AEPA
United States
Environmental Protection
Agency
Technology Transfer
Summary
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Report
Control and Treatment
Technology for the
Metal Finishing Industry
In-Plant Changes
-003-
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Technology Transfer
EPA 625/8-82-008
Summary Report
Control and
Technology
Treatment
for the
Metal Finishing Industry
In-Plant Cha
January 1982
nges
This report was developed by the
Industrial Environmental Research Laboratory
Cincinnati OH 45268
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This summary report was prepared for the Industrial Environmental Research
Laboratory's Nonferrous Metals and Minerals Branch in Cincinnati OH.
The Centec Corporation, Reston VA, prepared the report. The EPA Project
Officer is George Thompson.
The contact for further information is:
Nonferrous Metals and Minerals Branch
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati OH 45268
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Cincinnati OH, and
approved for publication. Approval does not signify that the contents
necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
COVER PHOTOGRAPH: Automatic rack machine for nickel-chromium
plating process
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Overview
•The metal finish
United States is
ng industry in the
subject to a variety
of changing business conditions.
Two of the most! significant factors
are the increasing costs of materials,
such as plating chemicals and
process water, and the environmental
considerations, which include
the need to control the discharge
of effluent waste streams and
the disposal of hazardous wastes.
The survival of rrany metal finishing
companies will c
The basic plating
immersing parts
solution and the
epend on how
effectively they ceal with the impact
of these changes and requirements.
operation involves
in a process
i rinsing off the
clinging film of plating chemicals,
which is known as drag-out. If
performed inefficiently, this opera-
tion wastes several pounds
(kilograms) per day of expensive
plating chemicals and creates
thousands of gal ons (liters) per day
of contaminated rinse water.
Inefficient operation, therefore,
significantly affects the interrelated
factors of material costs and
pollution control
By January 28, 1
job.shops that d
publicly owned t
must reduce con
984, electroplating
scharge to
•eatment works
lamination in the
rinse water and other process
wastewaters to federally regulated
levels. The disposal of treatment
residuals is gove
hazardous waste
promulgated in tie Resource Con-
servation and Re
covery.Act (RCRA).
Details of the 'wastewater and
solid waste regu
electroplating inc
in an earlier U.S.
Protection Agency (EPA) report.1
Because of rising
ned by the
regulations
ations for the
ustry are provided
Environmental
prices and
changing regulations, it is necessary
to reevaluate water pollution
control techniques and costs and
to examine methods for improving
raw material yields. In many
cases, changing
process can sign
:he manufacturing
ficantly alter
chemical losses a id water flow rates.
These in-plant cranges usually
involve techniques for reducing both
the drag-out removed from process
solutions and the amount of
water used in the rinsing process.
The overall effect is a reduction of:
• Chemical purchases
• Water use (resulting in lower
water and sewer costs)
• Wastewater treatment needs and
disposal costs
Although Federal law does not
require compliance with electro-
plating pretreatment standards until
January 28, 1984, in-plant
changes should be instituted
immediately. In addition to pro-
viding chemical savings and
reducing water use costs, in-plant
changes provide a basis for a
pollution control system design.
Waste treatment equipment needs—
whether wastewater concentrating
techniques, such as ion exchange,
or conventional end-of-pipe
treatment systems—often will be
reduced significantly by in-plant
changes. In some cases, electro-
platers will be able to reduce
flows to less than 10,000 gal/d
(38,000 L/d), thereby reducing their
pollution control requirements
as prescribed in the EPA pretreat-
ment standards.2 This report
describes the first steps a plater
should take to comply with either
wastewater or RCRA regulations.
The EPA publication. Economics of
Wastewater Treatment Alternatives
for the Electroplating Industry,*
addresses the costs of meeting water
pollution control requirements.
That report provides information on
reducing the costs of wastewater
treatment through in-plant modifica-
tions to the plating baths and
rinse systems. This summary report
expands that information through
additional discussion of waste
generation phenomena and abate-
ment measures involving in-plant
changes.
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Pollution Sources and
Characteristics
Contaminants in the effluent from
electroplating shops originate
in several ways. The most obvious
source of pollution is the drag-out of
various processing baths into sub-
sequent rinses. The amount of
pollutants contributed by drag-out is
a function of such factors as the
design of the racks or barrels carrying
the parts to be plated, the shape
of the parts, plating procedures, and
several interrelated parameters
of the process solution, including
concentration of toxic chemicals,
temperature, viscosity, and
surface tension.
With conventional rinsing tech-
niques, drag-out losses from
process solutions result in large
volumes of rinse water contaminated
with relatively dilute concentra-
tions of cyanide and metals. Rinse
waters that follow plating solutions
typically contain 1 5 mg/L to
100 mg/L of the metal being plated.
Most job shops operate several
plating lines that contain different
types of cleaning and electroplating
baths, such as zinc, copper, nickel,
cadmium, and chromium. The
combined rinse waters dilute the
concentrations of individual metals,
usually to less than 50 mg/L. The
results of a recent survey of effluent
from 22 electroplaters in the
Cleveland area are presented in
Table 1.
Another source of effluent con-
tamination is discarded process
solutions. These solutions are pri-
marily spent'alkaline and acid
cleaners used for surface preparation
of parts before electroplating.
The solutions are not usually made
up of metals; however, there are a
few cleaners that contain cyanide.
Plating baths and other process
solutions containing high metal
concentrations, such as chromate
solutions, are rarely discarded.
The amount of pollutants con-
tributed to the total pollution load by
discarded process solutions varies
considerably among plating shops. It
is not uncommon to find cyanide
and heavy metals in concentra-
tions of several thousand milligrams
per liter in spent solutions. This
contamination is caused by drag-in
from previous process cycles and
attack of the basic metals by the
chemicals in the cleaning solutions.
Table 2 presents an analysis of
some typical process solutions.
Accidental spills, leaks, and drips of
process solutions also can con-
tribute significantly to effluent
contamination. The plating room
usually is laid out so that the
entire area drains on the floor,
whichJs only an extension of the
sewer system leaving the facility. Al-
though it is not common for a
tank to spring a leak that would allow
the entire solution to leak away un-
detected, a slow leak amounting
to a solution loss of 10 to 20 gal/d
(38 to 76 17d) could go undetected
for months in many shops. Also,
it is not unusual to compensate
for evaporation losses in a process
tank by adding water to a process
solution with an unattended hose that
causes overflow of the solution
to the floor drains.
In some shops, the dripping of plated
parts is a significant source of
pollution. Process solution tanks
and rinse tanks often are separated
by a distance of several feet (meters)
or more. Carrying the racks of parts
between tanks will cause plating
solution or drag-out to drip on
the floor and enter the drain system.
Other sources of contaminants from
electroplating shops exist; how-
ever, they are not as universally
present as the preceding waste
sources. Additional pollution sources
include sludges from the bottoms of
plating baths generated during
chemical purification, backwash
from plating tank filter systems, and
stripping solutions.
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Table 1.
Effluent Characteristics of 22 Cleveland Electroplating
shops
Pollutant
Effluent concentration (mg/L)
Minimum Maximum Average
Cyanide, total
Nickel
<0.1
0.1
<0.1
0 1
04
<0 1
<0 1
95.9
47.2
52.2
178 0
101 4
3 0
24 3
14.4
4.7
5.7
20.2
19 3
04
43
The percentage contributed by each
pollution source to the pollutant
concentration of the final effluent
can vary substantially among
electroplating shops. For shops
whose primary process is the chrome
plate (copper-nickel-chromium),
drag-out usually will be the major
cause of metal loss. At facilities
that engage in large nickel plating
operations, more nickel is lost from
the operation of the chemical
purification filters and through the
sludge bottom dumps after purifica-
tion than througn normal drag-
out. The main contribution to
effluent metal concentration in zinc
or cadmium plating is often the
zinc or cadmium that is either
stripped off the danglers or rack tips
in the acid dip step of the clean-
ing cycle or removed from the
work in dichromating.4
Although some
higher contribut
from other sourqes,
every case, the
Table 2.
Analysis of Typical Spent Process Solutions That Are Dumped Periodically
shops may have a
on of pollutants
in almost
most significant pol-
lution problem is drag-out and
the resultant contaminated rinse
water. The size and cost of pollution
control equipment depend pri-
marily on wastewater volumetric
flow rate. Because the volumes of
rinse water are overwhelmingly
larger than the volumes of all other
waste sources, it follows that
contaminated rinse water is the
major source of pollution. A recent
survey in Cleveland showed that
the average rate of rinse water
discharged from 22 electroplating
shops was 18,500 gal/d (70,000 L7d),
whereas spent process solution
accounted for only 60 gal/d (230 L/d).
Electroplating shops should con-
centrate on drag-out and rinse
waters during the planning stages of
pollution control. The emphasis
of this report, therefore, is on the re-
duction of drag-out and rinse
water use. To provide a comprehen-
sive approach to in-plant control,
however, other sources of contami-
nation, such as accidents and
discarding of process solutions, will
be addressed.
Pollutant or parameter
Volume {gal} 3
Cyanide total (mg/L)
Nickel (mg/L)
Lead (mg/L)
Zinc (mg/L)
Sample solution
Alkaline cleaner
1
>5.0
2.5
0.2
W.O
>8.1
6.9
4.4
1.2
2
340.0
85.5
2.6
(a)
19.4
0.9
0
74.0
3
338.0
2.8
0.4
0.1
10.9
0.3
0.7
162.0
lectrocleane
390.0
1.3
0.8
36.5
1.9
5.2
1.9
10.5
1
65.0
(a)
6.4
39.2
12.1
128.0
11.6
365.0
Acid dip
2
50.0
n
0.1
10.8
0.1
0.6
0.1
5,240.0
3
165.0
(=)
1 ,990.0
n
n
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Drag-Out Minimization
For the typical electroplating job
shop, the drag-out of process
solutions and the subsequent con-
tamination of rinse waters are the
major pollution control problems. This
section explains the basic prin-
ciples of drag-out theory and
explores the function and applica-
bility of the various drag-out
minimization techniques in use
today.
Principles
Electroplaters are well aware that
drag-out varies considerably among
the various parts plated at their
shops. For example, the volume of
drag-out in rack plating differs
visibly from that in barrel plating.
When a barrel emerges from a
process tank, it usually carries with
it over 1 0 times more solution
than does a typical rack. In addition
to the obvious effects of rack
and barrel design and shape of parts,
there are more subtle factors that
affect the volume of drag-out.
These parameters include viscosity
and chemical concentration,
surface tension, and temperature.
