vvEPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
05/
Technology Transfer
Summary Report
Control Technology
for the
Metal Finishing Industry
Evaporators
JL.
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Technology Transfer
EPA 625/8-79-002
Summary Report
Control Technology
for the
Metal Finishing Industry
Evaporators
June 1979
This report was developed by the
Industrial Environmental Research Laboratory
Cincinnati OH 45268
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Final rinse station
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Overview
Plating chemicals, such as chromic
acid, nickel sulfate, and zinc cyanide,
are among many solutions used
throughout the electroplating industry
to produce decorative or protective
finishes on metal and plastic products.
Cleaning steps, plating baths, and
rinsing are similar in most plating
processes; however, proprietary varia-
tions occur in formulations and
process control parameters. All platers
must provide adequate rinsing after the
plating bath to remove excess plating
chemicals and contaminants that
could cause spotting or staining on the
product. The water from this rinse is
the major pollution control problem.
Actually the problem is twofold:
wastewater pollution control and loss
of valuable plating chemicals.
Pollution control is achieved by
chemical treatment or the elimination
of pollutants from the effluent. To
recover plating chemicals many platers
use evaporators, which not only
concentrate plating chemicals in the
rinse water for reuse in the bath, but
also reduce the amount of pollutants to
be chemically treated. The evaporator
method also can decrease the cost of
plating operations.
The technical and economic
advantages, including order-of-
rnagnitude investment costs,
operating costs, and cost saving
benefits, are illustrated in this re port for
evaporators used in electroplating
processes.
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Cyanide oxidation wastewater treatment module
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Conventional Wastewater
Treatment
Introduction
Four major factors contribute to costs
and selection of conventional
wastewater treatment systems (shown
in Figure 1), including:
Volumetric flow rates: The major
contributor to the volume of
process water containing pollu-
tants is the rinse water that comes
in direct contact with the
workpiece. The size and invest-
ment cost of systems to treat
this water are in direct proportion
to the wastewater flow rate.
Pollutant type: The treatment steps
needed to remove the pollutants
from the wastewater are deter-
mined by the type of pollutants
encountered. Neutralization and
clarification are common to the
treatment systems of most platers,
while separate systems for
chromium reduction and cyanide
oxidation are required only if the
plating operations use these
materials.
Pollutant loading: The costs for
treatment chemicals and the
quantity of solid waste (sludge)
increase in direct proportion to the
mass flow of pollutants in the
wastewater. Reduction of water
flow rate and pollutant loading will
decrease the use of treatment
chemicals, and pollutant loading
and choice of treatment chemicals
will determine the quantity of
solid waste for disposal.
Environmental regulations: Regu-
lations also affect the selection of
wastewater treatment systems as
well as the selection of treat-
ment chemicals.
Recovery systems, such as
evaporators, can reduce the operating
and investment costs for conventional
wastewater treatment processes by
substantially reducing the mass flow
rates of pollutants entering the
wastewater or by eliminating
pollutants that are difficult to treat.
Installing a recovery system often can
ease the burden of meeting
environmental regulations.
Operating costs for evaporators
depend to a great extent on the flow
rate of wastewater {feed to evaporator);
the savings depend on the value and
quantity of the plating chemicals
reclaimed from the wastewater. For
these reasons, this section briefly
describes the methods for reducing
rinse water rates. Also, the techniques
and chemical treatment costs for
conventional wastewater treatment
are presented to define the savings
attributable to recovery systems.
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Acid/alkali wastes
Acid
S02 or
NaHSO
Effluent discharge
CN
Solid waste
disposal
Figure 1.
Simplified Conventional Wastewater Treatment
Pollution Control: Conventional
Method of Wastewater Treatment
System Description. Currently many
electroplaters are meeting effluent
regulations by treating the wastewater
by means of chemicals that react with
the soluble pollutants to produce
insoluble byproducts. As shown in
Figure 1, the conventional wastewater
treatment systems can include
chromium reduction, cyanide oxida-
tion, and neutralization; these steps
are followed by clarification of the
wastewaterto remove the precipitates,
The dilute sludge is usually thickened
before disposal.
Chromium in wastewater is found
either as hexavalent chromium (Cr+6)
or as trivalent chromium (Cr+3}. For
chromium to precipitate out as a
hydroxide, the hexavalent chromium
must be reduced to the trivalent state.
The reduction is achieved by reaction
of either S02 gas or sodium
metabisulfite with hexavalent
chromium. Acid also is added to
control the pH between 2 and 3,
because the reaction proceeds rapidly
under this condition.
Cyanide in wastewater is oxidized to
bicarbonates and nitrogen by the
introduction of chlorine gas or sodium
hypochlorite. This procedure is carried
out in a split-tank configuration where
caustic or lime is added to control the
pH between 9 and 11 in the first stage
and at approximately 8.5 in the second
stage.
Effluents from chromium reduction,
cyanide oxidation, and acid/alkali
waste streams are mixed together in a
neutralizer. The pH is controlled
carefully at the point of minimum
solubility for that mix of metals, that is,
in the range of 8.0 to 9.5. The retention
time needed for effective precipitation
is usually 10 to 20 minutes.
The neutralizer can be either single
stage or multistage, depending on
whether there are rapid changes in
flow rates or pH. The pH is controlled
by adding caustic soda or sulfuric acid
as required.
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g$
^j^f^^^^^^^^j}l^^(n^^>-^-
Figure 2.
Investment Cost for Chromium Reduction Units
Following the neutralize^ the metal
hydroxides and other insoluble
pollutants are transferred to a clarifier
and separated from the water by gravity
settling. The rate of settling may be
increased by adding coagulating
agents, such as polymers, alum, or
ferrous sulfate. When the settling
characteristics of the suspended solids
are improved, the overflow from the
clarifier will contain lower levels of
pollutants.
The underflow from the clarifier is
transferred to sludge storage. Here,
thickening of the clarifier underflow
will reduce the volume of solids
required for hauling to disposal sites.
Cost of Conventional Wastewater
Treatment. Three major costs are
associated with wastewater treat-
ment techniques for pollution control:
Fixed costs associated with capital
investment
Chemical costs associated with
treatment of pollutants, varying
according to the types and mass
flow rates of the pollutants
Disposal costs associated with the
volume and chemical composition
of the sludge generated
Because recovery systems reduce
wastewater treatment costs, in many
cases these systems are economically
attractive. For instance, a plant may be
faced with choosing between a
wastewater treatment system needed
to handle the segregated rinse water
from a plating operation or a recovery
system that may eliminate the need for
that treatment system, as is often the
case with cyanide and chromium
plating rinse waters. The avoidance of
capital and operating costs for these
treatment systems may be offset by the
capital and operating costs of an
evaporative recovery system. Typical
investment costs for chromium
reduction and cyanide oxidation
systems are shown in Figures 2 and 3.
Although recovery systems can reduce
the size and investment costs of
neutralization and clarification
systems, the benefit is minimal and
usually is better achieved by reducing
the volumetric flowrate of wastewater
by in-plant modifications, such as
multistage rinse tanks. Improved
rinsing systems benefit both chemical
treatment and recovery systems.
