US EPA Office of Research and Development
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-99/008
November 2000
Technology Transfer
&EPA Capsule Report
Approaching Zero
Discharge in Surface
Finishing
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EPA625/R-99/008
November 2000
Capsule Report
Approaching Zero Discharge
In Surface Finishing
U.S. Environment Protection Agency
Office of Research and Development
National Risk Management Research Laboratory
Technology Transfer and Support Division
Cincinnati, OH 45268
Recycled/Recyclable
Printed with vegetable-based ink on
paper that contains a minimum of
50% post-consumer fiber content
processed chlorine free.
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Notice
The U.S. Environmental Protection Agency through its Office of Research and
Development funded and managed the research described here under contract
number 8C-R520-NTSX to Integrated Technologies, Inc. It has been subjected to the
Agency's peer and administrative review and has been approved for publication as an
EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with
protecting the Nation's land, air, and water resources. Under a mandate of national
environmental laws, the Agency strives to formulate and implement actions leading
to a compatible balance between human activities and the ability of natural systems
to support and nurture life. To meetthis mandate, EPA's research program is providing
data and technical support for solving environmental problems today and building a
science knowledge base necessary to manage our ecological resources wisely,
understand how pollutants affect our health, and prevent or reduce environmental risks
in the future.
The National Risk Management Research Laboratory (NRMRL) is the
Agency's center for investigation of technological and management approaches for
preventing and reducing risks from pollution that threaten human health and the
environment. The focus of the Laboratory's research program is on methods and their
cost-effectiveness for prevention and control of pollution to air, land, water, and
subsurface resources; protection of water quality in public water systems; remediation
of contaminated sites, sediments and ground water; prevention and control of indoor
air pollution; and restoration of ecosystems. NRMRL collaborates with both public and
private sector partners to foster technologies that reduce the cost of compliance and
to anticipate emerging problems. NRMRL's research provides solutions to
environmental problems by: developing and promoting technologies that protect and
improve the environment; advancing scientific and engineering information to support
regulatory and policy decisions; and providing the technical support and information
transfer to ensure implementation of environmental regulations and strategies at the
national, state, and community levels.
This publication has been produced as part of the Laboratory's strategic long-
term research plan. It is published and made available by EPA's Office of Research
and Development to assist the user community and to link researchers with their
clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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Acknowledgments
This guide was prepared by Peter A. Gallerani, Integrated Technologies, Inc., and
Kevin Klink, CH2M Hill. Douglas Grosse, U.S. Environmental Protection
Agency(USEPA), Office of Research and Development, National Risk Management
Research Laboratory (NRMRL), served as the project officer, co-author, and provided
editorial assistance. Dave Ferguson, U.S. EPA, NRMRL, served as the technical
advisor.
The following people provided technical review, editorial assistance, and graphic
design:
Dr. David Szlag USEPA, NRMRL
Paul Shapiro USEPA, Office of Research and Development
Joseph Leonhardt Leonhardt Plating Co.
Dr. John Dietz University of Central Florida
Carol Legg USEPA, NRMRL
John McCready USEPA, NRMRL
IV
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Contents
Notice ii
Foreword jjj
Acknowledgments jv
1 Introduction 1
2 Systematic AZD Planning 3
3 Process Solution Purification and Recovery Technologies 5
3.1 Diffusion Dialysis 5
3.1.1 Applications 5
3.1.2 Limitations 7
3.1.3 Secondary Stream(s) 7
3.1.4 Diffusion Dialysis Systems 7
3.2 Microfiltration 8
3.2.1 Applications 8
3.2.2 Limitations 9
3.2.3 Secondary Stream(s) 9
3.2.4 Microfiltration Systems 9
3.3 Membrane Electrolysis 9
3.3.1 Applications 9
3.3.2 Limitations 9
3.3.3 Secondary Stream(s) 9
3.3.4 Membrane Electrolysis Systems 9
3.4 Acid (Resin) Sorption 10
3.4.1 Applications 10
3.4.2 Limitations 10
3.4.3 Secondary Stream(s) 10
3.4.4 Acid (Resin) Sorption Systems 11
3.5 Electrowinning 11
3.5.1 Applications.. , 12
3.5.2 Limitations 13
3.5.3 Secondary Stream(s) 13
3.5.4 Electrowinning Systems 13
3.6 OtherTechnologies 13
4 Rinse Purification or Concentrate Recovery Technologies 14
4.1 Ion Exchange 14
4.1.1 Applications 16
4.1.2 Limitations 16
4.1.3 Secondary Stream(s) 16
4.1.4 Ion Exchange Systems 16
4.2 Reverse Osmosis 17
4.2.1 Applications 17
4.2.2 Limitations 18
4.2.3 Secondary Stream(s) 18
4.2.4 Reverse Osmosis Systems 18
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4.3 Vacuum Evaporation 18
4.3.1 Applications 18
4.3.2 Limitations 19
4.3.3 Secondary Stream(s) 19
4.3.4 Vacuum Evaporator Systems 19
4.4 Atmospheric Evaporation 20
4.4.1 Applications 20
4.4.2 Limitations 20
4.4.3 Secondary Stream(s) 20
4.4.4 Atmospheric Evaporation Systems 20
4.5 OtherTechnologies 20
5 Alternative Surface Finishing Processes and Coatings 22
5.1 Process Engineering and Re-engineering 22
5.2 Surface Finishing Properties 22
5.3 Surface Engineering 23
5.4 Surface Finishing Costs 23
5.5 Alternative Coatings and Processes 23
5.5.1 Alternative Electroplated and Electroless Coatings 23
5.5.2 Anodizing 23
5.5.3 Organic Coatings 24
5.5.4 Vapor Deposition 24
5.5.5 Thermal Spray 25
5.5.6 Hardfacing 25
5.5.7 Porcelain Enameling... 25
5.5.8 Metal Cladding and Bonding 25
5.6 Alternative Substrates 25
5.6.1 Alternative Substrate Treatments 26
5.7 Alternative Surf ace Preparation 26
5.7.1 Alternative Stripping Processes 26
5.7.2 Alternative Pickling and Descaling 26
5.7.3 Alternative Etching 26
5.7.4 Alternative Cleaning 26
5.7.5 Alternative Cleaning Equipment 27
5.7.6 Forming and Fabrication 27
6 Existing Processes, Conditions, and Practices 28
7 Conclusions 3°
8 References 31
Appendices
A. Systematic Approach for Developing AZD Alternatives 32
B. Installed Costs 37
vi
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Tables
1-1. Section/Topic References from Section 8 2
3-1. Technologies for Concentrated Surface Finishing Process Solution
Applications 6
4-1. Typical Ion Exchange Capacities for General Resin Types
(In milliequivalents per liter, meq/L) 16
4-2. Technologies for Surface Finishing Rinse Applications 21
6-1. General Approaches and Specific Techniques for Improving
Existing Process Conditions and Practices 29
A-1. Data Requirements for Characterizing Sources and Discharges 33
A-2. Common AZD Benefits 33
A-3. Common AZD Constraints 33
A-4. AZD Alternative Evaluation Criteria 35
A-5. Costs Savings and Benefits for AZD Actions 35
B-1. Installed Capital Cost Ranges for Typical AZD Project Approach
and Size Ranges 37
VII
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Figures
3-1. Diffusion dialysis schematic
3-2. Microfiltration system schematic
3-3. Membrane electrolysis schematic
3-4. Acid sorption system
3-5. Electrowinning system
4-1. Ion exchange system
4-2. Reverse osmosis system
4-3. Vacuum evaporation system
4-4. Atmospheric evaporation system
,..7
,..8
,10
.11
,12
,15
.17
.19
.21
VIII
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1. Introduction
This document provides technical guidance to surface
finishers, environmental managers and decision makers
on control technologies and process-changes for
approaching zero discharge (AZD). AZD is one of the key
themes underlying the Strategic Goals Program (SGP), a
cooperative effort among the U.S. Environmental
Protection Agency (EPA), the American Electroplaters and
Surface Finishers Society, the National Association of
Metal Finishers, and the Metal Finishing Suppliers
Association to test and promote innovative ideas for
improved environmental management within the metal
finishing industry. For more information on this program,
see http://www.strategicgoals.org/
In its broadest sense, "zero discharge" means no discharge
to any media. More commonly, zero discharge focuses on
zero wastewater discharge. This report presents
information and strategies for approaching zero discharge
for concentrated process fluids and associated
rinsewaters from surface finishing manufacturing. This
focus is intended to minimize discharges of spent and/or
underused process fluids. Specific SGP goals addressed
in this report are:
Improved use of process chemistry (SGP goal is
98% metals utilization on product);
Water use reduction (SGP goal is 50% reduction);
and
Hazardous waste emissions reduction (SGP goal
is 50% reduction in metals emissions to air and
water, and 50% reduction in hazardous waste
sludge disposal).
The following list provides a section-by-section overview of
this report:
Section 2: Systematic AZD Planning
This section and related Appendix A provide key
considerations for planning through implementation of any
AZD project. Without systematic planning and appropriate
implementation, an AZD project can fail or fall short of
overall potential. The techniques and technologies
presented in Sections 3 through 6 should be pursued within
a systematic framework. Specific approaches within these
general categories may be used independently or in
combination to meet specific AZD goals.
Section 3: Process Solution Purification and Recovery
Technologies
This section presents technologies for in-plant purification
and maintenance of surface finishing process solutions
and rinses. Pursuing this approach results in reduced
discharges through improved use of process solutions.
Section 4: Rinse Purification or Concentrate Recovery
Technologies
This section presents technologies for purification of rinses
for recycling to surface finishing processes. Pursuing this
approach can result in a combination of improved use of
process solutions and water.
Section 5: Alternative Surface Finishing Processes
and Coatings
Section 5 advances alternative surface finishing
processes and coatings. Most of the alternative surface
finishing processes and coatings can result in substantial
reductions in discharges compared to traditional
processes.
Section 6: Improving Existing Process Conditions and
Practices
This section presents techniques for modifying existing
process operations and plant practices. Reduced
discharges can result in modifications that provide for
better process optimization.
Section?: Conclusions
Section 8: References
Appendix A: Systematic Approach for Developing AZD
Alternatives
This is a supplement to Section 2 that presents a
systematic method to guide the identification, development,
and implementation of AZD actions.
Appendix B: Installed Costs
This appendix provides installed cost information.
Table 1-1 provides a topical section cross reference.
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Tabls 1-1 Section/Topic References from Section 8
Section Topic References
Section 2 Systematic
AZD Planning
Section 3 Process Solution Purification
and Recovery Technologies
Ion Exchange
Reverse Osmosis
Vacuum Evaporation
Atmospheric
Evaporation
Electrodeionization
Electrodialysis
Electrowinning
Nanofiltration
Polymer Filtration
« Ultrafiltration
Section 4 Rinse Purification
or Concentrate
Recovery Technologies
Section 5 Alternative
Surface Rnishing
Processes and Coatings
Section 6 Improving Existing
Process Conditions
and Practices
Section 7 Conclusions
Appendix A Systematic
Engineering Approach
Appendix B Installed Costs
9, 14, 15
2,3,4,5,6,7,9,10
2, 3, 4, 5, 6, 9
2, 3, 4, 5, 7
3, 4, 6, 7, 9, 17
5
3, 4, 5, 9, 17
3,6,9
5,9
4,5
3, 4, 5, 6, 9
11,12,18
1,2,4,5,13,15,16,17,
19
1,2,9
1,6,9
6, 9, 14, 15
1,5,7,8,9, 10, 17
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2 Systematic AZD Planning
Systematic AZD solutions can be developed by integrating
holistic source reduction planning, including considerations
for multiple sources, composite solutions, and life cycle
process and facility optimization. Nine key considerations
for systematic AZD planning are:
1. Is the AZD target a fixed endpoint or an
optimization point?
The type of AZD target frames the overall AZD
options and the planning approach. A fixed
endpoint could be below or beyond the most cost-
effective (optimal) AZD target. For example,
assume that for a particular wastewater stream,
the most cost-effective (life cycle) approach would
be to use single-stage reverse osmosis to recycle
water and reduce wastewater by 80%. Aless-than-
optimal AZD target might be to pursue a 50%
reduction goal, and a beyond-optimal AZD target
might be to pursue a 90% or 100% wastewater
reduction goal. These endpoint goals may be
based on specific drivers or constraints, such as
cost. As zero discharge is approached, the costs
for incremental discharge reductions can increase
significantly in proportion to the benefits achieved.
2. What tradeoffs are there between point source and
more combined reduction strategies?
Point source AZD strategies involve the use of
bath or rinse purification systems for individual
tanks or sources. Alternative strategies might
include combining compatible streams from
different processes for purification/recovery. This
could include use of single fixed location recovery
systems (e.g., centralized reverse osmosis/ion
exchange for recycling rinsewaters from several
process lines). Another combined strategy would
be to use a mobile system to perform intermittent
purification/recovery of several point sources. For
example, a single mobile diffusion dialysis system
might be used to purify/recycle several different
acid baths. Combined strategies may be more
cost-effective, due to economy of scale, unless
there are substantially increased plant interface
requirements. Point source systems may offer
more flexibility, redundancy, and reliability.
3. What tradeoffs are there between up-the-p'ipe
pollution prevention and end-of-pipe pollution
control?
Up-the-pipe systems can reduce end-of-pipe
system requirements. For example, bath
purification and water recycling can combine to
reduce wastewatertreatment system contaminant
loading and hydraulic sizing. In-plant systems
may also produce byproducts requiring waste
treatment or management.
4. What combination of technology, technique and
substitution would provide the best overall
solution?
Sections 3, 4, 5, and 6 present a range of
technologies, techniques and process substitution
strategies for AZD. Integrated approaches should
be considered as potential improvements over
single-approach solutions.
5. What future production and facility scenarios
should be considered?
AZD solutions should consider overall life cycle
and future production and facility needs. Potential
future requirements may lead to modified AZD
alternatives, or more allowances for change.
Defining future scenarios may lead to specific
phased implementation plans, or decisions to
accelerate/delay plans for facility renovation.
6. Are AZD solutions well defined?
Whether dealing with a single-point source, multi-
process or overall facility alternatives, all
significant impacts should be identified and
implemented to define requirements for a
comprehensive AZD solution. Those include
process byproducts, cross-media impacts, plant
interface and utility requirements, operations and
maintenance requirements. A particular approach
may be able to meet the primary AZD performance
requirement (e.g., 90% acid reuse) but may
present implementation problems caused by other
aspects (air discharge requiring ventilation
system, permitting, etc). Comprehensive definition
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of AZD alternatives is important to identify barriers
to implementation.