The viscosity of a plating process
solution can be described as its
resistance to motion or removal by
another liquid (in this case, rinse
water) because of the attractive
forces of the molecules of the solution.
The difference between high
and low viscosity can be demon-
strated with honey and water. A
much thicker film will form on
a knife dipped in honey than on one
dipped in water. Honey, therefore,
has the higher viscosity because
of its adhesive quality. The same effect
can be observed with plating
solutions. If two identical surfaces
are immersed in separate chromium
baths with concentrations of
53 oz/gal (397 g/L) and 33 oz/gal
(247 g/L), respectively, the lower
concentration bath will pror
duce 73 percent less volume of
drag-out.5
Surface tension is another physical
phenomenon that has a signifi-
cant effect in the plating shop. Accord-
ing to kinetic theory, molecules
of a liquid attract each other. At the
surface of a solution, such as a
plating bath, the molecules are sub-
jected to an unbalanced force
because the molecules in the gaseous
phase are so widely dispersed.
As a result, the molecules at the sur-
face are under tension and form
a thin, skinlike layer that adjusts to
create a minimum surface area.
The property of surface tension causes
liquid droplets to assume a spheri-
cal shape, water to rise in a capillary
tube, and liquids, such as water,
to move through porous materials
that they are capable; of
wetting.6
In the plating process, the volume of
solution that clings to a workpiece
surface depends largely on the sur-
face tension. The force of surface
tension appears to be most
effective at the bottom edge of the
part as it passes through and
leaves the process solution. This
force and the resultant volume
of drag-out appear to be greatly
affected by the orientation of the part
relative to the surface of the liquid.
Positioning parts so that only
a small surface area makes contact
with the liquid surface at parting
results in less volume of drag-out.7
The third major factor that influences
drag-out volume is the tempera-
ture of the process solution.
Temperature is interrelated with
viscosity and surface tension.
As the temperature of a plating solu-
tion is increased, its viscosity,
surface tension, and, therefore, drag-
out volume are reduced. As a
possible exception, when a part is
withdrawn too rapidly from a hot
process solution, evaporation may
concentrate the film and impede
drainage.8 This problem, however,
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can be overcome by reducing
withdrawal time and using a fog spray
rinse on the parts as they emerge
from the plating solution.
Techniques
Many devices and procedures can
be used successfully to reduce
drag-out. These techniques usually
are employed to alter viscosity,
chemical concentration, surface ten-
sion, velocity of withdrawal,
and temperature. Also used are
drag-out tanks for capturing
lost plating solution and returning
it to the bath.
Most drag-out reduction methods
are inexpensive to implement
and are repaid promptly through
savings in plating chemicals.
An additional savings many times the
cost of the changes will be
realized once a pollution control
system is installed. The reduced drag-
out will decrease the need for
treatment chemicals and, subse-
quently, the volume of sludge
produced. By reducing sludge
volume, many platers may be able
to meet the RCRA definition of a
small generator and thereby
take advantage of reduced regula-
tory requirements.
For some process solutions, return
of drag-out may be impractical.
For example, in the case of process-
ing baths that become steadily
depleted in use, the return of drag-
out would simply increase the
frequency of dumping.
Controlling Plating Solutions. As
a rule, as the chemical content
of a solution is increased, its viscosity
increases. The result is a thickening
of the film that clings to the work
withdrawn from the process solution.
Increased viscosity contributes
not only to a larger volume of drag-
out but also to a higher chemical
concentration of drag-out. The con-
sequent need for more rinse water
creates additional pollution control
problems.
Often plating bat is can be operated
at significantly lower concentra-
tions than those recommended by
chemical manufacturers. Research
on chromium plating9 indicates
that chromium deposits from solu-
tions containing jchromic acid
(CrO3) at only 3.3 to 6.6 oz/gal
(25 to 50 g/L) are acceptable. In the
experiments conducted, the
operating conditions were almost the
same as those for the standard
33.4-oz/gal (25(ig/L) bath. The
bright range was| narrower, how-
ever, with lower OrO3 concentrations.
Chromate films, which appeared
on the surface of deposits from the
dilute baths, cou d be removed by
dipping for a sho'rt period in the
plating solution.
Chemical manufacturers and sup-
pliers have become concerned
with the pollution control problems
of their clients—the platers. As a
result, research
nd develop-
ment efforts by tie chemical manu-
facturers have produced more
environmentally sound plating
solutions.
Cyanide plating baths have been a
major target of the chemical
manufacturers. The conventional
cyanide bath has been preferred for
many plating applications, such as
zinc and cadmium. Because of
stricter effluent limitations on
cyanide, however, an alternative to
high-concentration cyanide baths
is being sought. The chemical
manufacturers have experimented
and, in some cases, have developed
alkaline noncyan de or low-
cyanide baths and acid baths includ-
ing neutral chlorine solutions.10
Platers should in
evaluate the varic
disadvantages of
cal solutions. As
of control or the
/estigate and
us advantages and
the new chemi-
a rule, the acid
bath substitutes c o not offer the ease
overall satisfac-
tory operating conditions and deposit
quality that are available from the
cyanide bath. Fo
brass, and precious metals.
the cyanide plati
ig bath continues
to be the most cpmmonly used
solution.
zinc, cadmium.
Most of the substitute solutions also
are limited in application. For
instance, the acid copper bath, which
is not only widely accepted but
sometimes preferred over the cyanide
copper bath, cannot be used for
direct application to steel and zinc
die castings. A cyanide copper
strike is essential on zinc die
.castings, and either a cyanide
copper or a nickel strike is necessary
on steel before it enters the acid
copper bath. If these substitute
baths are applicable to a plater's
manufacturing conditions, however,
they may be a major factor in
the pollution control strategy.
For years wetting agents have been
used in process solutions to aid
in the plating process. These sub-
stances are used, for instance, in
bright-nickel plating to pro-
mote disengagement of hydrogen
bubbles at the cathode. Their use has
also found recent popularity as
an aid to drag-out reduction. A
wetting agent is a substance, usually
a surfactant, that reduces the
surface tension of a liquid, causing it
to spread more readily on a solid
surface. Atypical plating bath solution
has a surface tension close to that
of pure water at room temperature,
about 0.0050 ibf/ft (73 dyn/cm).
The addition of very small amounts
of surfactants can reduce surface
tension considerably—to as
little as 0.001 7 to 0.0024 Ibf/ft
(25 to 35 dyn/cm).10
Kushner5 estimates that the use of
wetting agents will reduce drag-out
loss by as much as 50 percent. He
recommends the use of nonionic
wetting agents that are not harmed
by electrolysis in the plating bath.
The safest maximum amount of
wetting agent should be used in the
bath. A check of the surface ten-
sion of the solution will determine
whether sufficient wetting agent has
been added. A stalagmometer or a
DelMuoy Tensimeter can be used for
this purpose.
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Kushner further suggests keeping
the concentration of all dissolved salts
at the minimum needed for proper
operation. To follow this recom-
mendation, the plater should not
permit substances to build up in the
plating bath, if it is possible to
control and maintain them at the
proper level. For example, cyanide
baths are permitted to build up
very high carbonate concentrations
even though the concentration level
could be controlled by treatment.
Such a buildup could increase drag-
out by as much as 50 percent5
Positioning on Rack. The metal
finisher's primary consideration in
the positioning of workpieces
on a rack is proper exposure of the
parts to the anodes for optimal
coverage and uniform thickness
of the electrodeposit. Drainage and
rinsability are important consid-
erations in racking. Damage to
the workpiece surface can be caused
by insufficient or inefficient rinsing,
and succeeding process solutions
can be contaminated by drag-in
of unremoved chemicals from the
previous solution.
Several rules apply to the position of
work on plating racks for drag-
out minimization. The basic principle,
however, is that every object can
be positioned in at least one way that
will produce the minimum of drag-
out This position could be deter-
mined by experiment, but unless a ,
significant number of similar items are
to be plated, it may be advisable
to follow the suggestions of Kushner5
and Wallace:7
• Tilt all solid objects with plane or
single-curved surfaces so that
drainage is consolidated, that is,
twist or turn the part so that
the clinging fluid will flow
together and off the part by the
quickest route.
• Rack all parts so that they are
extended more in area than
in depth; this will decrease the
average depth to which the parts
are lowered into a solution and, as
proven mathematically, will de-
crease the film thickness of the
drag-out
• If possible, avoid racking parts
directly over one another to pre-
vent lengthening the drainage
path of the plating solution.
• Avoid tablelike surfaces by
tipping the part, but not at the
expense of forming solution
"pockets."
• Orient parts so that only a
small surface area comes in con-
tact with the liquid surface as
it leaves the plating solution.
Workpiece Withdrawal. The velocity
at which work is withdrawn from
the process tank has a marked
effect on drag-out volume. The faster
an item is pulled out of the tank,
the thicker the drag-out layer
will be. The effect is so dramatic
that Kushner5 suggests that most of
the time allowed for withdrawing
and draining the item should be used
for withdrawal.
The velocity of withdrawal of
work from the process tank usually
can be adjusted with automated
equipment. If the metal finishing
cycle is operated by hand, however,
the withdrawal velocity is less
controllable. The best control method
is to place a bar or rail above the
process tank where the rack may be
suspended for drainage while
its predecessor is removed from the
rail and transported to the next
phase of the finishing cycle.
The withdrawal motion also affects
drag-out volume. When a rack is
jerked from a process solution,
surface tension forces do not have a
chance to operate and a much
larger volume of liquid will cling to
the surface. An automatic machine
that performs smooth, gradual
withdrawal usually will drag out less
solution per item racked than will
manually operated equipment.
Accurate predictions of the drag-out
volume to be saved by a given
reduction in withdrawal speed or by a
smooth withdrawal motion are not
possible. A savings may be expected,
but the degree will be determined
by the specific application.
Draining time over the tank may be
limited by the tendency of the
plated object to spot when the plating
solution dries on the surface. A
fog spray that uses water from the
first rinse is very effective in keeping
the surface from drying, accelerating
the drainage process, and maxi-
mizing the time available for
draining.
When considering the purchase of
new equipment, close attention
should be given to withdrawal
and drainage times. These factors
are especially important when
purchasing barrel plating equip-
ment. Slow barrel rotation during
withdrawal has reduced drag-out
volumes by as much as 50 percent.
Machines may be automated
readily to accommodate this type of
rotation at the time of design.8
Maintenance and Design of Racks
and Barrels. As an industry average,
maintenance of racks, fixtures,
and rack coatings has been poor.
Transport of chemicals inside
loose-rack coatings from one
process to another is!not uncommon.
