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Section 1
P2 Technology for Common Metal Finishing Processes

Table of Contents

1.1 INTRODUCTION
1.1.1 How the information is arranged

Section 1 contains an update to the original Bluebook, which is now presented in its entirety in Sections 2 through 8.

In the original Bluebook, technical information is arranged in sections covering speciﬁc topic areas, such as waste reduction, chemical recovery, and bath maintenance. Each section includes discussions of all of the technologies relevant to the topic, such as ion exchange, evaporation, or reverse osmosis. Information on each of the major plating baths or processes (nickel, copper, hard chrome, anodizing etc.) is dispersed throughout the technology discussions.
In Section 1, the order has been rearranged. Each plating bath is now discussed fully in a subsection of its own. Grouping the discussion topics according to bath type has two main advantages:

It enables users to develop a comprehensive strategy for upgrading each of the processes carried out in their facilities.
It helps users appreciate which of the different variants of each pollution control technology are appropriate for the speciﬁc process they are examining.

References cited in Section 1 are indentiﬁed in footnotes. References for Sections 2 through 8 are found in the Reference section at the end of the book.

1.1.2 Information sources

The information presented in Section 1 is derived from various sources, including:

the original Bluebook
technical journals
EPA and state environmental agency reports
supplier's literature
data collected from 1997 through 2008 from metal ﬁnishing facilities throughout the U.S. in conjunction with:
- the Strategic Goals Program (SGP, 1997 – 2001)
- the Benchmarking Metal Finishing study (conducted in 1999, updated in 2003)
- Surface Technology Environmental Resource Center (STERC) surveys (2002 – 2008)

The Benchmarking Metal Finishing study gathered information from 160 participants, who described process-speciﬁc pollution prevention practices that have worked in their shops. Most of these suggestions are practical and very cost effective to implement and many of them have been included in this update. This information is further supported by SGP data that were submitted by many of the survey participants and other facilities from 1997 through 2008.

In many cases, new technologies or variations of existing technologies have emerged since the publication of the Bluebook in 1994. A number of demonstration projects conducted since then have also provided very useful information for evaluating both technical capabilities and costs. This edition incorporates insights gleaned from the most recent sources.

1.1.3 Section format

Each process section has a similar format. Following a general overview, a set of basic pollution prevention objectives are presented in the left-hand column of a summary table, with each objective accompanied by a recommended strategy for attaining it. In general, the basic objectives include:

Reducing dragout and water use
Improving material utilization
Reducing human exposure

1.2 RINSING

Product quality and operating costs are both highly dependent on effective rinsing. Inadequate rinsing causes dragout carryover which contaminates downstream process solutions and can cause misplated parts. Excessive and/or inefﬁcient use of rinse water increases water use costs, including the costs of the raw water and chemical waste treatment. With any plating operation, there is an optimal range of rinse water use that can be attained by using the right equipment, conﬁguration and procedures.

A signiﬁcant portion of the original Bluebook is devoted to best practices for dragout reduction and rinsing (see Section 3). All of that information is still valid. Presented here is:

status report on water use trends,
good practices reminder,
refocus on total dissolved solids as a control parameter, and
update on rinse controller technology.

1.2.1 Water Use Trends

Water use by U.S. metal ﬁnishing shops has decreased signiﬁcantly over the past 30 years. In the late 1970s, EPA conducted wastewater sampling at electroplating facilities as part of the rulemaking process for wastewater standards ("efﬂuent guidelines"). Data from the supporting EPA document⁴ indicate that the average discharge for facilities in their sample population (25 facilities) was 109,000 gallons per day (or 28,340,000 gallons per year).
⁴(Development Document for Existing Source Pretreatment Standards for the Electroplating Point Source Category, August 1979 (EPA 440/1-79/003)

New federal wastewater limits, went into effect in 1984. Most U.S. plating shops reduced water use as part of their overall strategy to meet the new limits By reducing water use, facilities were able to save signiﬁcantly on both capital and operating costs for wastewater treatment, since both aspects of costs are closely related to the volume of wastewater ﬂow. These reductions were relatively easy to attain. Since there had previously been little or no regulatory or economic motive to lower water use, most facilities were using more water than necessary. Therefore, basic measures, such as installing flow restrictors and turning off free-running rinses during non-production times, cut water use by 50% or more for many shops. Some facilities had already installed counterﬂow rinse systems, but according to the EPA development document, in 1979 the majority of plants relied on single rinse tanks.

The Bluebook survey (1993) asked facilities which year they installed their wastewater treatment system. 70% of the respondents indicated that they had a system in place by 1985, and 95% had one installed by 1990. By 1994, the baseline year for the Strategic Goals Program (SGP)⁵, most plating shops had made decent progress on water use reduction, again mainly to reduce capital and operating costs for waste treatment. Exhibit 1-1 is a graph showing the average ﬂow for SGP facilities for 1994 and 1997 through 2006. A comparison of these ﬂow rates to those found in the EPA development document suggests that the industry signiﬁcantly cut water use by 1994, and made further gains through about 2003. The values here are not normalized and therefore don't account for changes in production. However, Exhibit 1-2 shows normalized values for approximately the same time period, and it suggests approximately the same trend (2004 to 2006 were not calculated because a much smaller number of facilities submitted SGP data in those years).
⁵(More information on the SGP can be found at www.strategicgoals.org)

How and why did facilities reduce water use from 1994 through 2003? The "how" is pretty easy to answer. Efﬁcient rinsing systems, consisting of multiple rinse tanks and control mechanisms can reduce water use by a factor of several times, compared to the practices that were common in 1979. Although industry made signiﬁcant progress by 1994, there was still plenty of low hanging fruit. Presumably, more and more facilities took advantage of this situation.

The "why" is more difﬁcult to answer. There are most likely a number of reasons for this improvement. Cost savings was the most likely motivator. Water and sewer charges increased during those 10 years, as publicly owned treatment works (POTWs) upgraded their facilities and passed along price increases to industry. This trend is reﬂected in the SGP data, which show a 24% increase in the median water/sewer price from 2000 to 2006 (from $3.64 to $4.50 per 1,000 gal).

Another potential stimulus for water use reduction is the "green" movement that has impacted nearly all sectors of U.S. industry. Metal ﬁnishers were sympathetic to the goals of the green movement, as evidenced by their widespread participation in the SGP (over 400 SGP companies). Another factor is the improvement in information access made possible by the Internet. Industry now has quick access to free pollution prevention and compliance assistance information such as the Surface Technology Environmental Resource Center (STERC)⁶. Also, the Internet makes it easier to locate and communicate with consultants and vendors that can assist with pollution prevention.
⁶(An EPA-sponsored compliance assistance center (www.sterc.org).)

So, what happened following 2003 (see Exhibit 1-1)? It is difﬁcult to say with certainty, because we are talking about a relatively short time period, and it may just be an anomaly of the data (fewer companies submitted SGP data after 2003 and perhaps a disproportionate percentage of low discharge facilities dropped out of the program). On the other hand, although it may be just coincidental, 2003 was the year that EPA decided not to impose the additional wastewater regulations on the electroplating industry that had been proposed under the Metal Products and Machinery Category.⁷ After a close call with some very stringent standards, some facilities may have eased up a bit. At any rate, the recent uptrend in water use will probably be erased over the next several years as industry is forced to squeeze down further on operating costs in order to remain competitive.
⁷(The Metal Products and Machinery (MP&M) regulation establishes national technology-based limits on pollutants in wastewater discharges from facilities that manufacture, rebuild or maintain metal parts, products, or machines. The proposed rule (Jan. 2001) included electroplating operations, however in 2003 this sector was dropped from the rule (see http://www.epa.gov/guide/mpm).)

1.2.2 Overview of Best Practices for Rinsing

The optimal rinse system will vary from facility to facility due to variables such as part size/shape, level of automation and other site-speciﬁc factors. However, in each situation, if each of the following elements is present, rinse water use will be close to optimal:

Multiple rinse tanks. Multiple rinse tanks plumbed in a counterﬂow conﬁguration can reduce water use by 90% or more. A simple calculator, available free on-line, can be used to evaluate the impact of various rinsing strategies (more on this below).
Water control. Any methodology that closely matches water use and workload will increase the efﬁciency of your rinse system. Flow restrictors as a standalone method are not sufﬁcient, except on automated lines that are in continuous use. Examples of viable control methods include timer and conductivity controllers and optical sensors, which are used on conveyor lines (more on controllers below).
Water circulation. If you have poor mixing in your rinses, you are wasting water. Proper plumbing of the tanks will minimize short circuiting. Air or mechanical agitation (e.g., pumping) will eliminate the problem altogether.
Standard procedures. Skipping stages in multiple tank arrangements is the most common rinse crime. Other forms of malpractice include sabotaging controllers and using supplementary water sources, such as a hose. Rules for rinsing should be established and enforced by management. Rinse tanks should be checked on a scheduled basis for total dissolved solids (TDS) concentration (more on TDS below).

Each of these factors is covered in detail in the original Bluebook. Some additional details on several of these items are presented in this update.

On July 1, 2008, EPA published the National Emission Standards for Hazardous Air Pollutants for Plating and Polishing Operations.⁸ This rule impacts many metal ﬁnishing operations (EPA estimates 2,900 facilities). It applies to facilities with plating, polishing or thermal spray processes that contain cadmium, nickel, lead, manganese and/or chromium. (The rule will not apply to chromium anodizing and electroplating, which are coverered under a previous rule.)
⁸(National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Final Rule, Federal Register July 1, 2008.)

With this regulation EPA did not establish emission limits for plating and polishing operations as they did with chromium plating, but instead required plating and polishing facilities to implement speciﬁc pollution prevention (P2) and control practices that are related to air emissions, dragout and rinsing:

minimize bath agitation when removing tank objects;
maximize dripping of bath solution back into the tank by extending drip time when removing the tank objects and using drain boards (i.e., drip shields);
optimize the design of barrels, racks, and parts to minimize dragout of bath solution, and
use a wetting agent or fume suppressant in non-cyanide electroplating baths.

Therefore, affected facilities should also incorporate theses practices into their dragout control and rinsing systems.

1.2.3 Water Use Benchmarks

How does your water use compare with that of other shops? One way to ﬁnd out is to compare your data with industry benchmarks. In 2000, NCMS published a benchmarking report⁹ (the benchmarks were updated in 2003) that covered various metrics, including water use, metals discharged, sludge generation, and energy use.
⁹(National Center for Manufacturing Sciences, Benchmarking Metal Finishing, June 2000. Available at www.sterc.org.)

A unique aspect of the study results is that they account for the types of plating processes employed. For example, there are separate benchmarks for hard chromium plating, decorative chromium plating, nickel plating, zinc rack plating zinc barrel plating, anodizing, etc. The benchmarks also account for differences between certain sectors such as the auto manufacturing and fastener plating sectors. Therefore, the benchmarks can be tailored to any plating operation, even when multiple processes are present.

