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Pollution Prevention and Control Technologies for Plating Operations
Section 3 - Chemical Recovery
3.2 ATMOSPHERIC EVAPORATORS
3.2.3 Applications and Restrictions
Exhibit 3-4 shows the three basic configurations used for application of atmospheric evaporators for chemical recovery. Application AE-1 is used mostly for elevated process baths (>120oF), AE-2 for moderate temperature baths (100 to 120oF) and AE-3 for ambient or low temperature baths (ambient to 100oF). In each case, an important aspect to the implementation of this technology is the incorporation of recovery rinsing. The quantity of recovery rinse solution that can be added into a bath equals the sum of the evaporation from the tank's surface and the evaporation caused by the atmospheric evaporator (i.e., ignoring any differences in drag-in and drag-out). For some processes, where the operating temperature is high and drag-out is sufficiently low, a closed-loop configuration can be employed. Several of the respondents to the Users Survey indicated that have they achieved a closed-loop (e.g., PS 003 for nickel, PS 213 for nickel and chrome). Generally, these shops used either a three or four stage recovery rinse. Lower temperature processes and those with high drag-out rates will usually require a free running rinse (or countercurrent arrangement) following the recovery rinse tank in order to maintain sufficiently clean water in the final rinse. The effects of rinsing configurations on recovery and plating quality are discussed in detail in Section 2.
The strategy with moderate temperature baths is to connect the atmospheric evaporator to a heated transfer tank with the idea that the solution in the transfer tank can be heated to a higher temperature than the maximum operating temperature of the plating process. For example, a transfer tank used with acid zinc can be heated to 140oF, whereas the maximum operating temperature of the process bath is about 90oF. By connecting the evaporator to the transfer tank, the evaporation rate will be approximately tripled. A number of shops responding to the Users Survey indicated that they were using heated transfer tanks (e.g., PS 098, PS 252 and PS 278).
With low or ambient temperature baths, where there is no appreciable surface evaporation from the process bath, a bleed from the process bath to the transfer bath is used to make the needed headroom. Alternatively, a second evaporator could be used. For some ambient baths that contain wetting agents, drag-in may exceed drag-out. The design capacity of the atmospheric evaporator must account for this difference plus the desired recovery rinse rate. In some cases, an atmospheric evaporator may be used simply to create the headroom needed to prevent discarding "extra" plating solution created by these conditions (ref. 355). One such application was found in the Users Survey (PS 214).
The Users Survey also showed that many shops were incorrectly using their atmospheric evaporators. Approximately 20% of the shops connected either an unheated recovery rinse or an unheated plating tank to the evaporator. Generally, these facilities experienced below average results. Performance experience is discussed in Section 3.2.6.
With all applications of atmospheric evaporators, the user should install water treatment in the form of ion exchange and/or reverse osmosis to remove the hardness and other impurities in the raw water that would otherwise accumulate in the process tank.
Recovery applications that were identified during the Users Survey are presented in Exhibit 3-5. The most common recovery application for atmospheric evaporators is nickel plating. On a combined basis, the nickel applications accounted for nearly 30% of all applications (includes bright nickel and electroless nickel applications).
When used in conjunction with hard chrome plating or other processes that have solution heating and cooling requirements and ventilation requirements, the atmospheric evaporator can serve as a recovery unit, cooling device and an air scrubber (ref. 299). Because the unit would be using waste heat from the tank for evaporating excess water, the evaporation process operates essentially free. The appropriate tank configuration is basically the same as shown in Exhibit 3-4, application AE-1. The plating tank's exhaust air would serve as the inlet air to the evaporator, which would remove its chromic acid mist. In such cases, the exhaust of the evaporator is usually connected to the existing ventilation system and the evaporator's blower is eliminated. Because of anticipated regulations for hard chrome plating air emissions, it is questionable that the evaporator's mist eliminator would adequately substitute for a future emission control device. At this time there are insufficient data to evaluate this application.
Atmospheric evaporators are not applicable in cases where the solution temperature cannot be raised above approximately 85oF, either in the process tank itself or in a heated transfer tank. Generally, these are solutions that either contain highly heat sensitive components or fume excessively when heated or aerated. Also, atmospheric evaporators should not be applied to any process solution which cannot be maintained through use of methods and/or technologies that replenish active bath ingredients or remove the contaminants that build-up as a result of recovery rinsing. When applied in these cases, the atmospheric evaporator hastens the disposal rate of the bath resulting in essentially the same mass of chemicals discarded as if recovery were not practiced. Methods of bath maintenance for process solutions are covered in Section 4.
Atmospheric evaporators should not be applied to solutions that foam significantly when air agitated (e.g., high cyanide baths and still nickels). Such solutions will foam in the evaporation chamber and render the system inoperable. A simple jar test is recommended by one manufacturer to determine if foaming is a potential problem. This test is accomplished by placing a sample of the solution in a jar, vigorously shaking it and then observing to see if the foam quickly disappears (ref. Poly Products file).