eng. Technol., Vol.23, pp. 449-455 (2000).

'Hydrogen Peroxide Enhances Removal of NOx From Flue Gases,' NASA Tech Briefs (Jan. 2005).
http://www.techbriefs.com/content/view/167/34/

Ken Vlach [deceased]
- Goleta, California

Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

A. Hi

We have used a commercial product called BioNOx Solver with success

thanks

Terry Tomt
- Auburn, Washington
2007

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