Chromium-bearing solutions, for
example, appear in plant effluent in
spite of treatment systems designed
to handle the normal chromium
discharge sources. These solutions
have been traced to:rinse tanks and
process solutions that are located
some distance from the chromium
discharge points. The chromium-
bearing solutions reach these remote
areas by way of loose rack coat-
ings. Increased attention to rack
maintenance not only will eliminate
this potential hazard but also will
contribute to a welcome reduction
in the number of workpieces
rejected because of poor contact.
Rack stripping plays an important
role in rack maintenance. The plater,
therefore, should organize rack
stripping as a separate operating
line. A separate rack strip line has a
number of practical advantages.
-------
It prevents the introduction of
possible contaminants to the plating
line (for example, chrome strip-
ping in the soak cleaners and electro-
cleaners). A separate rack strip
line also eliminates uncontrolled
spreading of solutions over the plant
floor and allows for more regular,
frequent, and efficient stripping.
This separate rack strip line should
be incorporated into the racking
operation. A racking workflow for all
plating should be organized in the
following cycle:
Rack off machine
Rack unloading
Rack stripping
Rack loading
Rack onto machine
The rack strip line should employ
multiple counterflow rinses and drip
tanks for maximum discharge
control.
When new racks and barrels are
purchased, the shape of the rack and
the coating material should be
examined closely. The shape should
not hamper the drainage of plating
solution, and the rack coating
must be a nonwetting material. The
size and shape of the holes on
the barrels also need to be considered
because they affect the rate of
drainage. Increasing the drainage
area with larger holes, when feasible,
can speed drainage and reduce
drag-out.
Simple Drag-Out Recovery
Commercially available equipment
for the recovery of plating bath
chemicals includes types that apply
such principles as ion exchange,
reverse osmosis, electrodialysis, and
evaporation. These devices usually
are applied to a single plating
bath where they concentrate the salts
in the rinse water, return them to
the plating bath, and recycle the
purified water to rinse tanks.
Before determining the costs and
benefits of recovery equipment, the
plater should consider several
Workpiece
Drip bar
Concentrated
solution
Figure 1.
Simple Drag-Oui
Recovery Devices
simple methods of drag-out recovery
that require muc h less capital
to implement. After using these
methods and establishing new drag-
out conditions, 'tie plater should
consider the applicability of additional
recovery through commercially
available units. A discussion of four
simple drag-out
follows.
recovery methods
Drain Board. A drain board is the
simplest method of drag-out
recovery. It can capture drips of
plating solution as racks and barrels
are transferred between tanks
(Figure 1). Not cnly do drain boards
save chemicals and reduce rinse
water requirements, they also prevent
unnecessary floor wetting.
The drain surface can be plastic
or metal. For acid solutions, the best
materials are vinyl chloride, poly-
propylene, polyethylene, and
Teflon®-lined steel. Stainless steel
should be used for hot alkaline
solutions.5 The drain surface
should be positic ned at an angle that
allows the platirg solution to
return to the bath.
Drip Tank. A drip tank is an ordinary
rinse tank that, instead of being
filled with water, simply collects the
drips from racked parts and barrels
after plating and before rinsing. The
drip tank is useful with work that
involves continuous dripping over a
period of time. Barrel plating,
therefore, is a better candidate than
rack plating for drip tanks. With
barrel plating, the barrel should be
rotated while it is suspended
over the drip tank to ensure maximum
drainage. When a sizable volume
of solution has been collected
in the drip tank, it can be returned to
the plating bath.
Using a drip tank tends to restrict the
potential use of a rinse tank. As
will be discussed, an additional rinse
tank used as a drag-out tank or in a
series arrangement may be more
beneficial. The determining factors
are the volume of drag-out and
the evaporation in the plating bath.
Fog Rinsing. Fog rinsing is used at
exit stations of process tanks.
A fine fog is sprayed on the work,
diluting the drag-out film and causing
a runback into the process solution.
Fog rinsing is applied when
process operating temperatures.
-------
high enough to produce a high
evaporation rate, allow replacement
water to be added to the process
in this manner. Fog rinsing prevents
dry-on patterns by cooling the
workpieces, but it may preclude the
use of a drag-out tank as a recovery
option. Forfog rinsing to be effective,
work must be withdrawn from
the process tank at a slow rate.
Drag-Out Tank. The drag-out tank
(Figure 2} is a rinse tank that
initially is filled with pure water. As
the plating line is operated, the
drag-out rinse tank remains stag-
nant; the salt concentration increases
as more work passes through the
rinse tank. Air agitation must be
used to aid the rinsing process
because there is no waterflow within
the tank to cause turbulence. The
presence of a wetting agent is
helpful.5
After a period of operation, the
diluted plating salts in the drag-out
tank can be used to replenish
the losses to the plating bath. If suffi-
cient evaporation has taken place,
a portion of the drag-out tanksolution
can be added directly to the plating
bath. Evaporation usually will
be sufficient with baths, such as
nickel, that are operated at elevated
temperatures. Low-temperature
baths have minimum surface
evaporation and their temperature
cannot be increased without
degrading heat-sensitive additives.
Recently, new additives, which
are not as readily degraded by heat,
have been developed for many
Evaporation
Workpiece
Plating
bath
Figure 2.
Recovery With a Drag-Out Tank
of these plating baths. These
additives might make operation of the
plating bath possible at higher
temperatures, facilitating drag-out
recovery by recycle techniques.
Usually the value of the recovered
chemicals is much greater than
the increased energy cost associated
with operating the bath at a
higher temperature.
As a rule, the use of a drag-out tank
will reduce chemical losses by
50 percent or more. The efficiency of
the drag-out tank arrangement
can be increased significantly by
adding a second drag-out tank. Use
of a two-stage drag-out system
usually reduces drag-out losses by
70 percent or more.
The applicability and benefits of drag-
out tanks are discussed in more
detail in the next section.
8
-------
Rinsing
The major pollution control problem
for electroplaters is process solu-
tion on workplaces being
dragged out and subsequently
rinsed with water. Many electroplat-
ing shops still employ single,
flow-through rinse tanks to remove
the clinging dissolved salts and
solids from workpieces. This method
of rinsing is extremely ineffi-
cient and, for a rcypical plating shop,
results in the generation of thou-
sands of gallons (liters) per
day of rinse wa
:er contaminated
with dilute concentrations of cyanide
and metals.
The enforcemert of pollution
control standards and the rising
costs of water E nd sewer use are
disrupting the conventional rinsing
practices of the
Traditional rinsii
are being replaced by more efficient
methods, such as parallel and
arrangements and
plating industry.
ig techniques
series rinse tan
drag-out rinses.
water use and tTe amount of pollu-
tants to be trea-
the next step in
A film of proces
is picked up in
that reduce
ed or discharged.
Principles
The purpose of
the surface of the workpiece for
•insing is to prepare
the plating process.
s solution that
he previous plating
step clings to the workpiece. Rinsing
must remove enough of this film to
ensure that the solution in the
next process tank will be effective
and remain uncontaminated.
To meet this objective,
must use a rinsing
includes:5
the plater
strategy that
Turbulent motion between work-
piece and water
Adequate period of contact
between wor
Presence of s
cpiece and water
ufficient water
during contact to reduce the con-
centration of
the salts that are
washed off tf e surface
These three principles apply to all
rinsing operations, including
those Using flow-through or still
rinse tanks.
Turbulence. Agitation is needed to
implement the first principle.
Agitation can be in the form of flow-
ing water, such as in conventional,
single, flow-through rinse tanks. This
form is inefficient, however,
because a very high flow velocity
is necessary to achieve the required
turbulence when water flows
into a rectangular tank.
Direct water flow can be used
efficiently with spray, fog, and flood
rinsing. With spray rinsing, the
workpiece is exposed to high-
velocity water jets. Spray rinsing
uses from one-eighth to one-fourth
the amount of water that would
be used for equivalent dip rinsing,5
but this method has limited
application because it is not effective
with recessed and hidden surfaces.
In both spray rinsing and fog rinsing,
water is applied to the workpiece
from nozzles. With fog rinsing,
however, the water is so highly
atomized that it approaches the con-
sistency of vapor. The fog rinsing
method uses less water than
the regular spray and is used most
often directly over the plating
bath to remove a major portion of the
drag-out before the workpiece
goes to the rinse tank.
With flood rinsing, the workpiece is
rinsed under a faucet that is con-
nected to an air entrainer or aspirator.
The air bubbles improve the effec-
tiveness of the water movement by
increasing the agitation and
displacing some of the plating solu-
tion from the workpiece surface.
The flood rinse usually is operated by
pressure on a foot treadle.
-------
Agitation also can be achieved by
moving the workpiece in the water.
This method is used on manually
operated hand rack lines, and
its effectiveness depends
on the conscientiousness of the
operator. It is possible to move the
workpiece mechanically, but,
in most instances, the bar would have
to be moved so rapidly that the
pieces would tend to fall off the racks.
The most common and efficient
means of creating adequate
turbulence is to apply forced con-
vection within the rinse tank by
pumping water, by propeller action,
or by blowing air through the
water. The first two methods are
used only for special purposes and
usually are not as efficient as the air
blower for agitating a rinse tank.
Pump rinsing, for example, has
been satisfactorily applied in wire
plating.
Of the forced-convection methods,
air bubbles usually produce the
best rinsing. Air bubbles create
sufficient agitation within the rinse
tank to dislodge the plating solu-
tion from the workpiece. Air usually
is filtered and then blown at the
bottom of the tank through a pipe
distributor. Air also can be forced into
the water by means of an air
entrainer on the water feed line.
Contact Time. The second principle
of a good rinsing strategy is to
allow an adequate period of contact
between the workpiece and the
water. For any particular instance,
this time will depend on the effective-
ness of the turbulence in the rinse
tanks. With good agitation and
a wetting agent, 5 s may be long
enough in the rinse water; if
there is little agitation and the
geometry of the work hinders forced
convection, even 10 min may not
be enough. Usually, however, when
good agitation is present, a con-
tact period of 10-15 s will be suffi-
cient. If the agitation is only fair,
30-60 s usually is sufficient.5
Rinse Water Volume. The final
principle of good rinsing is the pres-
ence of sufficient water during
the contact period for proper
reduction of the concentration of salts
that are washed off the surface.
Because water volume is the main
contributor to waste treatment costs,
this principle must be studied
closely.
As discussed, the conventional
method of rinsing uses the single,
flow-through rinse tank. A work-
piece covered with a thin film
of process solution enters the rinse
tank and the solution is removed
to an allowable limit before
proceeding to the next process
tank. The volume of rinse water nec-
essary to complete this process
depends mainly on the agitation
within the tank, the period of contact
and the maximum allowable
concentration of process solution on
the workpiece.