7. How does the surface finishing process chemistry
change with production?
One key dimension is understanding the
chemistry for each process step and how the
chemistry changes during production cycles,
including:
transfer or transformation of process
chemicals rendering them unavailable for
production and
generation of contaminants that reduce the
useful life of process chemicals.
Changes in process chemistry can necessitate the need to
purchase fresh or make-up process bath chemicals.
Similarly, the increased volume of waste process baths
and rinses requiring treatment results in more waste
treatment chemicals and corresponding increases in waste
generated.
8. What opportunities are there to use existing
systems? New systems?
Enhancements to existing systems may produce
significant benefits at low cost and overall effort.
Additional capital for new systems may result in
overall net beneficial gains in capacity,
productivity, reduced wastes, automation, and
space. Beneficial process changes may also
result from eliminating or consolidating processes.
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3. Process Solution Purification and Recovery Technologies
Purification of surface finishing process solutions allows
for extended use of bath chemistries while reducing wastes
and chemical purchases. Without solution purification,
process cycle times often increase over time as the result
of increased contaminant loading and decreased free acid
or alkali concentrations. This is especially true of stripping,
pickling, etching and cleaning process solutions. Process
contaminants are normally controlled through periodic
solution dumps and drag-out. Continuous, steady-state
bath maintenance can result in more constant production
rates and quality.
This section presents technology descriptions, applications
and limitations, secondary waste stream identification, and
system components and configurations for five process
solution purification technologies in surface finishing
applications:
1. Diffusion dialysis
2. Microfiltration
3. Membrane electrolysis
4. Acid (resin) sorption
5. Electrowinning
In addition, Table 3-1 features eight technologies,
considered to show promise for limited surface finishing
process solution applications:
1. Adsorption filtration
2. Crystallization
3. Electrodialysis
4. Ion exchange
5. Liquid ion exchange
6. Nanofiltration
7. Ultrafiltration
8. Vacuum evaporation
3.1 Diffusion Dialysis
Diffusion dialysis is a membrane separation process that
typically uses an anionic exchange membrane to transport
acid anions and protons from waste acid solutions into
deionized water streams. This process recovers useable
"free" acid commonly wasted when metals contaminant
buildup levels exceed processing criteria. Consequently,
the resultant acid bath is dumped. Such wasted free acid
consumes significant quantities of neutralization chemicals
and must be replaced in the process. Free acid readily
diffuses across the membrane in proportion to a
concentration gradient. Metal cations diffuse at a much
slower rate due to their positive charge and the negative
charge functionality of the anionic exchange membrane.
Typical acid recovery rates are 80-95% and typical metal
rejection rates are 60-95%.
Diffusion dialysis separations use a membrane consisting
of a series of alternating anion exchange membranes and
separators that form countercurrent fluid distribution paths.
Contaminated acid (feed) enters on one side and deionized
(Dl) water is fed via countercurrent on the other side.
Concentration gradients exist across the membranes. Free
acid is transported from the waste acid into the
countercurrent DI water stream via diffusion. Metals in the
feed liquor are rejected by the membrane to a large extent,
and are removed in the waste stream (retentate) for metal
recovery ortreatment. Free acid is collected in the Dl water
(dialysate) for acid recovery.Typically, the feed and exit
stream flow rates are approximately equal. Figure 3-1
shows the basic function of diffusion dialysis.
The concentration of recovered acid will normally be lower
than that of the feed acid, and make-up acid must be added
to bring the concentration up to the process level. When the
feed has a significant salt concentration, the concentration
of recovered acid can exceed the concentration of the feed
acid.
For diffusion dialysis processing, an increase in membrane
area per unit of acid flow increases the acid recovery rate.
If the flow rate of Dl water increases, the acid recycling rate
increases and the recycled acid concentration decreases.
3.1.1 Applications
Diffusion dialysis is a purification/recycling technology that
can be used to maintain or reclaim spent or contaminated
acids where acid concentrations are greater than 3% by
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Table 3-1. Technologies for Concentrated Surface Finishing Process Solution Applications
Technology
Description
Status
Applications and Limitations
Adsorption Filtration
Crystallization
Electrodialysis
Liquid/liquid Ion
Exchange
Ion Exchange
Nanofiltration
Ultrafiltration
Granulated polypropylene felt or other
lipophilic filter media is placed in filter
housings and used for removal of oils
by adsorption.
Various evaporation and cooling systems
are used to bring solutions to a super-
saturation point where solid crystals
form and can be separated from
solution.
Emerging
Anions and cations are removed from
solutions with an applied electric field in
cells with alternating anion- and
cation-permeable membranes.
Ionic contaminants are removed from
process solutions into immiscible
primary liquid extraction solutions.
Secondary liquid extraction solutions
are used to remove the contaminants
and to regenerate the primary extraction
solution.
Ions in solution are selectively removed
by exchanging positions with resin-
functional groups.
Membrane filtration manufactured for
larger size rejection (rejects molecules
larger than 0.001 to 0.008 microns)
than reverse osmosis. Preferentially
rejects some larger ions and passes
others.
Membrane filtration process that
passes ions and rejects macromolecules
(0.005 to 0.1 micron).
Commercial
technology;
limited surface
finishing
applications
Commercial
technology;
limited surface
finishing
applications
Commercial
technology;
limited surface
finishing
applications
Removes mineral oil derivatives from
aqueous cleaning solutions to less than
10 ppm range. Beneficial cleaner
components are not significantly
removed.
Operates over full pH range and at
temperatures up to 200 °F (95 °C).
Applicable for some etching or pickling
baths with relatively high metals
concentrations where controlled metals
removal and recovery are beneficial
(e.g., removal and recovery of copper
sulfate from peroxide-sulfuric etch
solutions).
Applicable for removal of carbonates to
maintain alkaline and cyanide plating
solutions. Used for acid pickling, aluminum
etchant and cyanide/alkaline plating bath
maintenance.
Used for regeneration of spent electroless
nickel baths. Sodium, iron and zinc cations
are removed through a cation membrane.
Sulfate and orthophosphite anions are
removed through an anion membrane.
Ammoniacal etch solutions have been
regenerated by removal of copper, with a
closed-loop extraction solution system.
Commercial
technology;
commonly
used in surface
finishing rinse water
applications
Limited concentrated
solution application.
Commercial
technology;
limited surface
finishing
applications
Commercial
technology;
limited surface
finishing applications
Used in some applications for tramp metal
removal from concentrated process
solutions. A typical application is the removal
of iron from chromium plating solutions.
Removal by ion exchange is not viable for
process solutions more concentrated than
ion exchange regenerant solutions.
Concentrated process solutions may also
degrade resins.
Used for separation of metals from spent
acid solutions, or from reverse osmosis
concentrates for acid purification/recycling.
Removes organics from process solutions.
For aqueous cleaners, removes more
contaminants compared to microfiltration,
but may also remove significantly more of
the beneficial cleaner constituents.
Vacuum Reduced pressure and elevated
Evaporation temperature combine to separate
constituents with relatively high
volatility from constituents with lower
volatility.
Commercial Contaminants with lower volatility than
technology; process solutions can be evaporated (e.g.,
limited use for removal of water from an acid solution).
surface finishing Evaporating the process solution.(e.g., acid
concentrates distillation) from contaminant phases with
(more commonly higher volatility can also purify process
used for rinse solutions. Multiple stages may be used to
applications) increase separation purity, to reduce energy
requirements, or to accomplish multiple
phase separations.
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Retentate
(rejected metals)
Metals -
. Dl water -KV,)
Heat exchanger
H+ Anions
-©(
7 Feed
(Contaminated acid)
Heat exchanger Filter
Anion
membrane
I Dialysate
(recovered acid)
Figure 3-1. Diffusion dialysis schematic.
weight. Diffusion dialysis is most typically
used where contaminant metals concentrations
are less than 1 gram per liter. Surface finishing
process solutions amenable to the use of
diffusion dialysis include:
Hydrochloric acid (HGI) pickle and
strip solutions
Sulfuric acid (H2SO4) anodize solutions
Sulfuric acid pickle and strip solutions
Nitric acid (HNO3) pickle and strip
solutions
Nitric acid/hydrofluoric acid (HNO3/
HF) stainless steel pickling solutions
Hydrochloric acid/sulfuric acid (HCI/
H2SO4) aluminum etch solutions
. Methane sulfonic acid (MSA) solutions
3.1.2 Limitations
Limitations in using diffusion dialysis to
recover surface finishing process acids
include:
Acids not highly dissociated (e.g.,
phosphoric acid) will not diffuse
across the membrane.
Complexed metal anions (e.g., fluoro-
titanium anions) can readily diffuse
across the anion exchange membrane
and are not efficiently separated from
the acid.
Cooling is typically needed if influent waste acid
temperature exceeds 122 °F (50°C).
Heating may be needed for low-temperature
influent waste acid. A temperature drop of 3.6 °F
(2°C) reduces the acid recycling rate by
approximately 1.5%.
Solvents can cause membrane swelling.
Strong oxidizing substances (e.g., chromic acid)
can cause membrane deterioration.
3.1.3 Secondary Stream(s)
The depleted acid waste stream (after diffusion dialysis
processing) is approximately equal in volumetricflowto the
waste acid influent. Depending on the application-specific
acid removal and metals rejection rates, the depleted acid
waste stream (retentate) typically contains 5 to 20% of the
acid and 60 to 95% of the metals from the influent waste
acid stream. This stream is usually sent to wastewater
treatment.
3.1.4 Diffusion Dialysis Systems
Typical diffusion dialysis system components include:
Membrane stack, including plate and frame,
membrane spacers, and special anion exchange
membranes
Feed and exit stream tanks and pumps
Process I&C and electrical
Acid pre-filter (some applications)
Dl water system (some applications)
-------
Heat exchanger (some applications)
Diffusion dialysis systems can be used for batch or
continuous flow applications. Small systems are often
configured as mobile units.
3.2 Microfiltration
Microfiltration (MF) is a membrane filtration technology that
uses low applied pressures in the range of 3-50 psi (20 to
350 kPa) with pore sizes in the range of 0.02 to 10 microns
to separate relatively large particles in the macromolecular
to micro particle size range (approximate molecular
weights > 100,000).
Microfiltration is typically configured in a crossf low filtration
pattern, rather than in a conventional, pass through
configuration. In the crossflow configuration, the feed
solution flows parallel to the filter media (membrane) and
splits into a permeate (filtrate) stream, which is the cleaned
solution that passes through the membrane, and a
concentrate (retentate or reject) stream that contains the
contaminants rejected by the filter membrane. A major
benefit for the crossflow configuration is that relatively
high-solids streams may be filtered without plugging.
Figure 3-2 presents a microfiltration system schematic for
process fluid purification applications.
Four primary operating parameters influence crossflow
filtration performance:
1. Trans-membrane pressure difference
2. Flow velocity parallel to the membrane
3. Temperature
4. Bulk stream component concentration
Increasing trans-membrane pressure will increase flux until
a constant flux point is reached. Increases in trans-
membrane pressure above the constant flux point will
result in a thicker, more compact particle layer at the
filtration surface.
The flow velocity parallel to the membrane influences the
shearing forces on the particle layer surface. High velocity
results in a constant layer thickness, and a more constant
flux. Generally, the higher the tangential velocity, the
higherthe flux. Higherflux results in a greater pressure drop
in the direction of tangential velocity, and higher energy
consumption. In most cases, temperature increases will
result in a lower viscosity of the liquid to be filtered. Lower
viscosity influences flow through and parallel to the
membrane. Temperature changes may cause components
to be precipitated or dissolved, greatly impacting flux.
Increases in bulk stream component concentration cause
a decrease in flux through the membrane. This effect can
vary, depending on the characteristics of the filtered
solution.
3.2.1 Applications
Microfiltration applications include:
Cleaner aqueous purification
Removal of oil and grease from process baths
Wastewater treatment applications (replaces
clarification and polishing filters)
Microfiltration has become a popular process for
continuous or batch maintenance of aqueous cleaning
solutions. Through proper membrane selection, it is
possible to remove both oily and solid solution
contaminants selectively from many proprietary industrial
cleaners. Chemical suppliers can assist in the selection of-
aqueous cleaners and microfiltration membranes to
optimize the separation of contaminants from cleaning
agents such as surfactants.
Feed
Oil
Concentrate
Oil
Solid .
phase
^
X"
^»
Sc
^
lid
^~
_____ ~~"
___ - '
Microfilter
Permeate _.
[to process)
Figure 3-2. Microfiltration system schematic.
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3.2.2 Limitations
Since cleaning agents are typically removed along with oil,
grease and dirt, the bath must be amenable to
replenishment with make-up chemical additives. Cleaners
with relatively high silicate concentrations are generally
less amenable to purification with microfiltration.
Aluminum cleaning solutions are typically not recycled with
microfiltration due to buildup of dissolved aluminum.
Recent advances in membrane technology may extend the
range of microfiltration application to silicated cleaners.
3.2.3 Secondary Stream(s)
The separated oil, grease and dirt from one or more
concentrated stream phases require waste treatment and/
or disposal. The relatively low-density oil phase is usually
skimmed off. The relatively dense dirt/grease phase is
removed in a separate bottom phase for disposal.
3.2.4 Microfiltration Systems
Typical microfiltration system components include:
Membranes and housings
Working tank (containing process fluid that is
circulated through the microfilter, and zones for
light and dense phase separations)
Oil and dense phase contaminant removal
systems (manual or automatic)
Membrane cleaning systems (chemical cleaning
and/or back pulsing system)
3.3 Membrane Electrolysis
Membrane electrolysis uses one or more ion-selective
membranes to separate electrolyte solutions within an
electrolysis cell. The membranes are ion-permeable and
selective. Cation membranes pass cations such as Cu and
Al, but reject anions. Anion membranes pass anions, such
as sulfates and chlorides, but reject cations.
Membrane electrolysis can regenerate process solutions
through two primary mechanisms: (1) Selective transfer of
ions from the process solution, across the membrane, into
an electrolyte solution and (2) Regenerating oxidation
states/ionic forms of key constituents in the process
solution through electrode electrochemical reactions.
A common configuration for removing cation contaminants
from surface finishing process solutions uses a cation-
specific membrane coupled with a two-cell compartment
drawing an electrical potential applied across the
membrane. One cell contains an anode with the anolyte
solution; the other contains a cathode with the catholyte
solution. Figure 3-3 presents a flow schematic for a two-cell
membrane electrolysis system.
