Pollution Prevention and Control Technologies for Plating
Section 3 - Chemical Recovery
3.4 ION EXCHANGE
3.4.3 Applications and Restrictions
Ion exchange is used for a variety of purposes in the metal finishing
shop, including: treatment of raw water; recovery of plating chemicals
from rinse water; purification of plating solutions; wastewater
treatment; and wastewater polishing. The following discussion
of applications and restrictions focuses on use of this technology
for chemical recovery from rinse water. The other applications,
with the exception of raw water purification, are discussed in
other interim reports.
Ion exchange is a useful technology for recovering plating chemicals
from dilute rinse waters. Two common configurations are shown
in Exhibit 3-22, application IX-1 is
referred to as metal scavenging. It uses only one type of ion
exchange resin, either anion or cation, depending on the charge
of metal or metal complex being recovered. Because this system
does not have both cation and anion resins, the rinse water will
not be fully "deionized" and cannot be reused as rinse
water for common rinsing purposes. In AE-2, both anion and cation
resins are employed and the rinse water can be recirculated in
a closed loop. With both of these configurations, rinse water
containing a dilute concentration of plating chemicals is passed
through an anion and/or cation column (or dual columns of the
same type) and the metals are removed from the rinse water and
held by the ion exchange resin. When the capacity of the unit
is reached, the resin is regenerated and the metals are concentrated
into a manageable volume of solution. Depending on the chemical
nature of the process, the regenerant (eluate) solution can be
returned directly to the plating tank for reuse, further processed
and returned, or the metals can be recovered by another technology
such as electrowinning (see Section 3.5).
The most common applications for these configurations of ion exchange
are with the recovery of copper, nickel, and precious metals.
Application IX-3 (Exhibit 3-23) is a
bath maintenance configuration that is only applicable to chromic
acid solutions. The chromium bearing rinse water is passed through
a cation column to remove trivalent chromium, and tramp metals
such as iron, nickel and aluminum and is then used as make-up
for evaporation in the bath. This application prevents the build-up
of contaminants in the bath, a process that is hastened by recovery
rinsing. Since hexavalent chromium is an anion, it is not removed
by the cation resin.
Drag-out recovery tanks (see Section 2) are used with ion exchange
systems whenever feasible. In operation, the drag-out tanks return
the bulk of the plating chemicals to the plating bath and an ion
exchange column captures only the residual quantities of chemicals.
The needed size of the ion exchange unit and its regeneration
frequency are therefore reduced.
Some of the respondents to the Users Survey misapplied the ion
exchange technology for chemical recovery. For example, PS 261,
operates a Watts nickel plating line with a 140°F bath and
a four stage counter flow rinse. This shop feeds the ion exchange
system from the first rinse, which is the most concentrated, and
returns the treated water to the last rinse. Instead, this shop
should utilize the configuration shown in Exhibit 3-22.
Application IX-1. The use of a recovery rinse will greatly reduce
the nickel load on the ion exchange columns.
As a recovery technology, ion exchange should be applied to dilute
rinse waters. It is not applicable to concentrated drag-out solutions
or plating baths (although it can be used as a bath maintenance
technology to remove tramp metals, see Section 4). A major limitation
of this process is that many plating baths are more concentrated
than the ion exchange regenerant. Therefore, it should not be
used in a "bleed and feed" system, where spent bath
is bled to the rinse water. The result in these cases is that
the chemicals are diluted in the rinse water, collected by ion
exchange, regenerated (using costly chemicals), and recovered
in a lower concentration than they started (ref. 40).
Ion exchange is applicable to a wide range of plating processes.
Exhibit 3-24 shows the applications
identified by the Users and Vendors Surveys.
Ion exchange is used much more frequently for metal recovery from
non-cyanide solutions than for cyanide solutions (e.g., cadmium
cyanide, copper cyanide, zinc cyanide), with the exception of
gold cyanide. When applied to cyanide solutions, the task of recovery
is more difficult due to the nature of the cyanide complex. As
explained by Reinhard (ref. 342), when a cation followed by an
anion column arrangement is used, the cyanide complex in the rinse
water is decomposed into free metal cations and cyanide anions
and these cations and anions are exchanged for H+ and OH- ions
in the corresponding resin beds. When the cation resin bed becomes
exhausted there is insufficient acidity left to decompose the
cyanide complex which is an anion and it will be exchanged for
an OH- ion in the anion resin bed. This presents a significant
problem since the cyanide complex will accumulate in the anion
resin and because of its high affinity to the resin matrix, regeneration
will not remove it. If this occurs, the resin may require replacement.
This problem is not a concern with gold cyanide applications because
in most cases the resins are incinerated during the recovery process,
rather than regenerated. Incineration of spent gold resin is an
economically acceptable practice because of the relatively small
quantity of resin used and its low cost in comparison to the gold
contained on the resin.
Another operational problem with cyanide solutions can be caused
by excessive free cyanide in the rinse water. If this condition
exists, the free cyanide will remove the heavy metal from the
cation resin and form a new metal-cyanide complex.
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