Water use benchmarks normalized to sales for common metal ﬁnishing processes are shown in Exhibit 1-3. The target benchmarks are the average water use for companies in the lower quartile (i.e. the best 25%).

The Strategic Goals Program (SGP) incorporated the benchmarks from this study into its program. Companies that participate in SGP automatically receive benchmark reports that show how well they measure up. These reports are particularly useful for identifying areas where signiﬁcant progress can be made. For example the reports indicated how much money could be saved annually by meeting the benchmarks. You can view benchmarking reports at www.strategicgoals.org (select facility reports from the menu, and then choose "Benchmarking Analysis" on the subsequent page).

1.2.4 Using Total Dissolved Solids (TDS) as a Control Parameter for Water Use

In order to optimize rinse water use, ﬁnishers need data. Speciﬁcally, ﬁnishers should know:

the quality of their raw water supply
the quantity of dragout generated by various parts and ﬁxturing used
target levels for rinse water quality and associated part cleanliness

A relatively easy and inexpensive means for generating these data involves measurement of total dissolved solids (TDS). In a matter of hours a ﬁnisher can generate data that are as useful as results generated by days of expensive consulting and laboratory work.

TDS is present in all natural waters. Typically, tap water has a TDS content of 100 to 400 mg/l, most of which is hardness (principally calcium and magnesium and associated anions). When water is used for rinsing, the TDS concentration increases signiﬁcantly due to the presence of the plating chemicals that are washed off the parts. Therefore, rinse tanks must be ﬂushed with "clean" water during processing. Good rinsing is necessary to prevent staining and other plating problems and to prevent cross contamination of process tanks. The "cleanliness" of rinse water can be measured by analyzing for TDS. The acceptable level of TDS varies depending on factors such as intermediate vs. ﬁnal rinsing, type of ﬁnish, type of parts, etc. For example, rinsing following an acid dip may be adequate at 1,000 mg/l TDS, whereas rinsing following bright plating may require a TDS of 50 mg/l or lower. Some tap waters may not be sufﬁciently low in TDS for rinsing after bright plating applications, thus requiring processing (e.g. by ion exchange or reverse osmosis) of the tap water prior to use.

TDS measurements are usually made in a laboratory by ﬁltering and then evaporating water and weighing the residue. A rapid TDS measurement can be made using speciﬁc-conductance measurements (a hand-held or "pocket" TDS instrument can be purchased for under $50). Such measurements indicate the capacity of a sample to carry an electrical current, which is in turn related to the concentration of ionized substances in the water, in units of milligrams per liter (mg/l). Most dissolved inorganic substances are in the ionized form and therefore contribute to speciﬁc conductance. Although this method provides only approximate TDS values, if it is properly used, it can serve as a guide to rinse water quality.

More sophisticated measuring devices can also be used to generate rinse water data with actual speciﬁc conductance readings, in units of microsiemens per cm (µS/cm) or the equivalent micromhos/cm (µmhos/cm). For practical purposes, you can convert µS/cm (or µmhos/cm) to mg/l of TDS by dividing by two (alternatively, multiple mg/l of TDS by 2 to arrive at µS/cm of speciﬁc conductance). For example, 1,000 µS/cm is approximately equal to 500 mg/l TDS.

A methodology for measuring dragout rates and rinse water quality can be found in Section 3.5.3. Once you have generated your own TDS data, you can use an on-line calculator to evaluate your rinse system and potential alternative conﬁgurations. The online tool is located at (www.sterc.org/subs/rinsecal.cfm). To use the tool, enter the process tank information and other parameters, and then select a type of rinse system. A diagram of the selected rinse system is then displayed. As an example, see the form and diagram in Exhibits 1-4 and 1-5, respectively.

1.2.5 Rinse Controller Update

Conductivity controllers. Conductivity controllers have been in use by the metal ﬁnishing industry for more than 25 years. A control system consists of three components:

a transformer box that houses the solid state circuitry that controls the system; and
a solenoid valve that opens and closes in response to signals from the circuitry.

Conductivity controllers work by responding to the actual conditions in the rinse tank. When dragout is introduced to the rinse tank, the probe senses a rise in conductivity. When the conductivity level rises above a set-point, the signal from the probe causes the circuitry to open the solenoid water valve. The value remains open until the probe senses a drop in conductivity below another set-point. Both of the set-points are operator-adjustable, to permit use over a range of desired water qualities.

Two types of conductivity sensors are available, conventional and electrodeless.

Conventional conductivity sensors consist of two electrodes that contact the water with a low-level electrical potential between them. The electrodes are sized and spaced to provide a known cell constant, which corresponds to a speciﬁc operating range that must match the conductivity range of the rinse water. Since the electrodes in a conventional sensor attract charged particles, they eventually become encrusted or fouled, causing the unit to malfunction. To ensure accurate conductivity measurements, sensors must be cleaned as frequently as every two weeks, and calibration checks must be performed monthly.

Electrodeless sensors, also referred to as inductive-type, eliminate the fouling problems associated with conventional units. The electrodeless sensor consists of two toroids, or wire loops, sealed within a nonconductive casing. The toroids use magnetic ﬁelds to interact with the rinse water without contacting it directly.

The ﬁrst toroid creates a magnetic ﬁeld that induces an electrical current in the water. The amount of current induced is proportional to the conductivity of the solution
The current in the rinse water gives rise to its own magnetic ﬁeld, which in turn induces a current in the second toroid. Thus the magnitude of the current in the second toroid is proportional to the conductivity of the rinse water.

Electrodeless sensors are easier to operate and maintain because they require less frequent cleaning (since the sensors do not directly attract ions, fouling is unlikely). An additional attribute of electrodeless sensors is their wide conductivity range as compared to conventional units.

Since the publication of the Bluebook (1994), there have been at least two good demonstrations projects that involved installation and monitoring of conductivity controllers.¹⁰,¹¹ Information from those demonstrations is discussed here.
¹⁰Waste Management and Research Center. Pollution Prevention Case Study: Conductivity Control System Technology, March 2000.
¹¹(Merit Partnership. Reducing Rinse Water Use with Conductivity Control Systems, December 1996. [The Merit Partnership is a joint venture between U.S. Environmental Protection Agency (EPA) Region 9, state and local regulatory agencies, private sector industries, and community representatives. The partnership was created to promote pollution prevention (P2), identify P2 technology needs, and accelerate P2 technology transfer within various industries in southern California.]

Merit Partnership. The Merit Partnership project involved the installation and monitoring of both conventional conductivity and electrodless controllers. A total of nine units were installed, three each from three different manufacturers. Six of the units were electrodless and three were conventional. The units were installed on rinses following acid activation, copper cyanide plating, nickel plating and decorative chrome plating. Eight of the nine units were wired to a common control panel, where operators could read conductivity measurements and adjust set-points. The other unit was mounted on a wall near the associated rinse tank.

Costs for the conductivity control units, including installation are shown in Exhibit 1-6.

The Merit Partnership demonstration was conducted over a three month time period. The water use and cost saving realized during the project are summarized in Exhibit 1-7. The study results indicate that the majority of savings are derived from reduced use of wastewater treatment chemicals. The study results noted that no adverse effects on product quality were observed during the demonstration.

Waste Management and Research Center. The Waste Management and Research Center (Champaign, IL) installed and monitored two electrodless (inductive loop-type) conductivity controllers at a plating shop that sought assistance in reducing their water use. The controllers were initially set at 1,000 µS and increased in 400 µS increments twice a week. This allowed the system to stabilize and gave production workers time to verify that there were no quality problems as a result of the increase in TDS. The ﬁnal conductivity set point was 3,600 µS. Over time, no product quality problems were observed and the facility achieved an average water savings of 87.5% per rinse system and an annual water use reduction of 2,808,000 gal ($10,883 saved annually on water and sewer costs). The cost of the controllers was $1,370 each. Additional material and installation costs brought the installed cost to about $2,000 for each unit.

Timer Controls. Timer rinse controls consist of a push-button switch, a timer mechanism, and a solenoid valve. These units operate in a manner similar to conductivity controllers. However, instead of regulating rinse water ﬂow on the basis of rinse tank water quality, the timer controls simply turn water on and off based on a preset time period.

In operation, a ﬁnisher lowers parts into the rinse tank and pushes a button (alternatively, a signal could be sent automatically by a momentary switch that is activated whenever a rack or barrel is lowered). The button or switch activates a timer and opens the solenoid valve for a preset time period. After that time period has expired the solenoid valve automatically closes.

The timer setting is selected through trial and error. It is best to select a time period that provides consistently clean rinse water, without excessive waste. Water quality can be periodically monitored using an inexpensive TDS tester, as discussed above. Once set, the time period is not changed unless the general trend of production changes.

The capital cost of timer rinse controls is similar to the price of conventional conductivity controllers. However, these units require less maintenance and generally have a longer lifespan.

Timer controls continue to be an underutilized technology, in part because there are few commercial units on the market. One currently available unit is shown in Exhibit 1-8. The timer mechanism is built into a NEMA 6 water-proof epoxy box with an external raised pushbutton that energizes the timer. The timers consist of a complete unit with continuous adjustment from 6 seconds to 10 minutes. An alternative range is from 1 minute to 100 minutes.

1.3 ALKALINE CLEANING
1.3.1 Overview

As recently as 1994 (the baseline year for the Strategic Goals Program), metal ﬁnishers often used halogenated solvents to remove oils and other contaminants from work pieces prior to plating. But over the next few years, the industry largely abandoned solvent cleaning, except for a few speciﬁc applications.

The handwriting was already on the wall in 1994. The Montreal Protocol (an international agreement limiting the use of substances that destroy the Earth's protective stratospheric ozone layer) required participating nations, including the U.S., to phase out most of the halogenated solvents then in use, including the widely used trichloroethane (TCA). In addition a National Emission Standard for Hazardous Air Pollutants (NESHAP¹²), covering vapor degreasing and immersion (cold) cleaning, was enacted that year. Companies operating existing equipment had until December 1997 to comply with the requirements.
¹²(The NESHAP was revised in May 2007.)

Much of the cleaning that previously had been done with solvent is now accomplished using alkaline cleaning, the subject of this section. Because it avoids the air emissions and hazardous waste problems associated with solvents, alkaline cleaning can be considered a pollution prevention technology.

Of the solvent cleaning that remains in use today, most is carried out by the aerospace industry, by military depots, by facilities whose applications require precision cleaning operations, and (to a lesser extent) by some facilities involved in general ﬁnishing. Even these applications have undergone some changes: most of these facilities use new solvents that have been designed for environmental compliance. The cleaning/degreasing equipment has also changed signiﬁcantly. It is often downsized, and is generally much more efﬁcient and less polluting (for example, being operated as totally enclosed systems, or being equipped with carbon adsorption capture devices).

