United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-94/148 September 1994
^ EPA Project Summary
Alkaline Noncyanide Zinc
Plating and Reuse of
Recovered Chemicals
Jacqueline M. Peden
A metal finishing process can create
environmental problems because it
uses chemicals that are not only toxic
but also resistant to degradation or de-
composition. A study was undertaken
at a zinc electroplating operation to
achieve zero discharge of wastewater
and total recycle of recovered precipi-
tates. The first step in this project was
to change an existing zinc cyanide (CN)
plating line to one that used an alkaline
noncyanide (ANC) zinc bath. The
project then investigated a closed-loop
system to treat plating rinsewater from
the ANC zinc plating line so the plating
chemicals were recovered and the wa-
ter purified. The goal was to return both
the recovered zinc hydroxide and the
clean water to the plating line for con-
tinued use. The system that was de-
signed and installed, at P&H Plating
Co., a Chicago area operation used pre-
cipitation by pH adjustment to remove
the zinc from the rinsewater. The pre-
cipitated zinc hydroxide was collected
on filters, dewatered using a filter press,
and stored for reuse in the plating line
as needed. Once filtered, the water was
recirculated to the rinsing portion of
the plating line. The recovery/recycle
system successfully purified the
rinsewater and facilitated the recycling
of the cleaned water and the precipi-
tated zinc hydroxide. Eliminating cya-
nide from the plating process meant
the line workers were dealing with a
less toxic plating bath, made compli-
ance with regulations easier, and re-
duced treatment and disposal costs for
the company. The recycling of the re-
covered water and the zinc hydroxide
further reduced the costs for treatment
and disposal. The replacement of this
single CN line with an ANC line re-
sulted in an annual savings to P&H
Plating of $14,000 from the elimination
of the need to pretreat the plating line
rinsewater to oxidize cyanide. The ad-
dition of the recovery/recycle system
increased the company's savings to
$62,000/yr. The reuse of 30% of the
recovered zinc hydroxide and 70% of
the treated rinsewater reduced annual
water usage and wastewater discharge
by 841,911 gal and reduced the amount
of sludge disposed annually by 14 yd3.
The payback period for the recovery/
recycle system is slightly less than 18
mo. Installation and use of this system
for other ANC plating operations would
result in reductions in wastes and in-
creased economic benefits similar to
those experienced by P&H Plating Co.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the WRITE program
demonstration that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
This cooperative effort between the U.S.
Environmental Protection Agency (EPA)
and the Illinois Hazardous Waste Research
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and Information Center (HWRIC) was un-
dertaken to evaluate the feasibility of us-
ing an innovative, closed-loop rinsewater
treatment system to precipitate plating
chemicals for recovery and reuse and to
produce purified water for recirculation to
the rinsing tanks and sprayers. The goal
of this study was to achieve zero dis-
charge of the wastewater and total re-
cycle of the recovered precipitate thereby
reducing the amount and toxicity of the
waste from an electroplating shop's zinc
plating operation. The CN plating line was
converted to one that used an ANC plat-
ing bath. A recovery/recycle (R/R) unit
was then designed, installed, and tested
to determine how completely the goals of
the project were being met. The effective-
ness of the R/R system in reducing the
process wastes was evaluated by: quanti-
fying the effectiveness of the removal of
the zinc through precipitation by pH ad-
justment (the basis of the recovery sys-
tem); determining the quality of the
precipitate and the treated water that were
recovered; comparing the plating quality
of the CN-based operation to that of the
ANC-based processes and analyzing the
costs associated with the change in the
process and the installation of the R/R
system. The R/R unit was designed, in-
stalled, and tested by engineers from the
Center for Neighborhood Technology
(CNT). Their initial analysis that the sys-
tem would be effective in reducing wastes
and facilitating recycling of the bath com-
ponents was confirmed by HWRIC engi-
neers who again tested the system after
more than 1 yr of operation. A slightly
modified unit that accommodates two plat-
ing lines, rather than one, is still in opera-
tion at P&H Plating Co. after more than
Syr.