The maximum allowable concentra-
tion becomes a very important
parameter when the other two param-
eters are satisfied. In fact, maxi-
mum allowable concentration is the
governing factor with respect
to water use. To understand
the importance of this parameter, it
will be helpful to begin the dis-
cussion of rinsing equations.
Equations
To determine the proper water flow
and to evaluate the advantages of
rinsing techniques (such as
parallel, series, and still tanks),
plating managers can use two equa-
tions or their equivalent nomo-
graphs.5 The relationship between
the concentration of salts in the
plating bath and the allowable con-
centration within the rinse is
referred to as the rinsing criterion, R.
Under conditions of complete
mixing, which are closely approached
when turbulence is achieved,
R can be determined by:
R = Cp/Cn (1)
where
Cp = concentration of salts in
process solution
Cn = allowable concentration in
rinse
When the volume of drag-out
entering the rinse is considered.
Equation 1 can be expanded to
calculate the required rinse rate:
Q=0(Cp/Cn) (2)
where
Q= rinse tank flow rate
9 = drag-out rate
The following simple' example
illustrates the use of Equation 2.
Sample Problem. A Watts nickel
plating solution contains a nickel
concentration of 11.3 oz/gal
(84.6 g/L). The drag-out rate is
0.05 gal (0.19 L) per rack, and the
production rate is 1 5 racks per hour.
What flow is necessary to main-
tain rinse tank nickel concentrations
of 50 mg/L and 25 mg/L?
Solution. First convert 11.3 oz/gal
to milligrams per liter using the
multiplication factor of 7,489:
11.3 oz/gal X 7,489 = 84,626 mg/L
Calculate the drag-out rate in
gallons per minute:
0 = 0.05(15/60)
= 0.013 gal/min
Then, apply Equation 2 using an
allowable rinse tank concentration
of 50 mg/L:
Q = 0.013(84,626/50)
= 22.0 gal/min
Using an allowable limit of 25 mg/L,
the required flow would be exactly
twice as much as for a 50-mg/L
concentration:
0=0.013(84,626/25)
= 44.0 gal/min
10
-------
Techniques
The most effective means of reducing
water use and waste treatment
costs is to alter rinsing techniques.
Changes can range from simple
piping alterations for recycling rinse
water to more complex changes,
such as installation of two or
three additional rinse tanks that are
arranged to combine the advantages
of series and recovery rinsing.
A discussion of current rinsing
methods follows. The discussion is
accompanied by examples of
using the rinsing equations to eval-
uate the various rinsing techniques.
Rinse Water Recycling. Use of a
simple method of water conservation
is becoming more widespread.
It involves the reuse of rinse water at
two or more rinse tanks where
the contaminants in the rinse water
after a processing step do not
detract from the rinse water quality
at another station. This method is
applied most often to the rinses
following acid dips and alkaline
cleaners. For example, instead of
using 5 gal/min (19 L/min) of
rinse water in each rinse tank [total
of 10 gal/min (38 L/min)], the rinse
water used following the acid
dip can be reused as rinse water
directly after the alkaline cleaner.
This practice will reduce the water use
for these two tanks by 50 percent
In most cases, contamination does
not appear to be a problem. In
fact the rinsing following the alkaline
cleaner appears to improve. The
diffusion part of the mass transfer
process is accelerated as the
concentration of alkaline material at
the interface between the alkaline
drag-out film and the surrounding
water is reduced by the chemical
reaction there. Also, alkaline
solutions usually are more difficult
to rinse off than acid solutions
because of the higher viscos-
ities, so neutralization aids in this
respect.5
Other recycling arrangements can be
employed where the less contami-
Drip guards tailored from inexpensive plastic pipe and installed over space
between countercurrent rinse tanks
nated overflow from critical or
final rinsing operations is reused
for intermediate rinse steps, such as
acid dips and a kaline cleaning
steps. The rinse water following
a nickel plating bath can be routed to
the rinse tank following the acid
dip. This rinse water, in turn, could
be routed to the alkaline cleaner.
Choosing the ODtimal configuration
requires analysis of the particular
rinse water neeps. Interconnecting
rinsing systemsj might make
operations more complicated, but
the cost advantage justifies the
extra attention required.
Multiple Rinse ranks. The bene-
fits from recycli ig rinse water
are limited bea use that method of
conservation cannot be applied
Methods exist,
to all rinse tank
however, that c
widely and that
dramatic water
an be applied more
result in more
savings. The three
A parallel rinse
most common methods are parallel
and series rinsing and the use
of still rinse tan
-------
Workplace ••—i
I
Drag-out
(a)
Workplace
Water
To
waste
treatment
Air agitation
Figure 3.
Three-Stage Rinse Systems: (a) Parallel and (b) Series With Outboard Arrangement
For two parallel rinse tanks:
_ r(84,626)(0.013)2'|1/2
Q~ L 50 J
= 0.54 gal/min per rinse tank, or
1.08 gal/min total flow
For three parallel rinse tanks:
r(84,626)(0.013)3]1/3
L 50 J
= 0.16 gal/min per rinse tank, or
0.48 gal/min total flow
Using a series, or countercurrent,
rinse tank arrangement, the plater can
achieve even greater water savings
than with the parallel system.
With the series feed (Figure 3b),
water flows into the rinse tank
farthest away from the plating tank
and moves toward the rinse tank
closest to the plating tank either by
gravity or by pumping. The work-
piece is dipped in the least pure water
first and in the cleanest water last.
A conductivity probe can be used
with a series rinse system to ensure
efficient operation. This water-
saving device controls a conduc-
tivity cell, which measures the level
of dissolved solids in the rinse
water and, when the level reaches a
preset minimum, shuts a valve
interrupting the freshvwater feed.
12
-------
When the concentration of dissolved
solids builds up to the maximum
allowable level, the conductivity
probe opens the valve. The probe is
especially valuable with an
irregular or varied work sequence
and probable fluctuations in the
level of dissolved salts in the rinse
system.
The quantity of chemicals entering
the final rinse will be significantly
smaller than that entering a
single-tank rinse system. The amount
of rinse water required for dilution
will be reduced the same degree.
Another equation can be applied to
solve for the required flow with
series rinsing.
Q = [(Cp/Cn)1/n + 1 /n]0 (4)
The use of Equation 4 is illustrated
in the following example. Using
the operating parameters from the
sample problem following Equation
2, determine the water flow rate
necessary to obtain a rinsing
concentration of 50 mg/L with two
series tanks and with three series
tanks.
For two series rinse tanks:
Q = [(84,626/50)1/2 + 1 /2]0.01 3
= 0.54 gal/min
For three series rinse tanks:
Q = [(84,626/50)1/3 + 1 /3] 0.01 3
= 0.16 gal/min
A rinse tank arrangement employing
a drag-out tank (Figure 2) is
another application of multiple
rinse tanks. This arrangement is
almost always associated with the
recovery of drag-out solution; there-
fore, it is only applicable to rinsing
following plating baths where
the bath is of significant value.
The use of drag-out tanks usually
results in less water savings than does
parallel or series rinsing. The
operational procedure used with
drag-out tanks is responsible for this
effect. The rinse water in the drag-
out tank increases in plating
100 r—
— 0.2
— 0.4
§
o
o
~t
o
RECYCLE RINSE RATIO (r)
ar = recycle rinse (gal/h) -=- drag-out (gal/h).
Note.—r (recycle rinse flow) = surface evaporation from bath, n = number of counter-
flow rinse tanks in recovery use. Cr= concentration in final stage of recovery.
Cp = concentration in plating bath.
Figure 4.
Percentage of Drag-Out Recovery With Rinse-and-Recovery System
returned to the
compensate for
salts concentration until a portion is
plating bath to
evaporative losses.
The concentration of salts in the
drag-out tank can reach as high
as 75 percent qf the plating bath
concentration. Consequently, a
significant water flow in the rinse
following the drag-out tank would be
necessary to meet the maximum
allowable concentration.
Figure 4 can be used to determine
flow rates with drag-out recovery.
The percentage recovery of drag-
out is first defined as a function of the
recycle ratio, r,
of recycled rins
which is the volume
3 divided by the
volume of drag-out. The recycle rinse
rate in the recovery rinse tanks
is equal to the evaporation rate.
Evaporation rates can be figured
using Figure 5. Equation 2 can be
used to determine the required
water rates for the final rinse once
the concentration in the final rinse is
known.
Figure 4 is used in the following
example. Using the operating param-
eters from the sample problem
following Equation 2 and a surface
evaporation rate of 3 gal/h (11 L/h),
determine the water flow neces-
sary in the free-flowing rinse to obtain
a rinsing concentration of 50 mg/L
using a single drag-out tank (as in
Figure 2) and with a two-stage
drag-out tank.
13
-------
0.5
0,3 —
0.2 —
0.1 —
g
o
O
£
cc
0.05 —
0.03 —
0.02 —i
0.01
80
100 120 140
BATH TEMPERATURE (°F)
160
180
Note.—Ambient conditions are 75° F, 75% relative humidity. Plating solution is 95% mole
fraction H2O.
Figure 5.
Surface Evaporation Rate From Plating Baths With No Aeration
= 3.8
The recycle ratio would be:
3 gal/h
r~"0.013 gal/minX60 min'
From Figure 4, a one-stage recovery
rinse and recycle system would re-
claim 78 percent; a two-stage system
would reclaim 97 percent. At
these recovery rates, the concentra-
tion ratios are 0.21 and 0.05,
respectively. Because the plating bath
concentration is 84,626 mg/L, the
concentration entering the final
rinse is 0.21 X 84,626 = 1 7,772
mg/L for a single drag-out tank
and 0.05X84,626 =4,231 rng/Lfor
a two-stage system. :
Applying Equation 2, using the
concentration of the last drag-out
tank as Cp, the required rinse rates
would be:
O, =0.013(17,772/50)
= 4.6 gal/min
Q2 = 0.013(4,231/50)
= 1.1 gal/min
A relatively new application of
multiple rinse tanks is the drag-in/
drag-out configuration (Figure 6).
With the drag-in/drag-out system, the
rinse tank preceding the plating
bath (drag-in tank) is connected to
the recovery rinse (drag-out tank)
following the bath; the recovered
drag-out solution is circulated
by a pump. The concentrations of
salts in the drag-in and drag-out
tanks remain about equal. When a
rack or barrel is processed, it drags in
plating solution to the plating
tank, thereby increasing recovery.