In most plants, a wastewatertreatment
system consisting of a neutralizer and
clarifier will be required whether or not
recovery systems are installed.
Treatment systems for handling spills,
or at least holding tanks necessary to
collect plating bath discharges, maybe
required for backup to the recovery
systems. For these reasons, the cost
benefits for recovery will be based
primarily on savings in treatment costs
associated with chemical use and
sludge disposal as well as on savings
in raw materials (plating chemicals).
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Figure 3.
Investment Cost for Cyanide Oxidation Units
Chemical costs for wastewater
treatment can be approximated based
on pollutant composition and mass
flowrates. A detailed discussion of
costs for conventional wastewater
treatment is presented in the EPA
publication, Economics ofWastewater
Treatment Alternatives for the
Electroplating Industry.*
The costs of wastewater treatment for
plating plants vary, depending on site-
specific conditions and types of
chemicals used for treatment. Table 1
lists the costs that will be used in this
report for plating chemicals, treatment
chemicals, and utilities. As a rule, the
chemical additives used by platers will
increase the bulk costs listed for pure
chemicals. To correct for site-specific
changes and for the changes in
recovery and treatment costs for
different raw material prices,
approximate costs may be obtained by
multiplying the values given in this
report by the ratio of actual plant costs
divided by costs shown in the table.
Because the costs of the chemicals
listed are for bulk quantities, the
savings attributable to recovery
techniques and the costs for
conventional wastewater treatment
may be higher.
aEPA 625/5-79-01 6, June 1 979.
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Table 1.
Raw Materials, Pollution Control Chemicals, and Utilities Used by the
Electroplating Industry
Item
^1978
CQSt
Plating chemtqals ($/lb): '
* Boric acid, H3B03- ,. . . ,..
Cadmium^liloride, CdCI2
=Chromic'a pi d. H2Cr04'
- 'Copper cyanide, Cu(CN)2 ,
Copper sulfate, Gu604' ., f...
Njckel chloride, NiCI2 ,
Nickel sulf^te, NiSO^ , ,. .
Sodium cyanide, NaCN ..
Zinc, as Zh metal . ... ,
Zinc cyanide, Zn(GN)2 ,
Water pollutiorj control chemicals j$/lb)-
Calcium hydroxide, Ca(OH)2 . ,
Calcium o^lde, CaO, quicklime ,
Chlorine, yCI2 ,
ferrous sulfide, Fe$, .'
Hydrochloric acid, 28% HCI . .
Sodium bisulfite, NaHS03
Sodium carbonate, 58% Na2C03
Sodium hydroxide, 98% NaOH equivalent
Sodium hypochlonte, NaOCl
'Sodium aulfide, Na2S .
Sulfur dioXlde, S02 .
Sulfunc acid, H2S04
Utilities ($)
Electricity (per kWh)
Steam (per 1,000 (b), energy source.
Natural gas . ,
No, 2 fuel oil
Water (per 1,000 gal}
Sewer fee
0,176
2-60
.Q.78
1.95 ;
o.sa -
"o<76
0,40
Q,31
14f
0.017
.0016
Q07j5
040
0,023*
013
"0.0^
0,08,
040
012
0,085
0023
0045*
210'
350
050-
060
Note Plating chemical costs are for bulk chemicals and d<£ not iricjude additive chemicals
for proprietary formulations / " " /*"
' ^ *«
SOURCE- "Current Prices of Chemicals and Related Materials " Chemical Marketing Reporter,
F.eb.,20, 1978 ,*+*'*.
Treatment cost using sodium
bisulfite, 2 Ib/h (0.9 kg/h)
X $0.69/lb=$1.38
Disposal costs at 4 percent solids
and $0.1 0/gal, 2 Ib/h (0.9 kg/h}
X$0.32/lb = $0.64
Total costs per hour would come to
$3.58.
If the cost for chromic acid plating
solutions is $1.00/lb and sludge
disposal costs are $0.25/gal, then the
cost will be as follows:
Replacement costs, 2 Ib/h (0.9
kg/h) X$1.00/lb = $2.00
Treatment cost using sodium
bisulfite, unchanged = $1.38
Disposal costs at 4 percent solids
and $0.25 gal,b ($0.64 at $0.10/
gal} X (25/10} = $1.60
Total costs per hour will then be $4.98.
Rinse Water. The major source of
contaminated wastewater is from the
rinse tanks, although further
contamination can come from leakage
and spills. Following the plating bath,
the chemicals adhering to the
workpiece (drag-out) must be removed
by rinsing. The spent rinse water
containing the plating chemicals must
be treated by conventional wastewater
treatment processes or must be
concentrated using recovery
techniques for recycle to the plating
tanks.
Table 2 shows typical operating costs
experienced in treating wastewater,
based on the mass flow of pollutants
and an average concentration of 100
mg/l. The total cost consists of three
key factors:
Replacement cost of plating
chemicals lost
Treatment cost for destruction
chemicals and polymers
Disposal costs for sewerage and
sludge handling
A plant either can calculate its own
actual treatment costs or can estimate
them using Table 2.
For example, if a rinse stream contains
the equivalent of 2 Ib/h (0.9 kg/h} of
chromic acid, then the hourly costs for
wastewater treatment will include:
Replacement cost, 2 Ib/h (0.9 kg/h)
X $0.78/lb=$1.56
For disposal of solid waste at different solids
concentrations, the costs in Table 2 can be
adjusted by: disposal cost per Ib at X% of
solids = (disposal cost per Ib at 4% solids)
X (4%/X%). This adjustment does not account
for sludge density changes.
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Table 2.
Economic Penalty for Losses of Plating Chemicals
Plating solution entering the rinse
tanks in many cases can represent as
much as 50 to 90 percent of the plating
solution consumed in the plating
process. The concentration of the
drag-out entering the first rinse tank is
identical to the plating bath solution.
Assuming good rinsing efficiency, the
concentration of the drag-out from a
rinse tank is the same as the rinse water
in that tank. The concentration of the
drag-out can be decreased
satisfactorily using one rinse tank and a
high rinse water flowrate; however,
additional rinse tanks usually are
included to reduce the quantity of rinse
water used.
The volume of water required for
rinsing will depend on many factors,
such as the volume and concentration
of drag-out, number of cpuntercurrent
rinse tanks, rinsing efficiency, and final
concentration of plating chemicals
permissible on the product after
rinsing. Usually, the concentration of
the plating chemicals adhering to the
workpiece after the final rinse will be in
the range of 5 to 50 ppm, although
some plating operations require final
concentrations of dissolved solids as
low as 1 ppm. Typical concentrations
of total dissolved solids in final rinses
are 1 6 ppm for chromium plating, 45
ppm for nickel plating and 50 ppm for
cyanide plating.
