Anodizing Emission Rates

Question:

I am trying to find information about anodizing emission rates. Our local air pollution control agency does not have any information on sulfuric acid anodizing tank emissions, but they do have information on chromic acid anodizing rates. They are looking for this figure in lb/hr. Any help is greatly appreciated.

Answer:

After some research, I believe that I have found an approach that would be acceptable to your local air pollution agency regarding estimated emissions from a sulfuric acid anodizing tank without doing expensive stack sampling. One of my references is EPA's AP-42 emission factors, where your local agency would obtain the chromic acid anodizing emission rate. AP-42 does not contain an emission factor for sulfuric acid anodizing. Based upon AP-42's Table 12-20-2 - Emission Factors for Chromic Acid Anodizing, the total chrome emission rate is 2.0 grains/hr/ft² or 0.00029 lb/hr/ft² while the total particulate emission rate is 4.2 grains/hr/ft² or 0.00061 lb/hr/ft². In reviewing the background document for AP-42, it stated that using surface area of the anodizing tank was the "best" parameter to correlate with the emission rate, and the average chromic acid concentration of the tanks tested was about 9 oz/gal.

The other reference I found was 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. This paper reported the work of EPA and the National Center for Environmental Assessment (NCEA) in the development of a means of determining risk assessment for workers in and local residents of finishing facilities. Based upon an extrapolation that this paper suggested, the following equation can be used to estimate the emissions from sulfuric acid anodizing tanks:

 

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

where EMa = emission rate of anodizing tank (lb/hr)

CCa = pollutant concentration in the anodizing tank (oz/gal)

CDa = current density of the anodizing process (amp/in²)

CEa = cathode efficiency of anodizing process, assumed at 95% based upon above presentation

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 3 amp/in² based upon above presentation,

CEcr = cathode efficiency of chromic acid anodizing process, assumed at 95% based upon above presentation,

ERcr = pollutant emission rate from chromic acid anodizing process, and

Aa = surface area of anodizing tank, ft²

 

For example, if the sulfuric acid concentration in the anodizing tank is 20 oz/gal, its current density at 0.1 amp/ft², and the tank's surface area at 100 ft², the estimated emission rate for sulfuric acid would be 0.002 lb/hr.

From this estimate, the sulfuric emission rate is extremely small. Because the air emissions from the anodizing tank is in the form of a mist, the local air agency may also want to know the estimated particulate matter emission rate. To estimate this you would substitute the total solids concentration (oz/gal) for CCa.

Last, even if you are not required to install a scrubber to control this low emission of sulfuric acid, I still recommend the use of a mesh pad de-mister with periodic washdown in order to protect your facility's roof and nearby property and vehicles from possible sulfuric acid mist fallout.