Emissions from sulfuric acid anodizing & regulations

A. Anodizing occurs because we split the water in the solution into hydrogen and oxygen, and the oxygen combines with the aluminum to oxidize it by forming aluminum oxides. So a lot of hydrogen gas is generated, creating mists -- both chromic and sulfuric anodizing processes gas a lot. The difference is that chromium is a known carcinogen with a long history of causing nasal perforations and chrome ulcerations. But the sulfuric acid is certainly irritating, and all anodizing tanks require local ventilation.

A. Benjamin,

I feel sorry for you as you are somewhat between a rock and a hard place.

If you go to a reputable mfgr. of mass transfer packing, they will ask WHAT are your concentrations of acids, WHAT are your airflows and then ask 'WHAT degree of efficiency of removal do you want?'. To the latter question were you to say l00%, then you'd have to sell the Company's Rolls Royce and Bentley ... after all, their job is to sell packing and they will be very helpful in telling you 'what' the diameter of your scrubber will be and 'what' the packing depth has to be and 'how much' alkaline reagent you'd have to use.

Coming back to all your 'chemicals', there is one point to consider regarding ambient or nearly ambient emissions of acids ...these emissions in MOST CASES are neither gaseous nor fumes. NEITHER. They consist of invisible low micron sized droplets. The exceptions are some of the many nitric compounds, but not all (a very unstable acid!)and partially HCl.... although with Assay applications these ordinary CT-120/2 eliminators meet specs re HCl and aqua regia.

In the Canadian province of Ontario the Air Management people (who parallel EPA on requirements) will demand to see a design drawing. They will then 'enter' this data into their computer and tell you ... it's OK ... it needs to be larger ... your stack height is too short ... you need to increase your alkali recirculation rate ... etcetera. In other words they'd be knowledgeably very helpful!

In British Columbia the authorities take a different tack. They don't care a hoot about the design ... only about the emissions and will demand on-site emission testing to comply with regulations.

In your case ... and may I suggest that all your ducting is made of PVC ... the approach I'd take (assuming you know all the airflow parameters) is to do some pre-empting work. If you did nothing at all, the authorities would come down on you like a brick shipyard. If you did something, something reasonably good, albeit inexpensive, you might well get away with it.

CASE HISTORY .... ALUMIDIZE Company (once called STAYSA) in Brantford, Ontario. l975 era. They were exhausting bright dip (at least 4% nitric) and sulfuric. The plant was situated in a narrow high valley necessitating quite a high stack with a good O.V.

I was experimenting to see if one could improve the capture efficiency of a l2 micron (blade type, horizontal airflow) mist eliminator by using a sprayed coalescer section (using PP mesh) before the eliminator. The sulfuric fumes, which are easy to control, bypassed the mesh and went directly into the eliminator. Because of the nature of bright dip, everyone said, EVERYONE, that one has to use an alkali to neutralize it.

A year goes by during which time the tough Air Management people OK the emissions, so I decide to revisit that plant and check on the alkali consumption as well as on the ventilation design, too.

The 'operator' of the alkali make-up tank is a recent Italian immigrant. I ask him how often is the alkali changed. 'No kapeech', he replies, 'only Aqua, aqua' !

So here we have a pretty common bright dip environment which passes all emission tests AND is located in a geographically bad news narrow valley ... and they are using W A T E R Ordinary tap water. Needless to say, thereon after all my bright dip designs used only 'aqua' and never alkali.

For the eliminators, try Munters in Florida. They have the German designed T-120/2 or try Allanco Environmental in New Westminster, B.C. who call their slightly improved design a CT-120/2.

I sincerely hope that this is of some help to you ... as with every battle (with the environment), a good pre-empt often works

Others follies teach us not
Nor much their wisdom preaches
Of sterling worth is what
Our own experience teaches. Author unknown.

Cheers !