The arrangement and number of rinse
tanks usually depend on economics
and space I imitations. Typically, one to
five rinse tanks are used. With a given
number of rinse tanks, a countercurrent
rinse system (in which the rinse water
flows in the opposite direction to the
product, as shown in Figure 4} will
minimize the consumption of rinse
water. In this arrangement, the first
rinse tank has the highest
concentration of plating chemicals.
The ratio of the rinse water volumet-
ric flowrate to the drag-out volumetric
flowrate is defined as the rinse ratio (r).
Atheoretical relationship between the
rinse ratio, the number of rinse tanks
and the resulting dilution in the rinse
tanks has been developed to predict
countercurrent rinse water require-
ments, as shown in Figure 5. The
values shown in Figure 5 are based on:
where
n = number of rinse tanks
C = plating bath concentration
Cn = concentration in n
(n= 1, 2, 3
r = rinse ratio
th
rinse tank
Figure 5 also compares the percent
recovery of the plating solution
contained in the rinse water with that
entering the rinse as drag-out on the
workpiece. This percentage is
potentially available for recovery and is
defined by:
Percent recovery
M X100%
Cp-
(2)
Usually it is assumed that the
concentration of the drag-out from the
final rinse tank is equal to the
concentration of the chemicals in the
final rinse tank.
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Figure 4.
Preferred Countercurrent Rinse Arrangement for Plating Process
100,000 e~
10,000
1,000
100
aRinse rate (gal/h) -s- drag-out rate (gal/h).
Notes.For countercurrent rinsing, C =
concentration in nth rinse tank; C = concen-
tration in process bath; n = number of rinse
tanks, n = 1, 2, 3,. . . Example: If the rinse ratio
were 10 in a two-tank rinse system, C /Cn would
be 100. A closed loop recovery system would
recover 99% of the drag-out plating chemicals.
10
100 1,000
RINSE RATIO3
10,000
100,000
Figure 5.
Rinse Water Dilution vs. Rinse Ratio for Multitank Countercurrent Rinsing
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Figure 6.
Rinse Ratio Effect on Percent Recovery of Drag-Out Chemicals from Rinse Systems
Figure 6 shows different levels of
chemical recovery over a range of rinse
ratios for recovery from one or two
rinse tanks. For example, if chemical
recovery were from one rinse tank only,
and the rinse ratio were 10, then it
would be possible to recover 90
percent of the drag-out chemicals from
the rinse tank. If the rinse ratio were
increased to 40, only an additional 7.5
percent recovery would be possible. If
recovery were from two rinse tanks and
the rinse ratio were 10, then 99 percent
recovery would be possible. If the rinse
ratio were increased to 40 in the two-
tank rinse system, then 99.9 percent
recovery would be possible.
To illustrate the application of the rinse
ratio and the drastic changes in
wastewater flowrates, assume that a
chromium plating bath concentration
(C ) is 40 oz/gal (300 g/l) and the
product specifications require a final
rinse concentration of 0.002 oz/gal
(1 5 mg/l). The drag-out rate is 1.5
gal/h (5.71 l/h) and a single-stage rinse
tank is installed. The ratio of C /Cn
(40/0.002) is 20,000 and, from Figure
5, the rinse ratio becomes 20,000.
Therefore, 30,000 gal/h (11 3,550 l/h)
of rinse water are required. If the plant
installs two additional rinse tanks to
have a three-tank countercurrent rinse
system, the rinse ratio, according to
Figure 5, is 27. The final rinse
concentration is maintained at 0.002
oz/gal (1 5 mg/l) using a rinse water
flowrate of 40 gal/h (151 l/h) (27 X 1.5
gal/h).
The concentration in the first rinse tank
that would discharge to the
wastewater treatment process is 1.5
oz/gal (11.2 g/l), as determined from
Figure 7. The total mass flow of
pollutants requiring treatment remains
unchanged, although the volume of
wastewater is reduced by a factor of
750. The investment cost for a
wastewater treatment system would
then decrease and the chemical
requirement for pH adjustment would
be less.
10
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100 r-
.1 l
10 20 , . ,100
ftlNSE RATIO (r)"\ '
=; 30
C- =, 2Q. oi/gal
'_ Note .Example; ft plating bath ooncerrtra^on
' JC > wefe 20 oz/ga-1 ?n6 the'riase .ratio wefe
20, pteting concentration in the first tinse ta^k
_ would-be-1 '
,1,000
Figure 7.
Determination of Plating Concentration in First Rinse Tank
11
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Evaporation
Introduction
Evaporation, reverse osmosis, ion
exchange, and electrodialysis are the
common technologies for recovery of
plating chemicals from rinse water.
Evaporation achieves recovery by
distil ling the wastewater until there is a
sufficient concentration of plating
chemicals to allow reuse in the plating
operation. Because of the thermal
energy costs of operation, evaporators
are not usually considered for recovery
of plating chemicals from dilute
wastewater. As discussed earlier,
changes in the rinse systems can
increase chemical concentrations in
wastewater to a level where
evaporative recovery can be con-
sidered.
Evaporation is becoming a popular
method that can offerfavorable returns
on investment and can reduce the
pollutant loading requiring treatment.
The technology is proven and its
application is expanding. The
evaporation system shown in Figure 8
can be used to recover plating
chemicals contained in the effluent
from a three-stage countercurrent
rinse. The feed to the evaporator is the
discharge from the first rinse tank. The
plating chemicals are concentrated in
the evaporator and returned to the
plating bath. The water vapor is
condensed and returned to the rinse
tanks.
Rising film evaporator: reboiler, vapor/liquid separator, and condenser
12
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Figure 8.
Closed-Loop Evaporative Recovery of Plating Chemicals from Drag-Out
Four types of evaporators are used
throughoutthe electroplating industry:
Rising film evaporators
Flash evaporators using waste heat
Submerged tube evaporators
Atmospheric evaporators
Site-specific conditions and the mode
of operation determine construction
materials and influence the selection
of one system over another.
Types of Evaporators
Rising Film Evaporators. Rising film
evaporators are built so that the
evaporative heating surface is covered
by a wastewaterfilm and does not lie in
a pool of boiling wastewater. A
complete unit usually consists of a
reboiler, separator, and condenser, as
shown in Figure 9. The reboiler is a
shell-and-tube heat exchanger in
which the heat from low pressure
steam5 to 15 Ib/in2 gauge (1,018 to
1,536 mm Hg absolute)or hot water
is transferred to the wastewater. The
wastewater may be circulated naturally
or forced through the tubes, or it may
be a rising film on the outside of the
tubes, depending on the evaporator
manufacturer. Because plating
chemicals are susceptible to
degradation at high temperatures,
evaporation is accomplished at
pressures of 1.3 to 7.5 Ib/in2 absolute
{67 to 388 mm Hg absolute), thereby
lowering the boiling point to 110° to
1 80° F (43° to 82° C): The operating
pressure may vary among
manufacturers and according to the
type of plating chemicals handled.
Reduced wall temperatures of the heat
transfer surface will reduce scale and
thermal breakdown of plating
13
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Figure 9.
Rising Film Evaporation for Chemical Recovery
chemicals. The wastewater leaves the
reboiler as a vapor/droplet mixture and
enters the separator.
