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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.8 MESHPAD MIST ELIMINATORS
3.8.1 Overview
Meshpad mist eliminators are used to recover plating chemicals that become entrained in the airstream that is exhausted from the surface of a plating tank. The primary application of this technology is with chromic acid baths, particularly hard chromium plating and chromic acid anodizing. From the Users Survey, 16 respondents (or 5.0%) indicated that they have employed this technology. Of these responses, there were 16 applications to hard chromium and 2 applications to chromic acid anodizing (two shops used the technology for both applications). The 16 hard chromium applications represent 19.2% of all shops that employ the hard chromium plating process. One manufacturer that responded to the Vendors Survey indicated that they have sold approximately 475 meshpad mist eliminators. Of these units, 63% are installed on hard chromium plating baths, 13% on sulfuric acid baths and the remainder on decorative chromium plating, chromic acid anodizing, caustic, cyanide, hydrochloric acid, and nitric acid baths (ref. Midwest Air Products file).
Meshpad mist eliminators are one of several technologies employed for the removal of plating chemicals from exhausted air. The other two technologies include liquid scrubbers and chevron mist eliminators. Of the three technologies, meshpad mist eliminators are considered to be the most efficient (ref. 213, 404). Other methods of control that may assist in reducing chemical emissions, but are not by themselves adequate control measures, include fume suppressants and polypropylene balls (both are used to cover the surface of the plating tank and reduce mist generation).
The EPA proposed air standards for decorative and hard chromium plating and chromic acid anodizing operations in December, 1993 (ref. 521). The standards are based on the use of scrubbers and meshpad mist eliminators.
Typically, a separate mist eliminator is used for each plating tank, although different configurations are possible. Meshpad mist eliminators are installed within the exhaust system ductwork, as near to the exhaust hood as practical, as shown in Exhibit 3-63 (39 kb). The mist eliminator enlarges the cross-sectional area of the duct which causes a reduced airstream velocity within this section. The reduced velocity permits the entrained droplets of plating solution to impinge on and adhere to the meshpads, thus removing them from the air stream. Having multiple pads in series increases the removal efficiency of the process. The accumulated plating chemicals are periodically washed from the pads (see Exhibit 3-64). This is usually accomplished with an integral water spray system. The liquid from pad washing drains to the bath.
The meshpad mist eliminator technology competes with wet scrubbers and chevron mist eliminators as a control device for chromic acid air emissions. However, with respect to chemical recovery, the meshpad has very little competition from scrubbers. While the recovery of plating chemicals, including chromium, from scrubber systems is possible, scrubber units generally serve multiple types of processes (e.g., cleaning and plating) and the entrapped chemicals are combined, and therefore less amenable to recovery. One recovery strategy that has been employed with scrubbers involves the use of an anion exchange column to collect hexavalent chromium from the scrubber water and to reformulate the resulting sodium chromate regenerant using membrane electrolysis. The resultant products are chromic acid, which is reused as a plating chemical, and sodium hydroxide, which is reused for the anion column regeneration (ref. 402).