Pollution Prevention and Control Technologies
for Plating Operations
Section 3 - Chemical Recovery
3.8 MESHPAD MIST ELIMINATORS
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.
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