Pollution Prevention and Control Technologies for Plating
Section 3 - Chemical Recovery
3.5.3 Applications and Restrictions
Exhibit 3-32 shows the three basic configurations
in which electrowinning was successfully applied by the shops
responding to the survey.
The most common configuration (EW-1) employs an electrowinning
unit connected directly to a drag-out tank. Alternatively, the
solution from the drag-out tank can be periodically transferred
to a holding tank that is connected to the electrowinning unit.
Either of these arrangements can be used with flat plate or reticulate
cathode units. The reticulate cathode types will maintain the
rinse system at a lower metal concentration (in some cases below
1 mg/l) but, because the cathodes are not reusable, the operating
costs will be higher. The operation of the flat plate cathode
types are more significantly affected by fluctuations in the metal
concentration of the electrolyte. Therefore, if the plating operation
causes sharp fluctuations in the drag-out tank concentration,
the user should consider the use of a side tank or a reticulate
type of cathode. The HSA cathode units should be directly connected
to the drag-out tank. This will permit them to maintain a low
steady state concentration of metal in the drag-out tank.
Electrowinning removes metal from the drag-out solution, but does
not remove all dissolved solids. For this reason, the drag-out
solution must be occasionally discarded or purged to prevent the
build-up of dissolved solids (e.g., acid). When this occurs, any
residual metal in the drag-out solution will be lost.
The metal recovery efficiency (i.e., the percentage of metal recovered
from drag-out) of the first configuration depends on two key factors:
(1) the average concentration of metal in the drag-out tank and
(2) the mass of metal in the purge. The concentration of metal
in the drag-out tank is important because it determines the mass
of metal that will be carried over by drag-out to the next rinse,
which is treated. This factor points out the weakness of the flat
plate cathode types. These units operate efficiently only when
the metal concentration is high (usually 1 to 5 g/l of metal).
Therefore, the drag-out tank must be operated until this level
is achieved, which in turn increases the loss of metal to the
free running rinse. The higher surface area of the reticulate
and HSA units allow the user to operate the drag-out tank at a
lower metal concentration and therefore reduce metal losses. Further,
these types of electrowinning units generate a purge with a lower
The second configuration (EW-2) is a combination of ion exchange
and electrowinning. This configuration potentially has a much
higher metal recovery efficiency than the first configuration.
It addresses both of the factors that impact metal recovery efficiency.
The ion exchange unit maintains a low metal concentration in the
final rinse, thereby almost eliminating drag-out losses. The ion
exchange unit concentrates the metal into a regenerant stream
and the electrowinning unit removes the metal. Residual metal
in the regenerant is of less concern than the first configuration
since it can be reconcentrated by the ion exchange unit. For the
same reason, a flat plate cathode will suffice for this second
In some cases, the reticulate cathode units can be substituted
for the second type of configuration. When such a unit maintains
the drag-out rinse in the low mg/l range, the metal recovery efficiency
of the process would approach that of the ion exchange/electrowinning
combination. Some recent operating data for a copper recovery
application using this configuration are presented in Section 126.96.36.199.
The third configuration shows the recovery of metal from a spent
process solution. Either the flat plate or reticulate cathode
type of unit can be used in this configuration. The reticulate
cathode type will provide greater metal recovery efficiency because
it can lower the metal concentration of the spent bath below that
of the flat plate. Because the reticulate cathodes are not reusable,
its higher recovery efficiency comes at an increased operating
Electrowinning is applied to a wide variety of chemical solutions
in the electroplating industry. The literature indicates that
the metals that are most commonly recovered by electrolytic treatment
are gold, silver, copper, cadmium, and zinc. The metal recovery
applications identified from the Users Survey are shown in Exhibit 3-33.
This exhibit indicates the number of survey respondents that applied
electrowinning to each of the processes and the average satisfaction
level of the technology for that application, based on a scale
of 1 to 5 (1 equals the lowest satisfaction level and 5 equals
For practical purposes, the degree to which a metal can be recovered
by electrowinning can be determined by its position in the Electromotive
Series (see Exhibit 3-34). Metals that
have more positive standard electrode potentials plate more easily
than the ones with less positive potentials. As an illustration,
the more noble metals, such as silver and gold, can be removed
from solution to less than 1 mg/l using flat plate cathodes whereas
with copper and tin, a concentration in the range of 0.5 to 1
g/l or more is required for a homogeneous metal deposit. Equations
for accurately estimating the potential for a given application
were presented by Brown (ref. 349) and Bailey and Chan (ref. 128).
It is interesting to compare the satisfaction levels in Exhibit 3-33
to the position of the metal in the electromotive series. The
satisfaction levels for silver, copper, cadmium and zinc cyanide
plating (the most common applications of the respondents) fall
into nearly the exact order as the metal's position in the electromotive
Although copper, cadmium and zinc have a lower position in the
electromotive series than precious metals and they received only
moderate to low satisfaction levels from survey respondents, this
is not to say that these applications cannot be successfully performed.
With the application of proper engineering and good equipment
selection these electrowinning applications can be highly successful,
as indicated by some of the respondents. For additional data,
Exhibit 3-35 groups potential electrowinning
applications based on their frequency of use and success in industry
and the general difficulty of the application. These rankings
are based on input from electrowinning vendors and information
from the literature. Included in this exhibit are a much broader
range of metals than those identified in the Users Survey.
Although there are limitations for electrowinning nearly every
metal, chromium is the only commonly electroplated metal that
is not recoverable using electrowinning. Nickel recovery is possible,
but it requires close control of pH and therefore is less frequently
performed than, for example, cadmium or copper. Also, Altmayer
suggests that nickel recovery is hampered by the absence of inexpensive
suitable inert anodes that do not give off chlorine gas and disintegrate
Solutions such as electroless plating solutions containing chelated
metals, reducing agents and stabilizers are more difficult for
the direct application of electrolytic recovery. However, there
was one survey respondent that indicated they were successfully
electrowinning nickel from a spent electroless solution (PS 188).
Another shop (PS 164) is in the process of starting up a unit
for the same purpose. One vendor (ref. 349) indicated that these
baths can be processed by electrowinning after undergoing pretreatment
(e.g., selective ion exchange) to break the metal-chelate bond.
Another reference suggests that reducing and oxidizing agents
can be combined to neutralize their effects; e.g., a printed circuit
board shop can mix spent micro-etch and electroless copper baths
and with proper pH adjustment create a solution that can be treated
by electrowinning (ref. I3 file). Another reference indicates
that electroless copper can be processed using electrowinning,
but that anode life will be short (ref. 99).
Fluoborate solutions (e.g., tin, tin-lead) are not commonly treated
using electrowinning due to their attack upon anode materials
including iridium oxide coated titanium and niobium. However,
one source (ref. 287) indicates that titania ceramic anodes coated
with iridium can provide a successful application. This material
and its application have been recently commercialized (ref. Kinetico
Certain corrosive solutions (e.g., certain etchants) may also
pose problems for electrowinning because metal that is plated
on the cathode may be etched off as quickly as it is plated (ref.
348). One reference suggests that increasing the current density
will partially overcome the etching action of ammonomical etches
when electrowinning copper from these solutions, but that complete
removal is difficult to achieve (ref. 99).
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