The anolyte solution is typically the spent process solution
requiring regeneration. Contaminant cations are removed
from the anolyte solution and transferred into the catholyte
solution. Anode and cathode reactions occur based on the
relative electronegativity and concentration of specific ions
in each solution, as water decomposes.
3.3.1 Applications
Membrane electrolysis has been used with chromic acid-
based solutions, including chromium plating, chromic acid
anodizing, etchants and chromating solutions. Trivalent
chromium can be beneficially reoxidized at anodes to
hexavalent chrome. Contaminant metals are transported
into the catholyte. Membrane electrolysis has also been
applied to various acid-based etchants, stripping and
pickling solutions to remove contaminant metals. Multi-cell
systems with special anolyte and/or catholyte solutions
have been used with highly corrosive acids, such as nitric
and hydrofluoric, to isolate the electrodes. Membrane
electrolysis can be used on a batch or continuous basis,
and is often configured as a mobile unit for smaller point-
source applications.
3.3.2 Limitations
Limitations of membrane electrolysis:
Special materials of construction and cell
configurations may be required for processing
highly corrosive fluids.
Fume collection and treatment may be required if
hazardous gases are generated in electrode
reactions.
Anionic metal complexes require pretreatment
prior to removal across a cation exchange
membrane.
Operating temperatures are typically limited from
60°Fto140°F.
Oil, grease and solvents can adversely affect
membranes.
Suspended solids and precipitates can clog
membranes.
3.3.3 Secondary Stream(s)
Contaminant metals are typically transferred from process
solutions into catholyte solutions. The catholyte solution is
periodically replaced. The spent catholyte solution is
usually a small percentage of the treated process solution
volume, and contains concentrated removed metals.
Spent catholyte solutions can be processed for metals
recovery or handled as waste.
3.3.4 Membrane Electrolysis Systems
Typical membrane electrolysis system components
include:
Cell with anolyte and catholyte compartment(s)
-------
Feed
~ To '
process
tank
I
From
process
tank
Anolyte
tank
(pro-
cess
solution
0 0
Filter
Anions
Cation:
Membrane
electrolysis
pell
Cation
membrane
Periodic waste
Figure 3-3. Membrane electrolysis schematic.
Ion exchange membrane(s)
Rectifier, buss bars, and bussing
Anolyte and catholyte process tanks, pumps, and
piping systems
Process instrumentation and controls (if needed)
Ventilation systems (if needed)
Membrane electrolysis systems can be configured as
multi-cell systems to enhance capacity. Three
compartment cells are used for special applications where
the electrodes must be isolated from the feed stream. A
range of selective and custom-made electrodes are
available for removal of special and noble metals.
3.4 Acid (Resin) Sorption
Acid (resin) sorption is a technology used primarily for
recovering acids from surface finishing etch and pickle
solutions. Configured similarly to ion exchange, resins are
designed to selectively adsorb mineral acids while
excluding metal salts (adsorption phase). Purified acid is
recovered for reuse when the resin is regenerated with
water (desorption phase). Figure 3-4 shows a general
process flow diagram for acid sorption.
3.4.1 Applications
Acid sorption is used to separate dissolved ionic metal
contaminants from acid baths. Applications include:
Sulfuric acid anodizing baths for aluminum
Sulfuric or hydrochloric acid pickling baths for
steel and galvanized steel
Sulfuric or nitric acid pickling, etching, or
brightening baths for copper or brass
Nitric/hydrofluoric acid pickling baths used for
processing stainless steel
Phosphoric and/or sulfuric acid baths for stainless
steel or aluminum electropolishing
Cation ion exchange acid regenerant solutions
3.4.2 Limitations
Acid sorption limitations include:
Not applicable for some highly concentrated
acids.
Should not be used on acids with anionic
complexes that sorb to the resin, thus reducing
acid recovery.
Application-specific temperature limitations should
not be exceeded (e.g., approximately 90°F for
nitric acid, and up to 160°F for sulfuric or
hydrochloric acid).
3.4.3 Secondary Stream(s)
The acid sorption process recovers only a portion of the
free or unused acid. It does not recover any of the combined
acid (salt). As a result, approximately 35 to 70% of the total
acid used is incorporated into a waste stream from the
process and will require treatment. Depending on the metal
involved, treatment will range from conventional
neutralization (pH adjustment with caustic) to metals
removal (e.g., precipitation).
10
-------
01 Water
Residual Acid
Waste
Resin
Columns
Feed
!
(Spent Acid) !
,1
j
.
Reclaimed
Acid
i
|
i
i
Feed
(Spent Acid)
J
Reclaimed
Acid
s
i
Figure 3-4. Acid sorption system.
3.4.4 Acid (Resin) Sorption Systems
Typical acid sorption system components include:
Resin columns with resin
Feed and discharge pumps, piping, and tanks
Process automation for adsorption and desorption
cycling
Prefiltration
Feed stream cooling
3.5 Electrowinning
Electrowinning (also called electrolytic metal recovery) is
an electroplating process used for recovery or removal of
metals from process solutions. Electrodes are placed in
electrolyte solutions with direct current power applied to the
cell. Electrochemical reactions occur at the electrode/
electrolyte interfaces. Cations migrate to and electrons are
consumed at the cathode (reduction). Anions migrate to
and electrons are supplied at the anode (oxidation). Metal
deposition rate is a function of the electrode area, current,
solution agitation rate, solution chemistry and temperature.
Metals deposited at the cathode are removed by
mechanical or chemical means and are reused or recycled.
Figure 3-5 presents a flow schematic for electrowinning.
Three basic approaches are used for the electrolytic
recovery of metals: conventional electrowinning, high
surface area and extractive methods.
Conventional electrowinning uses solid cathodes. The
recovered metal is removed in strips or slabs and can be
sold to a refiner or used in-house as an anode material.
Several variations of the conventional electrowinning
process are available to overcome electrode polarization
and low ion diffusion rates, which reduce recovery rates in
low concentration solutions. This is typically achieved by
reducing the thickness of the diffusion layer through
agitation of the solution or through movement of the
cathode. Conventional electrolytic recovery units are
usually operated on a batch basis.
High surface area electrowinning is most often used in
continuous rinsing operations, where low concentrations
are present. High surface area units extract the metal onto
cathodes made of fibrous material such as carbon.
Passage of a strip solution through the unit and reversal of
the current regenerate the fiber cathode.
Extractive electrowinning methods are used to remove
dissolved metals from solution, without regard to recovery.
Extractive methods may include using disposable
cathodes. Dummy plating, an important form of the
extractive method for surface finishing, is an electrolytic
treatment process in which metallic contaminants in a
surface finishing solution are either plated out (low-current
density electrolysis, LCD) or oxidized (high-current
. 11
-------
Recirculatiorr
ACACACACACA
C - cathode
A - anode
Scrap metal Recycle or
(periodically scrape wastestream
from cathode) (end ofi batch cycle)
Figure 3-5. Electrowinning system.
density/high anode: cathode ratio electrolysis, HCD). LCD
dummy plating uses an average current density of 5 amps
per square foot (ASF). Copper and lead are removed
preferentially at 2-4 ASF and zinc and iron at 6-8 ASF.
Increasing the overall cathode surface area and current
while maintaining the average current density can increase
the rate of removal. Solution agitation will improve the
overall efficiency of the process.
High-current density (HCD) dummy plating typically refers
to the practice of oxidizing trivalent chromium to
hexavalent chromium in chromium plating and chromic
acid anodizing solutions. It has also been used to gas-off
chloride as chlorine. This process requires the use of
anode:cathode ratios of between 10:1 and 30:1. Lead or
lead alloy anodes are typically used in the process. Current
densities ranging between 100-300 ASF are used. The
overall cathode and anode areas and current control the
rate of conversion.
Electrowinning uses insoluble anodes in all cases except
the specialized dummy plating previously described.
Anode materials include graphite, lead, lead alloys,
stainless steel, and coated (platinum, iridium, ruthenium
oxide) titanium, tantalum, tungsten, niobium, and
conductive ceramics. Anode material selection is based on
anode corrosion, overvoltage characteristics, available
forms and life cycle cost factors. Typical cathode materials
include stainless steel, steel, porous carbon, graphite,
metallized glass and plastic beads, and substrates
equivalent to the metal being deposited.
3.5.1 Applications
Electrowinning is applied to a wide variety of surface
finishing solutions. Electrolytic recovery works best in
concentrated solutions. In the electroplating industry,
metals most commonly recovered by electrowinning are
precious metals, copper, cadmium, zinc, tin and tin /lead.
The more noble the metal in solution, the more amenable
it is to electrowinning from solution. For example, with
adequate agitation, solution conductivity and temperature,
gold can be removed from solution down to 10 mg/L using
flat-plate cathodes.
Applications include:
Acid purification (e.g., sulfuric acid used for
copper wire pickling)
Recovery of metals from ion exchange
regenerants and reverse osmosis concentrates
Controlling metal concentrations in electroplating .
solution applications where the metal concentration
increases over time
Recovery of metals from spent surface finishing
solutions
Use of low-current density (LCD) dummy plating in
the purification of nickel plating, nickel strikes,
copper plating, cadmium plating, and trivalent
chromium plating
Use of electrolysis to oxidize concentrated
cyanide solutions
LCD dummy plating can be used in both a continuous and
batch mode. Continuous dummy plating is often practiced
in high-build sulfamate nickel plating applications such as
aerospace overhaul and repair operations and
electroforming. Batch treatment is usually performed in the
process tank and requires periodic down time. Continuous
treatment is usually performed in a side tank, which should
be sized to allow approximately 0.05 ampere-hours per
12
-------
gallon of plating solution. The side tank can normally be
connected into the process filtration loop.
3.5.2 Limitations
Chromium is the only commonly plated metal not
recoverable using electrowinning. Nickel recovery is
feasible, yet requires close control of pH. Minimum
practical concentration requirements vary for the specific
metal to be recovered and for cathode type. Systems with
flat-plate cathodes operate efficiently with metal
concentrations greater than 1 to 5 g/L. For copper and tin,
a concentration in the range of 2 to 10 g/L is required for
homogeneous metal deposits.
Metals recovery can be difficult to perform for solutions that
contain chelated orcomplexed metals, reducing agents, or
stabilizers.
3.5.3 Secondary Stream(s)
Solutions depleted by electrolytic recovery can often be
treated using ion exchange to reconcentrate the ions.
Piated-out metals can often be reused or sold as scrap
metal.
3.5.4 Electrowinning Systems
Typical system components include:
Electrowinning tanks
Electrodes
Feed pump
Ventilation system
Rectifier, buss bars, and bussing
A variety of cell designs is available to provide a range of
voltage drop, mass transfer, and specific electrode area
properties. Electrolytic cells that use metal fiber cathodes
can recover metals at significantly lower concentration
than can flat plate cathodes. Many techniques can be used
to improve the hydrodynamic conditions of the cell and
force convection. These include electrolyte agitation,
pumped recirculation, rotating electrodes, and fluidized
beds. As the complexity of the system increases, so do
capital cost and operation and maintenance costs.
3.6 Other Technologies
Table 3-1 (page 6) presents a description of eight
technologies with relatively limited existing surface
finishing concentrated process solution applications. Note
that some of these technologies, like ion exchange, are
used extensively for rinse water applications.
13
-------
4. Rinse Purification or Concentrate Recovery Technologies
Purifying and recycling process rinse water reduces water
use, wastewater generation and contaminant load from
influent water. Influent water contaminants must be
removed in water pretreatment systems, to prevent entry
into the processes. Purifying and recycling rinse water can
improve process rinsing quality, thereby improving
production. In some cases, it is possible to recover
concentrated solutions during rinse water purification.
This section presentstechnology descriptions, applications
and limitations, secondary stream identification, and
system components and configurations for four key
technologies for surface finishing rinse water purification
and process solution recovery:
Ion Exchange
Reverse Osmosis
Vacuum Evaporation
Atmospheric Evaporation
Brief summaries follow for another six technologies that are
either commercial, with relatively limited existing surface
finishing concentrated process solution applications, or are
emerging:
Electrodeionizaton
Electrodialysis
Electrowinning
Nanofiltration
Polymer Filtration
Ultrafiltration
4.1 Ion Exchange
Ion exchange is a chemical reaction where ions from
solutions are exchanged for ions attached to chemically
active functional groups, on ion exchange resins. Ion
exchange resins are typically classified as cation
exchange resins oranion exchange resins. Cation resins
usually exchange sodium or hydrogen ions for positively
charged cations such as nickel, copper and sodium. Anion
resins typically exchange hydroxyl ions for negatively
charged ions such as chromates, sulfates and chlorides.
Cation and anion exchange resins are both produced from
three-dimensional, organic polymer networks. However,
they have different ionizable functional group attachments
that provide different ion exchange properties. Ion
exchange resins have varying ion-specific selectivities
(preferences for removal).
The following chemical equilibrium equation describes a
cation exchange process:
zR - A + zB+ -> zR - B + zA+
R - = Resin functional group
A+ = R'esin-bound cation
B+ = Water phase cation
z = Number of equivalents
Ion exchange systems typically consist of columns loaded
with ion exchange resin beads. Process solutions are
pumped through the columns for treatment. Figure 4-1
presents a general flow schematic for ion exchange
purification of rinse water.
Key features of ion exchange column systems:
Ions are removed in a continuous flow system.
The ion exchange resins load in the direction of
flow until the entire column is loaded.
Resins can be regenerated, whereby acidic
solutions are typically used to remove metals from
cation exchange resins, and caustic solutions are
typically used to remove resin-bound salts. Rinse
solutions are used to remove excess regeneration
fluids from the columns.
The linear flow velocity through the resin bed
impacts the ion exchange rate.
The major types of ion exchange resins include:
Strong acid resins. A typical strong acid resin
functional group is the sulfonic acid group (SO3H).
14,
-------
Acid water
Cation
exchange
Feed
(rinsewater)
Figure 4-1. Ion exchange system.
Base water
Anion
exchange
Regeneration solution
to reuse o treatment
Regenera Ion solution
to reuse or treatment
Strong acid resins are highly ionized cation
exchangers. The exchange capacity of strong
acid resins is relatively constant over specific
functional pH ranges.
Weak acid resins. A typical weak acid resin
functional group is a carboxylic acid group
(COOH). Weak acid resins exhibit a much higher
affinity for hydrogen ions than do strong acid
resins, and can be regenerated using significantly
lower quantities of regeneration reagents.
