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Pollution Prevention and Control Technologies for Plating Operations


Section 3 - Chemical Recovery

3.8 MESHPAD MIST ELIMINATORS

3.8.4 Technology/Equipment Description

This subsection contains a description of commercially available meshpad mist eliminator equipment that is manufactured and/or sold by respondents to the Vendors Survey. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.

ChromeTech, Inc. developed and manufactures a Trimesh series mist eliminator (Patent Number 5,200,055) that has been applied to the recovery and control of hard chromium tank emissions. A diagram of the ChromeTech unit was previously shown in Exhibit 3-64.

The first stage is essentially a dry meshpad mist eliminator. Inlet air, heavily laden with chromium, flows through various laminations of woven polypropylene fibers which are contained in the first meshpad cartridge. Most of the chromium (typically 99%) is removed by impingement. As the condensed chrome collects, it is removed from the system through a drain network fitted to the bottom of the unit, and is automatically returned to the plating tank. The small percentage of chrome passing through the first meshpad represents the smallest droplets of the particle-size distribution curve, and is the most difficult to filter with a dry mechanism. Therefore, the second stage of the control device offers a wetted, scrubbing section. Water reservoired in a separate container beneath the device is pumped continuously during plating to the front face of the second meshpad, through atomizing nozzles provided. The pad offers a very large wetted surface area, and chromium mist still entrained in the airstream is dissolved into the recirculating liquid. At regular intervals, typically the end of each plating shift, the chrome that accumulates in this liquid can be returned to the plating tank by using it as the cleaning solvent for the front pad. A considerable amount of evaporation occurs in this stage, typically 2 gph to 5 gph. For this reason, the recirculation tank is fitted with a liquid level control system, and is automatically replenished with deionized water or clean tap water.

The third and final stage is similar to the first - i.e., a single, dry meshpad. The primary purpose of this section is to remove the scrubbing liquor that was administered to the airstream in the previous section. The efficiency of the Trimesh unit was tested by EPA (ref. 407, 408). The results of these two studies showed a consistent removal rate of 98.9 and 99.7%, respectively.

Midwest Air Products Co., Inc. manufactures a range of equipment for emission control including fume collection hoods with integral mesh pads (Terminator™), in-line mist eliminators (single and two-stage), mist eliminators utilizing either meshpads or chevron blades, and chromium scrubber units with meshpads (Enforcer III™). They also manufacture mesh pads designed as an integral mist eliminator stage within a packed bed wet scrubber. Two of their products are often combined in series to meet stringent emissions requirements. Diagrams showing their standard chromium scrubber unit (Enforcer III™) with an in-line mist eliminator and details of the mist eliminator and scrubber are presented in Exhibit 3-67 (40 kb). With the meshpad mist eliminator/chromium scrubber configuration, the in-line mist eliminator removes gross quantities of chromic acid and the scrubber unit further cleans the air. The mesh pad mist eliminators can be mounted in either a horizontal or vertical position. Vertical units incorporate a design that prevents washdown from draining down through the duct system. The standard meshpad is manufactured from polypropylene, however, Kynar meshpads are used for most chromic acid applications. As an alternative to the in-line mist eliminator, Midwest Air Products offers a tank exhaust hood that contains a mesh pad (Terminator™) (see Exhibit 3-67 40 kb). The scrubber unit, which is itself a combined scrubber/mist eliminator, is configured with either three or four mesh pads with each stage designed using specific parameters such as particle size, velocity, removal efficiency requirement and pressure drop. The first pad removes the larger droplets or particles of chemicals and the subsequent stages remove smaller particles. Each mesh pad section is equipped with removable spray headers for periodic washdown. The Enforcer III™ recirculation system is shown in Exhibit 3-68. The first pad is washed with effluent from the scrubber unit or rinse water and subsequent pads are washed with clean water. The effluent is periodically pumped to the plating tank to make up for evaporative losses and to recover the chromic acid.


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