Once the 1994 NESHAP was in place, there was an immediate and major reduction in use of solvent cleaning over the next three years, followed by a less dramatic, but still steady decline. Exhibit 1-9 shows data from the Metal Finishing Strategic Goals Program. These values are for quantities of solvent released to the air on a sales normalized basis.¹³
¹³(The SGP started collecting data in 1997, although most companies also submitted data for 1994, which served as a baseline. SGP data were collected by the STERC as recently as 2008, however there are insufﬁcient data to look reliably past 2005.)

With the declining use of solvent cleaning, alkaline cleaning is now typically the initial operation performed on the plating line. Typically, the process is carried out in two stages using (1) a soak clean followed by (2) an electroclean and rinse. The cleaning stage is important, both for the quality of the ﬁnished product and for the overall environmental impact of the plating operation. If cleaning is not properly performed, the result is sure to be

an increase in rejects (and consequent rework), and
contamination of process solutions,

two very signiﬁcant factors that increase waste generation.

Although alkaline cleaning can help prevent pollution, the cleaning baths and associated rinses can themselves be signiﬁcant sources of pollution. Cleaning baths are the most frequently discarded process solutions on most ﬁnishing lines. Spent cleaners and rinse waters, typically treated on site, generate signiﬁcant quantities of sludge. In addition, certain components of cleaners interfere with the precipitation of metals and can affect compliance with discharge regulations.

Exhibit 1-10 summarizes the pollution prevention and control recommendations for alkaline cleaning. The focus is on improved rinsing methods, bath maintenance, and strategies for on-site treatment. The remainder of this subsection provides supporting information for the recommended strategies, including new technology information not found in the original Bluebook.

1.3.2 DRAGOUT & WATER USE REDUCTION

Excessive dragout is costly because:

it increases water use
it results in the need for frequent additions of cleaner
it increases wastewater treatment costs

1.3.2.1 Dragout

As with any metal ﬁnishing process, it is desirable to minimize dragout rates from cleaning tanks to the greatest possible extent. Consider implementing the following process changes, which can be effective methods for reducing dragout from cleaners:

Modify rack or barrel design to improve drainage
Improve part orientation to minimize solution carryover
Operate cleaner bath at a higher temperature to reduce viscosity
Increase dwell times over tank
Rotate barrels over process tank
Use spray rinsing as parts emerge from the cleaner

1.3.2.2 Rinsing

Counterﬂow rinsing provides the cleanest parts with the lowest water use. In counterﬂow rinsing, the rinse process is carried out in several stages. Clean water is introduced at the ﬁnal stage, so the last water that the work piece contacts is the cleanest available. Water from the ﬁnal rinse then moves upstream for use in the next-to-last stage, and so on.

It is generally advantageous to use a counterﬂow rinse following alkaline cleaning. A two-stage conﬁguration is typically adequate, although a three-stage system can provide even greater reductions in water use. The rinse system should include some kind of automatic monitoring or control (such as a conductivity monitor or a timer) to ensure that operator inattention does not result in excess clean water consumption.

Benchmarking study participants reported several other successful rinsing practices, including:

Recovery rinsing. (See Section 3.4.2.3) Recovery rinsing is applicable to alkaline cleaning (although seldom used for this purpose), especially where bath maintenance (e.g., microﬁltration) is practiced. In general, when recovery rinsing is used, softened or deionized water should be used for rinsing. One benchmarking survey participant achieved a closed-loop cleaning operation by using three dragout tanks in a counterﬂow arrangement and a microﬁltration system that processes both the bath and recovery rinse. Closing the loop has an additional advantage. The cleaning solution chemistry includes chelating compounds such as EDTA that can interfere with metals precipitation. By closing the loop, ﬁnishers can minimize treatment problems caused by the chelating compounds.
Reactive rinsing. (See Section 3.5.3.4) During reactive rinsing, the rinse following acid dip is reused in the rinse tank following the alkaline cleaner. The acidic rinse helps to remove the alkaline cleaner ﬁlm better than plain water. However, some facilities warn that use of reactive rinsing sometimes results in precipitated solids in the rinse water, especially with silicated cleaners.

1.3.3 IMPROVED MATERIAL UTILIZATION

During the benchmarking survey several respondents provided suggestions with regard to material utilization and these are summarized below:

One company indicated that it reuses two-thirds of its used electrocleaner as makeup for its soak cleaner and, therefore, only uses fresh chemistry in the electrocleaner tank. Electrocleaner baths are more concentrated than soak cleaners due to the need for electrical conductivity. When spent, this company's electrocleaner has a sufﬁcient concentration of components to be reusable as a soak clean. However, some fresh solution must also be added. It should be noted that, as one survey respondent pointed out, some electrocleaners do not work well as soak cleaners due to different chemistries.
Several survey respondents indicated that they have implemented programs with more frequent bath analyses and that they analyze and subsequently adjust for individual bath components. One respondent analyzes cleaners once every 8-hour shift. A different respondent indicated that it has also increased analytical work to include contaminants such as oil and grease. This company uses these results to trigger bath disposal instead of routinely disposing of the solution based on a time schedule or amount of surface area processed.
One survey respondent suggested switching from powdered cleaners to liquid cleaners. Another respondent also recommends the switch because powdered cleaners may not completely dissolve in the bath, causing chemical waste and carryover of chemicals to the treatment system or sewer.
Several survey respondents have substituted biological cleaners for conventional cleaning baths. This technology uses a specially formulated cleaner chemistry (slightly alkaline pH), which permits the buildup of a microbial population that consumes oil and grease. One respondent indicated that they implemented use a microbial cleaner on one production line and due to the results plan to use it in all similar processes. A discussion of biocleaners is presented later in this section.
Cyanide is used in the formulation of some alkaline cleaners; however, its use has declined signiﬁcantly over the past 20 years. Several survey respondents indicated that they had recently eliminated use of cyanide cleaners as a P2 measure. One of these companies indicated they use bead blasters to supplement cleaning and although this is less efﬁcient than their old process, it has eliminated use of cyanide-containing cleaners.

1.3.4 BATH MAINTENANCE TECHNOLOGY UPDATE

Bath maintenance is an important aspect of both P2 and cost management. A well maintained bath improves the performance of the alkaline cleaning operation, which in turn reduces reject and rework, and extends the life of the bath. The result is better parts at lower cost, and less waste.

Maintenance includes chemical monitoring and timely additions of fresh chemicals, as well as the use of various technologies to remove contaminants. Knowing what to look for is an important aspect of monitoring. Most ﬁnishers limit the analysis of their alkaline cleaner bath to a single component, such as alkalinity or conductivity. When the alkalinity drops below the recommended operating level, they add cleaner solution. This practice fails to take into account that speciﬁc components of the cleaning bath are usually degraded or consumed at different rates. A single test will not provide sufﬁcient information.

Alkaline cleaning solutions are a mixture of chemicals, including surfactants, alkali salts, caustic soda, phosphates, and complexing agents. These baths build up concentrations of oil and solids during use. At some point, the cleaning efﬁciency of the bath is impaired and the solution is discarded, despite the fact that most of the bath's constituents are still usable. Alkaline cleaner bath maintenance technologies are primarily used to remove these suspended solids and oil. In many cases, heavy-duty cleaners must be replaced once per week.

Oil can be partially removed using an oil separator/skimmer or coalescer. Free oils can be removed by simple skimming. These methods work best with cleaner chemistries that are formulated to "split" oils. However, emulsiﬁed oils and colloidal solids are not affected by these devices. One benchmarking survey respondent reported adding a reagent to used cleaners that helps split emulsiﬁed oil. The oil layer is subsequently skimmed and the cleaner is returned to service.

Most solids can be removed by settling and/or cartridge ﬁltration. Large solids are removed by simple ﬁltration Colloidal solids and oil can be removed by microﬁltration. Several benchmarking survey respondents employ this technology. When implementing microﬁltration technology it is sometimes necessary to change cleaning chemistry, since microﬁltration works best with non-silicated emulsifying cleaners.

Many commercial microﬁltration systems used for this application employ ceramic ﬁlter membranes in a cross-ﬂow ﬁltration conﬁguration. These membranes are a relatively new development that permits application of microﬁltration to solutions and emulsions that are both heated and corrosive. The ceramic membranes are produced in a range of pore sizes that selectively permit a large percentage of the surfactants to pass through the membrane. Typical pore sizes are from 0.2 μ (microns) to 0.8 μ. Crossﬂow ﬁltration, as opposed to barrier or "dead-end" ﬁltration, permits the application of this technology to high solids-feed streams. With dead-end ﬁltration, all of the feed solution is forced through ﬁltration media by an applied pressure. With a high solids-feed stream, the pores of a deadend ﬁltration device plug. With crossﬂow ﬁltration, the ﬂuid to be ﬁltered is pumped across the membrane, parallel to its surface. By maintaining a high velocity across the membrane, the retained material is swept off the membrane surface.

EPA Testing of Microﬁltration. Under the EPA Environmental Technology Veriﬁcation (ETV) program (www.epa.gov/etv), an alkaline cleaner microﬁltration technology was evaluated under actual production conditions. The veriﬁcation test evaluated the ability of the Silverback™ Model 900 unit to remove oils and recover the alkaline cleaning chemistry.¹⁴
¹⁴(Cushnie, George, Evaluation of Silverback Model 900 Alkaline Cleaner Recycling Systems, April 4, 2000. (http://www.epa.gov/etv/pubs/06_tp_silverback.pdf).)

The Silverback™ Model 900 is shown in Exhibit 1-11 and an operational diagram is shown in Exhibit 1-12. The Membralox® Silverback™ Model 900 is an alkaline cleaner recycling system. This unit contains a Membralox® 7P19-40 module made up of seven alpha alumina elements, each with 19 lumens (channels) that are 4.0 mm in diameter. The inside of each channel is lined with an alpha alumina or zirconia membrane having a total surface area of 1.68 m² (18.1 ft²), and a pore size of 0.2 μ. The manufacturer indicates that the membrane is impervious to nearly all chemicals except for phosphoric and hydroﬂuoric acids, and can tolerate temperatures up to 200°F.

The alkaline cleaner used at the test facility is CLEAN-R-120GR, which is composed primarily of sodium hydroxide, phosphate, glycol ether, octylphenol, and an amine. In use, alkaline cleaner contaminated with oil enters a two-compartment stainless steel tank through a preﬁlter that removes large particulate material from the feed stream. Free oil accumulates in the initial compartment and can be removed on a periodic basis through a drain port located on the upper part of the tank. The liquid then moves to a second tank compartment through a sub-surface passage, thereby leaving the ﬂoating oils in the ﬁrst compartment. The liquid in the second compartment (referred to as the recirculation tank) is pumped through a microﬁltration ceramic ﬁlter (0.2 μ). The ﬁlter reject returns to the recirculation tank and the recovered alkaline cleaner ﬂows back to the cleaning process. At the demonstration site, the Silverback™ unit recovers 1.0 gpm of alkaline cleaner.