Dissociation of weak acid resins is strongly
influenced by solution pH. Weak acid resin
capacity is influenced by pH and has limited
capacity below a pH of approximately 6.0.
/
Strong base resins. A typical strong base resin
functional group is the quaternary ammonia group.
Strong base resins are highly ionized anion
exchangers. The exchange capacity of strong
base resins is relatively constant over specific
functional pH ranges.
Weak base resins. Weak base resins exhibit a
much higher affinity for hydroxide ions than do
strong base resins and can be regenerated using
significantly lower quantities of regeneration
reagents. Dissociation of weak base resins is
strongly influenced by solution pH; resin capacity
is influenced by pH and has limited capacity above
a pH of approximately 7.0.
Chelating resins. Chelating resins behave
similarly to weak acid cation resins but exhibit a
high degree of selectivity for heavy metal cations.
One common type of chelating resin is
iminodiacetate chelant resin. This resin has two
carboxylic acid functional groups attached to a
nitrogen atom that is attached to the resin
polymeric structure. The carboxylic acid groups
exchange with different cations, similar to a weak
acid resin. However, the nitrogen atom can also
form a ligand bond with metal cations, thereby
adding another cation capture mechanism.
Chelating resins are particularly selective for
'heavier divalent cations over monovalent or
trivalent cations due to the presence of two
desirably spaced functional groups.
The following lists illustrate relative ion-specific selectivity
preferences for common commercial ion exchange resin
types. The ions on each list are ordered from highest to
lowest selectivity.
Strong acid (cation) resin selectivity:
Barium> Lead> Strontium> Calcium> Nickel> Cadmium>
Copper> Zino lron> Magnesium> Manganese> Alkali
metals> Hydrogen
Strong base (anion) resin selectivity:
lodide> Nitrate> Bisulfite> Chloride> Cyanide>
Bicarbonate> Hydroxide> Fluoride> Sulfate
Weak acid (cation) resin selectivity:
15
-------
Copper>Lead> lron>Zinc> Nickel> Cadmium> Calcium>
Magnesium> Strontium> Barium> Alkalis
Chelating resin selectivity (iminodiacetate):
Copper> Mercury> Lead> Nickel> Zino Cadmium >
Cobalt> lron> Manganese> Calcium> Magnesium>
Strontium> Barium> Alkalis
Chelating resin selectivity (aminophosphonic):
Lead> Copper> Zino Nickel> Cadmium> Cobalt >
Calcium> Magnesium> Strontium> Barium> Alkalis
The exchange capacity of typical ion exchange resins can
be expressed in milliequivalent per liter (meq/L = ppm of
ions/equivalent weight per liter). Table 4-1 presents typical
exchange capacities for common commercial ion
exchange resins.
4.1.1 Applications
Ion exchange has been used commercially for many years
in water deionization, water softening applications, and
wastewatertreatment applications. There are widespread
applications for rinse water recovery and metals recovery
in the surface finishing industry. The most common
applications include recovery of copper (from acid copper
solutions), nickel and precious metals from plating rinse
water.
Ion exchange is an excellent technology for recovering
plating chemicals from dilute rinse waters. In the typical
configuration, rinse watercontaining a dilute concentration
of plating chemicals is passed through an ion exchange
column where metals are removed from the rinse water and
held by the ion exchange resin. When the capacity of the
unit is reached, the resin is regenerated and the metals are
concentrated into a manageable volume of solution.
For conventional chemical recovery processes, systems
are designed with either cation oranion beds, depending on
the charge of the ionic species being recovered. After
passing through the column, the treated rinse water is
discharged to the sewer or undergoes subsequent
treatment. In most cases, rinse water is recycled to the
process. Such systems include both cation and anion
columns to completely deionize the rinse water.
Drag-out recovery tanks can be combined with ion
exchange to reduce the required capacity of the ion
Table 4-1. Typical Ion Exchange Capacities for General Resin
Types (in milliequivalents per liter, meq/L)
Resin Type Exchange Capacity (meq/L)
Strong Acid (Cation)
Weak Acid (Cation)
Strong Base (Anion)
Weak Base (Anion)
Chelating (Sodium form)
1800
4000
1400
1600
1000
exchange columns. Using this configuration, the drag-out
tank(s) are followed by an overflow rinse that feeds an ion
exchange column. In operation, the drag-out tanks return
the bulk of the plating chemicals to the plating bath and an
ion exchange column captures only the residual chemical
load. This reduces the ion exchange system size
requirement.
4.1.2 Limitations
Common limitations for ion exchange:
Ion exchange may become impractical for use
with total dissolved solids concentrations above
500 ppm, due to the need for frequent
regeneration.
Resins have different effective pH ranges. For
example, iminodiacetate Chelating resin works
best in a slightly acidic range; selectivity is lower
at higher pH and below a pH of approximately 2.0.
Oxidants, solvents, organics, oil and grease can
degrade resins.
Suspended solids can clog resin columns.
4.1.3 Secondary Stream(s)
Regenerant chemicals can be selected to optimize the
products derived from the regeneration of ion exchange
resins. Chemicals are selected to produce salts that can be
directly recovered in the treatment process. Metals are
recovered via electrowinning and salts are recovered off-
site.
Depending on the chemical product specification of the
recovery process, the regenerant solution can be returned,.
directly to the plating tank for reuse, further processed, or
-the metals recovered by another technology, such as
electrowinning. The most common applications of this
technology are in the recovery of copper, nickel and
precious metals.
Countercurrent regeneration mechanisms can result in
significantly lower chemical use for regeneration^ the
regenerated zone is always maintained in a "clean"
condition. Co-current regeneration requires higher
chemical use and/or results in lower initial water quality as
the "regenerated zone" is left in a semi-contaminated state
following regeneration.
4.1.4 Ion Exchange Systems
Typical system components include:
Ion exchange columns with resin
Process pumps, piping and valves
Regeneration tanks, pumps and piping
16
-------
Regeneration chemicals and chemical mix
systems
Prefliters (to remove solids and organics)
Process controls (for automated
automated regeneration cycles)
or semi-
Depending on the application, various combinations of
anion, cation, and mixed-bed (anion/cation) resins may be
used.
4.2 Reverse Osmosis
Reverse osmosis (RO) is a membrane separation process
that separates dissolved salts from water using a
hydrostatic pressure gradient across a membrane. An
applied hydrostatic pressure exceeding the osmotic
pressure of the feed solution causes water to flow through
the membrane from the more concentrated feed solution
into the relatively low-concentration permeate solution.
This flow is the reverse of natural osmotic diffusion where
water would flow from the dilute .phase into the
concentrated phase. Dissolved solids are rejected by the
membrane surface. Many multi-charged ions can be
rejected at rates exceeding 99%. Single-charged ions
typically have rejection rates in the range of 90-96%.
Three important parameters impact the performance of the
RO process:
Recovery, defined as the percentage of the feed
that is converted to permeate
Permeate flux, or the rate at which the permeate
passes through the membrane per unit of
membrane surface area
Rejection, which describes the ability of the
membrane to restrict the passage of specific
dissolved salts into the permeate
The flux is determined by the hydrodynamic permeability
and the net pressure differential (hydrostatic pressure
difference between feed and permeate minus the osmotic
pressure difference) across the membrane. Higher
pressure differentials generally result in higher flux rates.
The applied pressure is generally between 400 and 800
PSI. In some specialized applications, pressures greater
than 1000 PSI are used. Permeate flux decreases over
time as an RO system is operated and the membranes
become fouled. Periodic cleaning of the membrane
restores flux. Cleaning should be initiated when a decrease
of 10-15% permeate flow, an increase of 10-15%
normalized differential pressure or a decrease of 1-2%
rejection is observed.
Rejection efficiency is specific to each component, and is
a function of concentration gradient across the membrane.
As the concentration gradient increases, the rejection
efficiency decreases. The leakage of various salts is a
function of the molecule size, ion radius, ion charge and the
interacting forces between the solute and the solvent. The
rejection of organic molecules is mainly a function of the
molecular weight and size of the molecules.
4.2.1 Applications
Reverse osmosis is used in the surface finishing industry
for purifying rinse water and for recovery of chemicals from
rinse waters. It has also been used to purify raw water for
the generation of high-quality deionized water in rinsing and
plating solutions. Figure 4-2 presents a reverse osmosis
flow schematic for rinse water applications.
Reverse osmosis applications involving the separation of
plating chemical drag-out from rinse water have been
applied mainly to nickel plating operations (sulfamate,
fluoborate, Watts and bright nickel). Other common
applications include copper (acid and cyanide) and acid
zinc. Recently, RO has been applied successfully to
n- £*;»»»
* ° «
High pressure
pump
Periodic
cleaning*
/ t (y\ ~~
/ * /--<
Pre-filter High pressure
pump
/^ Cone.
" " ^y recycle
(optional)
Reverse osmosis
Permeate
(to process^
i Periodic' cleaning
Concentrate waste
* *
Reverse osmosis
^^^
1
Concentrate per;c
(to reuse or treatment)
periodic cleaning waste
1
Permeate
recycle
(optional)
i Permeate
| (to process)
>dic cleaning waste
+
Figure 4-2. Reverse osmosis system.
17
-------
chromate rinse water. In the typical configuration, the RO
unit is operated in a loop with the first rinse following plating.
The concentrate stream is recycled to the plating bath and
the permeate stream is recycled to the final rinse.
Reverse osmosis is commonly used for water treatment
(with and without ion exchange) applications requiring
production of high-quality water from high total dissolved
solids (TDS) sources. Large-scale wastewater recycling is
evolving as an important application for RO in the surface
finishing industry. Microfiltration, ultrafiltration, nanofiltration
and reverse osmosis are often used in series to provide
pretreatmentfora range of filtration size separation needs
and to maximize performance of the total system.
Specialty membranes are available that offer an extended
pH range (1-13) and greater resistance to oxidizing
chemicals.
4.2.2 Limitations
Membrane performance of all polymer-based membranes
decreases overtime. Permeate flow (flux) and membrane
rejection performance are reduced. RO membranes are
susceptible to fouling by organics, water hardness, and
suspended solids in the feed stream or materials that
precipitate during processing. Installing prefilters can
control solids in the.feed stream. Changing operational
parameters, such as pH, inhibits precipitation. Oxidizing
chemicals like peroxide, chlorine and chromic acid can
also damage polymer membranes. Acid and alkaline
solutions with concentrations greater than .025 molar can
also deteriorate membranes.
In most applications, the feed solution will have significant
osmotic pressure that must be overcome by the
hydrostatic pressure. This pressure requirement limits the
practical application of this technology to solutions with
total dissolved solids concentrations below approximately
5000 ppm (with the exception of disc tube applications).
Specific ionic levels in the concentrate must be kept below
the solubility product points to prevent precipitation and
fouling. Ionic species differ with respect to rejection
percentage. Some ions such as borates exhibit relatively
poor rejection rates for conventional membranes.
4.2.3 Secondary Stream(s)
Reverse osmosis concentrate streams can be recycled to
the process, sent to reclamation, or managed as a
concentrated waste. Acid, EDTA and alkaline cleaning
solutions are used to clean RO membranes, depending on
the nature of the foulant. These cleaning solutions are
residual wastes that need to be managed.
4.2.4 Reverse Osmosis Systems
Typical system components include:
Pressure vessel(s) with application-specific
membrane modules
Applicable feed, pressure, and recycle pumps,
valves, and piping
Application-specific flow, level, temperature,
conductivity, and pressure instrumentation and
controls
Feed and discharge tanks and systems
Application-specific pretreatment systems (e.g.,
cartridge filtration, carbon, pH adjustment,
microfiltration, ultrafiltration, nanofiltration)
Selection of the RO membrane type depends on both the
application and the plating bath chemistry. RO membranes
are most common in spiral-wound or hollow fiber
configurations. More advanced systems use a disc tube
modular configuration.
Many systems are designed with two or more RO stages.
This design feature will allowthe concentrate stream from
the first stage to be passed through a second stage to
further concentrate the chemicals. The practical limit for
plating chemical concentration is up to 15 to 20 g/L (or lower
if compounds are neartheirlimitforprecipitation). In some
cases, insufficient surface evaporation in the plating tank
limits direct reuse of the RO concentrate stream. An
evaporator can be used to further concentrate the solution
or to supplement bath evaporation.
4.3 Vacuum Evaporation
Vacuum evaporators distill waterfrom process solutions at
reduced temperatures compared with atmospheric
evaporation. Vacuum evaporators work without the need
for an air stream feed or discharge. Vacuum evaporators
produce a distillate and a concentrate. The distilled water
is typically condensed and recovered as high-quality rinse
water. The concentrate contains process chemistry to
return to the appropriate process bath. Figure 4-3 provides
a process flow schematic for vacuum evaporation.
4.3.7 Applications
Vacuum evaporators are used for concentrating and
recovering process solutions and rinses, and are
particularly well-suited for specific applications where:
Air pollution control is a potential problem (Air
discharge is typically not an issue for vacuum
evaporation).
Relatively low evaporation temperatures are
needed to avoid problems with temperature-
sensitive products.
Alkaline cyanide solutions that build up
carbonates are present (atmospheric evaporators
would aerate the solution and accelerate the
buildup of carbonates).
Process solutions are sensitive to air oxidation.
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Liquid/
vapor^
separatorN
Condenser
Feed
Heat
exchanger
Vacuum^
source
Condensate
(distillate)
Concentrate
(return.to process)
Figure 4-3. Vacuum evaporation system.
Energy costs
evaporation.
are high for atmospheric
4.3.2 Limitations
Limitations associated with vacuum evaporation systems
include relatively high capital cost, and application-specific
potential for fouling and separation limitations.
4,3.3 Secondary Stream(s)
Vacuum evaporators produce a high-quality condensate
that can be reused as a process rinse and a concentrate
that contains the process chemistry to be reused,
recycled, or managed as a waste. Potential secondary
waste streams could be generated if there are periodic
system cleanout requirements to remove fouling
compounds.
4.3.4 Vacuum Evaporator Systems
Several types of vacuum evaporators are used in the
surface finishing industry: rising film, flash-type, and
submerged tube. Generally, each consists of a boiling
chamber under a vacuum, a liquid/vapor separator and a
condensing system. Site-specific conditions and the mode
of operation influence the system selection.
Energy for evaporation can be supplied either thermally
or mechanically. Two techniques have been applied
successfully to reduce the steam or electricity demand
for evaporation, multiple-effect evaporation and
mechanical vapor recompression. Both involve reusing
the heat value contained in the vapor from the separator.
