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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
June, 2003

California's South Coast Air Quality Management District Amended Rule 1469, Hexavalent Chromium Emissions from Chrome Plating and Chromic Acid Anodizing Operations

Q. California's South Coast Air Quality Management District recently issued its amended Rule 1469, Hexavalent Chromium Emissions from Chrome Plating and Chromic Acid Anodizing Operations. I would like to know how the emission limits compare to EPA's NESHAP since the limits are expressed differently in the two rules. The NESHAP limits are expressed as mg per dry standard cubic meter while the limits in AQMD's rule 1469 are expressed in mg per ampere hour. Example: the NESHAP for large new hard chrome operators is 0.015 mg/dscm. The limit for certain hard chrome facilities in the AQMD's rule is 0.0015 mg/AH. Is it possible to say which is more stringent?

A. I will limit my discussion to hard chromium electroplating operations, the topic of this forum.

In its chromium NESHAP, EPA has set concentration-based discharge limits for the controlled emissions of chromium from hard chromium electroplating operations. As you have stated, the mass of chromium (in milligrams) divided by the volume of exhaust air (in dry standard cubic meters) is limited to 0.015 for new sources.

Since the exhaust air volume for a hard chrome tank is typically proportional to the surface area of its liquid-to-air interface (the top of the plating bath), tanks with larger widths and lengths (the tank depth and gallonage don't matter) will generally be fitted with exhaust hoods that have higher volumetric flow rates than smaller tanks. Since this factor appears in the denominator of the mg/dscm fraction, these tanks with larger surface area, and higher exhaust rates, will find it easier to meet EPA's discharge limit.

It should be noted that many legitimate and effective designs for the exhaust ventilation systems for hard chrome plating tanks feature a reduced volumetric flow rate. An example would be a "push-pull" exhaust system that utilizes a small push-air manifold on one side of the tank to direct process fumes toward the capture slots on the pull hood. In a push-pull system, the reduced volumetric flow rate for the pull hood is independent of the surface area of plating tank or bath. It is apparent that any exhaust system design that incorporates a reduced air volume will be disadvantaged by EPA's concentration-based limitation.

It is also important to note that, unlike an hourly discharge rate (like pounds per hour for example), the concentration based limitation does nothing to discourage facilities from installing more plating tanks. A facility could have a hundred chrome plating tanks, instead of one, and still easily meet the EPA limitation, even though the mass of chromium discharged from the stacks may increase a hundredfold for a given time period.

The mass of chromium appears in the numerator of the EPA limitation, so it is advantageous to a) generate less chromium mist at the surface, and b) have a higher removal efficiency for the air pollution control device(s).

It is my opinion that, during the development phase of the standards, EPA struggled with the task of finding a suitable emission factor for chromium emissions from hard chrome plating tanks. As an example, a previous uncontrolled emission factor related the mass of chromium released to the atmosphere to the mass of chromium added to the plating tank as a makeup chemical. EPA later tried to relate uncontrolled chromium emissions to ampere-hours consumed during the plating process. I have performed numerous Initial Performance Tests (stack tests) on hard chrome plating tanks, where I have measured both uncontrolled (before control devices) and controlled (after control devices) emissions. I don't see any strong relationship between ampere-hours and emissions. In other words, if you double the plating amperes, the uncontrolled emission does not double. Many factors not considered by EPA influence the quantity and particle size of chromium mist released to atmosphere during plating. On the controlled side, the meshpad-based air pollution control devices act more like constant-output devices than constant-efficiency devices, and typically show much higher removal efficiencies when the input loading of chromium is high than when the input loading is low.

Nearly all modern air pollution control devices for chromium incorporate meshpad technology to removed entrained chromium from the exhaust air stream. These devices can easily meet EPA's discharge limit of 0.015 mg Cr/dscm. In fact, chromium concentration measured during a performance test may be an order of magnitude lower or better.

The California limitation of 0.0015 mg/AH is harder to meet. This is not a concentration-based limit, but rather, it relates the mass of chromium discharged to the ampere hours from the chromium electroplating power supplies. A favorable plating and pollution control situation would be to plate large parts that generate a small amount of mist (for their size) and a high efficiency air pollution control device. In explanation, larger parts typically are plated with higher amperage, which increases the ampere-hours in the denominator of the fraction. Conversely, an unfavorable situation would be to plate parts that generate a proportionally higher amount of mist for their surface areas and current levels. As an example, many cylinders for the printing industry are plated horizontally in large plating tanks. They are rotated continuously during plating, but only a portion of their surface area is actually submerged. This lowers the plating amperage, yet the evolved mist is very high due to the constant rotation which results in a wetted chromic acid film on the un-submerged portions. This type of plating will have difficulty meeting the mg/AH limitation.

It is possible to calculate the mg/AH value when the mg/dscm is known. For simplicity, I have not addressed the differences between acfm (actual cubic feet per minute) or conversions to "standard" conditions. Assume a volumetric flow rate of 6,000 dscfm (dry standard cubic feet per minute, i.e - "6,000 cfm"), a controlled emission rate of 0.015 mg/dscm and 2,000 ampere-hours/hr (i.e. - 2,000 amps delivered to plating parts for an hour).

(0.015 mg/1 m3)(1 m3/35.31467 ft3)(6,000 ft3/ 1 min)(60 min/1 hr)(1 hr/2,000 ampere-hour) = 0.07646 mg/AH.

In other words, the scenario above met the EPA limitation, but with 6,000 cfm's and 2,000 AH, resulted in a discharge that was about 50 times higher than the 0.0015 mg/AH limitation. Even if the 6,000 cfm tank was plating a large part at 10,000 amperes, and the controlled emission of chromium was still 0.015 mg/dscm), the resultant 0.01529 mg/AH would still be more than ten times higher than required.

In all fairness, most meshpad mist eliminators do much better than the 0.015 mg/dscm EPA limitation. Let's take another scenario whereby a 5,000 cfm tank is plating at 10,000 amps and discharging 0.0017 mg/dscm. This situation could actually occur if a large roll was plated vertically in a tank with modest width and length, and a very-high-efficiency air pollution control device was performing properly.

(0.0017 mg/1 m3)(1 m3/35.31467 ft3)(5,000 ft3/ 1 min)(60 min/1 hr)(1 hr/10,000 ampere-hour) = 0.00144 mg/AH.

In this case, both the EPA and California discharge limitations would have been met. However, it takes a very good control device to discharge only 0.0017 mg/dscm with a 10,000 amp plating scenario.

In conclusion, the 0.0015 mg/AH limitation is much more difficult to meet than EPA's 0.015 mg/dscm.



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