Two test runs of ﬁve days each were conducted during the ETV project. Test results show that the unit recovered greater than 95% of the cleaner chemistry, while average contaminant removal was 82% for total suspended solids (TSS) and 90% for oil.

Implementation of the technology reduced the disposal frequency of the alkaline cleaning solution at this facility from 15 times per year to two times per year. The overall volume of concentrated waste generated from alkaline cleaning was reduced by 67.5% and the weight of total solids in the waste products has been reduced by 58.9%.

Operating and maintenance (O&M) labor requirements were monitored during testing. The O&M labor requirement for the equipment was observed to be 3.75 hrs/wk. O&M tasks performed during the veriﬁcation test include daily inspections of the unit and weekly cleaning of the tank and membrane.

A cost analysis of the Silverback™ Model 900 was performed using current operating costs and historical records from the demonstration site. The installed capital cost (1999) of the unit was $43,000 (includes $36,000 for the unit, $5,000 for storage tanks, and $2,000 for installation costs). The annual cost savings associated with the unit is $32,064. The projected payback period is 1.3 years.

The test results show that the microﬁltration technology provides an environmental beneﬁt by extending the bath life of the alkaline cleaner, thereby reducing the amount of liquid and solid wastes produced by the cleaning operation without removing the cleaning constituents of the bath. The economic beneﬁt associated with this technology is low operating and maintenance labor and a payback period of approximately 1.3 years. As with any technology selection, the end user must select appropriate cleaning equipment and chemistry for a process that can meet their associated environmental restrictions, productivity, and cleaning requirement.

Biological Cleaners. A unique approach to bath maintenance that may work for some facilities is biological cleaners, which are an alternative to conventional alkaline cleaners. Typically, these are mild alkaline emulsifying solutions that use biological digestion to remove residual oils from baths that operate at relatively low temperatures. The system is essentially self-regulating, since the microbial activity will adjust itself to the amount of removed oil and grease present in the system. Bio-cleaner systems operate at temperatures between 104°F and 131°F, with a pH range of 8.8 to 9.2, conditions which provide a viable environment for microorganisms. The microbes present in the cleaner system are those normally found in industrial oils and greases.

The cleaning process takes place in two separate operations:

When parts come in contact with the solution, the oil and impurities are emulsiﬁed into micro-particulates
The micro-particulates are then consumed by microorganisms which are present in the bath

Although the bio-cleaner technology has been has been tested (see results below), it is a relatively new technology and should be approached with prudence. These cleaners introduce a biological process into the shop – a type of process that is otherwise foreign to most ﬁnishers. Successful use of this technology requires a different approach to bath operation and maintenance from that used with more common cleaner formulations. If the industry's use of this technology expands, implementation will become easier for each facility as operating experience is communicated through journal publications and reports.

EPA Testing of a Bio-Cleaner. An EPA ETV project evaluated the effectiveness of one particular bio-cleaner, the BioClean™ system.¹⁵ The facility selected for the test was a zinc plating shop. At this facility, the cleaning solutions from four separate cleaning baths are pumped continuously into a holding tank that feeds the BioClean system (see Exhibit 1-13). After BioClean treatment the cleaning solution is returned, by gravity, into the holding tank, and is then pumped back into the cleaner tanks. This operation is run in a continuous mode, with level guards on the cleaner tanks that prevent overﬁlling.
¹⁵(Eskamani, Gus, Environmental Technology Veriﬁcation Report: Evaluation of BioCleanUSA, LLC Biological Degreasing System for the Recycling of Alkaline Cleaners ETV (http://www.epa.gov/etv/pubs/06_vr_bioclean.pdf))

The demonstration project resulted in the following conclusions:

Implementing the BioClean System reduced the annual amount of alkaline cleaner waste requiring treatment from 34,400 gallons to 6,940 gallons.
The O&M labor requirement for the equipment was observed to be 2 hrs/wk.
The installed capital cost (1998) of the unit was$47,569 (includes $27,625 for the BioClean unit, plus $19,944 for installation to four work-centers). The annual cost savings associated with the BioClean System was $86,192. The projected payback period was 0.6 yrs.

1.3.5 TREATMENT OF SPENT CLEANERS

Eventually, as the active ingredients are depleted, all alkaline cleaning solutions need to be discarded. Dumping a cleaner bath directly into a wastewater treatment system will usually upset the treatment process, and may result in a discharge violation for one or more metals. To avoid such problems, consider implementing one of these methods:

Bleed into General Flow. To implement this option a shop would typically install a holding tank in the waste treatment area that is large enough to hold one or more cleaner tank dumps. Drain valves on the cleaner tanks would be piped to this holding tank. It would be necessary to schedule tank dumps to adequately space out the expected volume of spent cleaner. Once a cleaner dump is transferred to the holding tank, it would be bled at a predetermined rate (e.g., 1%) to the treatment process using a chemical metering pump.
Pretreatment of Spent Cleaner Before Combining with General Flow. To implement this option, a shop would typically install a batch treatment system, rather than the holding tank discussed in the previous option. A conical bottom tank equipped with a mixer and chemical feed systems is recommended for this purpose. The spent cleaner is chemically treated, solids are removed through a valve in the bottom of the tank, and the treated liquid is metered into the general wastewater ﬂow.
Hauling Spent Cleaner to a Treatment/Disposal Site. To implement this option, a facility would transfer spent cleaner from the cleaning tanks to drums suitable for holding or transporting hazardous waste (e.g., 55-gal. DOT-approved drums). Spent aqueous cleaning solutions may or may not be hazardous, depending its pH and toxic metal content.¹⁶ The drums would be stored on-site and transported by a licensed transporter to an appropriate treatment or disposal site. If the spent cleaner were determined to be a hazardous waste, it would be handled in compliance with EPA's hazardous waste manifest system.
¹⁶(Federal rules for determining if a waste is hazardous can be found at 40 CFR 261. State rules may supersede federal rules (state hazardous waste rules can be downloaded at: http://www.envcap.org/hwrl).)

A project was conducted at a zinc plating shop to evaluate and compare these options.¹⁷ The evaluation also included a microﬁltration system for bath maintenance. Exhibit 1-14 shows a cost comparison of the four options for five sizes of cleaning operations, ranging from 1,500 gal./yr. to 30,0000 gal/yr. of spent cleaner generated. For labor calculations it was assumed for options 1, 2 and 3, that cleaning baths are replaced every two months and that the cleaner is not replaced with option 4.¹⁸ The costs shown in Exhibit 1-13 are the sum of annualized capital costs and operating costs. Exhibit 1-15 shows an example of a breakdown of these costs for one of the five cleaning operation sizes (12,000 gal./yr.).
¹⁷(Cushnie George, Chalmer, Paul, Marsh, Greg and Ferguson, David, Management of Spent Alkaline Cleaners, Plating and Surface Finishing, Nov. 2005 (www.sterc.org/pdf/psf2005/110522.pdf))
¹⁸(Therefore, the cleaning tank capacities (could be one or more tanks) for the ﬁve sizes of operations are 250 gal. (1,500/6 = 250), 500 gal., 1,000 gal., 2,000 gal., and 5,000 gal.)

The cost analysis in Exhibit 1-14 shows that for all but the largest cleaning capacity evaluated, bleeding spent cleaner into the wastewater is the most economical option. For the largest capacity, bath maintenance is the more economical option. Although the bleed method is usually less expensive to implement, in many cases, this option is not practical or feasible because wastewater ﬂow is insufﬁcient (and/or too variable) to dilute the spent cleaner to a point where it no longer interferes with the treatment processes. (Based on experimentation, a maximum 1% bleed rate was identiﬁed; see Exhibit 1-16). It is also important to keep in mind that intentional dilution is unlawful, so adding clean water to the waste-water ﬂow to achieve a treatable spent cleaner dilution rate is not an option.

Getting back to the cost analysis in Exhibit 1-14, if bleeding spent cleaner at a sufﬁciently low rate is infeasible, impractical or just too risky, then either batch treatment or bath maintenance are the most cost effective options. Batch treatment is more economical with small volumes of spent cleaner. With larger cleaning tank volumes, bath maintenance becomes more economical: when the generation rate of spent cleaner is between 6,000 and 12,000 gallons, the replacement costs for cleaner begin to outweigh the annualized cost of bath maintenance equipment. With a 30,000 gal. cleaner capacity, bath maintenance is clearly more economical than batch treatment.¹⁹ In every case analyzed, hauling the spent cleaner was not economical, primarily due to the high labor cost associated with drumming and managing the waste.
¹⁹(This cost analysis does not account for differences in cleaning chemistry that exist at most facilities (e.g., different cleaners for different alloys, soak clean vs. electroclean, etc.). However, some facilities have been able to implement bath maintenance by ﬁrst reducing the number of different cleaners used. By doing so, they are able to use the same bath maintenance equipment for multiple cleaning tanks.)

For this analysis, microﬁltration was used to establish the costs and beneﬁts of bath maintenance. The underlying assumptions used for cost calculations were based on the previously discussed EPA ETV demonstration project.

Conclusions. Bleeding spent cleaner into the general wastewater ﬂow is risky business, considering the low dilution rate that is required and the normal variability of wastewater ﬂows. At the selected facility, a single cleaner dump has a volume of 1,500 gal. and would require 260,000 gal. of wastewater in order to dilute the spent cleaner to 1%. With an average daily wastewater ﬂow of only 13,000 gpd, it would take 20 days to bleed the spent cleaner into the waste treatment system. Facility management considers such a slow bleed rate to be a non-practical solution and further, because of the risk of non-compliance associated with the bleed method, they have chosen to employ batch treatment. Their decision is corroborated by the results of the cost analysis shown in Exhibit 1-14.

Batch treatment of spent cleaners is an economical alternative for small operations. However, for facilities that use a sufﬁcient volume of cleaner, bath maintenance is a more attractive alternative. The breakeven point appears to occur approximately at 12,000 gal. of spent cleaner generated per year. At this point, the sum of cleaner replacement cost savings and other savings begin to outweigh the annualized capital cost of bath maintenance equipment and other associated costs.

1.4 ACID DIPPING AND PICKLING

Acid pickling refers to processes aimed at removal of scale. (Scale is a surface oxide that is formed when a metal, such as steel, is cooled during the transformation from molten into solid form.) Acid dipping generally refers to processes that remove the alkaline ﬁlm carried from the previous cleaning step, and that activate the surface of parts prior to plating. The most common acids used for this purpose are sulfuric, hydrochloric, nitric and phosphoric acids. The most signiﬁcant waste products generated from acid dipping and pickling are:

Spent solutions
Rinse water
Wastewater treatment sludge
Acid fumes
Scrubber water blow-down

Pollution prevention and control efforts for these processes should be aimed at process changes that reduce pollution and worker hazards, rinsing considerations, bath maintenance/recovery methods, and other waste reduction opportunities. Exhibit 1-17 presents a pollution prevention strategy for acid dipping and pickling.