Multiple-effect evaporators are vacuum evaporators in
series with different boiling points, operated at different
vacuum levels. The solution to be concentrated is fed into
the boiling chamber of the first effect and external heat is
introduced to volatilize the water. The water vapor is then
condensed at a different vacuum level and the energy is
used to heat the subsequent vacuum chamber. Energy is
used several times in multiple stages. Multiple-effect
evaporators are practical for larger scale applications and
those in which high-boiling point elevations make vapor
compression ineffective. These systems can be
configured so that a final effect can create crystal solids.
Increasing the number of effects increases energy
efficiency, but also increases the capital cost of the
system. Optimization involves balancing capital versus
operating costs.
The second technique is the use of a mechanical
compressor. These evaporators are similar to single effect
units, except that the vapor released from the boiling
solution is compressed in a mechanical or thermal
compressor. This compressed water vapor condenses,
yielding latent heat of vaporization, which is used to
evaporate more waterf rom the Concentrated liquid. These
types of evaporators can concentrate to about 50%
dissolved solids, and evaporative capacities range from
200-2400 liters (50-600 gallons) per hour. Vapor
19
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recompression evaporators are the highest-capital-cost
evaporators, but are the most energy efficient.
4.4 Atmospheric Evaporation
Atmospheric evaporators use an air stream to strip water
from a process solution. The process solution is pumped
through the evaporator where it contacts the air stream
blown through the evaporator. The humidified air stream is
discharged to the atmosphere. The evaporation chamber is
usually filled with a packing material to increase the air-to-
water evaporation surface. Depending on the process
solution and air conditions, heating the process solution
and/or the air stream may be necessary to achieve
sufficient evaporation.
4.4.1 Applications
Atmospheric evaporators are relatively basic, uncomplicated
low-capital-cost systems. Atmospheric evaporators are
used in conjunction with plating bath rinses to recover
process chemistry removed by drag-out. Two basic
approaches where atmospheric evaporators are used to
help achieve chemical recovery:
The solution from a heated plating tank is fed to
and concentrated by an atmospheric evaporator
and returned to the plating tank. This increases the
quantity of recovery rinse water that can be
transferred to the plating tank.
The recovery rinse water is fed to the evaporator,
concentrated, and returned to the plating tank.
Figure 4-4 illustrates a process flow schematic for
atmospheric evaporation. One application particularly well-
suited for atmospheric evaporation is drag-out recovery for
hard chrome baths, where heat is suppljed to the
evaporator by the hard chrome bath. This achieves drag-
out recovery and removes excess heat from a high-
. amperage plating bath.
4.4.2 Limitations
Atmospheric evaporation systems have several limitations:
When the feed process stream and/or air stream
needs to be heated, atmospheric evaporators
typically have a high energy use.
The discharge air stream may require treatment to
avoid discharge of hazardous substances.
In some applications, there is a risk of
overconcentration and fouling the evaporatordue
to salting-out.
Surfactants or wetters used in plating baths can
cause foaming problems in the evaporator.
Some bath constituents may be susceptible to
heat degradation or may be oxidized by exposure
to air.
Aeration of the process solutions can cause
carbonates to build up.
Recovery results vary depending on changing
process conditions and air stream conditions.
4.4.3 Secondary Stream(s)
Humidified air streams are vented from atmospheric
evaporators. These streams may present air emission
problems.
4.4.4 Atmospheric Evaporation Systems
Common atmospheric evaporator system components
include:
Process solution feed pump
Blower to draw air into and move through the
evaporator with sufficient exit velocity
Heat source
Evaporation chamber in which the water and air
can be mixed
Mist eliminatorto remove any entrained liquid from
the exit air stream
Typical commercial units have evaporation rates of 10 to
30 gph, depending on the size of the unit and.operating
conditions (e.g., solution temperature). For larger
applications, multiple atmospheric evaporators are used in
parallel.
4.5 Other Technologies
Table 4-2 presents comments for six technologies that are
commercial technologies with relatively limited surface
finishing rinse applications, or that are emerging
technologies:
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Liquid/
vapor
to atmosphere
Feed
Heat
exchanger
Inlet
' Air '
T > ambient
P< ambie it
Liquid/
vapor
Separator
Figure 4-4. Atmospheric evaporation system.
Concentrate
(return.to process)
Table 4-2. Technologies for Surface Finishing Rinse Applications
Technology Description
Status
Applications and Limitations
Electrodeionization
Electrodialysis
Electrowinning
Nanofiltration
Polymer Filtration
Ultrafiltration
Ions are removed using conventional ion
exchange resins. An electric current is
used to continuously regenerate the
resin (instead of regeneration chemicals).
Anions and cations are removed from
solutions with an applied electric field in
cells with alternating anion- and cation-
permeable membranes.
Electrodes are placed in solutions,
and direct current power is applied.
Electrochemical reactions occur at
the electrode/electrolyte interfaces.
Cations migrate to, and. electrons
are consumed at the cathode (reduction).
Anions migrate to, and electrons are
supplied at the anode (oxidation).
Membrane filtration that operates at
larger pore sizes (rejects molecules
larger than 0.001 to 0.08 microns),
and lower typical pressure (50 to 400 psi)
than reverse osmosis. Preferentially rejects
some ions and passes others.
Chelating, water soluble polymers
selectively bind target metals in aqueous
streams.
Membrane filtration process that passes
ions and rejects macromolecules
(0.005 to 0.1 micron).
Commercial
technology;
limited surface
finishing applications.
Commercial
technology;
limited surface
finishing applications.
Commercial
technology;
widely used for
removal of metals
from surface finishing
concentrated solution;
limited applications
for rinses.
Commercial
technology;
limited surface
finishing applications.
Effective for relatively high-purity water
purification/recovery applications, including
polishing treatment of reverse osmosis
permeate to meet process
rinse purity requirements.
Electrodialysis has been used in the surface
finishing industry to recover nickel salts from
rinse water.
Electrolytic cells that use a metal fiber
cathode have demonstrated the ability to
remove less-noble metals, such as copper
and cadmium, from recirculated rinses to
concentrations in the range of 10 to 50 mg/L.
High-surface-area units are used to recover
metals from cyanide-based plating process
rinses (e.g., cadmium, copper, zinc, and
brass). These units remove metal ions to
low concentrations and also oxidize the
cyanide in the rinse water.
Recovery of metals and water recycle for
rinse waters.
Emerging
Commercial
technology;
limited surface
finishing applications.
Selective metals removal from rinse waters.
Removal of oils, colloidal silica, particles, and
proteins from rinse waters for reclamation.
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5. Alternative Surface Finishing Processes and Coatings
Process substitution is a form of process optimization,
where environmentally cleaner process alternatives
replace existing processes in part or in whole. Process
substitution, however, will not automatically result in
cleaner manufacturing. In assessing alternative processes,
the first step is to review existing processes for
opportunities to optimize those processes. Adaptation of
new processes is generally desirable when implemented
on a progressive basis, but can often fail when
implemented without careful planning basis. Process
change can be complicated and deserves careful
evaluation. New processes may present unexpected
problems that are relatively difficult to manage compared
to known problems with existing processes.
Successful process substitution requires well-defined
goals. This involves identifying the drivers which impact
the manufacturing process. Too often, process
substitution is begun prematurely without a clear
understanding of the drivers and constraints.
One common misconception is that toxic use reduction
(TUR) and toxic waste reduction (TWR) are synonymous.
TUB may require process change; whereas, TWR often
favors process optimization. Toxic use reduction has a
primary goal of reducing the amount of hazardous raw
materials used. This goal can be accomplished by a variety
of methods that follow substitution, reuse and recycling.
Toxic waste reduction seeks to reduce hazardous waste
generation by optimizing the existing process, rather than
a wholesale process substitution. Both techniques can
result in more efficient processes by incorporating
recovery and/or recycling options.
Another common misconception in process substitution is
that substitute technologies must embody all of the
finished surface properties of the existing finished surface.
Alternatives should possess the properties critical to that
application. The application should be assessed to
determine critical and non-critical properties. The result will
be a wider range of possible alternatives.
Athird common misconception is that alternatives must be
universal. There are rarely any universal substitutes.
Instead, several alternatives may be required to satisfy a
range of applications.
5.1 Process Engineering and
Re-engineering
Process engineering and re-engineering costs and
success can be impacted by a variety of factors, including
chronology. Mature processes are often significantly
constrained by product specifications and the expected life
of the process. A process that will be phased out within a
few years will undoubtedly receive little new investment for
engineering or capital. Many mature processes are also
constrained by risk factors. A failure of a new coating in a
jet engine can result in human tragedy. Process changes
can require significant testing and associated costs.
How changes impact overall product and process flow is
also an important consideration. Manufacturing flow is
becoming more cellular and this results in the addition of
multiple smaller-scale wet processes to support the
manufacturing cells. Key questions to consider for overall
impacts:
How will new process(es) affect manufacturing
flow?
How will the new process(es) fit into the overall
facility?
5.2 Surface Finishing Properties
Finished surface properties and variability with different
process factors are important to evaluate to consider
substitute processes. Decorative and functional properties
are affected by key process parameters such as solution
concentration, bath additives, bath temperature, and
current density. Careful consideration of these parameters
in the context of process substitution or optimization is key
to successful process change. Coating adhesion, pre-
processing requirements, and plating characteristics over
a range of operating parameters all can vary for a process
substitution.
Questions to consider for potential substitutions include:
How will processing times be affected?
Are different pre-processing steps required?
How will the pre-processing treatment steps affect
the substrate?
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Will additional procedures, such as grinding or
coating, be required to achieve the final surface
requirements?
5.3 Surface Engineering
Surface finishing is an integrated system. Surface
engineering that considers the entire product manufacturing
cycle will result in much greater choices and efficiency.
Successful process change is the product of a logical
evaluation of not only the immediate process, but also how
this process fits into the scheme of the entire
manufacturing picture.
5.4 Surface Finishing Costs
Overall costs to produce a finished surface determine the
efficacy of a process substitution. Coatings are generally
applied to reduce net product cost and/or enhance product
properties. Copper-plated aluminum may be used instead
of solid copper to reduce cost or weight. In many
applications, both the substrate and the coating are critical
to the application. The substrate provides material strength
or other properties, while the coating provides corrosion
resistance or cosmetic appeal. Polished stainless steel is
often used in place of chromium-plated trim. Chromium-
plated plastic may be a viable lower-cost alternative to
stainless steel.
5.5 Alternative Coatings and Processes
There is a wide range of alternative processes and coatings
available. Some are common surface finishing techniques
in which the range of application is being expanded by
subtle change. Others are newer solutions that have
applicability in specific situations, such as vacuum and
thermal spray techniques. Alternative coatings and
processes include:
1) Alternative Electroplated and Electroless Coatings
2) Anodizing
3) Organic Coatings
4) Vapor Deposition
5) Thermal Spray
6) Hardfacing
7) Porcelain Enameling
8) Metal Cladding and Bonding
9) Alternative Substrates
10) Alternative Substrate Treatments
11) Alternative Surface Preparation
12) Alternative Stripping Processes
13) Alternative Pickling and Descaling
14) Alternative Etching
15) Alternative Cleaning
16) Forming and Fabrication
5.5.1 Alternative Electroplated and
Electroless Coatings
Much process change is focused on alloy plating
techniques. For some applications, alloy plating can
replace chromium, cadmium and other metals. These
solutions require more attentive control and are typically
more difficult to recover from rinse streams, primarily due
to the selectivity of recovery processes for specific atomic
species. The control and recovery factors, if not properly
handled, can lead to increases in labor, processing times
and product defects.
Alloy plating processes include nickel/tungsten/boron, tin/
nickel, tin/zinc, zinc/nickel, copper/tin/zinc, and zinc/
cobalt alloys. Nickel composite processes are important in
applications requiring lubricity and/or wear resistance.
Aluminum electroplating has replaced cadmium in some
applications. Zinc and cadmium (cyanide and non-cyanide)
are the most important sacrificial .coatings available.
Copper (cyanide and non-cyanide) plating is ubiquitous.
Trivalent chromium has replaced hexavalent chromium in
many decorative plating applications and is becoming
more important in functional applications. Nonchromate
conversion coatings are evolving rapidly, and in some
cases offer better performance than chromates. Silver
(cyanide and non-cyanide), tin and lead-free tin-alloy
plating alternatives have also been developed and are
gaining acceptance in some applications. Electroless
alternatives include nickel, copper, gold, silver, cobalt,
platinum, palladium, ruthenium, and palladium, ruthenium,
rhodium, and iridium alloys. Hot dipping processes include
zinc, aluminum,, tin and lead. Mechanical plating
processes include zinc, cadmium, zinc/cadmium, tin/
cadmium, tin, copper, and lead.
5.5.2 Anodizing
For many years, sulfuric acid anodizing and chromic acid
anodizing have been used widely in industry. One of the
main benefits of chromic acid anodizing is that residue in
lap joints or blind holes is not corrosive to the part.
However, in this age of strict specifications, the staining
these residues cause leads to rejection on a cosmetic
basis. In addition, the residue represents lost material
valuable to the process. Coupled with risk from its health
hazards, chromic acid anodizing is declining. In many
applications it can be replaced by sulfuric/boric acid
anodizing.
Other alternative anodizing processes include sulfuric,
sulfuric/oxalic, sulfuric/boric, phosphoric, oxalic, sulfamic
(NH2SO2OH), malonic (CH2(COOH)2), and mellitic
23
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(C6(COOH)6) chemistries and numerous variations thereof
with special additives.
5.5.3 Organic Coatings
Organic coatings have always been an alternative to
electroplating, and recent formulations and application
methods have increased the substitution of organic
coatings fortraditional electroplating applications. Higher-
efficiency coating techniques, such as powder coating and
electrocoating, have made organic coatings very
attractive. In addition, many solvent systems that
eliminate the traditional VOCs are appearing. Some
drawbacks occur with two-part epoxy finishes, due to their
limited application times, ("pot life") after mixing. Another
difficulty that can arise, particularly with thermal cure
coatings, is a loss of viscosity priorto final cure. This effect
can result in non-uniform coatings. Sometimes, partial cure
can reduce this effect. Many of the more efficient coating
techniques are capital-intensive for equipment. Since
newer coatings are quite expensive, it is easier to justify
equipment on the basis of savings these improvements
yield. Organic finishing can produce bright, metallic-
looking coatings, as well as a variety of colors and textures.