Industry Partner
P&H Plating Co. is one of 174 plating
operations in Cook County, Illinois. It is a
large job shop that operates 16 hr/day, 6
days/wk. It uses barrels, hoists, and racks
to move parts through the plating pro-
cess. Although this project concentrated
on zinc plating, the shop is capable of and
does plate nickel, brass, copper, and cad-
mium on a variety of surfaces. The facility
contains a waste treatment area that re-
ceives effluent at an average rate of 150
gal/min. For this project, the company
made two changes to one of its barrel
plating lines. It replaced a CN plating line
with one that used an ANC bath and in-
stalled the CNT-designed R/R system. An
evaluation of the effectiveness of these
changes continued over 2 yr.
Problem Statement
Achieving waste reduction in the elec-
troplating industry is important because
the wastes can create many environmen-
tal problems. The chemicals used are not
only toxic but also resistant to degrada-
tion or decomposition. Many opportunities
are open to electroplaters to reduce their
wastes. Simple housekeeping techniques
such as slowing down the movement of
parts through the operation to increase
drain time can significantly reduce the
amount of plating bath chemicals carried
over into the wastewater. Other options
include recovering bath chemicals, clean-
ing and recycling rinsewater, using less
toxic chemicals when possible, and using
technologies such as ion exchange to
clean and maintain plating baths. Adopt-
ing these types of process modifications
are generally not only environmentally ad-
vantageous but frequently result in eco-
nomic benefits.
Federal and state regulations set limits
on the amounts of toxic plating line con-
stituents that can be discharged. The costs
to comply with these regulations can be
considerable. Cyanide is a particularly dif-
ficult contaminant to treat; it readily com-
bines with metals, particularly iron, to form
stable complexes that may not be de-
stroyed by standard treatment methods. It
poses substantial health and environmen-
tal risks if not handled correctly or treated
completely. Enforcement efforts on both
the state and federal levels are well coor-
dinated, and prosecution of violators has
increased. Electroplaters must develop an
integrated approach to waste management
that meets compliance standards and in-
cludes waste reduction as a vital compo-
nent.
The CNT Recovery/Recycle
(R/R) System
The relatively simple principles involved
in the design of the CNT R/R system are
similar to those for a standard wastewater
flocculation treatment to remove metals
before disposal of water and sludge
wastes. Ideally, 100% of the zinc in the
CNT R/R system's rinsewater would be
recovered and returned to the plating bath.
Additionally, all rinsewater would be re-
cycled. The projected result would be sub-
stantial savings for the company in plating
chemicals and water from this rinsewater
purification that both recovers and reuses
as well as treats.
In the plating process at P&H, there is
the usual parts pretreatment or cleaning,
followed by the plating process, a spray
rinse, and finally, submersion into two
counterflow rinse tanks. Although clean-
ing requirements for ANC plating are gen-
erally more stringent than those for CN
plating, no change was required to the
pretreatment portion of the line at P&H.
The company had already installed a very
stringent cleaning component to their plat-
ing lines to ensure good parts cleaning
and, presumably, better plate quality. The
CNT R/R system was plumbed from the
spray rinse tank into which the counter
flow tanks ultimately overflowed. The
rinsewater flows into a tank where the pH
is measured and automatically adjusted
to a pH between 10 and 10.5.
This pH monitoring and control tank is a
continuous flow stirred reactor (CFSR). It
is designed to stimulate precipitation by
sparging with compressed air entering from
the bottom of the tank. The flow rate
through the CFSR is set at 10 gal/min.
The precipitate/water slurry next flows into
a flat-bottomed clarifying tank. To facili-
tate the settling process, the tank is baffled.