1.4.1 Water Use Reduction

Excess water introduced by drag-in can dilute acid baths, especially if they are operated at ambient temperatures. Excessive dragout is costly because it results in increased rinse water use, more frequent acid replacement, and increased wastewater treatment costs. To reduce drag-in and dragout, efforts should focus on:

Rack design
Part orientation
Dwell times

Conductivity controls or pH controllers can regulate rinse water use. For ﬁnal rinses following acid dips, a pH range of pH 5 to 6, or a conductivity of 3,000 to 5,000 µmho, is generally considered acceptable. Timer rinse controls can also be used, with a timer setting adjusted so that it produces similar results.

A two-stage counterﬂow rinsing conﬁguration following acid dipping or pickling will typically sufﬁce. Some benchmarking survey respondents use reactive rinsing, where the rinse following acid dip is reused in the rinse tank following the alkaline cleaner. The acidic rinse helps to remove the alkaline cleaner better than plain water. However, two survey respondents warned that use of reactive rinsing could result in precipitated solids in the rinse water. One survey respondent employs drag-in/dragout recovery rinsing and reports that it conserves acid.

1.7.2 Bath Maintenance

Bath monitoring and timely additions will improve performance and reduce waste generation. The efﬁciency of baths can be measured by a weight loss test. Acid baths are commonly contaminated with soils and dissolved metals, both of which reduce performance. Filtration is commonly used for removing particles. Dissolved metals can be removed by acid sorption and diffusion dialysis (see discussion below), although these technologies are generally not cost effective for small operations. Electrowinning can be used for removing copper and zinc from dilute sulfuric baths (10% by volume).

Acid Sorption. Acid sorption is described in the original Bluebook (see Section 5.5), where a diagram shows how the technology can be applied to an acid dip process. Since the publication of the Bluebook (1994) the U.S. Department of Defense has investigated use of this technology for their metal ﬁnishing depot activities (e.g., aircraft overhaul).²⁰ The DoD demonstration tests were conducted using a 1-inch diameter by 12-inch length, packed ion exchange resin column. A set of 10 experimental runs was carried out using concentrated sulfuric, hydrochloric, nitric, hydroﬂuoric, and phosphoric acids in combination with a variety of metals, including iron, nickel, chromium, zinc, and aluminum. The tests represented typical conditions from pickling, cleaning, activation, anodizing, and stripping procedures. Acid recovery averaged over 80 percent for all the acids that were tested. The process achieved recovery efﬁciencies in excess of 95 percent for the acids of greatest interest, such as hydrochloric, nitric, and sulfuric acids. Metal removal efﬁciencies were not reported.
²⁰(Adsorption Treatment Systems to Recover Mineral Acid Solutions)

The DoD demonstration offered the following economic analysis, which may be useful to those considering this technology.

The purchase of an acid puriﬁcation system for the Jacksonville Naval Aviation Depot was compared to off-site disposal of spent acid. A summary of annual operating costs for the two options is shown in Exhibit 1-18. The following assumptions were used:

Throughput: 175 lb/hr of acid
Operation: 50 days/yr (400 hrs/yr)
Acid recovery efﬁciency: 60-95%
Average waste density: 15.0 lb/gal
Labor (burdened): $79/hr

Economic Analysis Summary:

The total installed cost is $29,153, including adsorption equipment ($19,100), mobile kit ($1,557), multi-tank manifold ($796), resin ($2,690), and set-up ($5,000).
Operating costs for an adsorption unit is $38,461 vs. $92,500 for disposal.
The calculated payback period for investment in the equipment/process: 7 months, using a 15-year analysis, 10% discount rate, and straight line depreciation over 10 years.
The calculated annual savings is $54,039 in disposal costs.

Diffusion Dialysis. Diffusion dialysis is described in the original Bluebook (see Section 5.8). A diagram of an operational system for acid dip bath recovery is shown in Exhibit 1-19. This diagram illustrates the use of both continuous and batch operation. In the continuous mode, acid is withdrawn directly from an acid bath, and the recovered acid stream is returned directly to the bath. The stream containing the contaminants and unrecovered acid is sent to an industrial wastewater treatment system (IWTP). An advantage of operating in the continuous mode is that labor requirements are minimal. Once the operation has been established, the unit runs with only occasional oversight. Another advantage is that continuous processing of the bath acid results in a bath with consistent composition. The primary disadvantage of the continuous mode of operation is that the required processing rate is generally about three times the normal acid disposal rate for the bath, and the waste generation rate is correspondingly larger. In general, continuous operations are only applicable to acid baths with inventories greater than several thousand gallons. At smaller volumes, a batch operation, as shown, is more practical.

In 1999, the Environmental Protection Agency Region I supported research to test a diffusion dialysis acid recovery system. The resulting report documented eight applications of diffusion dialysis at four companies; the results are summarized in Exhibit 1-20.

²²Bonner, Francis J., and Donatelli, Alfred A., Diffusion Dialysis and Acid Recovery in Metal Operations, The Toxics Use Reduction Institute, University of Massachusetts Lowell, 2001.

The sizing of a dialysis system can be based on the volume of spent acid previously produced; the "rule of thumb" requires that, at a minimum, the volume of spent acid that was previously discarded should be recycled once through the dialysis unit over the same period of time it took to generate the spent acid. For example, if a 300-gallon acid bath is being disposed of once per month, then a 10 gallon per day (gpd) diffusion dialysis system would be recommended. The standard processing rate for diffusion dialysis systems is 0.025 gal/hr/sq ft (approximately 1 liter/hr/sq meter) of available anion exchange membrane surface area. To obtain the membrane area needed to process large volumes, the membranes are stacked between gasketed hydraulic ﬂow spacers. These membrane stacks are usually standardized over a range of differing processing capacities.²³
²³(Delaney, Sean and Bailey, Daniel, Acid Recycling at Plating for Electronics, Products Finishing Magazine.)

Comparison of Diffusion Dialysis and Acid Sorption. Diffusion dialysis and acid sorptiont compete directly for acid recovery applications. Based on vendor literature and journal articles it appears that there are approximately equal numbers of diffusion dialysis and acid sorption units in commercial use (about 500 to 750 of each).

Acid sorption is the older of the two technologies, having been ﬁrst used around 1963. Early applications of acid sorption used available ion exchange resins and equipment. In 1975 a novel ion exchange technique called reciprocating ﬂow ion exchange was applied to acid recovery, which essentially commercialized the acid sorption process. This method of ion exchange is characterized by

ﬁne particle size resins
countercurrent elution
short column heights
ﬁxed, over-packed resin beds
short cycles

and a number of other features. The process is ideal for treating small volumes of concentrated solutions with a minimum of dilution or ﬂuid intermixing in the resin column. Reciprocating ﬂow ion exchange has seen wide application in the metal ﬁnishing industry for the recovery of a wide variety of metals. By employing this method, the performance of the acid sorption process was signiﬁcantly improved.²⁴ There is only one major North American manufacturer of this technology (Eco-Tec, Ontario, Canada).
²⁴(Pajun, Paul and Harrison, J., Anodizing Acid Puriﬁcation using Resin Sorption Technology at Pioneer Metal Finishing, AESF Compliance Week, January 1997.)

Diffusion dialysis was developed in the early 1980's in Japan and commercialized around 1990. There are at least three U.S. manufacturers of this technology.

Diffusion dialysis and acid sorption were objectively evaluated by Concurrent Technologies Corporation (Johnstown, PA) using bench scale equipment to compare their ability to remove metal contaminants and recover free acid from a contaminated mineral acid bath.²⁵ This comparison was based on one speciﬁc application: recovery of hydrochloric acid (HCl) from iron (Fe) contaminated hydrochloric acid solutions.
²⁵(Kinnel, Wayne A. and Roberts, David S., Mineral Acid Puriﬁcation Opportunities, Concurrent Technologies Corp., May 1996.)

For the test, solutions were prepared in a laboratory to closely approximate actual spent hydrochloric acid activation and cleaning baths. The results of testing indicated:

Diffusion dialysis achieved acid recoveries averaging 97% and iron rejection averaging 86%.
Acid sorption achieved acid recovery and iron removal efﬁciencies of approximately 96% and 45%, respectively.

The tests show that for this application, the two technologies achieve almost identical HCl recovery efﬁciencies; however, diffusion dialysis achieves greater percent rejection of the contaminant metal in the feed and therefore is the more effective technology. It should be noted that for other applications, different results may be obtained.

In terms of capital cost, the two technologies are similar; however, replacement costs for the diffusion dialysis membrane will exceed replacement costs for acid sorption resin over the lifetime of the equipment.

1.4.3 Process Conditions

Higher concentrations of acid and higher operating temperatures dissolve more of the substrate metal. Since dissolved metal will cause the bath to lose effectiveness, lower acid concentrations and lower bath temperatures are preferred from a pollution prevention (P2) standpoint. The less metal that is dissolved into the bath, the longer the life span of the solution. In addition, higher operating temperatures require use of energy, which is also a source of pollution. However, lower is not always better. Lower temperatures and acid concentrations increase the time necessary for pickling and oxide removal, and the ﬁnisher must consider the time constraints of the process. There is usually an operating range in which both time constraints and P2 objectives can be met.

1.4.4 Inhibitors

Inhibitors can be added to pickling baths to retard or stop the etching caused by the acid solution, thus providing a P2 beneﬁt. Synthetic inhibitors made up of organic compounds are in common use today with pickling operations. As may be expected, the use of inhibitors increases the time needed to remove scale. When the concentration of inhibitor is increased, less base metal is dissolved and the required processing time increases, especially when low-concentration acids are employed. Yet P2 favors lower acid concentrations. The metal ﬁnisher must be prepared to deal with this trade-off.

1.4.5 Chemical Substitution

Mineral acids are effective at removing scale and activating metal surfaces. Because of acid fumes (e.g., hydrochloric and nitric acids) and safety concerns, however, substitutes are sometimes sought. Chemical substitution was implemented by several benchmarking survey respondents, including the use of acid salt substitutes for nitric, hydrochloric and sulfuric acids. Examples of acid salts used for descaling (pickling) include sodium ﬂuoride, ammonium persulfate, and ferric sulfate. The acid salts can be mixed with other chemicals to give a more effective cleaning action. For example, hydrogen peroxide is sometimes added to acid salt solution to improve surface activation.²⁶
²⁶(Tan, A. C., Tin and Solder Plating in the Semiconductor Industry: A Technical Guide, Springer Semiconductors, 1993.)