Coating techniques include spray coating, powder coating,
electrocoating, autodeposition and dip processes. Organic
coatings are made up of polymers or binders, solvents,
pigments and additives. Polymers or binders include
natural oils, alkyds, polyesters, aminoplast resins,
phenolic resins, polyurethane resins, epoxy resins, silicon
resins, acrylic resins, vinyl resins, cellulosics and
fluorocarbons. Pigments are classified as colored, white,
metallic and functional. Solvents are classified as active
solvents, dilutants and thinners. Additives include
surfactants, colloids, thickeners, biocides, fungicides,
freeze/thaw stabilizers, coalescing agents, defoamers,
plasticizers, flattening agents, flow modifiers, stabilizers,
catalysts and anti-skinning agents. Organic coatings
require careful screening to optimize life cycle cost factors.
5.5.4 Vapor Deposition
Vapor deposition offers finishing alternatives in some
specific and ever- expanding applications. The equipment
used is relatively expensive, can require high-level
operators and is sensitive to contamination. Vapor
deposition is, generally, a high-vacuum process. The
systems are sensitive to humidity, often require controlled
environments and are vented with dry nitrogen or
compressed air. Some cycle times are quite short, while
others can take several hours.
In a typical vapor deposition system, two or more stages
of vacuum pumping are required (rough and high vacuum).
Mechanical roughing pumps can achieve pressures of 10-
100 to millitorr (760 torr = atmospheric pressure). High
vacuum pumps can achieve pressures in the range of
10-s-10'a torr. These systems operate by isolating the
vacuum chamberf ram the pumps, enabling the parts to be
handled without complete system shutdown. A typical
operating cycle involves set-up, rough pumping, high-
vacuum pumping, coating, venting and part removal. Cycle
times can be as short as a few minutes or may require
several hours. Internal systems to coat the parts vary
depending on the coating material and performance
requirements. There are three basic types of vapor
deposition systems: chemical vapor deposition, physical
vapor deposition, and ion vapor deposition.
Chemical vapor deposition (CVD) is a heat-activated
process that relies on the reaction of gaseous chemical
compounds with heated and suitably prepared substrates.
The CVD process can produce a variety of high-density,
high-strength, and high-purity coatings. The process has
exceptional throwing power, and complex components can
be coated successfully. Most materials readily
electroplated are not suitable for CVD because these
metals are not normally available as CVD-compatible
halide salts.
In CVD, an inert gas is bled into the system after pump-
down, to a few torr of pressure. A high voltage is applied to
the gas to create a reactive plasma. Depending on the
material used forcoating, it may be evaporated directly and
then ionized in the plasma, or ions may combine with a
second gas (oxygen or nitrogen, for example) and
molecules will cool and crystallize upon striking the part
surface. The compound formation reaction usually occurs
in the plasma. Plasmas are very energetic, and care must
be exercised to prevent overheating the parts. Typical
coatings include refractory compounds and refractory
metals.
Physical vapor deposition (PVD) uses similar equipment
and operating procedures. PVD describes a broad class of
vacuum coating processes, wherein material is physically
emitted from a source by evaporation or sputtering,
transmitted through a vacuum or partial vacuum by the
energy of the vapor particles, and condensed as a film on
a substrate. Chemical compounds are deposited by
selecting an equivalent source or by reacting the vapor
particles with an appropriate gas. Three primary
characteristics of all PVD processes are: source-
generated coating emissions, vapor transport through a
vacuum, and condensation on a substrate.
The PVD process is generally limited to thinner coatings of
1-200 microns. Fixturing is critical because the process is
"line-of-sight." Stationary, rotary and rotary with planetary
motion fixtures are used to produce uniform coatings on
complex parts. The process is capable of producing
. coatings with extraordinary decorative and functional
properties.
Ion vapor deposition (IVD) was originally developed as an
ion plating process for aluminum. The properties of IVD
aluminum coatings are nearly identical to aluminum. The
IVD process takes place in an evacuated chamber where
an inert gas is added to raise the pressure of the chamber.
The gas (typically argon) becomes ionized when a high
negative potential is applied to the parts to be coated. The
24
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positively charged ions bombard the negatively charged
parts and provide final cleaning. Aluminum or other metals
are melted and vaporized in the chamber, and some metal
vapor is ionized, coating the parts. The IVD process
produces dense, highly adherent and uniform coatings over
complex parts. Hydrogen embrittlement is notafactor. IVD
eliminates solid-metal embrittlement of titanium and does
not reduce the fatigue strength of aluminum.
Other more specialized vapor deposition processes are
vacuum metallizing and sputtering. Vacuum metallizing
consists of evaporating a metal or metal compound at high
temperature in an evacuated chamber. The vapor
condenses on a substrate in the chamber, at a relatively
low temperature. Vacuum metallized coatings are typically
very thin, 0.2-20 microinches. Decorative products are
typically coated with protective organic topcoats. In
principle, virtually all metal and metal compounds can be
deposited as coatings. In practice, the process has been
generally limited to aluminum, selenium, cadmium, silicon
monoxide, silver, copper, gold, chromium, nickel-
chromium, palladium, titanium and magnesium fluoride.
Sputtering is a specialized PVD process. Virtually all
metals and compounds can be sputter-coated. This
technique generates an energetic particle that strikes a
target of the coating material, ejecting a molecule that
strikes and cools on the part. Sputtering is very much a
"line-of-sight" process and has difficulty coating surfaces
perpendicular to the target plane. Coatings are generally
thin (angstroms to microns), although thicknesses greater
than 25 microns are possible. A variety of PVD processes
exist, including:
Diode and triode sputtering
Planarand cylindrical magnetron sputtering
Direct current (DC) and radio frequency (RF)
sputtering
Electron beam evaporation
Arc evaporation
5.5.5 Thermal Spray
Thermal spray is a process that deposits a molten and
semi-molten matrix, including metals, metal alloys and
ceramics on substrate materials. The process does not
normally change the mechanical properties of the
substrate. Spray materials can be in the form of rod, wire,
cord or powders. Materials are heated to a molten or semi-
molten state and then atomized or projected onto the target
substrate. Heating is accomplished by a variety of means.
As sprayed particles strike the substrate, they flatten and
form thin platelets.
Coating techniques include flame spray, high-velocity oxy/
fuel (HVOF), electric arc, plasma spray, and detonation
gun. A wide range of coating compositions is possible with
thermal spray. Substrate heating is normally minimal.
Surface preparation is usually limited to cleaning and
roughening. Masking and fixturing are important for
effective coating. Fixtures become coated during the
process and will require frequent stripping, using strong
acids to maintain the fixtures. Thermal spray is a line-of-
sight process and coating of complex components can be
difficult. Automation of the process with robots or
specialized machinery is almost always required for a
quality coating. Post-coating finishing, such as grinding, is
usually required to obtain desired surface finish because
the as-sprayed surface finish is often rough.
5.5.5 Hardfacing
Hardfacing produces a buildup of material in specific areas
to improve wear resistance or to reclaim worn parts.
Mechanical finishing techniques such as grinding,
polishing and lapping may be required to achieve the
desired work surface. Hardfacing materials are generally
applied by a variety of welding methods. Very thick layers
can be built up with manual or automated equipment.
Thermal spray and hardfacing coatings include tungsten
carbide, high chromium irons, martensitic alloy irons,
austenitic alloy irons, martensitic, semi-austenitic and
pearlitic steels, chromium-tungsten alloys, chromium-
molybdenum alloys, nickel-chromium alloys, chromium-
cobalt-tungsten alloys, nickel-based alloys, and
copper-based alloys.
5.5.7 Porcelain Enameling
Porcelain enamels are highly durable, alkaliborosilicate
glass coatings bonded by fusion to a variety of metal
substrates at temperatures above 800°F. Porcelain
enamels differ from other ceramics by their predominantly
vitreous nature. Porcelain enamels have good chemical
resistance, good corrosion protection, good heat
resistance, reasonably good abrasion resistance and good
decorative properties. Porcelain enameling is commonly
applied to food processing equipment, cooking and serving
utensils, jet engine components, induction heating coils,
transformer cases, mufflers, home appliances, and
architectural materials.
5.5.5 Metal Cladding and Bonding
Metal cladding and bonding applications include stainless
steel (SS) to copper and aluminum cookware, titanium and
SS to copper and aluminum buss bars, electroplated
plastic to aluminum and steel automotive components.
5.6 Alternative Substrates
Alternative substrates can replace many coatings.
Stainless steel is commonly substituted for chromium-
plated steel. Copper replaces copper-plated aluminum.
Anodized aluminum replaces chromium-plated steel in
some applications. As the environmental costs of
.manufacturing processes are considered in overall product
cost, more-expensive materials can often be justified.
Alternative substrates include stainless steel, copper and
copper alloys, aluminum and magnesium, titanium,
25
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tungsten and molybdenum, superalloys, plastics,
composites, and powdered metals.
5.6.1 Alternative Substrate Treatments
Alternative substrate treatments can be used to extend the
application range of coated and uncoated substrates.
Many substrate treatments are available including vacuum
impregnation, heat treatment, ion implantation, laser
hardening, carburizing, electron-beam hardening, nitriding,
flame hardening, carbonitriding, boronizing, chromizing,
induction hardening and high-frequency resistance
hardening. With the exception of vacuum impregnation
these treatments usually convert the surface structure of
the metal substrate by the addition of small amounts of
various elements at elevated temperatures. The crystal
structure of the surface is altered and the new surface
provides additional hardness, wear resistance or
toughness, depending on the specific treatment used.
These treatments often allow the substrate to function with
little additional surface treatment, thereby reducing the
plating or surface finishing steps required.
5.7 Alternative Surface Preparation
Many alternative surface preparations can be used to
extend the application range of substrates or to
complement treatment and coating processes. Surface
preparation, prior to plating and finishing, is also an
important surface finishing stepfrom a pollution prevention
standpoint. First, these techniques remove loose metal
chips and also deburr parts. If left on the part, burrs can
dissolve preferentially in many of the pretreatment
solutions, adding to metal loading. They also alter the local
current density and can lead to additional rejects. Finally,
these surface preparations can'impact compressive
stresses on the part, which improve fatigue strength. This
can improve plating adhesion and lengthen the service life
of the part. Alternative surface preparations include
chemical polishing and bright dipping, electropolishing,
mass finishing, abrasive flow machining, abrasive blasting
(dry and wet blasting), shot peening, mass finishing, and
thermal deburring and deflashing.
5.7.1 Alternative Stripping Processes
Stripping process alternatives are usually focused around
the substitution of non-maintainable strips for maintainable
strips. Many of the non-maintainable strippers are heavily
chelated to improve their useful processing capacity.
Chelating chemicals bind metals to prevent immersion
deposition and smut formation, and separation of these
metals to extend bath life can be difficult. In addition, these
materials aredifficulttotreatforwaste. Maintainable strips
are often more predictable, because some metal loading is
maintained in the solution. This constant loading removes
the highs and lows of processing rates related to low or high
dissolved metal concentrations.
Abrasive stripping alternatives are normally applied to soft
organic coatings and are not generally applicable to
inorganic coatings. Chemical strippers used on organic
coatings create considerable waste volumes. The
strippers cause swelling of the coating to destroy the
surface bond. The strippers are often hazardous, while the
cured coating is not necessarily. Abrasive blasting or water
jet blasting can efficiently remove these coatings, generate
less waste (often non-hazardous), and can be configured
with materials that do not abrade the part surface.
5.7.2 Alternative Pickling and Descaling
There is a variety of alternative pickling and descaling
processes, most of which are acid-based. Alkaline
processes are normally electrolytic and have historically
contained cyanide. Non-cyanide alkaline descalers are
heavily chelated and can cause wastewater treatment
problems. Chromic acid is normally used for nonferrous
alloys. Nonchromic alternatives almost always contain
nitric acid, and NOJs becoming an increasing difficult
environmental control problem. Acid salts are commonly
substituted for mineral acids, and ammonium bifluoride is
substituted for HF to minimize safety concerns. Ammonia,
however, can cause wastewater treatment problems.
Potassium permanganate is used for descaling wire and
other steel products, and molten salt descaling is used for
a variety of applications.
5.7.3 Alternative Etching
Aluminum finishers often used etching as a cleaning
process, which generated a considerable amount of
unnecessary waste. Caustic etching is a source of
considerable waste. Substrates are often over-etched,
generating excessive dissolved aluminum. Aluminum
preparation for anodizing, chromating or electroplating
does not always require etching. Acid etching is commonly
substituted for alkaline etchants in aluminum finishing and
yields a lighter etch. Non-etch cleaners are available. The
printed wiring board (PWB) industry uses a variety of
etchants. Considerable effort has been focused on fully-
additive processing as an alternative to subtractive
processes. PWB etchants include peroxide, ammoniacal,
cupric chloride, ferric chloride, and chromic acid.
5.7.4 Alternative Cleaning
Proper cleaning and preparation forfinishing is critical. With
the environmental controls required for vapor decreasing,
a variety of technologies have re-emerged. Typically, the
lower vapor pressure cleaners (aqueous, semi-aqueous,
etc.) were not used because of the additional rinsing and
drying steps required compared to vapor decreasing.
Generally, these cleaners have higher soil-bearing
capacities than vapor-phase solvents. Efficient rinsing is
important, because residues can interfere with subsequent
processes or can corrode the part surface. Selection of
alternative cleaning equipment, however, is often more
important than the chemistry. Many of the alternative
cleaning technologies can be operated at elevated
temperatures to improve evaporative drying. Hot rinses
and air knives are often used to assist drying. With some
metals, rust-inhibiting additives are used to prevent
corrosion from occurring before final processing is
completed. A variety of cleaning equipment is available.
26
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Alternative cleaning processes include aqueous, semi-
aqueous, solvent, abrasive blasting and vacuum de-oiling.
5.7.5 Alternative Cleaning Equipment
Various equipment features are added to improve the
activity of the cleaner on soils. These features include
mechanical agitation, sprays, eductors (fluid jets) and
ultrasonics. All act to mechanically remove soils and
particles, as well as carry fresh solution to the part surface.
Vapor degreasers are still used in strategic applications,
particularly in ultra-precision cleaning. The new solvents
are expensive (some approaching $100/gal) and are used
in totally enclosed machines. Many shops that used
traditional vapor degreasing have switched back to
trichloroethylene while other alternatives are investigated.
Alternative cleaning equipment includes immersion, spray,
spray-under-immersion, ultrasonic, vapor degreasing and
hermetically sealed vapor degreasers.
5.7.6 Forming and Fabrication
Improving forming and fabrication processes can often
reduce surface finishing requirements. Improved casting
methods can reduce surface porosity, improve surface
finish, and reduce surface contamination and inclusions.