A recirculating pump is used to pull water
from the clarifying tank through the dual
filtering system to remove suspended hy-
droxide. The filtered water then either flows
to a storage tank for reuse or to the waste
treatment area for disposal. The precipi-
tate that has settled in the clarifying tank
is removed and combined with that col-
lected on the filters. This composite hy-
droxide is placed in a filter press to remove
as much water as possible. The water
that is removed is returned to the precipi-
tation reactor and the dewatered hydrox-
ide is analyzed and stored for future use
or disposed if not needed. Figure 1 shows
the system components and the path of
the water and the solids as they flow
through the system.
R/R System Performance
Although the system had problems, it
proved to be a highly efficient and eco-
nomically advantageous addition to the
plating operation. The quality of the zinc
hydroxide precipitate and the purified wa-
ter were sufficient for recycling. Careful
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Product
Treatment
or
disposal
Rinse Four-part plating process
Can be returned to plating bath
meter Counter flow rinses
Precipitate collection
Filter press
Figure 1. P&H Plating alkaline noncyanide plating line with CNT designed recovery/recycle system.
and regular analysis of the essential bath
chemicals is recommended to ensure con-
taminants are not introduced through these
recycling efforts.
The CNT R/R system has been in con-
tinuous operation for over 3 yr. Mainte-
nance consists of proper care of the
mechanical parts and periodic replacement
and cleaning of the filters. Zinc hydroxide
is removed from the rinsewater with an
efficiency averaging 84%. The amount of
precipitate recycled to the bath depends
largely on the number of jobs requiring
use of that line. The company estimates
that it recycles 30% of the zinc hydroxide
it recovers. The quality of the purified wa-
ter is generally acceptable for recycling or
for discharge. Approximately 70% is used
in the rinsing operation or to replenish the
plating bath. The system works well pro-
viding both environmental and economic
benefits to P&H.
Plating Quality Comparison
Although the reasons for plating are
sometimes purely ornamental, more fre-
quently the plate is for protection. Fin-
ishes may be bright or dull, and it may not
always be possible to achieve the desired
luster with the ANC system. Advances in
the last decade have, however, provided
less toxic bath alternatives that produce
parts more like the bright, shiny objects
that result from a CN-based operation.
Ultimately, whether the plate is satisfac-
tory or not is up to the customer, but two
commonly used standard tests can be per-
formed to evaluate quality—thickness and
corrosion resistance. When parts plated
on both the CN and ANC lines were com-
pared for these standards, both processes
produced parts with the desired thickness
and acceptable corrosion protection.
Toxicity Comparison
Had there been no economic benefit
from the change to ANC plating, the re-
duction in health and environmental risk
resulting from the elimination of cyanide
from the process would have been suffi-
cient to warrant its adoption. CN plating
requires extensive treatment before dis-
posal and uses chemicals hazardous to
human health. Chemical substitution to
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achieve source reduction, as was done
for this project, not only reduced process
costs but also the company's liability be-
cause of the reduced toxicity of the chemi-
cals being handled and disposed.
The exposure of shop workers to toxic
chemicals presents the most serious
health and safety problems for the elec-
troplating industry. Although no occupa-
tional illness has been documented for
electroplating operators, they are routinely
exposed to hazardous substances known
to cause serious health problems. The
use of cyanide, generally considered the
most potentially dangerous of the electro-
plating chemicals, is carefully monitored
and employees are trained to use it prop-
erly. Combining employee education with
substitution of less toxic chemicals may
provide the least costly and most produc-
tive control of workplace hazards. Re-
placing cyanide plating solutions with
noncyanide baths is strongly recom-
mended.
Economic Comparison
When the R/R system is installed on
the ANC line, the economic benefits be-
come significant. An economic compari-
son for the change to ANC and then for
that change plus the addition of the R/R
unit is provided in Table 1. An annual cost
savings of $14,000 is achieved by switch-
ing from a CN to an ANC process with a
savings of $62,000 when the R/R system
is also added to the line; this savings
takes into account the annual cost of op-
eration of the R/R system which is
$10,900.