1.4.6 Uses for Spent Baths

Spent acid baths can be used as a wastewater treatment reagent for neutralizing highly alkaline wastes. Spent sulfuric baths containing iron can be used with chromium wastewater as a reducing agent. This practice may result in higher sludge production rates as compared to the use of conventional reagents such as sodium bisulﬁte.

1.5 ANODIZING

The most common electrolytes used for anodizing are:

Chromic acid (Type I anodizing)
Sulfuric acid (Type II anodizing)
Sulfuric and oxalic acids (Type III anodizing)

The use of chromic acid raises the most signiﬁcant environmental issues. One of the most important changes in anodizing that has taken place over the past 25 years is a signiﬁcant reduction in the use of the chromic acid process. This change has occurred primarily because of environmental and health concerns, and the consequent regulations. The most common alternative chosen to replace chromic acid anodizing has been the sulfuric acid process. Chromic acid, however, is still the method of choice for certain applications. One example in particular where chromic acid anodizing remains popular is coating aircraft parts, especially those with recesses (e.g., spot welded assemblies and rolled seams), which trap electrolyte. Lingering sulfuric acid in recesses will cause corrosion and possible part failure. One beneﬁt of the chromic acid process is that residual chromic acid does not appreciably attack aluminum alloys. In addition, the chromic acid process has a less deleterious effect on fatigue life overall than does sulfuric acid anodizing. Nevertheless, aircraft manufacturers continue to seek alternatives to chromic acid. During the past 15 years, the aerospace industry has used a substitute process with a sulfuricboric acid (SBAA) electrolyte to further reduce the use of the chromic acid process.

P2 efforts associated with sulfuric acid anodizing have capitalized on opportunities related to rinsing, anode use, bath maintenance, substitution and energy use. Exhibit 1-21 summarizes the following discussion regarding pollution prevention strategies for anodizing.

The anodizing process involves several additional operations, including pretreatment (e.g., etch) and post-treatment (e.g., sealers and dyes). Pollution prevention opportunities also exist for these ancillary processes.

1.5.1 Dragout and Water Use Reduction

A good rinsing conﬁguration for this process is a two-stage counterﬂow rinsing arrangement. As with all processes, efforts should be directed at reducing dragout, which has a direct bearing on rinse water use. Rinse water control can be accomplished using pH controllers, conductivity controls, or a timer rinse control system. For ﬁnal rinses following anodizing, a commonly accepted conductivity range is 2,000 to 4,000 µmho.

Some ancillary processes performed in anodizing shops are covered under the Plating and Polishing air pollution regulation (e.g., nickel acetate and chromate seals).²⁷ For the affected processes (those that use cadmium, chromium, lead, manganese and/or nickel), shops must implement the required P2 practices; among which the following are related to dragout control and rinsing:
²⁷(National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Proposed Rule, Federal Register March 14, 2008 (pp. 14126 – 14151).)

minimize bath agitation when removing tank objects
maximize dripping of bath solution back into tank by extending drip time when removing the tank objects and using drain boards (i.e., drip shields)
optimize the design of barrels, racks, and parts to minimize dragout of bath solution

1.5.2 Sulfuric-Boric Acid Anodizing

The Boeing Company developed and qualiﬁed the boric sulfuric acid anodize (SBAA) process in 1990 as a direct replacement for the chromic acid anodizing²⁸ (CAA) process used on aluminum production pieces. The SBAA process consists of a sulfuric-boric acid anodizing bath and a dilute chromate sealer bath (about 70 mg/l chromic acid). SBAA is a commercially available process that provides a protective coating meeting all military and industrial speciﬁcations that apply to the chromic acid anodizing process.
²⁸(Boeing Corporation (http://www.boeing.com/companyofﬁces/doingbiz/environmental/chromic.html).)

The SBAA process was tested by the Naval Air Systems Command at NADEP North Island, and has been approved by the military as an alternative to the chromic acid anodizing process. The SBAA process has since been implemented at various military depots. Mil-C-8625C (the Mil-Spec that governs all anodized coatings) was revised to include the SBAA process as an option for aluminum anodizing.²⁹
²⁹( Joint Services Pollution Prevention and Sustainability Technical Library, Sulfuric/Boric Acid Anodizing, April, 2003.)

The following are processing characteristic of BSAA as compared to chromic acid anodizing (Boeing):

Paint adhesion -- As good or better than chromic acid.
Processing time -- 20 minute process, 30-60%less time than chromic acid.
Energy Requirements -- Requires lower temperature and voltage, more energy efﬁcient than the chrome-based process.
Facilities Requirements -- Equipment may require material upgrade.
Sealing -- Dilute chromate seal must be used. De-ionized water seal is not acceptable BSAA will take longer to reach the same extent of hydration as Chromic Acid Bath Performance.
Long term bath performance is more reliable
Licensing -- BSAA requires patent license. No fee if all parts are for Boeing.
Other -- In dusty environments, addition of sodium benzoate or benzoic acid to BSAA prevents fungus growth.

1.5.3 Cathode Use

Sulfuric acid anodizing can be carried out using either lead or aluminum alloy cathodes. One benchmarking survey respondent reported substituting aluminum cathode rods for lead cathodes as a P2 measure. In addition to eliminating lead, the advantage of aluminum cathodes is that they require less current and less cooling, due to the greater current carrying capacity of aluminum as compared to lead. Aluminum alloy cathodes will also outlast lead cathodes, which will slowly dissolve unless an electrical charge is maintained on the cathodes when the tank is not in operation.³⁰
³⁰(Schaedel, Fred C, Try a Holistic Approach to Improve Anodizing Processes, Products Finishing Magazine, Dec., 2006.)

1.5.4 Process Control

The anodizing process is referenced by more than 75 major aerospace, military and commercial speciﬁcations, some of which have overly generous ranges for acceptable chemical concentrations, operating parameters, (e.g., temperature) and impurity levels. Operating the anodizing process within narrow, more desirable parameter ranges will reduce pollution by minimizing rejects and rework, extending the life of baths, and lowering energy use. For example, free sulfuric acid concentration ranges for the various speciﬁcations range from 5.0% to 28.4%, although the ideal range is 10 to 11% for Type II and 11.0 to 15.5% for Type III (Schaedel).

Correct use of additives and modiﬁers can signiﬁcantly boost efﬁciency and energy savings. One benchmarking survey participant reduced energy use with Type II anodizing by using a bath additive that allowed the bath to operate at a higher temperature. This reduced the electrical requirements for cooling the anodizing solution. Much of industry operates Type II baths at 70 °F. When a good additive and modiﬁer are used, the bath can be run as high as 85 °F (Schaedel). The same survey respondent indicated that use of a lower-frequency pulse rectiﬁer reduced energy consumption for their hard anodizing process. This same equipment is also applicable to Type II anodizing. Special secondary half-wave rectiﬁers can also provide an energy savings with Types II and III anodizing (Schaedel).

Bath additives can also extend the life of nickel acetate seals. One such additive adds wetting and dispersing agents to improve seal quality and prevent smut and white powder formation on parts. It also provides a hydrophobic anodic coating that helps water to run off parts, thus minimizing spotting.³¹
³¹(Merit Partnership, Extending Metal Finishing Bath Life, Dec. 1997.)

One small anodize shop (23 employees) documented saving of $12,130 per year through improved process solution monitoring and control (Merit Partnership). A breakdown of their cost savings is shown in Exhibit 1-22. In addition to implementing various control measures, their improvements included installing a ﬁlter system on the nickel acetate bath.

Sealers and Dyes. The Merit Partnership (a joint venture including U.S. EPA Region 9, state and local regulatory agencies, private-sector industries, and community representatives) successfully demonstrated the use of reverse osmosis (RO) for recovery of nickel acetate and black dye.³² In each operation (see Exhibit 1-23), the overﬂow from the ﬁrst-stage rinse tank is pressurized by a high-pressure feed pump. Particulates are removed from the feed solution by two 1-micron cartridge ﬁlters. The feed solution then ﬂows in series through spiral wound modules containing thin-ﬁlm composite RO membranes (two modules for nickel acetate and four modules for black dye operations). After separation, a portion of the concentrate stream is returned to the process bath in order to recover process chemicals. The remaining portion of the concentrate stream is conveyed to the recirculation tank, where it is temporarily stored before being recirculated through the RO unit. Permeate is conveyed to the second rinse tank and is reused as rinse water. A small amount of fresh deionized water from an outside source is added to the process baths in order to make up for evaporative water losses.
³²(Merit Partnership, Reverse Osmosis Applications for Metal Finishing Operations, Jan. 2002 revision.)

The installed cost of the nickel acetate RO unit was $45,000. Annual operations and maintenance costs are $541 (mainly electricity and RO membrane cleaning). Cost savings for this installation are shown in Exhibit 1-24. The payback period for the RO installation was approximately 2 years, not including depreciation of the RO unit (i.e., tax savings).

1.5.5 Special Practices for Chromic Acid

The operation of chromic acid anodizing is affected both by environmental and by safety and health regulations directed toward the control of airborne emissions from the process:

National Emissions Standards for Hazardous Air Pollutants (NESHAP) (40 CFR. §63.6) for chromium regulates air emissions to the environment.
OSHA's chromium permissible exposure limit (PEL) regulates the amount of chromium in the air within the shop (29 CFR Parts 1910, 1915, et al.).

P2 practices can help meet these two regulations simultaneously. Chromium is released to the air from anodizing operations during the process, as gases created during electrolysis at the anode and cathode rise through the solution, break at the surface and release an aerosol to the air above the tank. A common P2 practice involves the use of fume suppressants that lower surface tension and/or create a foam blanket that reduces the amount of chromic acid mist released to the air.

There are two basic types of fume suppressants: wetting agents and foam blankets. The difference between foam blankets and wetting agents is the way in which they reduce emissions:³³
³³ (Riordan, Barrett J., Karamchandanl, Rohit T. Zitko, Larry J., and Cushnie George C., Capsule Report Hard Chrome Fume Suppressants and Control Technologies, United States Environmental Protection Agency Technology Transfer Ofﬁce of Research and Development, Washington, DC 20460, EPA/625/R-981002 December 1998.)

A foam blanket fume suppressant generates a layer of foam across the surface of a solution when current is applied to that solution. The foam layer physically suppresses the mists produced on the surface of plating baths. Foam blankets do not prevent the formation of chromic acid mist – instead, they trap the mist under the blanket.
Wetting agents lower the surface chemistry of plating baths. This causes gases to escape at the surface of the solution with a diminished "bursting" effect, reducing mist formation.

Another approach to preventing chromium losses to the air is to use tank covers that enclose the top of the anodizing tank. One commercial unit (Emissions Elimination Device, EED) incorporates membranes that allow gases to pass freely, while effectively blocking the escape of water vapor and chemical mists (see Exhibit 1-25). Prior to opening the tank cover, an evacuation system removes chromium-laden air, ﬁlters it, and returns the air to the plating room. Tank covers of this design eliminate the need for fume suppressants and a scrubber system.^{34, 35}
³⁴(Washington, DC 20460, EPA/625/R-981002 December 1998. Graves, Beverly, Encapsulating Hard-Chrome Plating Tanks, Products Finishing Magazine, Aug., 1999.)
³⁵(Products Finishing Magazine, December, 1997.)

