Reducing Hexavalent Chromium Emissions

The plating industry is one of the most regulated industries in the U.S. Based on past practices, strict regulatory compliance was necessary and warranted. These past practices have cast a dark shadow across the entire industry and have caused regulations to become more stringent.
Consequently, the plating industry must be as forward thinking as possible in terms of meeting regulatory demands and standards set by the EPA and other regulating bodies. Two areas that the plating industry must focus on are the allowable discharge standards for air and water. It is no secret that at some point zero discharge is not going to be a buzz word for the future, but a desired standard.
Techmetal, Inc., Dayton, Ohio, a chromium plating shop with numerous tanks, operations (manual and automatic) and applications, wanted to make certain it would be able to meet the possible zero discharge limit. However, Techmetals found that virtually every conventional pollution control device could exceed certain limits as a result of equipment failure.
Techmetal reviewed many professional articles on EPA standards for chromium emissions, testing data and development of the MACT standard. After its research, Techmetals still had three concerns: Moving parts will break down, causing allowable discharge limits to be exceeded; The technology used to conduct MACT testing still emits chromium particles; and Some states and localities have concentration-based and/or risk-based rules that are more restrictive than the MACT standard. Because of these concerns, the chromium plater searched for a device that could produce zero chromium emissions.
Schematic diagram of the EED.
Conventional ventilation. Techmetals uses hexavalent chromium, which extends the life and enhances the performance of a variety of manufactured parts. Chromium coated parts offer excellent corrosion resistance and hardness and a low coefficient of friction, characteristics deemed invaluable by the defense and aircraft industries.
Unfortunately, hexavalent chromium is highly toxic and a known carcinogen, especially when carried through the air as a vapor. Transport of chromium particles occurs during plating because a number of by-products are generated, such as hydrogen and oxygen gases, water vapor and chromic acid mist. Hydrogen and oxygen gases are generated due to the inefficiency of electrolytic reaction during chromium plating. Chromium plating is about 14 to 18 pct efficient when compared to acid zinc plating, which is 93 to 96 pct efficient. Chromic acid mist is generated by the bursting action of the hydrogen and oxygen gases escaping at the surface of the solution and air interface.
With conventional ventilation systems, the chromic acid mist is carried away by air moving across the tank via a blower or push-pull device. In addition, since the tank is open to the atmosphere, stray air currents add to the mobility of these particles. Typically, a push-pull system is designed to move these particles away from the plating tank to a scrubber where the air is cleaned of chromic acid and discharged.
Conventional ventilation systems, such as packed-bed scrubbers, composite mesh pads and fiber bed mist eliminators, are commonly installed by plating shops in order to comply with air emission standards and regulations set by the EPA. These systems lower the amount of hexavalent chromium released into the atmosphere. However, Techmetals was looking for something that eliminated emissions. It decided to use an emission elimination device (EED) developed by Responsible Alternatives, Inc., Dayton, Ohio. An EED is a specially engineered hood system that contains hexavalent chromium mist without interfering with the normal chromium plating operation.
Emission Elimination Device.
Techmetal placed an adapter on top of the tank walls with openings for bussings, utilities and other electrical conduits. A hinged cover was connected to the adapter ring. A sealing gasket material was applied between the tank and adapter and the hood and adapter. A vacuum pump process was connected to the cover to evacuate any chemical mists or fumes that might remain in suspension after the plating process had ceased and the cover remained closed. Gases from the vacuum system were exhausted through a filtering system near the plating tank.
When this tank cover system is used over a chromium tank, chromic acid mist particles simply rise and fall back into the solution because of gravity and the absence of forced air. The hydrogen and oxygen gases generated during the chromium plating process escape through the system's membrane, water condenses on the inside walls of the enclosure and the condensate trickles back into the plating solution. In setups such as Techmetals, where chromium mist extends to the height of the buss bars, the water droplets continuously clean the bussing.
Emission standards for platers. Data was collected from two chromium plating facilities using this system (Table I). These results are not from stacks or scrubbers, because the system eliminates them. The data reflects testing around the process tank with workers wearing testing equipment. Exposure times will vary, but even when compared to OSHA standards, the system provides results that are 10 to 30 times below the regulatory standards.
TABLE I—Air Sampling Results for Chromium at Two Plating Shops California Plating Shop Chromium Conc.(mg/m3) Permissable Exposure Limits(mg/m3) OSHA CalOSHA NIOSHA ACFGIH 0.0003 at filter 0.1 0.05 0.001/0.025 0.05 Ohio Plating Shop Person No. 1 0.0007 at filter ACGIH TLV 0.05 Person No. 2 0.0004 at filter ACGIH TLV 0.05
Energy cost savings. Techmetals conducted its own energy savings study using kilowatt/hour rate structures developed by the local utility company. Table II displays the on-site results when three of the plating shop's tanks were fitted with a cover system. Savings will vary depending on the geographic location, percentage of tanks with the system and whether stack testing was eliminated or reduced.
TABLE II—Operating Cost for a Three-Tank System Emission Elimination Device System (EED) Conventional Ventilation Three-Tank System Original Equipment andInstallation Cost $130,000 $107,000 Annual Operating Cost $6,866 $69,167 Total First Year Cost $136,866 $176,167
Testing the EED. Even though Techmetals eliminated the need for testing, the EPA still required that "stack" testing be done. As a result, Techmetals' staff had to develop its own procedure and application for testing in order to receive the EPA's approval.
The procedure took several steps. For initial compliance testing, Techmetals' staff performed a smoke testing procedure based on total tank surface area. The smoke test verified the design and placement of all seals and that the system's membrane was working. The next step was to initiate a continuance/compliance monitoring program, which required logs of daily, weekly and monthly inspections. Then Techmetals developed a recommended replacement program for all critical equipment, membranes, seals and tie-downs. Finally, Techmetals' reporting requirements were determined by the local air regulatory authority.
Techmetals had several concerns about using the system. Since the system conserves energy, additional cooling of the solution may be required. However, this was not a problem at Techmetals since it had adequate cooling capabilities. Another concern was that air agitation had to be eliminated. Techmetals used air for solution agitation and mobility to enhance plating conditions. It has now installed the appropriate pumps and hardware to perform solution agitation without air. The system also conserves water. Therefore, Techmetals needed a system designed to evaporate water in the rinse tank. Techmetals' final concern was correctly designing the system.
Despite these concerns, the plating shop has achieved a capital cost pay back of nine months per unit based on energy savings, while establishing a zero emissions standard for chromium.
source .pfonline.com/articles/129705.html