Sulfuric Emissions from Electrocleaning

Question:

My company operates a non-production electroforming line that vents into the plant environment. One of the cleaning steps involves electrocleaning in a 30% sulfuric acid tank that we would like to vent externally, however, we can’t seem to find a way to calculate an emission rate which is required to determine permit applicability.

I reviewed your September, 2001 Pollution Control article on sulfuric acid emissions from anodizing, but the same formula can’t be applied to this situation.

We have also reviewed USEPA’s AP42 which gave an SCC code of 3-09-010-15 for acid dip but did not have an emission factor; even if there was the factor, I'm not sure we could use it because there is a current running through our tank.
Here is information on our tank:

 

• 1.5 × 1.5 × 1.5 ft (approx. 25 gal)
• a typical part has 64 sq inches of surface area
• each cycle is three min at 40 amps

 

Any suggestions short of testing? C.M.

Answer:

Before answering your specific question, let me make several introductory comments.
First, USEPA and states would consider your entire electroforming line a “source,” not just the electroclean tank. Therefore, I recommend that you need to evaluate all process tanks.

Second, just because a “source” vents to the inside of a facility, and, hence, indirectly to the outside, does not mean it is exempt from air pollution permitting. EPA rightly asserts that pollutants released inside a facility will, eventually, be emitted to the outside.

Thirdly, the availability of “good” emission data for many metal finishing processes is sketchy at best.
Lastly, you are right regarding the acid dip tank in AP-42.

Please take another look at the September, 2001 article on anodizing emissions. It may “work” for your case.

Let’s recap the equation, which was extrapolated from chromic acid anodizing emissions as reported in a presentation at the 2000 AESF/EPA Conference for Environmental Excellence, “Characterizing Site Specific Source Emissions for EPA’s Risk Assessment Tool for the Metal Finishing Industry” by S. Schwartz and M. Lorber; the research was sponsored by USEPA and the National Center for Environmental Assessment.

EMa = (((CCa × CDa) / CEa) / CEa) / ((CCcr × CDcr) / CEcr) × ERcr × Aa

where

EMa = emission rate, lbs/hour,
CCa = pollutant concentration, oz/gal,
CDa = current density, amps/sq inch,
CEa = cathode efficiency,
CCcr = chromic acid concentration, assumed at 9 oz/gal based upon AP-42 background document
CDcr = current density of chromic acid anodizing process, assumed at three amp/ sq inch based upon presentation CEcr = cathode efficiency of chromic acid anodizing process, assumed at 95% based upon presentation
ERcr = pollutant emission rate from chromic acid anodizing process, assumed at 0.00029 lbs/hr/sq ft based upon AP-42 Table 12-20-2
Aa = surface area of tank, sq ft.
Why could this emission estimate “work” for you?

1) Electrocleaning is similar to anodizing in that current is used without plating, and its electrolysis of water creates hydrogen and oxygen bubbles, resulting in the misting of solution.
2) Pollutant concen- tration (sulfuric acid, or total solids, aka par- ticulates) is known or can be estimated.
3) Current density is known.
4) Catode efficiency is known or can be estimated.
5) Surface area of electroclean tank is known.

Considering your very small tank and short cycle time, I would be surprised that the source would require permitting unless your state requires a permit application for your particular process. The vast majority of states have “de minimis” emission thresholds below which permits or registrations are not required. Here in Ohio, if a source is not specifically regulated (your process is not) and has a pollutant emission rate of less than 10 pounds per day or a hazardous air pollutant (sulfuric acid is NOT a HAP) of less than 1 ton per year, it is exempt from permitting.

Let’s see what results this emission estimate comes up with if we assume:
CCa = 70 oz/gal of sulfuric acid, (my rough estimate of 30% sulfuric by weight)
CDa = 0.63 amps / sq inch ( 40 amps / 64 sq inches )
CEa = assume 50%
Aa = 2.25 sq ft ( 1.5 x 1.5 )
EMa = ((70 × 0.63) / 50% ) / ((9 × 3) / 95%)) × 0.00029 × 2.25 = 0.002 lbs/hr of sulfuric acid.

Hence, if this process ran 24 hours per day, 365 days per year, its potential-to-emit could be about 0.05 lbs/day and 17.5 lbs/year. This very low emission rate could explain why the process operators have been able to “tolerate” working around the tank.

A final note: Even though your process may not require air pollution permitting or registration, be very considerate on how you exhaust this process. Also, you may want to consider a mesh pad de-mister with periodic washdown in order to provide protection of your facility’s roof and nearby property and vehicles from possible sulfuric acid mist fallout.