Chromium plating is an essential process for the U.S. Department of Defense (DoD) but, during operation, byproduct gases are created which rise to the surface as bubbles that burst and create a mist of chromic acid. This project explored two concepts for reducing these hexavalent chromium emissions from electroplating tanks by preventing the bubbles from reaching the surface: (1) Venturi/Vortex Scrubber Technology (VSST) and (2) the Pushed Liquid Recirculation System (PLRS). The VSST recycled electroplating fluid through a unit that subjected the fluid to changes in flow direction to coalesce and separate entrained gases from the liquid. The PLRS employed jets placed just below the liquid surface of the electroplating bath to create a surface flow pattern that diverts rising gases to a collection hood, thereby reducing the effective tank surface area requiring ventilation by conventional control devices.
The VVST concept was abandoned during this demonstration due to insurmountable design flaws. It was substituted by the PLRS, which was demonstrated at the Marine Corps Logistics Base (MCLB) in Albany, Georgia. The PLRS successfully reduced the ventilation flow rate at MCLB Albany by 63 percent to 75 scfm per square foot and maintained emissions directly above the center of the tank below the OSHA Permissible Exposure Limit (PEL) of 52 micrograms per cubic meter.
DoD spends an estimated $2 million per year or more for chromium scrubber operations (not including scrubber wastewater treatment). The PLRS ventilation reduction translates into a 48 percent savings in annual expenditures and an estimated 36 percent life-cycle savings over a 10-year period. In addition, a 25 percent capital cost reduction would be realized for any future ventilation equipment installation. Capital costs for conventional scrubbers range between $50,000 and $1 million. Cost reduction percentages will be fairly consistent regardless of facility size. The potential for implementation of PLRS technology at the 21 DoD facilities and more than 1,500 private shops that conduct hexavalent chromium electroplating and/or anodizing operations is high. This is because PLRS has advantages over alternative approaches to emissions reduction which use sealed tank covers, automated tank covers, push-pull systems, and mist suppressants.
The full cost benefit of the PLRS will be realized when control/ventilation systems such as packed bed scrubbers and composite mesh pad units are replaced by smaller units (due to their lower capital cost). The smaller systems are thought to be able to maintain National Emission Standard for Hazardous Air Pollutant (NESHAP) chromium stack emissions standards, although this was not tested during the demonstration. Realistic lifetime estimates for these systems are around 10 years because of the corrosive nature of the chromic acid droplets they are designed to separate.