1.5.6 Bath Maintenance

Chromic acid anodizing. Common bath contaminants associated with chromic acid anodizing include:

Chloride – causes burning or pitting of parts: maintain below 0.3 g/l.
Sulfate – gives rise to opaque appearance of coating: maintain below 0.5 g/l.
Trivalent chromium – forms complexes that are ineffective for anodizing: maintain below 4 g/l.
Aluminum – forms complexes that are ineffective for anodizing: maintain below 3 g/l.

Chloride and sulfate are often introduced via water that is added for evaporative makeup, and for rinsing parts. Using deionized water for these purposes can minimize chloride and sulfate introduction. Precipitation using silver oxide will remove sulfate if it builds up beyond tolerable limits. Limiting the total cathode surface to between one-ﬁfth and one-tenth of the total anode area will minimize the production of trivalent chromium. This can be accomplished by shielding part of the cathode area with wire-reinforced glass plates. When trivalent chromium levels exceed tolerable limits, high current density electrolysis can be used to convert trivalent chromium to the required hexavalent form.

Various technologies have been marketed for removing dissolved aluminum, including ion exchange, ion transfer (i.e., non-ion-speciﬁc membranes including ceramic porous pots and units with polyﬂuorocarbon membranes), and membrane electrolysis (i.e., ion-speciﬁc membrane). Each of these technologies is discussed in Section 5. Of these three technologies, only ion exchange is presently a viable method for removing dissolved aluminum at a rate that prevents buildup of aluminum in the bath. Membrane electrolysis does an effective job; however only one company produced a suitable unit and, although they are still servicing existing units, they are no longer selling new equipment.³⁶ Ion transfer has only a marginal effect on dissolved aluminum concentration and it cannot maintain a chromic acid anodizing bath below tolerable limits. An EPA Environmental Technology Veriﬁcation (ETV) test of an ion transfer unit was conducted at a chromic acid anodizing aerospace facility in 2000.³⁷ The results of the test showed that the unit does remove some dissolved aluminum from the bath, although the rate of removal was insufﬁcient to prevent the bath from having to be discarded. During the ten week test, the unit was only able to remove about 25% of the aluminum that dissolved into solution during the test period.
³⁶ (Personal communication with Dan Vaughan of Ion Sep Corporation, Rockland, DE.)
³⁷ (Start, Chris, Environmental Technology Veriﬁcation Report, Evaluation of US Filter Corporation's RETEC® Model SCP-6 Separated Cell Puriﬁcation System for Chromic Acid Anodize Bath Solution, VR-P2MF-10-02, September 2001.)

Although ion exchange is an effective technology for maintaining chromic acid anodizing baths, it generates a signiﬁcant volume of waste during resin regeneration. The waste stream is typically treated on-site using neutralization and precipitation, which generates a hazardous waste sludge (F019).

Additional information on chromic acid puriﬁcation can be found in the hard chromium plating section.

Sulfuric Acid Anodizing. Bath maintenance is an important aspect of sulfuric acid anodizing. The anodizing process causes an increase in the concentration of dissolved aluminum. In conventional practice, the bath, or a portion of the bath, is replaced with fresh electrolyte. The spent solution is then typically neutralized and treated using a hydroxide precipitation process, a practice that creates sludge.

A fairly signiﬁcant number of anodizers have implemented bath maintenance as an alternative to solution dumping. The most widely used technologies for this purpose are acid sorption and diffusion dialysis. Both technologies are discussed in the Acid Dipping process section.

Although they work by very different mechanisms, acid sorption and diffusion dialysis produce approximately the same results. Exhibit 1-26 shows operating performance data for an acid sorption unit applied to sulfuric acid anodizing baths.³⁸ For this installation, 96% of all free sulfuric fed to the unit is recovered for reuse. For every pound of aluminum removed by the unit, 2.5 pounds of sulfuric acid is lost to waste. This translates to a 70% reduction in sulfuric acid purchases and caustic soda neutralization costs when compared to a dump-decant scenario.
³⁸(Jan, Paul Pajunen, P.E., Jim Harrison; Anodizing Acid Puriﬁcation Using Resin Sorption Technology at Pioneer Metal Finishing, 18th AESF/EPA Conference, 1997.)

A benchmarking survey participant operates a hard coat anodize tank with a maximum aluminum content of 3.75 g/l. When that limit is exceeded, they use the spent hard coat bath as replacement for a sulfuric acid anodize bath, a larger bath that requires a lower acid concentration and has a maximum allowable aluminum concentration of 18 g/l. When the concentrated hard coat bath is used, water is also added to dilute the acid, and the aluminum concentration is proportionally diluted to about 3 g/l. With periodic dumping of the sulfuric acid tank to a treatment system and replacing the sulfuric acid bath with the used hard coat solution, the aluminum sulfuric acid bath never exceeds the 18 g/l limit. The hard coat solution contains additives that are not needed in the sulfuric acid process, but these additives have not caused any operational problems with the sulfuric acid anodizing process. This practice of reusing the hard anodize bath has been in effect for about 15 years.

1.6 CADMIUM PLATING

The number of cadmium plating operations in the U.S. and worldwide has declined signiﬁcantly during the past 20 years because of the considerable environmental and health concerns associated with cadmium metal. In some parts of the world, such as Europe, cadmium-plated parts cannot be imported or sold. Most cadmium electroplating has been replaced by zinc electroplating. Other popular alternatives include vacuum deposition of aluminum, and zinc and tin alloy plating.

Nevertheless, cadmium provides several distinct advantages over zinc; therefore, its use continues for certain applications. Cadmium:

provides superior corrosion protection in marine environments
is less apt to cause hydrogen embrittlement
has a dense and adherent oxide ﬁlm that does not form bulky corrosion products
provides a conductive coating with good lubricity, suitable for fasteners (particularly in aerospace applications)

Seventeen benchmarking survey respondents (12.8%) perform cadmium plating. Two indicated that they are in the process of discontinuing cadmium plating. In an effort to retain customers while eliminating cadmium, one job shop offers to perform R&D for customers to ﬁnd alternative coatings. Cadmium plating is currently 6.4% of this shop's total metal ﬁnishing sales ($9.5 million).

Exhibit 1-27 summarizes the following discussion regarding pollution prevention strategies for cadmium plating.

1.6.1 Water Use Reduction

Rinsing following cadmium plating presents challenges to the metal ﬁnisher. Due to the relatively low temperature of the cadmium bath, the opportunity to use dragout recovery rinsing is limited. However, some companies from the benchmarking survey do use dragout recovery rinsing or spray rinsing to capture and return dragout to the process tank. A company using spray rinsing is reportedly able to recover all dragout by spraying over a dead rinse and returning that solution. This company, which plates zinc die castings (rack and barrel), has experienced a buildup of zinc in its cadmium plating tank, which is attributed to spray rinsing of the barrels. The zinc contamination problem may force the company to make design changes.

The drag-in/dragout tank arrangement is applicable to cadmium plating. Generally, a good strategy is to focus on dragout minimization and to implement counterﬂow rinsing and good water use control. Dragout reduction can be achieved by:

Increasing dwell time
Rotating racked parts for better drainage
Redesigning barrels to reduce the volume of trapped solution

Operating the cadmium plating bath at the lowest practical metal level will also lower dragout losses. Water use control can be accomplished by using ﬂow restrictors, conductivity controls, shut-off valves, and timer rinse controls. For final rinses following cadmium plating, a commonly accepted conductivity range is 500 to 700 µmho.

The Merit Partnership investigated ways of improving rinsing efﬁciency and reducing chemical losses for cadmium plating at a job shop.³⁹ The original and modiﬁed rinse systems for cadmium plating and chromate conversion coating from their report are shown in Exhibit 1-28. Overall, this system reduced the mass of metal and wastewater discharged to treatment by about 50%. The calculated payback of the modiﬁcations was 1.7 years.
³⁹ (Merit Partnership, Modifying Tank Layouts to Improve Process Efﬁciency, Oct. 1996.)

On July 1, 2008 EPA published the National Emission Standards for Hazardous Air Pollutants for Plating and Polishing Operations.⁴⁰ This rule impacts many metal ﬁnishing operations (EPA estimates 2,900 facilities). It applies to facilities with plating, polishing or thermal spray processes that contain cadmium, nickel, lead, manganese and/or chromium. (The rule will not apply to chromium anodizing and electroplating, which are covered under a previous rule.)
⁴⁰ (National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Final Rule, Federal Register July 1, 2008.)

1.6.2 Bath Maintenance

Cadmium baths tend to increase in metal concentration over time, due to the difference between anode and cathode plating efﬁciency. Some platers compensate for this increase by adding other bath constituents to maintain recommended ratios. This procedure results in signiﬁcantly higher levels of cadmium in the dragout. An alternative method is to control the cadmium metal concentration in the bath using a combination of insoluble anodes (carburized steel balls) together with soluble anodes. Other accepted bath maintenance methods include ﬁltration and carbonate control (using chilling system).

1.6.3 Chemical Recovery

Various chemical recovery technologies have been used with cadmium electroplating. The most frequently used technologies are electrowinning, ion exchange and evaporative recovery (vacuum type). Electrowinning and ion exchange applications were demonstrated by EPA on a full scale installation.⁴¹ Exhibit 1-29 shows the cadmium recovery system conﬁguration. Water from the ﬁrst rinse tank is passed through a ﬁlter (8 gpm) to remove suspended solids that would otherwise clog the resin bed. An anionic resin captures the cadmium-cyanide complex, and the water is returned to the second rinse. Samples of the rinse water collected before and after reovery were analyzed. The feed into the exchange unit contained about 2 to 8 mg/l of cadmium, 1 to 35 mg/l of cyanide, and 200 mg/l total dissolved solids (TDS). The ﬂow out of the ion exchange unit was generally below 0.01 mg/l of cadmium and cyanide, and contained about 150 mg/l TDS. The pH of the rinse water was around 11.5, both before and after ion exchange. The resin is regenerated, using 15 to 20% sodium hydroxide, when the capacity of the resin bed is reached, and the regenerant (about 660 gallons per year) is processed in an electrowinning unit. The cadmium metal recovered by the electrowinning unit (about 70 pounds per year) is reused in the plating tank as anode material. Operating costs for this installation are shown in Exhibit 1-30. The capital cost of the system was $15,725 and the payback period for the system was less than one year.
⁴¹ (Gavaskar, Arun R.; Olfenbuttel, Robert F.; and Jones, Jody A., Cadmium and Chromium Recovery from Electroplating Rinsewaters, EPA Risk Reduction Engineering Laboratory, EPA/600/SR-94/050, April 1994.)