The drag-in/drag-out system finds
application with plating baths that
have a low evaporation rate. The re-
cycle ratio, which determines
recovery efficiency, is calculated as
the volume of recycled rinse
plus the volume of drag-out divided
by the volume of drag-out The recycle
ratio, therefore, is greater with
a drag-in/drag-out system than a
common recovery tank. If the
evaporation rate is low, the difference
between the recycle ratios for
common recovery and drag-in/
drag-out systems is significant.
When evaporation ratios are high, the
difference is less.
To illustrate the benefits of a drag-in/
drag-out system, consider adding
a drag-in tank to the recovery
system just discussed. The recycle
ratio would become:
_3 gal/h+0.78 gal/h _
r~ 0.78 gal/h ~~ ~4'8
14
-------
Reoirculate
Workplace •
Recycle
Plating
bath
Rinse
Figure 6.
Drag-In/Drag-Out Recovery Arrangement
From Figure 4, a one-stage recovery
rinse and recycle system would re-
claim 83 percent The increase in
percentage recovery is only 5
percentage points. Considering the
cost of an additional tank and pump,
this change is not likely to be
cost effective.
If the evaporation rate were only
1 gal/h (4 L/h), the recycle ratio for a
recovery rinse system would be:
ing from chromium solution, which
is notoriously difficult to rinse.
By simply making the first rinse after
chromium plate
a stagnant rinse
r =
1 gal/h
0.78 gal/h
= 1.28
For a drag-in/drag-out system, the
recycle ratio would be:
1 gal/h+0.78 gal/h
: 0.78 gal/h
= 2.28
The percentage recovery, in this
case, would increase from 51 percent
to 68 percent by adding a drag-in
tank.
Chemical Rinsing. The technique
of chemical rinsing has been used by
the metal finishing industry for
many years. One of its earliest
applications was to eliminate stain-
containing sodium bisulfite, the
drag-in of hexavalent chromium was
converted to trivalent chromium.
The rinsability cf the workpiece
in the second rinse was improved
considerably by changing the chem-
ical nature of the film on the work-
piece in the sta gnant rinse and by
reducing film concentrations before
attempting to rinse by diffusion.
The same principles are frequently
employed in "neutralizing" dips.
The application of chemical rinsing to
plant effluent treatment, known
in the industry as integrated waste
treatment, has been described by
Lancy11 and Pinner.12 Aside from the
environmental benefits, this type
of chemical rinsijng also prevents the
majority of heavy metal solids
formed in the chemical rinse from
reaching the succeeding water rinses
by removing these materials in an
external settling vessel. Removal
of these solids is accomplished by
flowing the che
to a treatment r
nical rinse solution
sservoir. The over-
flow from the reservoir is pumped
back to the rinse tanks, forming
a complete closed-loop system.
Chemicals are added to the reservoir
to provide a controlled excess of
reagent in the solution. The reservoir
acts as a combined reaction and
settling tank Because of the presence
of a controlled excess of reagents
in the chemical rinse tank, toxic
materials and heavy metals are
removed from the metal finishing
sequence and are prevented from
entering the subsequent water rinse.
At the same time, rinsing is
improved because the diffusion layer,
which is present during conven-
tional water rinsing, is broken down
by the chemical reaction.
Rinsing Recovery Systems. The
information provided on drag-out
and rinsing principles can be formu-
lated into a strategy for simple
recovery systems using multiple
rinse tanks and a minimum of addi-
tional equipment. Examples will
be presented for various plating
applications and considerations, such
15
-------
as space limitations. Before waste
treatment equipment is installed,
the implementation of these systems
will generate substantial savings
in plating chemicals and water.
After waste treatment equipment is
installed, the system can continue to
operate and will provide further
benefits by reducing waste treatment
costs.
The tank arrangement in Figure 2,
which consists of a drag-out tank
followed by a flow-through rinse tank,
is the simplest recovery system.
The drag-out rinse collects a signi-
ficant portion of the process solution
carried on the parts, rack, or
barrel. Periodically, the strong
solution in the drag-out tank
is returned to the plating tank. The
volume returned is limited to
the volume made available in the
process tank by evaporation.
The efficiency of a drag-out tank
recovery system can be improved
significantly by the addition of
a rinse tank. The additional
tank could be used as a second
recovery tank to decrease chemical
losses further, or it could be used
in a series arrangement by connect-
ing it with the overflow rinse
(Figure 3b). The latter change would
provide a water savings but would
not reduce chemical losses.
Various other rinsing configurations
could be developed by adding
tanks. The choice of a best arrange-
ment is difficult because of the trade-
offs involved between further
reducing chemical losses and
further reducing the rinse flow rate.
Obviously, the value of the lost
chemicals is a significant cost.
Chemical losses also result in addi-
tional rinse water and waste treat-
ment chemical requirements and
more sludge.13
Although complex, the evaluation
and selection of a multiple rinse
tank system can be accomplished by
analyzing each rinsing configura-
tion. Such an evaluation involves
using the equations and graphs
presented in this section and corn-
Stationary drain board under rack passing between tanks
paring cost factors, such as water,
sewer, and waste treatment. The
results of the evaluation will enable
the plater to determine whether a
multiple rinse tank arrangement
is beneficial and to identify the
most appropriate configuration. It is
important to find this optimal
point because the space needed for
additional rinse tanks is limited.
The application of the complex
rinsing evaluation will be presented
through the use of examples.
Two widely used electroplating
baths—a concentrated chromium
and a Watts nickel bath—will be con-
sidered. These baths were selected
because they differ with respect to
two important factors affecting the
selection of an optimal rinsing
configuration: operating temperature
and bath concentration. The
concentrated chromium bath normally
is operated at 110° F (43° C) with a
chromium concentration of
200,000 mg/L The Watts nickel
bath normally is operated at a
temperature of 140° F (60° C) with a
nickel concentration of 85,000
mg/L. The higher evaporation rate
and lower plating bath metal concen-
tration make nickel a better
candidate for recovery. A summary
of the operating conditions for the
two baths is presented in Table 3.
The cost evaluation of complex
rinsing systems must'include all
significant operating and investment
costs that are affected by the
inclusion of additional rinse tanks
and either flow or chemical loss
reduction. Some site-specific costs,
such as plating room rearrange-
16
-------
Table 3.
Operating Parameters for Watts Nickel and Concentrated Chrprnjurn.Baths
Bath
Parameter
Wat
Concentration {mg/L} 8
Maximum allowable concentration in final rinse (mg/L)
:s nickel
5,000
140
25
50
Concentrated
chromium
200,000
110
25
35
Table 4.
Cost Variables for Evaluating Rinsing Options
Parameter
Treatment chemicals:
Chromium (Cr):b
$/lb Cr
Nickel (Ni):°
$/lb Ni
Sludge disposal:1*
$/lb Cr
$/lb Ni
Plating chemicals:
$/lb Cr
$/lb Ni
Cost
200
240
041
028
018
0 21
1 48
1 15
4.20
6.35
1 ,080
"Cost is for flanged, open-top, mild steel tank lined with polyvinyl chloride; cost does not in-
clude installation, which is highly site specific.
^Treatment chemicals for chromium: sulfur dioxide, 2 Ib/lb Cr; sulfuric acid, 0.2 lb/1,000 gal
wastewater; sodium hydroxide (NaOH), 1.5 lb/1,000 gal wastewater; NaOH, 2.3 Ib/lb Cr; floc-
culant, 0.1 lb/1,000 gal wastewater. ..-
"Treatment chemicals for nickel: NaOH, 1.0 lb/1,000 gal wastewater; NaOH, 2.0 Ib/lb Ni; floc-
culant, 0.1 lb/1,000 gal wastewater.
d$0.25/gal at 4% solids by weight.
Note.—All costs, except those for treatment chemicals', are in 1981 dc liars. Costs for treat-
ment chemicals, originally in 1979 dollars, were updated to reflect average 1980 prices using the
Monthly Labor Review Producer Price Index for industrial commodities •
SOURCE: U.S. Environmental Protection Agency, Environmental Regul
The Electroplating Industry, EPA 625/10-80-001, Aug. 1 980.
3tions and Technology:
ment, must be considered in the
analysis. Because these costs
will vary from shop to shop, however,
they will not be included in this
analysis. Instead, only those costs
that are common to all shops will be
considered (Table 4).
The actual cost
analysis. For ins
sewer cost ($2/
fo"r individual plants
will differ from 1hose used in the
:ance, the water and
1,000 gal, in 1981
dollars) varies considerably
among municipalities. Platers are
urged to insert the costs that best
reflect their situations, including
any additional costs, before proceed-
ing with their analyses.
For the two examples, 15 different
rinsing configurations were
considered (Figure 7). Configura-
tion 1 is the basic single overflow
rinse. Configurations 2 through 4 are
the three possible options using
two rinse tanks. Configurations 5
through 9 use three rinse tanks, and
Configurations 1 0 through 1 5
employ four rinse tanks.
Each of the 15 configurations can
be analyzed using the methods pre-
sented earlier in this report.
Examples already have been pre-
sented for Configurations 1 through
7. Such arrangements as Con-
figuration 8 are more complex, but
they can be divided into simpler
problems and analyzed using Equa-
tions 2 and 4 and Figure 4. The
following example will illustrate this
method.
A Watts nickel plating solution
contains a nickel concentration of
85,000 mg/L The drag-out rate is
1 gal/h (4 L/h), or 0.01 7 gal/min
(0.063 L/min), and the evaporation
rate (Figure 5) is 2.75 gal/h
(10.41 L/h). Determine the per-
centage of nickel solution that is
saved and the flow required to meet
a maximum allowable nickel con-
centration criterion of 50 mg/L
in the final rinse using Configura-
tion 8.
First, calculate the recycle ratio:
recycle rinse 2.75
r= •
drag-out
1
- = 2.75
Then, using Figure 4, find the per-
centage recovery for a single-
stage drag-out tank, which is 77
percent. At this recovery rate, the
concentration ratio is 0.23. Now
calculate the concentration
entering the final rinse:
0.23 X 85,000 mg/L = 1 9,550 mg/L
17
-------
ONE RINSE TANK
Configuration 1:
Single overflow
TWO RINSE TANKS
Configuration 2: Two-stage
parallel
Configurations: Two-stage
Configuration 4: One drag-
out. one overflow
THREE RINSE TANKS
FOUR RINSE TANKS
Configuration 5: Three-stage parallel
Configuration 10: Four-stage parallel
Configuration 6: Three-stage series
Configuration 11: Four-stage series
Configuration 7: Two drag-out, one overflow Configuration 1 2: Three drag-out, one overflow
Configuration 8: One drag-out, two-stage Configuration 13: Two drag-out, two-stage series
series
Configuration 9: Drag-in/drag-out, one
overflow
Configuration 14: One drag-out, three-stage series
Configuration 15: Drag-in/drag-out, two-stage
series
Note.—Decreasing heights of shading show that metals concentrations decrease.