The separator can serve two functions:
it can separate the water vapor from
the heavier plating solution and it can
provide a reservoir for the
concentrated plating solution to
collect until returned to the bath. One
manufacturer employs a separate
concentration tank to serve the same
function as the reservoir in the
separator. The water vapor exits
through the top of the separator. A
recirculation loop continuously
recirculates the separated plating
solution through the reboiler and
separator until the concentration
increases to a preset level. At this
point, the wastewater flow is
momentarily valved off, allowing the
concentrated plating solution to return
to the bath or hold tank. This step takes
only a few minutes, then the
wastewater flow to the reboiler is
resumed. Some systems operate with a
continuous feed and do not require this
valve cycling.
The vapor leaving the separator is
condensed in a shell-and-tube heat
exchangerand the distillate is returned
to the rinse tanks or to other plant uses.
Cooling water can come from cooling
towers or reservoirs, or once-through
process water can be used.
Flash Evaporators Using Waste Heat.
Flash evaporators, as shown in Figure
10, are of the same basic design as the
rising film evaporators. The main
difference is that plating solution from
the bath is recirculated continuously
through the evaporator. This
recirculation reduces overall energy
requirements for evaporation and
cools the bath solution as well.
14
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Figure 10.
Flash Evaporation for Chemical Recovery
During the plating operation, the
temperature of the plating bath
increases because of the electrolytic
process. Cooling coils are installed to
maintain the bath temperature. An
alternative approach for high
temperature baths is to withdraw a part
of the plating solution for feed to flash
evaporators. The plating solution from
the bath is at a temperature above its
boiling point at the pressure
maintained in the evaporator. A small
part of the plating solution therefore
vaporizes in the separator, and thus
reduces the temperature of the
solution and provides heat to the
wastewater being evaporated. If the
evaporator is operating at a pressure of
1.3 Ib/in2 absolute (67 mm Hg
absolute) and the temperature of
plating bath is 135° F (57° C), the
plating solution will flash cool to 110°
F (43° C). The 110° F (43° C) plating
solution is recycled to the plating bath
to maintain the bath temperature at
135° F(57° C).
To save 1 pound (0.45 kg} of steam in
the flash evaporator, approximately 5
gallons (1 9 liters} of plating solution
must be flash cooled by 25° F (14° C) in
the evaporator. If a rising film
evaporator required 1,000 Ib/h (454
kg/h) of steam, a flash evaporator could
achieve the same concentration rate
using 800 Ib/h (363 kg/h} of steam if
1,000 gal/h (3,785 l/h)based on
5 gal/lb steam X 200 Ib/h (91 kg/h
X 41.85 I/kg steam)of plating solu-
tion is fed to the unit. The amount
of heat removed from the bath cannot
exceed the cooling duty. Therefore,
the plating bath cooling requirements
are at least 200,000 Btu/h (50,400
kcal/h)200 Ib/h (91 kg/h) steam
X 1,000 Btu/lb (555 kcal/kg) steam.
Because the quantity of heat provided
by the flash cooling of the plating
bath solution is small compared to the
latent heat provided by steam heating,
flash evaporation usually is limited
to large, high temperature plating
installations.
15
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Figure 11.
Submerged Tube Evaporation for Chemical Recovery
Submerged Tube Evaporators. Sub-
merged tube evaporators operate on a
slightly different principle from that of
film or flash evaporators for supplying
thermal energy to the wastewater. As
shown in Figure 11, steam, at 5 to 1 5
Ib/in2 gauge {1,01 8 to 1,536 mm Hg
absolute), or hot water, at approxi-
mately 1 60° F (71 ° C), is supplied to
heating coils that are immersed in the
boiling wastewater. A single-effect
unit consists of one vessel that
includes internal heating coils for
evaporation, a moisture separator, and
cooling coils for condensing water
vapor. The wastewater is not
recirculated, unlike that of rising film
and flash evaporators.
Submerged tube units are designed to
operate under a pressure as low as 0.7
to 1.3 Ib/in2 absolute (36 to 67 mm Hg
absolute). This pressure is created by
diverting part of the cooling water
through an eductor external to the unit.
The distillate is recycled to the rinse
tanks or for other plant uses. The
plating solution is recycled to the
plating tanks after the desired
concentration is reached.
The cost for submerged tube
evaporators usually is lower than that
for rising film or flash units, primarily
because of the integrated evaporation/
condensation single-unit design. The
steam or thermal demand is the same
as that for rising film evaporators.
Atmospheric Evaporators. Atmos-
pheric evaporators, as shown in Figure
1 2, do not recover the distillate for
reuse and do not operate under a
vacuum. Wastewater is evaporated by
using it to humidify airflowing through
a packed tower. The humidified air is
exhausted to the atmosphere; this
procedure eliminates the need for
cooling water and a condenser. The
concentrate from the reservoir is
circulated continuously through the
packed tower at a rate of approxi-
mately 50 gal/min (1 89 l/min).
Contaminated rinse water enters the
system and mixes with the concentrate
in the reservoir. The wastewater is
recirculated through the shell-and-
tube heat exchanger, where steam is
used to raise the temperature of the
solution to approximately 1 60° to
170° F(71°to 77° C). Ambient air is
drawn in through the packed tower,
where it becomes saturated with water
vapor and is exhausted to the
atmosphere. Because the exhaust air
removes some of the heat supplied by
the wastewater heat exchanger, this
type of unit requires approximately 20
percent more steam than do
evaporators of other designs.
Deionized water is added to the rinse
tanks to make up for all of the water lost
to the humidified air. Deionized water
is required to minimize the scale
deposit from evaporation.
16
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Figure 12.
Atmospheric Evaporation for Chemical Recovery
Operating Requirements for
Evaporators
Introduction. The major operating
costsforevaporators include operating
labor, maintenance, and utilities. The
utility requirements include steam or
hot water, cooling water, and
electricity. Instrumentation and con-
trols may be electrical or pneumatic; if
pneumatic, an air supply will be
required. Fixed costs, such as
depreciation and interest on borrowed
capital, also are incurred.
Operating Labor. Evaporators usually
are trouble-free systems that require,
typically, less than one-half hour per
shift of operator attention. Evaporator
operation may be manual or automatic.
A manual system is started up by
opening the steam supply and the
wastewater inlet valves and starting
the recirculation, vacuum, and cooling
water pumps. In some systems, the
vacuum pump and distillate pump are
the same; in others, the cooling water
pump also creates the vacuum through
an eductor. The system is shut down by
closing the wastewater inlet and steam
supply valves.
Some automatically controlled
systems are equipped with concen-
tration sensor/controllers that monitor
the plating concentration. When the
desired concentration is reached, the
automatic controls close the
wastewater inlet valve and return the
concentrate to the plating bath or
holding tank. The systems are sized so
that the return of the plating chemicals
to the bath occurs approximately once
per shift. Other systems are designed
to operate continuously with no
interruption to the wastewater feed.
