Pollution Prevention and Control Technologies for Plating
Section 3 - Chemical Recovery
3.3 VACUUM EVAPORATORS
3.3.3 Applications and Restrictions
Two common configurations for the application of vacuum evaporators
to chemical recovery are shown in Exhibit 3-10.
In configuration VE-1, a closed-loop process is achieved using
a four stage rinse system with the feed from the first rinse being
concentrated by the evaporator and directed to the plating bath.
A solution purification step is shown, which removes bath contaminants
that would buildup in the bath due to the closed-loop process.
Typical solution purification technologies used for this purpose
include ion exchange and/or carbon filtration. PS 125 employs
this configuration using a cation exchange unit to remove contaminants
from its decorative chromium plating rinse water/drag-out. PS
124 has a similar arrangement. PS 082 installed a cation exchange
unit and electrolytic purification unit that are connected to
a storage tank. The use of multiple-stage rinsing is nearly always
required with evaporator applications in order to minimize the
quantity of water to be evaporated. The survey respondents used
a minimum of two and a maximum of four rinsing stations. The second
configuration (VE-2) shows an open process, where a small portion
of the drag-out is not recovered. Also shown in VE-2, is a direct
bleed from the bath to the evaporator. This may be required for
ambient or low temperature baths, where there is a limited surface
evaporation rate and insufficient "head-room" in the
plating tank to return the concentrated drag-out/rinse water.
Vacuum evaporators are applied to the recovery of a wide range
of plating solutions. They are especially applicable in situations
where atmospheric evaporators are either technically or economically
impractical. This includes: (1) the recovery of heat sensitive
chemicals (e.g., cyanide plating baths); (2) the recovery of chemicals
that are sensitive to air oxidation (e.g., cyanide plating baths
or the stannous tin bath); (3) low or ambient temperature plating
solutions where there is no appreciable surface evaporation; (4)
the recovery of solutions that contain volatile components; and/or
(5) where high evaporation rates (e.g., >20 to 40 gph) are
necessary to achieve recovery and atmospheric evaporators become
too expensive (i.e., energy cost) to operate (ref. 299).
The results of the Users Survey and Vendors Survey showed that
vacuum evaporators are applied to a range of plating and finishing
solutions. These identified applications are shown in Exhibit
Although vacuum evaporators may provide an energy savings over
the atmospheric types of evaporators, neither one is economically
practical to purchase or operate where large volumes of low concentration
solutions are involved. In those cases, ion exchange or reverse
osmosis are the more cost effective methods of recovery (ref.
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