A. Well vented or not, I'd give ten to one that it's the anodizing tanks not the heater.

A. Or the deox/desmut tank. It is extremely difficult to capture 100% of the vapors off of the anodizing tank. Sometimes the first rinse tank will get to a pH of 1-2, so that might be a source also. What about drips?

! I've been wrong many times before and will be again, Mark. And I appreciate that reactions between water and organics can cause an acid condition--as is often seen in the solvent reservoirs of vapor degreasers. But I can't concede to things that are contrary to a lifetime of experience until the evidence gets reasonably compelling. I've been to very few anodizing and plating shops where the atmosphere isn't corrosive, ventilation or no, space heaters or no. But I can't recall any instance of a home, commercial installation, or industrial facility which did not use acid yet had a corrosive environment.

I am absolutely not saying you are wrong. You have presented enough evidence that I very much appreciate the feedback, and will keep it in the memory banks for the future. But I honestly can't concede 'case closed' based on one instance and one test; winds shift and I have seen the acid plumes from exhaust stacks sucked back into an air make-up unit one day and not the next.

A. I believe that you can make some acid in a heater flame if there are chlorohydrocarbons in the burner air. Like HCl, for example. Do you have any degreasers or using some solvents in your shop? Can you test for chlorides?

A. Good afternoon:

The OSHA website lists permissible exposure limits (PEL) based on an 8 hour time weighted average for several common mineral acids. Go to

www.osha.gov/SLTC/pel/index.html ⇩

I think the info you're looking for is in Table Z1 and Z2 on this web page. Hope this is what you're looking for. And I hope you don't have to spend big $ for industrial hygiene testing.

Ed. update: The above link is broken. Thankfully, the Internet Archive preserved a copy here
Please consider a donation to The Internet Archive.

A. Mark,

You said that your blower is at ground level and that your exhaust fans are on the roof, 35 feet up.

So my question is ... how high are the exhaust fan stacks? They should be at least 7 feet, presupposing there are no other and higher buildings nearby. Otherwise the roof eddy zone effect could cause some of the fumes to waft downwards. Anyhow, food for thought ... and my regards to your dawg.

A. We had air monitoring done in 2001 for an 8 hour period.

Limits were

1.0 mg/m3 permissible for Sulfuric
2.0 mg/ m3 permissible for Nitric
2.0 mg/m3 permissible for Sodium Hydroxide

My operators tested at 0.13 mg/m3 for Sulfuric None detected for Nitric 0.032 mg/m3 for Sodium Hydroxide

I run an etch recovery system and the caustic fumes can be pretty intense at times with the little exhausting we have that is why I had the testing completed. I hope this helps a little.

A. Hi Dipesh !

Firstly, the air mgt standards for pollution control will surely VARY ... they vary sometimes in Canada from Province to Province and I'd imagine that India would certainly follow suit ... but yours are probably not nearly as onerous as the U.S. EPA specs. Check with your local authorities. OK?

The first prime requisite is good ventilation, i.e., a well designed ventilation system starting with the hood. The second one deals with air pollution ... and I'd suggest that you use a good 10 micron(capture) blade type horizontal mist eliminator. Don't use stainless. Use PVC. Don't waste your money on expensive mass transfer scrubbers.

A. Hi Alex,

May I suggest that first of all you have a gander in the archives at # 43/96, # 67/33, # 149/44

Then, perhaps, you might like to contact the local Air Management people who surely, surely could be of some help.

In a nutshell, there are two major chemicals ... sulfuric and nitric.

Sulfuric is dead easy and very inexpensive to collect. The nitric can present problems but I was always lucky because the fumes are low concentration and hence the exhaust airflow is 'high' on oxygen (i.e., air) and hence, as you know better than I (I hope), the bad NOx fumes ideally need a good (long) retention time plus oxygen so that they can revert to a scrubbable mode.

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A. The amount of fumes from sulfuric acid anodizing will be governed by the size of the load and the amount of air agitation used. Generic values mentioned in the ventilation spec, engineering books and the Metal Finishing Guide book work quite well.