Pollution Prevention and Control Technologies for Plating
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
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
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).
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