MIL-HDBK-1165
e) Ion exchange. Ion exchange is the same method by
which water softeners work. For plating rinse-waters, beds of
"ion exchanging" resins retain ions (charged particles) of
plating chemicals (metals) which have been dragged into the
rinse-water from the plate bath solution, and exchange them
(release into the water) with other ions harmless to the rinse-
water's application. The retained metals in the resin are then
extracted by regeneration. During regeneration, a strong acid or
base is used to recover the plating ions by exchanging them for
the original resin ions. Depending on the purity of the
recovered metals, they can be added back into the plating
solution. Ion exchange systems can be set up to regenerate
automatically, but still require more operator attention and
maintenance than other recovery technologies.
f) Reverse osmosis. Reverse osmosis (RO) uses
membranes instead of resins to separate metal salts from rinse-
water. Rinse-water is purified by forcing it through the
membranes at high pressure, leaving behind the metals. RO
membranes are sensitive to oxidizing chemicals and extremely low
or high pH's, and are susceptible to fouling in concentrated or
hard water solutions.
g) Evaporation. Evaporation simply involves boiling
off water from the contaminated rinse-water, condensing the
purified water vapors for reuse in the rinse tank, and returning
the concentrated leftover metal solution to the plate bath.
Evaporation requires thermal energy to operate and thus needs a
rinse-water solution of sufficient original plating metal
concentration to be cost effective.
h) Diffusion dialysis or electrodialysis. Diffusion
dialysis or electrodialysis are more recently developed methods
than the technologies listed above, and their effectiveness is
still being researched and refined. Dialysis, like reverse
osmosis, uses membranes to separate metals from rinse-water.
Instead of high pressures, the separation occurs by either
placing an electrical charge on the membranes, or by the
phenomenon of diffusion. In electrodialysis, charged membranes
allow ions of the same charge to pass through. By alternating
negative and positively charged membranes, the incoming metals
can be separated from the rinse-water. Electrodialysis requires
relatively little maintenance and can operate continuously
without regeneration. In diffusion dialysis, solutes (metals,
acid) move from areas of high concentration to areas of low
concentration based on their individual diffusivity, or ability
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