Improved rolling, forging, drawing and stamping can reduce
burrs, and likewise reduce surface contamination and
inclusions. Improved substrates can also dramatically
reduce surface finishing costs by reducing surface
preparation. Furthermore, standardizing and maintaining
coolants and lubricants can substantially reduce in-
process and final cleaning requirements. Depending upon
the cleaning process used, petroleum-based lubricants
may be easier to clean and/or separate from cleaners.
Coordination between forming and fabrication operations
and surface finishing operations can pay enormous
dividends.
27
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6. Existing Processes, Conditions, and Practices
A range of process conditions and practices can be
changed to reduce the generation of waste. General
approaches are described below, with specific examples
listed in Table 6-1:
(1) Improve facility conditions and housekeeping.
General facility conditions and housekeeping
practices can be improved to help reduce process
bath contamination and overall facility waste
generation.
(2) Reduce process solution drag-out. Reducing
process solution drag-out directly reduces
process solution contaminant loading to rinse
baths and subsequent wastewater treatment
systems. Since drag-out removes process bath
contaminants as well as beneficial chemicals, the
impacts on bath contamination buildup and
potential bath purification requirements should be
considered when evaluating any drag-out
reduction alternative.
(3) Improve rinsing (reduce drag-in). Improving rinsing
performance reduces the carry-over of process
bath constituents into subsequent process steps.
Improving rinsing efficiency and reducing wasteful
rinsing reduces wastewater and conserves water.
(4) Improve process solution control. Improving
process solution control helps maintain production
consistency and reduces wastes from less
efficient processing, shorter process bath life, or
longer processing times. Methods for process
solution control range from simple process
operations and maintenance procedures to more
sophisticated systematic or even automated
chemistry monitoring and controls.
(5) Select and maintain process materials to
minimize contamination. Process equipment can
contribute to waste generation if not properly
selected and maintained for the application.
Corrosion-resistant equipment should be selected
for new or replacement applications. Maintenance
procedures should be developed and followed to
maximize equipment life and minimize corrosion,
and to avoid spills or upsets.
(6) Enhance process procedures. A number of
process modifications can reduce waste
generation. Each process step should be
considered for potential beneficial changes.
28
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Table 6-1.
General Approaches and Specific Techniques for Improving Existing Process Conditions and Practices
General Approach
Improve Facility Conditions and Housekeeping
Specific Technique
Reduce Drag-Out
Improve Rinsing
Improve Process Solution Control
Enhance Process Procedures
Control material purchases to maximize material use and
minimize waste
Control air contaminants
Plan and implement programs to avoid spills and minimize
wastes from cleanup operations
Use proper rack and barrel design and
maintenance
Reduce plating bath viscosity (reduce temperature,
concentration, surface tension)
Capture drag-out before rinsing
Install fog or spray rinses
Use drag-out tanks to return chemicals to process tanks
Use multiple drag-out tanks to increase the chemical
recovery rate
Adjust part withdrawal and drip times to minimize drag-out
Control the rate and time of water flow to match process
needs
Turn off rinse water when not in use
Use spray rinsing to mechanically remove chemicals and
contaminants
Use countercurrent rinsing
Use cascade or reactive rinsing
Track water use
Promptly remove materials that fall into the tanks
Filter baths to remove suspended solids
Use carbon filtration on baths, where effective, to remove
contaminant organics
Use conductivity and pH monitoring to detect chemical losses
Implement statistical process monitoring and control
Implement real-time system monitoring and control
Use good cleaning and surface preparation techniques and
part inspections to minimize bath contamination and part
rework
Define water quality standards and use feed water of
appropriate purity
Document and follow good operating procedures
Mask areas not to be processed
Eliminate obsolete processes
Use both soluble and insoluble anodes in the same bath to
balance cathode and anode efficiencies
Remove anodes from idle baths where this will reduce metals
buildup (e.g., cadmium and zinc anodes)
Nickel plate copper buss bars to reduce the rate of corrosion
and bath contamination
29
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Conclusions
A variety of management practices and technologies are
available to enable surface finishing manufacturers to
approach or achieve zero discharge. Individual or
combined actions consisting of source reduction, process
water recycling, and process substitution need to be
considered to determine the best approach for specific
applications. Understanding process chemistry and
production impacts are essential to the identification,
evaluation, and implementation of successful AZD
actions. Systematic methods can be used to help
managers move effectively through the planning, decision-
making, and implementation phases. Systematic
considerations can be included in AZD planning to optimize
integrated process, environmental, and facility benefits.
Benefits from implementing AZD projects can include:
reduced costs, waste generation, and chemical usage,
increased regulatory performance, and enhanced facility
operations. However, as zero discharge is approached, the
costs for incremental discharge reductions can increase
significantly in relation to the benefits achieved.
Suggested areas for additional development to help
advance AZD initiatives include:
Water and rinse water quality standards
Process solution contaminant standards
Process pollution prevention and control
technology verification data linked to specific
applications
Installed cost and operations and maintenance
(O&M) cost survey data corresponding to AZD
implementation
30
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8. References
1) Benchmarking Metal Finishing, National Center
for Manufacturing Sciences, June 2000.
2) Metal Finishing 2000 Guidebook and Directory
Issue, Elsevier Science, Vol. 98, No. 1, Jan. 2000.
3) 21 st AESF/EPA Pollution Prevention and Control
Conference, Jan. 2000.
4) Products Finishing2000 Directory and Technology
Guide, Gardner Publications.
5) Proceedings: AESF/EPA Conference for
Environmental Excellence, Jan. 1999.
6) Gallerani, Peter A., P2 Concepts and Practices for
Metal Plating and Finishing, AESF/EPA Training
Course, 1998.
7) "19th AESF/EPA Pollution Prevention and Control
Conference," Jan. 1998.
8) "18th AESF/EPA Pollution Prevention and Control
Conference," Jan. 1997.
9) Reinhard, Fred P. and Kevin L. Klink, Innovative
Recycling and Maintenance Technologies for
Surface Finishing Operations, 18th AESF/EPA
Conference on Pollution Prevention and Control,
Jan. 1997.
10) "17th AESF/EPA Pollution Prevention and Control
Conference," Feb. 1996.
11) Wick, Charles and Raymond F. Veilleux, Tool and
Manufacturing Engineer's Handbook, "Materials,
Finishing and Coating," Volume 3,4th Edition.
12) "Surface Cleaning, Finishing and Coating," Metals
Handbook, Volume 5, 9th Edition.
13) Altmayer, Frank, Pollution Prevention and Control
-An Overview, AESF Press, 1995.
14) Higgins, Thomas E., Pollution Prevention
Handbook, Lewis Publishers, 1995.
15) Byers, William, et al., Howto Implement Industrial
Water Reuse: A Systematic Approach, Centerfor
Waste Reduction Technologies - American
Institute of Chemical Engineers, 1995.
16) Lowenheim, FrederickA., Electroplating,Technical
Publications, Ltd., Arrowsmith, 1995.
17) Cushnie, George, Pollution Prevention and
Control Technology for Plating Operations,
National Center for Manufacturing Sciences,
National Association of Metal Finishers, 1994.
18) US Department of Commerce, Advanced Surface
Engineering, NISTGCR 94-640-1.
19) Gallerani, Peter A., Summary Report: Minimization
of Metal Finishing Wastes at Pratt and Whitney,
Jan. 1993.
31
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Appendix A
Systematic Approach for Developing AZD Alternatives
Systematic AZD planning can be achieved by integrating process facilities. Opportunities should be identified where
holistic and specific source reduction assessment, discharge reductions can solve discharge-related
including considerations for multiple sources, composite problems, or can otherwise benefit production operations
solutions, life cycle design and facility optimization. and overall costs.
Systematic AZD solutions can be relatively easy to
develop, or may require extensive data collection, scenario
development and failure analysis. Accurate data collected
from technologies and process changes are needed to
evaluate case-specific application potential. Evaluation
tools such as process modeling and demonstration
projects help to determine future needs. A combination of
in-plant and outside expertise helps to focus AZD
implementation encompassing a range of applicable
solutions. The level of effort needed to pursue systematic
solutions enables the decision makers to weigh the
potential value gain toward implementing a systematic
AZD solution.
Step 1. Establish Goals
Establishing goals provides a foundation for an AZD
project. As the project progresses, goals and priorities can
be revisited and adjusted as deemed appropriate. Although
the objective is to proceed with minimal changes, the
process can lead to new information that results in
decisions towards beneficial changes in goals and
priorities. For goals to be implemented effectively, they
should be specific, appropriate and measurable.
Goals directly or indirectly related to AZD are to:
_ Achieve discharge reduction targets.
Stay within budgets and meet payback
timeframes.
_ Achieve regulatory compliance and beyond-
compliance targets.
_ Improve process consistency and quality.
_ Improve plant space use.
_ Meet schedule targets.
Step 2. Identify Opportunities
AZD opportunities can be identified for individual process
solutions, process lines, multi-process lines, or entire
General steps to identify viable AZD opportunities:
List discharges and sources for potential
reduction. Wastewater treatment and waste
disposal data need to be reviewed to provide
information on discharges from process baths and
rinses. Discharges can be associated with
specific upstream, in-plant sources. Depending on
the available data and the level of complexity of
the facility processes, it may be necessary to
perform a plant-wide process survey to identify
specific waste sources.
Characterize sources and discharges. The type
and magnitude of each identified discharge should
be estimated, including constituents, mass and
volumetric rate, and variation with time (for
projected production type and level). Table A-1
presents data objectives for characterizing
sources and discharges. This can include a
combination of measurements and analyses,
calculations, and modeling.
Identify drivers and benefits. Identifying drivers
and benefits for source-specific zero discharge
opportunities helps provide a basis for setting AZD
goals. Benefits associated with source-specific
reductions include a range of net cost and no-cost
gains or improvements that would result directly or
indirectly from actions implemented to reduce
waste discharges. Table A-2 lists common AZD
benefits. Drivers are major benefits that represent
primary reasons for implementing source-specific
AZD alternatives.
Identify impediments. Constraints represent
limitations that apply to source-specific AZD
considerations. Table A-3 lists some common
constraints. After constraints are identified,
assess whether measures could be implemented
to remove the constraints. For example, if a
capital-cost ceiling is identified for a project, it
might be possible to finance the capital project
with no increase, or even a reduction in net short-
32
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Table A-1. Data Requirements for Characterizing Sources and Discharges
Waste quantity
and characterization data
Production information
Wastewater treatment
chemical additions
Water supply characteristics
Chemical use and wastes
from water pretreatment
processing
Bath dumps, wastewater discharge flow and analyses, waste chemicals and raw materials
information on spills or process upsets, and residuals and byproducts from process purification
and treatment systems (including routine operations and wastes from periodic cleaninq and
maintenance) a
Process sequences and bath chemistries, process specifications, component data (type, size
throughput, and output), bath chemistry additions, rectifier amp-hours totals (specific to production
periods), part rejects and reprocessing
Chemical quantity and frequency of addition
Total dissolved solids, conductivity/resistivity, hardness, temperature, pH, specific ions
Chemical quantity and frequency of addition
Table A-2. Common AZD Benefits
Overall Benefits
Specifications
Reduced costs
Reduced waste generation
Production improvements
Reduced chemical use
Increased regulatory performance
Enhanced facility operations
Enhanced environmental
and production performance
Capital costs (e.g., overall capital costs for new projects that include new or expanded wastewater
treatment systems may have reduced capital costs due to more focus on in-piant discharge
reductions), operating costs, and life-cycle costs
Wastewater, wastewater treatment residuals, bath dumps, process bath and rinse treatment
wastes
Reduced rejects, reduced processing variability, improved product consistency and quality
updated/increased capacity
Surface finishing bath chemicals, wastewater treatment chemicals, process maintenance treatment
chemicals
Compliance or beyond-compliance performance, wastewater discharges, air emissions, hazardous
wastes, and toxic chemical use
Space availability, energy consumption, safety, and reduction of water use
Corporate goals, customer requirements, environmental metrics
Table A-3. Common AZD Constraints
Constraints Specifications
Financial
Process
Facility
Equipment
Operational
Regulatory
Schedule
Data
Capital spending limits, payback timeframe requirements
Limitations on changing processes
Space or location limitations that could impact plant modifications or the addition of new systems
Existing equipment that must be used or must stay at a fixed location
Limited operator availability and capabilities
Multimedia permit requirements or triggers
Difficult requirements for project milestones and completion
Limited process data and ability to gather project data
33
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term cash flow due to operational cost savings.
Removing a short-term capital ceiling constraint
could lead to significant net cost savings over the
long term.
Step 3. Identify and Screen
Alternatives
A preliminary range of plausible alternatives for AZD
should be identified to address specific AZD opportunities.
To gain afull perspective on potential costs and benefits of
AZD, it is often beneficial to consider alternatives that
range from simple process modifications to more
comprehensive process enhancements. General types of
AZD alternatives include:
Process solution purification systems (Section 3)
_ Process rinse water recycle systems (Section 4)
_ Alternative processes (Section 5)
_ Improving existing processes and operations
(Section 6)
Priorto more detailed evaluations, preliminary alternatives
should be screened to eliminate those that have serious
flaws. This will result in a range of specific alternatives for
detailed evaluation. Another important level of consideration
for AZD alternatives are systematic approaches, which
provide enhanced solutions for multiple AZD opportunities,
and integrate overall facility and life cycle considerations.
Section 2 discusses systematic AZD approaches.
Target alternatives need to be defined to provide scenarios
that can be evaluated in accordance with application-
specific requirements. The information needed to build a
scenario for an AZD alternative varies substantially with
the type of alternative. It is important to consider the
complete implementation requirements, including plant
and process interfaces, and construction and operations
requirements.
Step 4. Evaluate Alternatives
Alternatives that have passed initial screening should be
thoroughly evaluated using a consistent, systematic
evaluation method encompassing evaluation categories
and evaluation metrics. Typical evaluation categories are
described below. Table A-4 lists specific evaluation criteria
for each category.
Technical Feasibility. Technical feasibility
evaluations typically involve identification of
essential implementation and operational
performance criteria for an alternative. Relevant
information is collected and evaluations are
performed to determine how each alternative
corresponds with the technical criteria. For some
applications, it may be necessary to perform more
extensive evaluations, including bench and/or
pilot testing to determine technical feasibility. The
technical criteria vary depending on the type of
alternative and specific impacts on the facility and
its processes and operations.
Cost. Cost evaluations include developing capital
and operations and maintenance (O&M) estimates
for AZD alternatives and comparing costs for
existing operations. Projections for futur§
production requirements need to be included as a
basis for cost estimating.