Figure 7.
Rinsing Configurations
18
-------
Table 5.
Evaluation of Chromium and Nickel Rinsing Systems
1-gal/h drag-out
Configuration Water use
gal/min
1,000 gal/yr
Plating chemicals lost
(lb/yr)a
2-gal/h
Water use
gal/min
1 ,000 gal/yr
drag-out
Plating chemicals lost
(lb/yr)a
Chromium plating
1
2
3
4
5.
6
7
8
9
10
1 1
12. .
13
14
15
97.1
i 2.6
1 .3
46.6
0 9
02
27 2
09
28.6
0.5
0.1
16.5
0.7
0.3
0.7
24,236
649
324
11,638
225
50
6,789
225
7,131
125
25
4,133
175
75
175
6,950
6,950
6,950
3,336
6,950
6,950
1,946
3,336
2,085
6,950
6,950
1,204
1,946
3,336
2,085
188.6
5.0
2.5
127.0
1.5
0.3
102.0
2.1
80.0
1.0
0.1
84.0
1.9
0.6
1.6
47,074
1,248
624
31,699
374
75
25,459
524
19,968
250
256
21 ,080
474
150
412
13,900
13,900
13,900
9,313
13,900
13,900
7,500
9,313
5,838
3,900
13,900
6,151
7,500
9,313
5,838
Nickel plating
1
2
3
4
5
6
7
8
9
10
1 1
12. .. .
13
14
15
. 28 9
14
0 7
6.4
0.6
0.2
2.0
0.4
62
04
0 1
0.9
0.3
0.2
0.3
7,213
349
175
1,587
150
50
500
100
1,555
100
25
225
75
50
75
2,950
2,950
2,950
650
2,950
2,950
177
650
649
2,950
2,950
91
177
650
649
56.6
2.7
1.4
26.9
1.6
0.2
15.7
1.0
17.6
0.7
0.1
8.2
0.7
0.3
0.8
14,127
674
349
6,714
400
50
3,918
250
4,383
176
25
2,071
174
75
195
5,900
5,900
5,900
2,484
.5,900
5,900
1,383
2,484
1,829
5,900
5,900
859
1,383
2,484
1,829
'Pounds of metal (chromium or nickel) per year.
Next, apply Equation 4, using
1 9,550 as Cp:
Q = [(1 9,550/50)1/2 + 1 /2]{1 /60)
= (19.8+0.5)(0.017)
= (20.3}(0.01 7)
= 0.35 gal/min
Using a similar approach of break-
ing down the complex problem
into two lesser problems, the
flow and percentage recovery of
most rinsing systems can be deter-
mined. After fin Jing these param-
eters, a cost analysis can be
performed usinc
and the optimal
can be identifie i.
the data in Table 4,
configuration
To illustrate the use of the cost
analysis, the 15iconfigurations have
been analyzed for a concentrated
chromium bath and a Watts nickel
bath. The operating parameters for
the baths were
Two analyses a
shown in Table 3.
e performed for
each bath. The first assumes a drag-
out rate of 1 ga /h (4 L/h) and the
second assumes a drag-out rate of
2 gal/h (8 L/h). The results are
presented in Tables 5 through 7.
The following conclusions can be
drawn from the cost analysis of
the chromium and nickel rinsing
systems:
• Single overflow rinses require
extremely high flow rates to meet
good rinsing criteria, even at
low drag-out rates.
19
-------
Table 6.
Chromium Rinsing System Costs
_ .. Flow
Con!lg- rate
uratlon (gal/min)
1'.....
2
3
4»
5
6
7»
8
9* ...
10
11
12 .
13 ....
14
15
97.1
2.6
1.3
46.6
0.9
0.2
27.2
0.9
28.6
0.5
0.1
16.5
0.7
0.3
0.7
Water and
sewer at
$2/1,000 gal
48,472
1,298
648
23,276
448
100
13,578
450
1 4,262
250
50
8,250
350
150
350
Additional
rinse tanks at
$240/tank
0
240
240
240
480
480
480
480
480
720
720
720
720
720
720
Cost ($)
Treatment chemicals at
$0.41 /IbCr
1-gal/h
2,850
2,850
2,850
1,368
2,850
2,850
798
1,368
855
2,850
2,850
494
798
1,368
855
$0.28/1,000 gal
wastewater
drag- out
6,786
182
91
3,259
63
14
1,901
63
1,997
35
7
1,155
49
21
49
Sludge at
$1.48/lbCr
10,286
10,286
10,286
4,937
1 0,286
10,286
2,880
4,937
3,086
10,286
10,286
1,782
2,880
4,937
3,086
Plating
chemicals
lost at
$4.20/lb Cr
29,190
29,190
29,190
14,011
29,1 90
29,190
8,173
14,011
8,757
29,1 90
29,1 90
5,057
8,173
14,011
8,757
De-ionized
water
0
0
0
1 ,080
0
0
1,080
1 ,080
1 ,080
0
0
1 ,080
1 ,080
1 ,080
1 ,,080
Total
97,584
44,046
43,305
48,171
43,317
42,920
28,890
22,389
30,517
43,331
43,103
18,538
14,050
22,287
14,897
2-gal/h drag-out
1«
2
3
4'
5 ......
6
7«
8
g«
10
11
12"
13
14
15
188.6
5.0
2.5
127.0
1.5
0.3
102.0
2.1
80.0
1.0
0.1
84.0
1.9
0.6
1.6
94,148
2,496
1,248
63,398
748
150
50,918
1.048
39,937
500
50
42,1 60
948
300
842
0
240
240
240
480
480
480
480
480
720
720
720
720
720
720
5,699
5,699
5,699
3,818
5,699
5,699
3,075
3,818
2,393
5,699
5,699
2,522
3,075
3,818
2,393
13,181
349
175
8,876
105
21
7,129
147
5,591
70
7
5,902
133
42
115
20,572
20,572
20,572
13,783
20,572
20,572
11,100
13,783
8,640
20,572
20,572
9,103
11,100
13,783
8,640
58,380
58,380
58,380
39,115
58,380
58,380
31,500
39,115
24,520
58,380
58,380
25,834
31,500
39,115
24,520
0
0
0
1,080
0
: o
1,080
1 ,080
1 ,'oso
0
0
1,080
1,080
1 ,'oso
1 ,080
191,980
87,736
86,314
130,310
85,984
85,302
105,282
59,471
82,641
85,941
85,428
87,321
48,556
58,858
38,310
"High rinse rate required with this configuration to meet maximum allowable concentration in the final rinse (35 mg/L Cr)'may|preclude its use.
Note.—All costs, except those for treatment chemicals, are in 1981 dollars. Costs for treatment chemicals, originally in 1979 dollars, were updated
to reflact average 1980 prices using the Monthly Labor Review Producer Price Index for industrial commodities.
Multiple rinse tank arrangements
can provide significant cost
savings.
With low recycle ratios (1.37 or
less), drag-out recovery is imprac-
tical because of high water
requirements, unless three or more
drag-out tanks are used (see
upper half of Table 6, Configura-
tion 11) or series rinsing follows
drag-out (see upper half of
Table 6, Configurations 12 and
13).
With recycle ratios of 2.75 or
greater, all additional tanks should
be used as drag-out tanks rather
than in parallel or series
arrangements.
Drag-in/drag-out systems
usually must include series rinsing.
These systems become cost
effective with recycle ratios at or
below 1.25 (see lower half of
Table 6, Configuration 1 5).
Plating baths that operate at lower
temperatures than those described
in the foregoing examples have
lower evaporation rates ancl offer
fewer opportunities for rinse.
recovery. If the bath can be operated
at an elevated temperature, even
though it is not required, the cost of
added energy usually will be more
20
-------
Table 7.
Nickel Rinsing System Costs
„ ... Flow
Config-
. a rate
uration . ., . .
(gal/min)
Water and
sewer at
$2/1,000 gal
Additional
rinse tanks at
$240/tank
Cost ($)
Treatme|nt chemicals at
$0.18/lbNi
$0.21/1,000 gal
wastewater
Sludge at
$1 .1 5/lb Ni
Plating
chemicals
lost at
$6.35/lb Ni
De-ionized
water
Total
1-galAi drag-out
1a
2
3
4
5 .
6
7
8
9
10
11 ....
12 ....
13 ....
14 ....
15 ....
28.9
1.4
0.7
6.4
0.6
0,2
2.0
0.4
6.2
0.4
0.1
0.9
0.3
0.2
0.3
14,426
698
350
3,174
300
100
1,000
200
3,110
200
50
450
150
100
150
0
240
240
240
480
480
480
480
480
720
720
720
720
720
720
531
531
531
117
531
531
32
117
117
531
531
16
32
117
117
2-gai/
r
2
3
4a
5
6
T
8 ....
9a. .
10 ....
11 ....
12 ....
13
14 ....
15
56 6
2 7
1 4
26 9
1.6
02
157
1 0
1 7 6
0 7
0 1
8 2
0 7
03
0 8
28,254
1,294
698
13,428
800
100
7,836
500
8,766
348
50
4,142
348
150
390
0
240
240
240
480
480
480
480
480
720
720
720
720
720
720
1,060
1,060
1,060
447'
1,060
1,060
249
447
329
1,060
1,060
155
249
447
329
1,515
73
37
333
32
11
105
21
327
21
5
47
16
11
16
3,393
3,393
3,393
748
3,393
3,393
204
748
748
3,393
3,393
104
204
748
748
18,733
18,733
18,733
4,127
18,733
18,733
1,123
4,127
4,127
18,733
18,733
578
1,123
4,127
4,127
0
0
0
1,080
0
0
1,080
1,080
1,080
0
0
1,080
1,080
1,080
1,080
38,598
23,668
23,284
9,819
23,469
23,248
4,024
6,773
9,989
23,598
23,432
2,995
3,325
6,903
6,958
i drag-out
2,967
142
73
1,410
84
11
823
53
920
37
5
435
• 37
16
41
6,785
6,785
6,785
2,856
6,785
6,785
1,590
2,856
2,1 04
6,785
6,785
987
1,590
2,856
2,104
37,465
37,465
37,465
15,773
37,465
37,465
8,782
15,773
11,614
37,465
37,465
5,455
8,782
15,773
11,614
0
0
0
1,080
0
0
1,080
1,080
1,080
0
0
1,080
1,080
1,080
1,080
76,531
46,986
46,321
35,234
46,674
45,901
20,840
21,189
25,293
46,41 5
46,085
12,974
12,806
21 ,042
16,278
aHigh rinse rate required with this configuration to meet maximum allo n/able concentration in the final rinse (50 mg/L Ni) may preclude its use.