Making the switch from CN to ANC re-
quires disposal of the existing 1,800 gal
CN bath at a cost of $20/gal for a total
cost of $36,000. This must be calculated
as part of the capital investment for this
project. Adding the R/R system increases
the capital investment by $51,822 for a
total of $87,822. These capital investments
and the operational expenses listed in
Table 1 were entered into a spreadsheet
program that calculates a number of eco-
nomic indices. The calculations were
based on 1992 costs. They use an infla-
tion rate of 4%, a discount rate of 7.7%, a
depreciation schedule of 7 yr, and a project
life of 10 yr. It was assumed for all calcu-
lations that the line operates 8 hr/day, 5
days/wk for 50 wk/yr. Table 2 contains the
results of the spreadsheet calculations.
This analysis shows the recycling option
provides the greater economic benefit, the
shorter payback period, and the larger
return on investment.
Discussion
As a normal part of a plating line, a
rectifier is used to convert alternating cur-
Table 1.
Comparison of Annual Operational Costs for CN Process, ANC Process without R/R Unit, and ANC Process with R/R Unit at P&H Plating
Co.
Process Operation
CN Costs ($)
ANC Costs ($)
ANC + R/R* Costs ($)
Bath makeup
Bath maintenance
Water usage
1. Use @ $7.56/7480 gal
2. Sewering @ $5.59/7480 gal
Wastewater treatment
1. Cyanide oxidation
2. Metal precipitation
3. Labor @ $15/hr
Sludge disposal @ $209/yd3
Total
1771
22325
1213
897
14000
69000
7500
2600
119306
1860
21225
1213
897
0
69000
7500
2600
104295
1860
19425
364
269
0
20700
2250
1820
46688
* Assumes 70% water and 30% zinc hydroxide recycled
Table 2. Comparison of Economic Indices for the Alkaline Noncyanide Plating Process with
and without the Recovery/Recycle System
Index
ANC
Option
ANC + R/R
Capital Investment
Payback Period
Net Present Value
Implied Rate of Return
$36,000
3yr
$57,500
27.0%
$87,822
1.5 yr
$281,122
71.9%
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rent to the direct current needed for the
plating process. Some type of cooling is
needed to prevent the rectifier parts from
overheating and thus damaging or de-
stroying the entire unit. The rectifier on
the test plating line is cooled by water
although air-cooled models are used on
other zinc plating lines at P&H. Tap water
used in this noncontact cooling system is
not discarded but is rather added to the
counter flow rinsewater tanks on the plat-
ing line. Because of this continuous addi-
tion of fresh water to the plating line, a
holding tank to collect the R/R treated
water was installed. Water in this tank can
be used in the rinsing operation and as
makeup water for replenishing the plating
bath. The tank is designed with an over-
flow valve that opens and sends excess
water to P&H's wastewater treatment area
for further treatment before discharge. Al-
though this means that the objective of
zero discharge has not been met and that
this precipitation process is partially serv-
ing as a pretreatment step, the company
estimates that the amount of water from
this line that needs treatment has been
reduced by more than 60%. Additionally,
the cost of the precipitation pretreatment
is less than would be the cost of pretreat-
ing the cyanide containing rinsewaters.
The compliance criteria that need to be
met by the final treatment are only those
for zinc, since there is no longer cyanide
in this rinsewater. The maximum discharge
levels for metals are generally easier to
achieve than are those for cyanide. This
results in reduced treatment costs and
fewer compliance problems.