Single-Effect Evaporators: Utility
Requirements. Most evaporators used
in the plating industry are single-effect
units, as illustrated earlier. Single-
effect evaporators operate with one
reboiler or evaporator section. The
watervapor is condensed orexhausted
to the atmosphere. Approximately 1.1
pound (0.5 kg) of steam is consumed
in evaporating each pound of water
from the plating solution. Various
methods can be employed for reusing
the thermal energy of the water vapor
to reduce the thermal energy demand;
however, additional capital invest-
ment is required.
Figure 1 3 shows the utility
requirements for single-effect
evaporators as a function of waste-
water flowrates to the evaporator.
Because the typical application
requires evaporation of over 98
percent of the water, the evaporator
ratings are based on wastewater flow-
rates. Changes in concentration of the
plating chemicals do not affect the
utility demand significantly if the flow-
rate remains constant; however, the
plating chemical concentration will
have a significant effect on the
economics and cost savings.
17
-------
Figure 13.
Utility Requirements for Single-Effect Evaporation
18
-------
Figure 14.
Double-Effect Evaporation for Chemical Recovery
The electrical demand is associated
with power requirements of the
vacuum pump, recirculation pump, and
feed pump. As a rule, the cooling water
rates are based on a temperature rise of
25° F (14° C) across the condenser.
For example, from Figure 13, if the
wastewaterflowrate to the evaporator
is 80 gal/h (303 l/h), the steam rate is
730 Ib/h (331 kg/h) for 1 5 Ib/in2 gauge
(1,536 mm Hg absolute) steam. The
electrical demand is 2.9 kWh and the
cooling water rate is 56 gal/min (21 2
l/min). For atmospheric evaporators
where no cooling water is used, the
steam rate would be at least 20 percent
higher.
Methods for Reducing Steam Rates.
Two techniques have been applied
successfully to reduce steam demand
for evaporation; both involve reusing
the heat value contained in the
vapor from the separator.
The most common technique is to use
a double-effect evaporator, as shown
in Figure 14. In this system, approxi-
mately 50 percent of the wastewater is
concentrated in the first effect using
steam. The vapor from the separator of
the first effect enters the second-effect
reboiler and condenses to provide the
thermal energy required to reach the
final concentration of the plating
solution. Rising film, flash, and
submerged tube evaporators can be
employed in this manner; however, the
capital costs are significantly
increased because of the need for an
additional reboiler and separator. The
19
-------
Figure 15.
Utility Requirements for Double-Effect Evaporation
steam and cooling water rates for the
double-effect unit (Figure 1 5) are
approximately 50 percent of those
required for the single-effect unit (see
Figure 13).
Some platers using double-effectunits
achieve an additional benefit by
recovering two different plating baths
simultaneously. Care should be taken
in employing this arrangement
however, because there is a possibil-
ity of cross-contaminating baths.
The second technique is to use a
mechanical compressor (Figure 16}.
The water vapor from the separator
enters the suction of the compressor
where its temperature and pressure are
increased. The vapor is then
desuperheated and enters the reboiler.
The mechanical vapor recompression
system requires no external steam or
cooling water; this characteristic
eliminates the need for capital invest-
ment in boilers and cooling towers.
Currently the application of this system
is limited to alkaline plating solutions
because acid carryover can cause
corrosion damage to the compressors.
20
-------
Double-effect evaporator: first-effect vapor separator and second-effect reboiler
21
-------
Figure 16.
Mechanical Vapor Recompression Evaporation for Chemical Recovery
The initial investment cost for Figure 1 7 shows the electric utility
mechanical vapor recompression is requirements for mechanical vapor
the highest among equivalent capacity recompression units as a function of
single- and double-effect units; capacity.
however, this technique has the lowest
operating cost.
The application of mechanical vapor
recompression and multiple-effect
evaporators should be considered
when the additional investment costs
can be economically justified by the
reduced operating costs.
22
-------
MA. J' - 4 "- I. > Jr., .1 VrM^..,.T- .1
Figure 17.
Utility Requirements for Mechanical Vapor Recompression Units
Closed- Loop and Open-Loop Systems
for Evaporation
The capacity of the evaporator will
depend on the rinse flow rates and the
level of recovery desired. As discussed
earlier, the mass flow rate of the plating
chemicals entering the rinse tank is
based on drag-out. This rate sets the
total raw material savings. The concen-
tration of the plating chemicals
entering the evaporator will depend on
the rinse ratio and the number of rinse
tanks for a fixed drag-out
concentration and rate. Because the
utility requirements and evaporator
capacity are primarily a function of
rinse water flowrate, it is important to
reduce the flowrate to the evaporator
by using multiple rinse tanks, thereby
increasing the concentration of plating
chemicals. An optimum closed-loop or
open-loop system can be developed to
reduce the costs for evaporation.
If three or more rinse tanks are used,
the rinse ratio usually will be low
enough to make closed-loop recovery
economical. The closed-loop system
allows maximum recovery of plating
chemicals. For example, if a plant
operating a chromium plating bath at a
concentration of 45 oz/gal (337 g/l)
and having a three-tank rinse system
maintains the final rinse concentration
at 0.002 oz/gal {1 5 mg/l), the required
rinse ratio would be 28 (Figure 5). If the
drag-out rate were 1 gal/h (3.78 l/h),
then a closed-loop evaporator system
would require a minimum capacity of
28 gal/h (106 l/h). Recovery of 99
percent of the drag-out would be
possible.
A similar plant having the same plating
bath and final rinse tank
concentrations, but having only two
rinse tanks, would require a rinse ratio
of 1 50 (Figure 5). At the drag-out rate of
1 gal/h (3.78 l/h}, the minimum
evaporator capacity would be 150
gal/h (568 l/h}, or a little greater than
five times the capacity of a three-tank
rinse system. The same 99 percent
plating recovery would result.
The plant with the two-stage rinse
system would have two alternatives; it
could install another rinse tank or it
could operate the two rinse tanks as an
open-loop recovery system (Figure
1 8). The disadvantage of an open-loop
system is that the rinse water rate of the
final rinse tank (which requires
chemical treatment) will increase
considerably. The overall recovery of
the two-stage system, however, will be
only slightly lower than that achievable
with the three-tank rinse system.
At a rinse ratio of 28 in the first rinse
tank, there is 96.5 percent recovery of
plating chemicals from drag-out. The
rinse ratio for the second tank is set to
maintain the final rinse concentration
of 0.002 oz/gal (1 5 mg/l). This rate is
calculated by determining the concen-
tration in the first rinse tank (from
Figure 7), which, at a rinse ratio of 28
and a plating bath concentration of 45
oz/gal (337 g/l), is 1.6 oz/gal (12 g/l).
Figure 5 will give the rinse ratio
23
-------
Figure 18.
Open-Loop Evaporative Recovery of Plating Chemicals from Drag-Out
required in the second tank to achieve
a dilution ratio of 800 (Cp/Cn = 800).
If the drag-out rate were 1 gal/h (3.78
l/h),the rinse water flowrate needed in
the second tank to achieve the final
discharge concentration of 0.002
oz/gal (1 5 mg/l) would be 800 gal/h
(3,028 l/h).