Regulatory. Regulatory evaluations identify
applicable regulations and performance
requirements that determine how specific AZD
alternatives fall short of, achieve, or exceed the
requirements and guidelines.
Company/staff acceptability. Company and staff
acceptability evaluations assess how AZD fits a
specific application from the operator to corporate
management. Evaluations of AZD alternatives
can be based on a range of qualitative (e.g., high,
medium, or low) or quantitative (e.g., 1 to 10)
metrics specific to each evaluation category. A
matrix format is often useful to summarize
alternative evaluations. In some cases, evaluation
results are combined from different categories into
a single overall score for each alternative. This
involves development of a consistent scoring
basis and category-specific weightings to allow for
calculation of an overall score.
Decision analysis methods may assist in evaluating large,
complex AZD applications when alternatives depend on
highly variable production scenarios and when expected
costs and benefits vary greatly. Powerful decision analysis
software tools are available that generate ranges of
probabilistic outcomes and delineate key variable impacts.
Step 5. Select Action(s)
Specific actions need to be selected based on the best
individual or combination of alternatives to satisfy short
and long-term needs. Actions can be implemented in one
or more phases to satisfy project-specific resource
limitations, sequencing to maintain operations, and
priorities for achieving maximum results. Lastly, project
funding and implementation support should be secured to
allow for implementation within desired projecttimeframes.
StepG. Implement Action(s)
Successful AZD solutions require good implementation to
gain the desired reductions and cost benefits. Technical,
strategic, and management experience is important
throughout each general implementation phase to achieve
overall success with AZD projects. General implementation
phases include:
1. Develop a design basis. Fixed and variable facility
data, site factors, production requirements, and
process parameters are essential in the design
34
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Table A-4. AZD Alternative Evaluation Criteria
Evaluation Category
Technical Feasibility
Specific Criteria or Parameter
Ability to maintain process chemistry, recovery percentages for water recycle or
chemical recovery, reliability, operability, facility space and interface requirements^ level of
technology and applications development, byproducts and waste generation
Engineering, process modifications or new process systems, pollution prevention and control
systems, facility costs, utility connections and plant interfaces, training, start-up and commission-
ing, general construction and implementation, permitting and regulatory, plant space cost of money
internal coordination and administration costs
Operations labor for new or modified process systems, utility costs, chemicals and consumable
materials, analytical requirements, and management of byproducts from new process
maintenance systems
Permitting, data collection, record keeping and reporting, and compliance requirements and
guidelines, voluntary and beyond-compliance programs
Company policies, standards, and goals relevant to AZD including preferences and dislikes of AZD
decision makers, owners, and implementers for specific AZD alternatives
Cost: Capital
Cost: Operation
& Maintenance (O&M)
Regulatory
Company and Staff Acceptability
SSiVe T f eV^?rS ln? Ude- Capital and 0&M cost savi"9s with ^D alternatives compared to existing operations. Table A-5
lists common capital and O&M cost savings and benefits that may result from implementing AZD actions.'
Table A-5. Cost Savings and Benefits for AZD Actions
Major Capital Cost Savings and Benefits " ~
Downsized wastewatertreatment requirements
Salvage value or reuse of existing equipment
Gain in plant space (e.g., with downsized wastewater treatment systems)
Gain in tax credits
Additional project funding resources
Major O&M Cost Savings and Benefits
Reduced rework and rejects
Increased revenue and/or cost to produce -product quality improvements, increased throughput or yield and
decreased effort to produce
Reduced waste management costs bath dumps, wastewater treatment and discharge
Reduced process bath chemistry costs
Reduced water supply and pretreatment costs
Reduced insurance and liability costs
Gained revenues from byproducts
35
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factors. Pilot testing may be necessary to verify
technical assumptions.
2. Prepare the design. The type and level of design
(ranging from performance-based design to
detailed design) should be appropriate for the
application.
3. Procure systems and services. Implementation
needs to be appropriate for the application.
4. Perform installation and startup. Careful planning
and coordination in implementing new systems
minimizes disruptions to existing operations.
5. Perform ongoing implementation. New systems or
procedures need to operate in accordance with
well-defined plans (O&M plan or procedure
protocol).
6. Monitor operations and identify improvements.
Pre-defined monitoring identifies potential problems
or possible enhancements that might require
additional process changes.
Step 7. Follow-up Monitoring and Actions
Scheduled monitoring of AZD systems will determine
whether the actions taken are being executed properly and
that the desired results have been achieved. Changes
needed to optimize or enhance the system may be
identified through ongoing monitoring of actions and
results.
36
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Appendix B Installed Costs
This Appendix provides a limited overview of typical
installed capital costs forcontrol technologies and process
changes for approaching zero discharge (AZD) in the
surface finishing industry. Many approaches are presented
in this document for process solution purification and
recovery, rinse purification/concentrate recovery, alternative
surface finishing processes and 'coatings, and for
improving existing process conditions and practices. Table
B-1 presents representative ranges of installed capital
costs that are size- and approach-specific. Described
below are representative projects that would be expected
to fall within the Table B-1 cost ranges for project sizes
defined as small, medium, large, and very large.
B.1 Process Solution Purification and Recovery
(Section 3)
Examples corresponding to the process purification and
recovery cost ranges in Table B-1:
Small
A 100-gallon per day throughput microfiltration
system to maintain an alkaline cleaner bath at a
target contaminant concentration at less than 250
ppm soil, with 100 g/day contaminant loading.
A diffusion dialysis or resin sorption system to
purify 20 gallons per day of Type 2 sulfuric acid
anodize bath with an aluminum concentration of 10
g/L, recovering more than 80% of free acid.
Medium
A 500-gallon per day throughput microfiltration
system to maintain three alkaline cleaner process
baths at less than 250-ppm soil with overall 500-g/
day contaminant loading.
A diffusion dialysis or resin sorption system to
purify 100 gallons per day of Type 2 sulfuric acid
anodize bath with an aluminum concentration of 10
g/L, recovering more than 80% of free acid.
Moderately large
A 500-gallon per day throughput microfiltration
systemto maintain three alkaline cleaner process
baths at less than 250-ppm soil with overall 500 g/
day contaminant loading.
A diffusion dialysis or resin sorption system to
purify 250 gallons per day of Type 2 sulfuric acid
anodize bath with an aluminum concentration of 10
g/L, recovering more than 80% of free acid.
Large
Diffusion dialysis system to reclaim 5,000 gallon
per day of hydrochloric acid 7 wt % waste
hydrochloric acid steel pickle stream with 4 wt %
iron. Reclaimed acid to have less than 1 wt % iron.
Membrane electrolysis system for maintaining a
full production, 50,000 gallon chromate conversion
coat bath, to remove and maintain total
Table B-1: Installed Capital Cost Ranges for Typical AZD Project Approach and Size Ranges
Installed Capital Cost (in thousands of dollars)
Project Size
AZD Approach
Process Technique
Water Purification/Recycle
Bath Purification
Process Replacement
Small'
<5
<20
<20
20 to 100
Medium2
5 to 20
20 to 100
20 to 100
100 to 500
Moderately Large3
20 to 100
100 to 500
10010500
500 to 2500
Large"
100 to 500
500 to 2500
500 to 2500
2500to 10000
A point-source-purification system for a single small to medium-sized surface finishing bath with low to moderate contaminant loading
A point-source-purification system for multiple small to moderate-sized baths with low to moderate contaminant loading.
3 A bath maintenance system for a relatively large process bath or process baths with low to moderate contamination or several bath
maintenance systems for a medium-sized shop.
"Several multi-tank bath maintenance systems for a moderate to large shop or single bath maintenance systems for very large process
tcin ks.
37
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Small
contaminant (iron, aluminum, and copper) Medium
concentration below 1 g/L and to reoxidize trivalent
chromium to hexavalent chromium.
B.2 Rinse Purification/Concentrate Recovery
(Section 4)
Examples corresponding to the rinse purification cost
ranges in Table B-1:
Small
Single rinse point source ion exchange system,
equipped with a manual regeneration system, for
water purification/recycle of a 1 to 3 gpm nickel
plating rinse system with 300 ppm TDS influent
and 10 ppmTDS rinse water purification required.
Medium project:
A 5 to 20 gpm ion exchange system or reverse
osmosis system for purification/recycle of several
nickel plating rinses with 300 ppm TDS influent
and 10 ppm TDS rinse water purification required.
The ion exchange system would be equipped with
an automatic, PLC-controlled regeneration system.
The reverse osmosis system would be PLC-
controlled.
Moderately Large
A 50 to 100 gpm ion exchange system or reverse
osmosis system for centralized purification/
recycle of several compatible metal finishing
process rinses with 100 ppm TDS influent and 10
ppmTDS rinse water purification required. The ion
exchange system would be equipped with an
automatic, PLC-controlled regeneration system.
The reverse osmosis system would be PLC-
controlled.
Large
A 100 to 1000 gpm combined reverse osmosis
system followed by ion exchange system for
centralized purification/recycle of several
compatible metal finishing process rinses with 100
ppm TDS influent and 2 ppm TDS rinse water
purification required. The overall system includes
complete redundant reverse osmosis modules
and ion exchange resin 'beds to maintain highly
reliable continuous operations. The ion exchange
system would be equipped with an automatic,
PLC-controlled regeneration system. The reverse
osmosis system would be PLC-controlled.
B.3 Alternative Surface Finishing Processes and
Coatings (Section 5)
Examples corresponding to the alternative surface
finishing processes and coatings cost ranges in Table
B-1:
Change anodizing chemistry on large line from
chromic acid to sulfuric/boric acid.
Moderately Large
Small electrocoat line for 10,000-sq ft per day
production.
Limited production single chamber scale batch
vapor deposition system.
Large
Electrocoat line for 100,000-sq ft per day
production.
Fully automated production scale continuous
vapor deposition system.
B.4 Improving Existing Process Conditions and
Practices (Section 6)
Examples corresponding to the Table B-1 cost ranges for
improving existing process conditions and practices:
Implement more frequent bath chemistry
monitoring and maintenance for single process
bath.
Install fog or spray rinses on several small tanks.
Add water conductivity controller for single rinse
water make-up.
Small
Change anodizing chemistry on small or medium-
sized line from chromic acid to sulfuric/boric acid.
Medium
Add automated chemistry monitoring and
maintenance for a single electroless nickel bath
Add water purification system for 5-gpm city
water supply with 100-ppm TDS to provide
<10-ppm TDS influent process water.
Add water conductivity controller for
automatic rinse water make-up for several
tanks.
Moderately large
Add automated chemistry monitoring and
maintenance for several electroless nickel baths
Install 50-gpm water purification system for
city water supply with 100-ppm TDS to provide
<10-ppm TDS influent process water.
Install three triple countercurrent rinse
systems with 200-gallon tanks.
Add automated feed and bleed systems for
maintaining more uniform process chemistry
for four, 500-gallon metal finishing process
tanks with a combined total of seven different
38
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process make-up chemistries and three
segregated bleed waste streams.
Large
Install six triple countercurrent rinse systems with
500-gallon tanks.
Install 500-gpm water purification system for city
water supply with 100-ppm TDS to provide <10-
ppm TDS influent process water.
Add automated feed and bleed systems for
maintaining more uniform process chemistry for
ten 500-gallon metal finishing process tanks with
a combined total of 12 different process make-up
chemistries and four wastewater treatment
streams.
Factors Influencing Installed Costs
Installed costs for process systems can vary significantly
depending on many site-specific and other project-specific
factors that can affect system, installation, and operational
requirements. These factors should be considered when
extrapolating costs from other installations or when
estimating installed costs from equipment only. Project
budgets are often set prior to consideration of these
factors, frequently leading to insufficient funding and
corresponding delays and cost increases for overall project
implementation. Some common factors that can
significantly impact implementation costs for process or
production systems for AZD projects include:
Relative inlet and required outlet concentrations
for process systems. Differences in overall
percent removal and final concentrations may
require supplemental pretreament or post-
treatment unit processes. It is important to
adequately characterize process streams and
their inherent variability, and define the outlet
process fluid purity requirements for estimating
specific process system requirements and costs.
Materials of construction. Equipment material
requirements may vary significantly based on the
chemicals and concentrations handled, along with
required effluent process fluid purity requirements.
If premium materials of construction are required,
costs can increase greatly compared to typical
systems costs with standard materials of
construction. If the need for specialty materials is
not recognized until after installation, this can
result in significant costs for equipment
maintenance and early replacement.
Site installation. Installation requirements and
costs may vary significantly, even for identical
process systems, installed at different project
locations. Site installation costs can range from a
fraction to a total multiplier of equipment costs.
Some modular, skid-mounted systems can be
easily set in place and quickly connected to
utilities and fluid inlet and outlets. Other process
systems or locations can require a combination of
facilities modifications or expansions, utility
upgrades, seismic restraints, or other significant
site work to complete installation.
Start-up and commissioning requirements.
Process system or client-specific requirements
can vary substantially for start-up and
commissioning, ranging from a few hours to
weeks. The extent of these requirements and the
degree of proper installation and systems
application implementability can significantly
impact the duration and effort required to meet
these requirements. Unanticipated installation
and operation difficulties can result in significant
schedule and cost impacts for systems start-up
and commissioning.
Process system redundancy/reliability.
Application-specific redundancy and reliability
requirements can impact the number of parallel
and/or series process trains, with corresponding
multiple or additive system costs.
Level of automation. Local and centralized
instrumentation and control requirements for'
process automation can vary significantly. Where
automation can significantly increase initial
capital costs, overall life cycle costs can be
reduced due to improved production efficiencies.
Equipment quality. Process systems may vary
significantly in quality and corresponding capital
costs. In evaluating comparative systems it is
important to consider the comparative life cycle
costs for systems. Higher-priced, better-quality
systems may provide longer and more trouble-free
service life, thereby reducing overall life cycle
costs.
Location specific configuration requirements.
Facility-specific footprint space and height
requirements may require custom system
configurations and corresponding custom system
design and fabrication, increasing costs compared
to standard off-the-shelf systems.
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Allowances for production expansion and
flexibility. When considering AZD-related
replacement process systems or process
systems that interface with primary production
systems (e.g., water recycle or process bath
maintenance systems) it is important to consider
and plan for production expansion and potential
production changes. Proper allowances for
production expansion and/or changes may result
in significant increases in short-term AZD-related
capital costs but may reduce future costs related
to systems modification, expansion, or replacement
in response to production increases and/or
changes.
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