The recovery efficiency for zinc hydrox-
ide from the CNT system averages 84%;
however, because of the variability in the
numbers and types of plating jobs and the
general fluctuations in business (all of
which play a role in the amount of zinc
hydroxide needed for the line), the com-
pany does not recycle all of the precipi-
tate that is recovered. Additionally, the
recovered zinc hydroxide is not a totally
suitable substitute for the zinc ingots tra-
ditionally used to add zinc to the plating
bath. All of the precipitate produced is
passed through the filter press, which
greatly reduces its volume. A portion,
which varies depending on production
needs, but generally averaging 30%, is
returned to the plating bath. The remain-
ing 70% is stored for later use or dis-
posed as a hazardous waste. While it
would be possible to petition to delist this
waste, the amount being produced is less
than 5% of the metal waste that the com-
pany produces and must routinely dis-
pose. Since much of the other metal waste
was from cyanide-based lines and prob-
ably contained residual amounts of cya-
nide, even after treatment, it was disposed
as a hazardous waste. Currently, it is more
economically advantageous to simply add
the zinc hydroxide to that waste and dis-
pose of it as hazardous rather than sepa-
rating it, storing it, and applying for
delisting.
The company has recently converted
all of its plating lines from CN-based to
ANC-based. This change will eliminate the
need for cyanide destruction in the
company's treatment operations, will elimi-
nate one potential compliance problem,
will eliminate the potential health risk as-
sociated with cyanide exposure, and will
ultimately save the company money. The
change was completed more quickly than
originally anticipated. Now that it is com-
plete, it may become more economically
inviting to pursue the delisting process for
the zinc hydroxide that isn't being recycled.
Also, the elimination of the cyanide lines
will allow use of the precipitated zinc hy-
droxide to replenish other zinc plating
baths, which is an option currently under
consideration at P&H.
Conclusions And
Recommendations
The CNT-designed R/R system proved
quite successful in meeting the project
objectives. By converting from a CN-based
to an ANC-based plating bath, the com-
pany eliminated one step from the treat-
ment cycle, i.e., the destruction of cyanide.
The removal of cyanide from the system
also reduced the risk to employees by
eliminating a highly toxic substance from
their work environment. The purification of
the wastewater proved so successful that
nearly all of the recovered water could be
recycled as well as much of the precipi-
tated zinc hydroxide. As a result of install-
ing this recovery unit, annual operational
costs (including treatment and disposal
costs, raw material purchases, and water
usage fees) were reduced by $62,000.
The costs to design and install a R/R
system like the one in use at P&H Plating
is recovered during the first year and a
half of operations. Additionally, recycling
of the recovered materials reduced water
usage and discharge by 841,911 gal and
reduced the sludge disposed by 14 yd3/yr.
These savings apply to a single line, but
the test unit could easily accommodate a
second line's waste, which would double
these waste reduction figures. This sys-
tem is simple and functions well. It is in
use at P&H Plating today, and its use in
similar electroplating operations would re-
sult in economic benefits comparable to
those described in this project summary.
Finally, there are still many pollution
prevention opportunities for the electro-
plating industry left to explore. Documented
case studies of process modifications and
technology evaluations that lead to source
reduction are available, but more are
needed. Distribution of this information to
the appropriate audience can be achieved
with the assistance of trade groups for the
electroplating industry. Continued associa-
tion with these industry organizations is
essential to identify new pollution preven-
tion options and to promote adoption of
those that have proven successful. This
continued interaction can only benefit all
parties involved. Economic benefits could
be substantial to companies willing to work
toward source reduction. The environmen-
tal benefits that could result from the re-
duction in toxicity and volume of this
industry's wastes would be significant and
would be reason enough to continue to
support research that will bring about those
reductions.
The full report was submitted in partial
fulfillment of Cooperative Agreement
#CR815829 by the Hazardous Waste Re-
search and Information Center under the
sponsorship of the U.S. Environmental Pro-
tection Agency.
-------
Jacqueline M. Peden is with Hazardous Waste Research and Information
Center, Champaign, IL, 61820.
Paul M. Randall is the EPA Project Officer (see below).
The complete report, entitled "Alkaline Noncyanide Zinc Plating and Reuse of
Recovered Chemicals," (Order No. PB94-205549; Cost: $19.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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$300
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EPA/600/SR-94/148
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