If an open-loop system with a two-
stage rinse is used, the volume of
wastewater processed by the evapora-
tor is the same as the volume
processed in a three-stage closed-loop
system28 gal/h (106 l/h). Although
the utility requirements are the same
for evaporation, the open-loop system
will result in slightly less chemical
recovery and in increased rinse water
and chemical treatment costs.
For plants that cannot reduce the
rinsing rates, open-loop systems
provide the opportunity to use
evaporative recovery in an economi-
cally feasible manner. As a rule, the
most economical approach is to add
additional rinse tanks to minimize the
rinse rate required in a recovery rinse
and in a free rinse, where chemical
treatment of the rinse water is required.
24
-------
Economics
Investment Costs for Evaporators
The investment costs for evaporators
depend on the capacity, design, and
materials of construction. For each
type of evaporator, the major
difference in investment costs
depends on the materials of con-
struction. Most evaporators are
supplied as package units and only
require the hook-up of utilities before
startup.
Evaporators currently are marketed
with a wide range of construction
materials to resist the corrosiveness of
many of the plating chemicals. The
more popular materials include
titanium, tantalum, borosilicate glass,
fiberglass-reinforced plastic (FRP),
stainless steel, and polyvinyl chloride
(PVC). Carbon steel can be used for
condensers when there is no chance
for rust contamination of the distillate.
Figure 1 9 shows the approximate
installed costs for complete package,
single-effect, rising film evaporators
excluding a bath purification system.
The investment costs for submerged
tube evaporators are approximately 30
percent lower, primarily because of the
integrated evaporation/condensation
single-unit construction.
Chromium plating of bicycle parts
25
-------
Figure 19.
Capital Cost of Single-Effect Rising Film Evaporators
For rising film evaporator capacities
above 100 gal/h (378 l/h), the
investment costs for double-effect
units will be approximately 30 percent
higher than for single-effect systems.
The investment costs for mechanical
vapor recompression evaporators are
approximately 50 percent higher than
for single-effect units. Maintenance
costs also are higher for mechanical
vapor recompression units. The costs
for installation of package evaporators
range from 10 to 25 percent of the
hardware costs. Installation costs for
larger evaporators will be a lower
percentage of the hardware costs
because essentially the same utility
hook-ups are required and only line
sizes will change. The economics for
evaporative recovery systems depend
primarily on savings in plating
chemicals and wastewater treatment
costs. These savings will be used with
the investment costs for rising film
evaporators to illustrate the
economics.
Chemical and Treatment Cost
Savings
Figures 20 and 21 show the net
savings attributable to recovery of
three typical plating solutions, using
the cost basis of Tables 1 and 2, for
each 100 gal/h (378 l/h) of waste-
water concentrated in single-effect
and double-effect evaporators. Net
savings is determined by adding the
raw materials savings for plating
chemical replacement and the
reduction in chemical treatment costs,
including costs for sludge disposal,
and then subtracting the utility costs
(electricity, steam, and cooling water}
of the evaporator. The figures also
show the impact on net savings of
different plating chemical costs.
To illustrate the effects of the concen-
tration and the evaporator capacity on
net savings, assume that a chromium
plating line has a drag-out rate of 1.5
gal/h (5.7 l/h}, a closed-loop rinse
system with a rinse ratio of 100, and a
plating bath concentration of 50 oz/gal
(375 g/l). From Figure 7, the concen-
tration of rinse water entering the
evaporator is 0.5 oz/gal (3.7 g/l} and
the flowrate is 1 50 gal/h (568 l/h)
1.5 gal/h (5.7 l/h} drag-outX rinse ratio
of 100. From Figure 20, the net savings
is approximately $1.50/h ($1 /h X
1 50/100}, assuming a plating replace-
ment cost of $0.78/lb, curve D.
Approximately 1.5 gal/h (5.7 l/h) of
plating chemicals are returned to the
bath. If the rinse system is modified to
reduce the rinse ratio to 20, the
concentration of plating chemicals
entering the evaporator increases to
26
-------
Figure 20.
Plating Chemical and Wastewater Treatment Savings for Single-Effect Evaporative Recovery Systems
27
-------
Figure 21.
Plating Chemical and Wastewater Treatment Savings for Double-Effect Evaporative Recovery Systems
28
-------
2.5 oz/gal (1 8.7 g/l), Figure 7, and the
feed rate decreases to 30 gal/h (114
l/h)1.5 gal/h (5.7 l/h) drag-out X
rinse ratio of 20). From Figure 20, the
net savings for recovery increases to
$6.30/h ($21/h X 30/100}. Approxi-
mately 95 percent of the plating
chemicals contained in the drag-out is
returned to the plating bath at 1.5 gal/h
(5.7 l/h), but the utility costs for the
evaporator are reduced because the
evaporation rate is decreased by
approximately 1 20 gal/h (454 l/h). The
installed cost for a rising film evapora-
torto recoverthe drag-out is decreased
from $90,000 for a 1 50-gal/h (568-l/h)
unitto$42,500fora30-gal/h(114-l/h)
unit, as showh in Figure 19.
In Figures 20 and 21, the intersection
of plating solution curves with the
vertical axis gives the utility cost of
evaporating 100 gal/h (378 l/h) of
water, based on the fuel and utility
costs presented in Table 1. The
intersection of the plating solution
curves with the horizontal axis gives
the minimum plating concentration
necessary for the cost savings to equal
the evaporator utility costs. At plating
solution concentrations lowerthan the
minimum needed to offset evaporator
operating cost, the recovery system
will yield a net operating loss.
Because steam requirements are
approximately 50 percent lowerforthe
double-effect units, the increased
investment, maintenance cost, and
operating complexity sometimes can
be justified from net savings. For
example, from curve D in Figures 20
and 21, at a chromic acid concentra-
tion of 1.0 oz/gal (7.5 g/l), the double-
effect evaporator would have a net
savings of $7.50/h compared with
$5.50/h for a single-effect unit for each
100 gal/h (378 l/h} of wastewater
Table 3.
Basis for Estimating Costs for Investment Options and Economics
Item
Basis
Operating expenses:
Operating labor3-
Supervision6 . . .
Maintenance ,,,,..
General plant overhead
Depreciation (10-yr straight line)
Taxes and insurance.. ......
Federal taxes
Interest on borrowed capital' ..
Utility charges:
Electricity
Cooling water
Once through city water .
Cooling tower
Steam, based on No. 2 fuel 6il
Annual operating tirrte ,,. .
Annual profit before tax ($/yr).." ' .
Annual profit after tax ($/yr)
ROI, /
Gash flow (.$/yr) ...
Payback period (yr).