Note.—All costs, except those for treatment chemicals, are in 1981 dc liars. Costs for treatment chemicals, originally in 1979 dollars, were up-
dated to reflect average 1980 prices using the Monthly Labor Review Producer Price Index for industrial commodities.
than offset by the benefits of
recovery. Drag-in/drag-out systems
offer perhaps the best situation for
implementing the techniques
presented here When the recycle
ratio is low. Nevertheless, when
the evaporation
rate is very low, the
most cost-effective solution may be
concentration o1
rinse itself, such as by an evaporator
or by ion exchange, before replace-
ment in the plating tank.14
the drag-out
21
-------
Plant Assessment
Procedures
A plant assessment is the initial step
in a pollution control program. It
involves a thorough analysis of
the operations of a metal finishing
plant that relate to pollutant sources
and water use. The information
generated during a plant assessment
is used in evaluating the applica-
bility of in-plant changes for reducing
chemical loss and water use.
A plant assessment includes the
following steps:
• Inspect the plating room layout
• Review plant operating practices.
• Examine process water use.
• Conduct sampling and laboratory
analysis to characterize waste
streams and to determine drag-
out rates.
• Identify the frequency, volume,
and characteristics of batch
dumps.
A plant assessment can be per-
formed by the plater or by a qualified
engineering consultant. Although
the plater has the advantage of
thoroughly knowing the manufactur-
ing process, the consultant can
frequently provide a fresh and
impartial view of the plant and often
can identify overlooked possi-
bilities. If the plater performs the
assessment, a laboratory can be
hired to analyze the samples. Most
laboratories qualified to perform
the analyses charge between $12
and $20 for each heavy metal
parameter that is analyzed and
from $20 to $40 for cyanide
analysis.3 When the plant assess-
ment is performed by a consultant,
the complete service ranges from
$4,000 to $10,000, depending on
the size and complexity of the shop
and the extent of the survey and
subsequent evaluation.
In nearly every case, the benefits of
a plant assessment will far out-
weigh the amount of time and money
"Costs in this section are given in 1981 dollars,
except for treatment costs, which are in
1980 dollars.
expended. Usually the assessment
will be repaid in less than 1 year
through savings in chemicals and
water.
Inspect Plating Room Layout. The
first step in a plant assessment is
relatively simple; it involves the
preparation of drawings showing
the layout of the plating room(s).
For many platers, this task will have
been performed already.
The drawings should be made to
scale showing the location of all
relevant equipment and tanks. Each
tank should be numbered and
labeled with its contents (for
example. Tank 1, soak cleaner; Tank
2, rinse). Individual plating lines
should be identified, such as zinc
barrel and chrome plate rack. Also,
water feed lines, gutters, sumps,
and sewer lines should be indicated.
On the water lines, all control
valves and flow regulators should
be identified.
Review Plant Operations. All opera-
tions of the plating room that
relate to chemical or water use—
including the plating-sequences for
each plating line—should be
reviewed and documented. Often
sequences will vary on a particular
line because of differences in plating
requirements and specifications.
Each major variation should be
listed.
Estimates of production for each
line should be developed. Production
can be measured either by hours of
operation or by production units,
such as number of square feet
(square meters) plated or number of
parts, racks, or barrels to pass
through a particular plating line or
sequence.
Other, more specific information
that is needed on the plating opera-
tion can be gathered through
observation during manufacture.
If automatic lines are used in plating,
rinsing and draining times should
be measured. If manual hoist or hand
22
-------
lines are used, the efficiency of the
operators in rinsing and draining
should be noted. Also, if the tank
arrangement requires that racks or
barrels be transported between
tanks that are not located next to one
another, the relative amount of drip-
ping onto the floor should be
recorded.
Examine Process Water Use. A
survey of water use is a basic step in
the plant assessment because the
capital costs of water pollution
abatement equipment depend
primarily on the volume of water
used. Wherever water is used,
therefore, accurate measurements
must be taken. To increase the
accuracy of these data, the plater
should start by reviewing past water
bills to determine an expected
water use rate in gallons (liters) per
day and per minute. The plater
should then measure and record the
actual water use at each process
step. Because most of the water at a
plating shop is used in rinsing,
rinse flow rates must be measured.
Then, a comparison of actual flow
versus metered flow (that is, water
bills) can be made. This compari-
son is referred to as a water balance.
Table 8 shows an example of a
water balance in which the plant had
individual water meters installed
on each line. (Whereas many
plants have meters only at the water
source, individual meters should
be considered for their value in
monitoring and controlling water
use.) The actual, or measured, flow
rates are within an acceptable
limit (15 percent or less), indicating
that there are no unforeseen water
losses.
Often, however, measured water use
differs considerably from water
bills. One potential source of variance
is faulty water meters. This
problem can be resolved by request-
ing the local water authority to
certify the water meter. A discrep-
ancy is more likely to result from
Table 8.
Process Water Survey Sample
Plating line
Water use
Gallons per minute:
Measured ....
Difference:
Gallons per 8-h shift
Metered f
Difference:
Barrel
21 5
20 0
1 5
7
10 320
9 600
720
7
Anodizing
100
9 5
0 5
5
4800
4 560
240
5
Zinc rack
automatic
240
22 5
1 5
6 25
1 1 520
10 800
720
6 25
Hoist
23 6
22 0
1 6
6 8
1 1 328
10 560
768
6 8
Still
9 5
8 9
0 6
6 3
4 560
4272
288
6 3
periodic water i se for washing
down floors or simply from hoses
being allowed to run without regard
to waste.
Water uses other than rinsing also
may contribute jto a difference in
measured versus metered flow. For
many plating shops, these non-
rinsing uses of water include fume
scrubbers, water-cooled rectifiers,
heat exchangers, boilers, heating
and cooling coi
and welders.
s, air conditioners.
Water flow rate measurement at
rinse tanks can be performed simply
using one of several methods. First,
a bucket with a predetermined
volume can be set under the over-
flow from a rinse tank, provided
rinse water is discharged through an
accessible vertical pipe. The time it
takes to fill the bucket is measured
and the flow rate calculated. Often
this method cannot be used because
of the location or construction of
the discharge pipe.
An alternative for measuring water
flow rate is to shut off the incoming
water and remove a specific amount
of water from thje rinse tank [5 to
10 gal (19 to 38 L)]. The water is
then turned back on, and the time it
takes the water level in the tank
to return to its overflow height is
measured.
A third method of measurement in-
volves depressing a 5-gal (19-L)
bucket into the flowing rinse so that
the water reaches the top of the
bucket but does not enter it This will
cause 5 gal (19 L) of water to be
depleted from the rinse tank immedi-
ately. Once the overflow from the
rinse tank appears normal, the
empty bucket should be removed and
the time it takes the water to resume
an overflow should be measured.
If the second or third method is
employed, the test should be repeated
several times and the results
averaged. Averaging is recommended
because it is difficult to ascertain
visually the exact time at which
the flow over the overflow dam or
weir reaches a stabilized flow condi-
tion. It is especially important to
repeat the test if the rinse tank has a
high flow rate or if air spargers are
used to create turbulence.
Conduct Sampling and Analysis. The
usual procedure for sampling at a
plating shop is to sample the final
effluent as local pollution control
authorities often do. During the plant
assessment, the final effluent is
sampled for the same reason, that is,
to determine which pollutants are not
in compliance with the regulations.
The effluent, however, is only one of
several points for sampling and
23
-------
analysis during the plant assessment
Samples also should be taken of all
individual rinse tanks, overflows,
batch dumps, and, to some extent,
plating solutions. Plating solutions are
sampled when calculations of
drag-out are needed. These addi-
tional samples will be used to isolate
sources of pollution, to calculate
chemical losses, and to evaluate the
potential benefits of drag-out
reduction and flow minimization
techniques.
Sampling can be performed by taking
a single, or grab, sample of the
effluent or rinse water. A grab sample
gives an instantaneous reading of
the conditions of the water. If
conditions are variable (for instance,
if plating is intermittent, causing
fluctuations in pollutant concen-
trations), the sample is not likely to
be representative. When variability is
significant, it is necessary to com-
posite samples over a period of time
(usually one sample every 15 to
30 rnin over a single shift or a single
day) and to analyze the com-
posite to determine the average
conditions.
Compositing is recommended for
samples taken of final effluent
and individual rinse tanks. Grab sam-
ples will suffice for batch dumps
and plating solutions.
Electroplaters conducting their own
plant assessments will need
sampling containers. Often the con-
tainers can be secured through the
laboratory performing the
analysis. If they are not available
from that source, appropriate con-
tainers can be purchased from an
analytical supply house. Because the
analysis will be limited to pH,
metallics, and cyanide, either
plastic or glass containers can be
used. The plastic containers cannot
be used if trace organics are to
be measured. The appropriate plastic
containers are made of polypropy-
lene or polyethylene and should have
a volume of approximately 0.26
gal (1 L).
If metals are to be analyzed, it will be
necessary to collect at least 0.1 3
gal (0.5 L). Usually this sample
will be enough to analyze for the six
federally regulated metallic pollu-
tants in the common metals electro-
plating subcategory.1 If cyanide
is to be analyzed, an additional 0.1 3
gal (0.5 L) of sample must be taken
and placed in a separate container.
Cyanide is relatively unstable and
must be "fixed" once it reaches
the laboratory.
Samples should be delivered to a
laboratory as quickly as possible to
retain the accuracy of the analy-
sis. Speed is especially critical with
cyanide samples, which should
reach the laboratory within 24 hours.
The sample of the final effluent
should be taken at a point where all
rinse waters and other plating
wastes are combined but before the
introduction of domestic and
other process wastes. Often this
point is not accessible to manual
sampling, so it may be necessary to
rent a compositing device.
The final effluent analysis includes
pH, all metals being plated,
base metals, regulated metals, and
cyanide. For shops plating over
10,000 gal/d (38,000 L/d), the analy-
sis usually includes pH, chromium,
copper, lead, cadmium, zinc,
nickel, iron (a base metal), and
cyanide; the cost of this effluent
analysis usually ranges from $11 5
to $180.