PC6 rate of return {<
$8/h
$10/h ' ,'
6% of-total investment cost > ;
0,58 X (operating labor + supervision + mainte-
nance labor), maintenance labor = 03?Tx
maintenance cost ,
10% of total investment'
2% of total investment
48% of annual before-tax profit
10% on unpaid balance
$Q045/kWh ^ "
$0.50/1,000 gal
$0.10/t,OOOgaJ
$3 50/1,000 Ib t\ f -
5,000 h 4 ' ,
Operating cost reduction (savings) .resulting from
investment minus increase infixed and operating
cost for new system
Annual before-tax profit X 0.52
Annual profit after taxes divided by totaf installed
^investment - ->
Annual profit'after taxes plus depreoatipn ^
Total installed investment divide^ by cash flow '
Interest rate On capital recovery rpfthe'tmal invest-
^msnf based on the annual cash flow
Evaporator operating labor requirement assumed at 0 5 h per shift (average expense $2",5'00/yr)
'Taken at 50% of operating labor cost, or $1,250'
evaporated. By comparison, a mechan-
ical vapor recompression unit evapor-
ating 100 gal/h (378 l/h) would have a
utility cost of approximately $1/h as
determined from Figure 1721 kWh
(75,600 kJ) X $0.045/kWh. Because
of the lower operating cost, the net
savings would be $2.75/h ($3.75 -
$1.00) greater than that of a single-
effect unit.
Return on Investment
The economics for recovery can be
based on the net savings including
chemical recovery and reduction in
wastewater treatment costs, as
illustrated in Figures 20 and 21.
Moreover, if the recovery system can
reduce the investment cost in waste
treatment hardware, then this savings
should be factored into the economic
analysis.
In addition to utility costs, operating
labor and fixed costs such as deprecia-
tion, taxes, and insurance will be
incurred by the plant. Table 3 shows
the basis for the derivation of the return
on investment (ROI).
29
-------
There will be two situations to consider
when the potential economic benefits
of evaporative recovery are evaluated.
In the first case, where a closed-loop
evaporative recovery system is
installed in a chromic acid or zinc
cyanide plating line, the investment
cost for treatment hardware can be
reduced or possibly eliminated
(Figures 3 and 4). The second case
assumes that the chromium and
cyanide treatment systems also must
be installed or already have been
installed. In the second case the
economics must be based on the total
investment costs for the evaporator; in
the first case the economics can be
based on the investment costs for the
evaporators minus any investment cost
savings for waste treatment hardware
resulting from installation of the
evaporator. For nickel sulfate recovery,
the investment in an evaporative
recovery system will not result in any
significant reduction in the investment
required for neutralization and clarifi-
cation hardware.
Before-tax profit is calculated by
determining the net savings, as
illustrated in Figures 20 and 21, and
subtracting the additional costs, such
as operating labor, maintenance labor,
and depreciation (Table 3). The annual
after-tax cash flow then may be
estimated by subtracting 48 percent
Federal taxes from the before-tax profit
and adding the depreciation expenses.
Once the annual after-tax cash flow is
known, the capital expenditure
justified for a desired discounted cash
flow (DCF) rate of return can be
obtained from Figure 22. Or, if the total
lit""-,---- -,
f-^jL^.4^
t&fe*^*
rf-^Sf-
lit -i
* ! -Sfc
v* c* & "---=4- -
Figure 22.
Capital Expenditures Justified at DCF Rates of Return for Annual After-Tax
Cash Flow
30
-------
'600
500
400
300
200
100
' 75
50
25
P'
-25
-50
0
50 100 ' <" -I5Q "
FEED'fcATE TO*EVAPORATQR
"200 ,-, < ' '
'Notes, Cn =* plating concentration to evaporator, Operating Qost savings were* based,oh
replacement, treatment (with S02),,and disposal costfactpr^ presented, ioTaW^-"^nfty
cost =,$3.75/1 00 gal/h, evaporated. Operation =* 5,GOOVyr. Additiortal Qp'4ratirtg eosj ,
.calculated u$ing basis m Table 3. , ' * , ,' , \ J ' ',-'-", >
installed investment cost and annual
after-tax cash flow are already known,
the resulting rate of return can be
obtained from Figure 22. For example,
if the capital cost were $75,000 and
the annual after-tax cash flow were
$25,000, then a 30 percent rate of
return would be possible. If the
company were to desire a minimum of
20 percent rate of return on its
investment, and the annual after-tax
cash flow were already calculated to be
$25,000, then the maximum capital
investment would be $105,000.
Figures 23 through 25 show the annual
before-tax profit for evaporative
recovery for the three plating solutions
typically processed as a function of
concentration and flow rates entering
single-effect evaporators. The total
operating time is set at 5,000 hours.
The before-tax profit for recovery of
100 gal/h (378 l/h) of a rinse water
containing 1 oz/gal(7.5g/l) of chromic
acid is$5,000annually(see Figure23).
Figure 23.
Annual Bef ore-Tax Prof it for Single-Effect Evaporative Recovery of Chromic Acid
31
-------
If another plant with the same drag-out
rate had a rinse ratio that increased the
evaporator feed rate to200gal/h (757
l/h}, the chromic acid concentration
would reduce to 0.5 oz/gal (3.7 g/l).
The annual before-tax profit would
then become a negative $30,000.
With a given quantity of plating
chemicals in a rinse stream,
minimizing the volume of rinse water
that must be evaporated is critical in
justifying the recovery system
investment. As a rule, if theflowrate to
the evaporator is too high and cannot
be reduced by installing additional
rinse tanks, it will be advantageous to
consider an open-loop system and to
accept a lower percentage of recovery.
Figure 24.
Annual Before-Tax Prof it for Single-Effect Evaporative Recovery of Nickel Sulfate
Solutions
32
-------
Figures 26 through 28 show the after-
tax DCF rate of return for recovery for
single-effect, rising film evaporators.
For example, if 1 50 gal/h (567 l/h) of
wastewater flow were to contain 3
oz/gal (22.5 g/l) of chromic acid, then
the installation of a single-effect, rising
film evaporator to process this flow
would result in a DCF rate of return of
100 percent (Figure 26). The ROI's
would be similar for submerged-tube
and flash evaporators. Site-specific
economics for these evaporators and
for double-effect and mechanical
vapor recompression systems can be
estimated by following the procedures
given in the case study in Section 5.
Figure 25.
Annual Before-Tax Profit for Single-Effect Evaporative Recovery of Zinc Cyanide
Solutions
33
-------
Figure 26.
DCF Rates of Return for Evaporation Equipment to Process Chromic Acid
Wastewater
34
-------
Figure 27.
DCF Rates of Return for Evaporation Equipment to Process Nickel Sulfate
Wastewater
35
-------
Figure 28.
DCF Rates of Return for Evaporation Equipment to Process Zinc Cyanide
Wastewater
36
-------
Case Study
Plant Situation
An electroplater operates a chromic
acid bath at a concentration of 40
oz/gal (300 g/l). The drag-out rate from
the plating bath is 2 gal/h (7.57 l/h).
Three rinse tanks are used, and the
rinse water flow is adjusted so that the
concentration in the final rinse tank is
0.003 oz/gal (22.5 mg/l). The plant
operates 5,000 hours annually. The
economics for installing an evaporator
in a closed-loop system are compared
to that of an open-loop system. A 25-
gal/h (94.6-l/h) evaporator is
evaluated for the open-loop system
based on the electroplater's limited
budget,
The rinse rate (evaporator feed rate) is
determined as follows:
Rinse rate = r X drag-out
= 24 X 2 gal/h
= 48 gal/h
The percent recovery of plating
chemicals from drag-out is determined
using Equation 2:
Percent recovery
X 100%
0.003 oz/gal
40 oz/gal
X 100%
= 99.99%
Case 1: Closed-Loop System
Figure 29 diagrams evaporative
recovery in a closed-loop system,
The rinse ratio (r) required to obtain C3
= 0.003 oz/gal is determined using
Equation 1:
C
40
0.003
= 24
Figure5 also may be used to determine
r, at n = 3 and C/C = 13,333.
37
-------
Figure 29.
Case 1: Evaporative Recovery in a Closed-Loop System
The plating concentration in the first
rinse tank (concentration to the
evaporator) is determined using Figure
5, where n = 1 and r = 24,
c
40 oz/gal
24
= 1.67 oz/gal
Using curve D in Figure 20, the net
savings for evaporation is determined:
annual net savings = ($1 2.50/h)
X (48/100) X 5,000 h/yr =
S30,000/yr.
Determined from Figure 23, the annual
before-tax profit = $9,600/yr.
For a single-effect, rising film
evaporator, from Figure 19, total
installed cost = hardware cost
X 1.25 = $40,000 X 1.25 =
$50,000.
The DCF rate of return for the
investment is determined using Figure
22. For a capital expenditure of
$50,000 and an annual after-tax cash
flow of $10,000 (after-tax profit +
depreciation), the DCF rate of return is
15 percent. (After-tax profit = 0.52
X before-tax profit.)
From Table 3, the payback period is
determined as: payback = $50,000/
($10,000/yr) = Syr.
38
-------
Figure 30.
Case 2: Evaporative Recovery in an Open-Loop System
Case 2: Open-Loop System
Figure 30 diagrams evaporative
recovery in an open-loop system.
The rinse ratio (r) is determined forthe
first rinse tank, given a rinse rate of 25
gal/h (evaporatorfeed rate) and a drag-
out rate of 2 gal/h:
r =
rinse rate
drag-out rate
25 gal/h
2 gal/h
12.5
The concentration of thefirst rinse tank
(also the concentration entering the
evaporator) is determined as follows:
For a 25-gal/h single-effect, rising film
evaporator, from Figure 19, total
installed cost = hardware cost X
1.25 = $32,000 X 1.25 = $40,000.
Using curve D in Figure 20, the net
savings for evaporation is determined.
Hourly net savings = $27.50/h at
100 gal/h X (25 gal/h * 1 00 gal/h)
= $6.88/h. Annual net savings =
$6.88/h X 5,000 h/yr = $34,400/yr.
Determined from Figure 23, the annual
before-tax profit for the evaporator =
$15,000/yr.
The rinse ratio (r) is determined for the
second and third (final) rinse tanks,
given Cp = 3.2 oz/gal and C3 =
0.003 oz/gal:
3.2 oz/gal V*
0.003 oz/gal/
= 33
For the last two rinse tanks,
Rinse rate = r X drag-out
= 33 X 2 gal/h
= 66 gal/h
Therefore, 66 gal/h of wastewater
must be treated.
C =
U" r
_ 40 oz/gaj
12.5
= 3.2 oz/gal
39
-------
Wastewater discharge flow and pH recorders
The concentration of chromic acid to
be treated (also the concentration of
the second rinse tank) is calculated by:
_ 3.2 oz/gal
33
= 0.10 oz/gal
The DCF rate of return for the invest-
ment of $40,000 for the evaporator,
and a before-tax profit of $1 5,000/yr,
is determined as follows: After-tax
profit (before-tax profit tax at 48
percent) = $15,000/yr - $7,200/yr
= $7,800/yr. After-tax cash flow
(after-tax profit + depreciation) =
$7,800/yr + (0.1 X $40,000) =
$11,800/yr. From Figure 22, the DCF
rate of return for $40,000 investment
and $11,800/yr after-tax cash flow is
25 percent.
From Table 3, the payback period is
determined as: payback = $40,000/
($11,800/yr) = 3.4 yr.
Summary
The economics of the open-loop
recovery system are superior to the
economics of the closed-loop recovery
system for the following reasons:
The investment cost of evapora-
tion equipment for the open-loop
recovery system is $10,000 less
than that for the closed-loop
recovery system because of the
difference in flowrates.
The ratio of profit to investment
cost of evaporation equipment is
greater for the lower capacity
system.
The use of an open-loop evaporation
system has the advantage of reducing
the volumetric flowrate of the rinse
water stream to be evaporated. The
result is a higher concentration in the
first rinse tank (feed to evaporator),
with the further benefit of a lower utility
cost for the evaporator (utility cost is
proportional to the total volume of
liquid evaporated).
Although the closed-loop evaporator
recovers all the chromic acid, the
incremental savings is not great
enough to offset the advantages of the
open-loop configuration. If evapora-
tive recovery is not used, then the cost
of wastewater treatment by chemical
destruction, in addition to chromic acid
replacement, would be $40,000
annually.
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Abbreviations
actual ft3 actual cubic feet
Btu British thermal unit
°C degree centigrade
°F degree fahrenheit
ft3 cubic feet
g grams
gal gallons
Gg gigagrams (109 g)
GJ gigajoules (109 J)
gr grains (1.4 X 1CT4 Ib)
hm3 cubic hectometer (1 X 106 m3)
h hour
J joule
kg kilogram (103 g)
kW kilowatt {103 watt)
kWh kilowatt-hour
I liter
Ib pound
m meter
Mg megagrams (1 O6 g)
MW megawatt (1 O6 watt)
normal m3 normal cubic meter (0° C)
Pa pascal
ppm parts per million (wt)
ppmv parts per million (volume)
stdft3 standard cubic foot (60° F)
stdft3/min standard cubic feet per minute (60° F)
Tg teragrams (1.012 g)
TJ terajoules (1 O12 J)
yr year
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Environmental research and
development in the metal finishing
industry is the responsibility of the
Metals and Inorganic Chemicals Branch,
Industrial Environmental Research
Laboratory, Cincinnati OH. The U.S.
Environmental Protection Agency
hired the Centec Corporation, Fort
Lauderdale FL, under Contract No. 68-
03-2581 to prepare this report. Mr.
George S. Thompson is the EPA
Project Officer.
EPA thanks the following companies
and organizations for providing
information and technical review:
Advance Plating Company; American
Electroplaters' Society; Battelle
LaboratoriesColumbus, Ohio;
Corning Glass Works; LI-CON,
Incorporated; Pfaudler Company; and
Wastesaver Corporation,
Photographs were supplied by
Schwinn Bicycle Company, Chicago
IL, and Sommer Metalcrafts Corpora-
tion, Crawfordsville IN.
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 orcommercial products
constitute endorsement or
recommendation for use.
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