Rinse tanks are sampled to deter-
mine whether a smaller overflow rate
is possible and to isolate the
sources of pollution. Also, sampling
of rinses following soak cleaners
and acid dips may provide data
showing concentration levels of
pollutants below the standards; such
rinses would not need further
treatment.
Because pollutant concentrations
in the rinses fluctuate, a composite
sample is advisable. A composite
sample can be taken by running
plastic tubing from a rinse tank to a
collection container and placing a
clamp on the tubing. By creating
a siphon with the tube and con-
trolling the flow with the clamp, a
sample can be taken over the time
period of a shift without constant
attention. Care must be taken, how-
ever, to avoid positioning the end of
the tube in a "dead" zone of the
rinse tank, such as a corner. If the
tank is not aerated and fully turbulent,
the end of the tube should be
placed in or near the overflow.
A complete analysis usually is not
necessary for rinse samples
because it can be assumed, based on
knowledge of the manufacturing
process, that some metals or
cyanide are not present. By review-
ing the plating operation, a signi-
ficant savings in analytical work
can be achieved. The analysis,
however, should include any metals
and cyanide if they are present
anywhere on the plating line where
the rinse is located, even when the
rinse precedes the plating of a
specific metal. Plating solution often
remains on racks, especially if they
are worn, resulting in contamina-
tion of process solutions, such
as soak cleaners. The concentration
of chromium, for instance, can
build up to such a level in a
soak cleaner that the rinse following
the soak cleaner has a chromium
concentration above 5 mg/L
Drag-out measurements are per-
formed at plating tanks to determine
the amount of metal or cyanide
contributed by these sources. This
information is used to evaluate
the applicability and potential bene-
fits of drag-out minimization
techniques and innovative rinsing
systems.
Because drag-out volume is affected
by the size and shape of the parts
and racks or barrels, measure-
ment of drag-out on a particular line
24
-------
with significant variability of
these factors will have little meaning.
Where variability is extreme, the
plater should refer to typical
drag-out rates found in the literature
instead of producing meaningless
data.
The method of drag-out measure-
ment described in this report
differs slightly from those presented
by Kushner.5 The first step in the
drag-out measurement is to stop the
flow in the rinse tank following
the plating tank and empty its con-
tents. Jf the rinse tank is a still rinse
or drag-out tank, it should be con-
sidered as a plating bath and the
rinse following it should be used.
After filling the rinse tank with
clean water, the water should be
shut off and a sample should be
taken and marked "Time 0." A sample
should also be taken from the
plating tank (or drag-out rinse, if
applicable). The plating line should
then be operated. After three to
five racks have gone through,
a second sample of the rinse should
be taken and the number or racks
and elapsed time recorded. The line
is again operated and several
more racks are plated and rinsed. A
third sample is taken and the
number of racks and elapsed time
are recorded.
The plating solution sample and the
three rinse tank samples should
be analyzed for the plated metal. The
drag-out is then calculated
based on unit production [gallons
(liters) per part or rack] or time
[gallons (liters) per hour] for the two
increments and is averaged.
Identify Batch Dump Parameters.
Process solutions, such as alkali
cleaners and acid dips, become ex-
hausted after a period of use.
These solutions are routinely
dumped to the sewer, usually on a
weekly or monthly schedule.
They often contain significant
concentrations of heavy metals and,
therefore, will require treatment
by January 28, 1 984. Also, these
solutions are by lature either
very basic or acidic and can cause
a significant pH fluctuation in
the total effluent
directly to the sewer. Such
changes in pH often result in non-
compliance with
Attention should
batch dumps con
impact on waste
when discharged
ocal pH standards.
be given to
Derning their future
treatment
chemical requirements and their
current impact otji compliance
with local pH standards. In terms of
analytical parameters, batch
dumps should be analyzed for cyanide
and all metals that have a poten-
tial for accumulating in the solution.
A separate sample should be taken to
determine pH ad
ments.
The adjustment of pH on batch
dumps is perform
an acidic (usually
ustment require-
3d by adding either
sulfuric acid) or
basic (usually caistic soda) solu-
tion until an acceptable pH is
reached. Local regulations
often require that the pH of dis-
charges be in the range of 6.0 to
9.0. The Federal general pre-
treatment regulat
the pH be above
ons15 require that
5.0.
By determining th e relative strengths
of the alkaline cleaners and acid
dips, the plater often can develop a
batch dump schedule that uses
these solutions to neutralize
each other. This practice will provide
a significant savings in pH adjust-
ment chemicals.
Application. The information gathered
during a plant assessment pro-
vides the plating shop with the input
needed to evaluate the opportun-
ities for rinse recovery. These
data are useful ir
in-plant changes
as those designed to reduce total
wastewater flow
10,000 gal/d (38
to less than
,000 L/d).
assessing other
as well, such
Case Study
The following case study is an
example of the procedures for per-
forming a plant assessment
and evaluating optional in-plant
changes. The data are based on an
actual plant's operation; however,
an incomplete data base necessitated
the assumption of certain param-
eters. The plating shop in the
example performs mostly zinc,
cadmium, and tin plating as well as
chromate conversion coatings.
The analysis focuses on the shop's
automatic cyanide zinc barrel line.
Data Gathering. The initial step in
the plant assessment or data
gathering phase is to inspect the
plating room and develop drawings
showing the location of all
relevant equipment and tanks. A
drawing of the cyanide zinc line is
presented in Figure 8.
After sufficient information is
gathered to prepare drawings, the
operating practices of the plant
should be observed. In this
example, only one work sequence is
used. It involves the following
steps:
• Barrels are filled with parts at
the loading area.
• The barrels proceed through each
process and rinse tank (Tanks 1
through 22) in a straight-line order.
The observed time at each
station was 2 min; the draining
time was 15s.
• Finished parts are unloaded at the
end of the line. The observed
production rate was 1 2 barrels per
hour.
• The empty barrels are trans-
ported back to the loading area.
It was observed that once the barrels
were removed from one tank, they
were not allowed to drain fully
before immersion in the subse-
quent tank. Another potential
problem noted was the lack of air
agitation in the rinse tanks
following acid dip and zinc plating.
25
-------
Load barrels
1
Unload barrels
Soak cleaner
o
Hot rinse (22]
Soak cleaner ( 2
City water I
Rinse
(0.1 mg/L)'
6 gal/min
City water ^^^
Wastewater *^^
Series rinse (21,
Series rinse
(25.0 mg/L)a
3 gal/min
Wastewater
9
V
Electrocleaner
0
Chromate (19J
Electrocleaner
©
Chromate (18]
c
Series-rinse
(<0.1 mg/L)a
4 gal/min
City water i
Wastewater
City water
Series rinse
0
c
Series rinse (17
Series rinse (16
Acid bath
©
"Concentration of zinc.
C
Series rinse (15
Series rinse
(300.0 mg/L)a
2 gal/min
Wastewater
Cyanide zinc plating bath
(1.34-gal/h drag-out at 86° F)
City water
Series rinse (12,
rffwTii, i \«_ . M, n.v4%r!i^jM gnegysttgyiT'lg'S cd'j^fp^*" "v% ft"^fgt
Wastewater
Figure 8.
Cyanide Zinc Line Before Plant Assessment
26
-------
r-iJM
-------
moving the final rinse tank following
the acid bath and converting
this rinse and the three-stage series
rinse arrangement following
plating to a two-stage recovery rinse
and two-stage series rinse. Using
Alternative 1, approximately 34
percent of the drag-out could be
recovered. Air spargers also would be
added to the rinse tanks to in-
crease the mixing and improve the
efficiency of rinsing. The addition of
air spargers would lower the water
use rate after zinc plating to
0.3 gal/min (1.1 I/mm). The rinse
system following the acid dip is
unchanged in Alternative 1.
Alternative 2 employs the drag-in/
drag-out rinsing configuration. The
drag-in tank (Tank 12) was
originally the final rinse following
the acid dip. To eliminate the need
for additional water in the acid dip
rinsing system, air spargers are
added to Tanks 10 and 11. Similarly,
air spargers are added to the
rinses in the new zinc plate rinsing
system.
The drag-in/drag-out rinsing con-
figuration provides 77 percent
recovery of plating chemicals. This
relatively high rate is primarily a
result of increasing the recycle ratio
from 0.35 in Alternative 1 to 1.35
in Alternative 2.
Tables 9 and 10 summarize the
water use and cost analysis of the
rinsing options. For this plating line,
the best choice is the drag-in/drag-
out rinsing system. The major
benefits include a savings in
plating chemicals and waste treat-
ment and sludge disposal costs.
Table 9.
Evaluation of Case Study Rinsing Systems
Water use
Rinse system
Plating
chemicals
lost(lb/yr)
gal/min 1,000gal/yr Zinc Cyanide
Current
Alternative 1 .
Alternative 2.
5 1,248 1,280 1,477
3.3 824 845 975
3.3 824 294 340
Table 10.
Evaluation and Cost Comparison of Case Study Rinsing Alternatives
Rinse system
Parameter
Current Alternative 1 Alternative 2
Flow rate (gal/min)
Cost ($):
Waterand sewerat$2/1 ,000 gal wastewater. . . .
Additional rinse tanks at $240/tank
Treatment chemicals at:
$1 .33/lb cyanide
$0.1 8/lb zinc
$0.21/1,000 gal wastewater
Sludge at $1.15/lb zinc
Plating chemicals lost at $1 .89/lb zinc
De-ionized water
Other3
5
2,496
0
1,964
230
262
1,472
2,419
0
0
3.3
1,648
0
1,296
152
173;
972
1,597
1 ,080,
160
3.3
1,648
0
452
53
173
338
556
1,080
280
Total cost.
8,843
7,078
4,580
aCost of retrofitting tanks with air and repiping spargers ($200 per tank depreciated over 5 yr).
Note.—All costs, except those for treatment chemicals, are in 1981 dollars.: Costs for treat-
ment chemicals, originally in 1979 dollars, were updated to reflect average 1 980 prices
using the Monthly Labor Review Producer Price Index for industrial commodities.
Recommended changes to the zinc
barrel line are presented in Figure
1 0. The investment cost of the
changes will be approximately $6,840,
which includes installation of a
de-ionized water unit ($5,400),
retrofitting of tanks with air spargers
and repiping ($1,400), and flow
control devices ($80). Considering
the reduced water and sewer
costs, reduced treatment and dis-
posal costs, and the savings in
plating chemicals, the benefits of
in-plant changes total $7,123/yr.
28
-------
Figure 10.
Recommended Rinsing Arrangement for Cyanide Zinc L
-------
References
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Source Category Pretreat-
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30
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