A second demonstration of electrowinning for cadmium was conducted by the Merit Partnership using a high surface area electrowinning unit (Exhibit 1-31). A heater was installed in the second rinse tank (ideally it would have been installed in rinse tank 1, but that was a plastic tank) and it was operated at 115 °F. The heater evaporates water, which concentrates the cadmium, and thereby improves the efﬁciency of electrowinning. After two months of operation, the unit had recovered 4.4 pounds of cadmium. The installed cost of the electrowinning system (including heater) was $9,010. The projected annual savings for the installation was $1,564, mainly due to reduced treatment costs. The estimated payback
period was 8.7 years.

1.6.4 Non-Cyanide Cadmium Plating

Concerns over occupational health and safety, waste treatment costs, regulatory compliance requirements, and potential liability have encouraged the metal ﬁnishing industry to investigate non-cyanide plating technologies. The earliest and most complete cyanide substitution that has taken place in the industry is the conversion from zinc cyanide to zinc chloride or zinc alkaline. Cadmium plating solutions that do not use cyanide are less common, but are commercially available. These include cadmium neutral or acid sulfate and cadmium acid ﬂuoroborate plating solutions. Successful conversion to non-cyanide cadmium plating has been documented.⁴² At this installation, the facility indicated that many of the anticipated difﬁculties or perceived shortcomings of a non-cyanide plating electrolyte did not materialize. Reported positive aspects of the non-cyanide bath included:

The effect of operating temperature was underestimated. Although the temperature never exceeded the maximum recommended in the operating guide, signiﬁcant changes in the deposit brightness, uniformity, coverage and color
(staining) occurred above 80°F. Also, brightener consumption tripled, which led to the installation of a cooling coil.
There are a total of four additives for this process, making it somewhat difﬁcult to interpret Hull cells and make necessary additions.
The initial cost of equipment for this process can be high, due to the need for zirconium anode baskets.

⁴² (Products Finishing Staff, Elimination of Cadmium Cyanide Waste Treatment, Products Finishing, Oct. 1997.)

1.6.5 Non-Chromate Conversion Coatings

Chromate conversion coating is a type of post-treatment applied to passivate cadmium and other metal deposits, to inhibit corrosion, and to promote coating adhesion. The traditional process uses toxic hexavalent chromium compounds. Recently, the industry has developed less toxic alternatives in order to comply with substance restriction legislation such as RoHS. One alternative is trivalent chromate conversion, which under testing by the Navy has shown equal or better effectiveness compared to hexavalent chromium coatings in a 1,000 hour neutral salt fog test.⁴³ Work by the Navy is continuing and includes evaluation of non-chromium conversion coatings.
⁴³ (Matzdorf, Craig, Trivalent Chromium-based Post Treatment for Sacriﬁcial Coatings, AESF/EPA Conference for Environmental and Process Excellence, Feb. 2003.)

1.7 COPPER PLATING

Operating environmentally efﬁcient copper electroplating processes begins with the selection of the plating bath. Twenty years ago, most copper plating was carried out using cyanide baths. However, acid copper baths have become the predominant choice for meeting most metal ﬁnishers' needs.

Unfortunately, acid copper baths cannot be used to plate copper directly onto steel or zinc (both of which are commonly plated). In these cases, a copper cyanide strike (thin deposit) can be used, followed by an acid copper solution. This strategy greatly reduces the quantity of cyanide needed. Alternatively, copper pyrophosphate strike solutions are employed by some shops to avoid the cyanide bath when plating steel substrates. In addition, proprietary alkaline, non-cyanide copper plating processes may allow the use of a single solution to replace the two-step copper strike-plate process. However, the use of such solutions is not widespread. Exhibit 1-32 summarizes the following discussion regarding pollution prevention strategies for copper plating.

1.7.1 Water Use Reduction

Minimizing dragout, using multiple rinse tanks, and controlling water use can achieve water use reduction. To reduce dragout, efforts should focus on:

Rack design
Part orientation
Dwell times

To meet rinsing needs and lower water use, a two-stage counterﬂow rinsing conﬁguration will typically sufﬁce following copper plating.

Although dragout tanks for recovery are typically not employed with copper plating, they can be used with baths operated above 100°F. One benchmarking survey respondent reported using recovery rinsing on copper plating. On continuous production lines, where rinse water needs are relatively stable, ﬂow can be controlled easily by periodically monitoring rinse water conductivity and manually adjusting ﬂow, using a valve. If workﬂow is sporadic, a conductivity control or timer rinse control should be considered. These devices help to match rinse water use and production ﬂow. For ﬁnal rinses following copper plating, a commonly accepted conductivity range is 200 to 500 µmho.

1.7.2 Bath Maintenance and Control

Copper electroplating baths are rarely discarded. Common bath maintenance methods include ﬁltration (removes suspended solids), carbon treatment (removes organic contaminants), low current density electrolysis (removes dissolved contaminants), and carbonate freezing or carbonate precipitation (removes excessive carbonate from cyanide baths).

Organic residues from bufﬁng compounds, cleaners, tank and rack materials and other organics can contaminate the bath. Continuously ﬁltering the plating solution through a ﬁlter chamber packed with approximately 0.7 grams of activated plating-grade carbon per liter of plating solution (0.1 oz per gallon) will remove small amounts of some organics.

Low concentrations of hexavalent chromium, which can be dragged into the bath on the racks, will produce dull and spotty deposits. Chromium can be reduced quickly to its trivalent form and precipitated by the addition of proprietary reducing agents directly to the plating bath. Hexavalent chromium also can be converted to the inactive trivalent state by using high-current-density electrolysis, or by adding sodium hydrosulﬁte or sodium stannite. There is disputed evidence that trivalent chromium left in the plating bath can be reoxidized by air back to hexavalent chromium. The best procedure is to precipitate the chromium and remove it by ﬁltering.

Zinc contaminants will plate out simultaneously with the copper, causing brittle and brassy looking deposits. Zinc can be removed by dummying the bath at 0.2–0.4 A/sq dm. Dummying also removes many other metallic contaminants.

Alkaline cyanide solutions are relatively more difﬁcult to control than acid sulfate solutions. Cyanide also involves special handling and treatment procedures because of its acute health hazards and waste disposal requirements. Recommendations for solution control include:⁴⁴

Use high-purity, oxide-free anodes for high-efﬁciency decorative copper plating. Lower quality anodes can be used in strike and medium-efﬁciency Rochelle-salt cyanide-copper plating baths.
Use copper balls or nuggets in steel baskets, or slab anodes. Intermingle plain steel anodes with the copper anodes to control the buildup of copper in the bath.
Bag the anodes (PP, nylon) in order to contain insoluble particles coming from the anodes.
Keep the anode/cathode ratio between 1:1 and 2:1.

⁴⁴ (Snyder, Dondald L., Choosing and Troubleshooting Your Copper Electroplating Process, Products Finishing Magazine.)

Acid copper sulfate baths are easier to maintain:

Use quantitative analysis to control copper, sulfuric acid and chloride.
Add proprietary addition agents, which control brightness, ductility, and leveling, on the basis of amp-hr. Also regulate their addition by using Hull cells and a copper analysis.

1.7.3 Material Usage

Converting from copper bar anodes to baskets of copper nugget or ball anodes can improve plating efﬁciency and material usage rates. Because the copper surface is maintained when nuggets or balls are used, consistently high plating speeds can be maintained. Bars typically cost about 10% more than nuggets, and not all of the bar material is consumed in the plating process. Although the work may be distributed evenly, the plating process does not deplete the copper evenly along the length of the rod. There is typically less erosion in the vicinity of the hook than at the opposite end. In time, this produces a spear shape that needs replacing. Typically, bars are replaced when about 15% of the copper material remains. Bars also hang from hooks in such a way that not all of the copper material is fully immersed in the plating bath. Inevitably, there will be unused material at the top. In basket operations, copper material is fully submerged, and all of the material is consumed. As the copper dissolves, the baskets are replenished, leaving no scrap material. Selling the unused bar material back to a copper distributor as scrap recovers some of the value of the original metal, but the unused copper still represents a sizable cost. A typical scrap return of $2.00 per lb still produces a scrap cost of $1.00 per lb, not taking into account the costs related to handling and transportation.^{45

45} (Panagiotakis, Gregory, The Case for Anode Baskets, Products Finishing Magazine, Nov. 1995.)

1.7.4 Chemical Recovery

Various chemical recovery technologies have been used with copper electroplating. The most frequently used technologies are ion exchange, electrowinning, and evaporative recovery. The most common approach to copper recovery is a combined system using both ion exchange and electrowinning.

An efﬁcient IX/EW conﬁguration is shown in Exhibit 1-33. A dragout rinse is used to recover the bulk of the copper and therefore allows for installation of smaller and less expensive IX and EW recovery equipment. The ion exchange unit, which is best applied to dilute rinse waters, maintains a low metal concentration in the ﬁnal rinse. The ion exchange unit concentrates the metal into a regenerant stream, and the electrowinning unit removes the metal.

1.8 DECORATIVE CHROMIUM PLATING

Although some of the same strategies used for hard chromium plating are applied to decorative chromium plating, several key factors have caused companies to take some approaches that are speciﬁc to decorative chrome. These factors include:

Lower plating bath temperature (<125°F)
Higher average dragout rates
Widespread use of a substitute plating solution (trivalent chromium).

The advantages to using the trivalent solution include the following:

Trivalent chromium baths reduce human exposure to toxic hexavalent chromium compounds.
During operation, the trivalent bath does not generate mist, as the hexavalent bath does.
Trivalent baths also have a signiﬁcantly lower chromium concentration and lower viscosity, both of which reduce the quantity of chromium lost due to dragout.

Some benchmarking survey respondents have successfully implemented trivalent chromium plating. But decorative chromium also presents some environmental challenges. For example, the lower bath temperature and higher dragout rate make implementing closed-loop rinsing more difﬁcult than with hexavalent chromium plating. Exhibit 1-34 summarizes the following discussion regarding pollution prevention strategies for decorative chromium plating.

1.8.1 Water Use Reduction

A good formula for environmental success with decorative chromium plating includes dragout minimization, direct recovery, and counterﬂow rinsing with good water use control. Dragout can be minimized by orienting parts on racks such that they drain completely, slowly withdrawing parts from the process solution, suspending the parts over the process tank for drainage (see draining/rinsing over the plating tank), and using a dragout recovery rinse followed by a multiple-stage counterﬂow rinse.

Another approach to dragout minimization is to operate the bath at a lower, but still acceptable, chromium concentration. A number of survey respondents have implemented most or all of these P2 elements.

Water use control is often achieved simply with ﬂow restrictors due to the continuous nature of workloads. However, for decorative chromium lines that are not operated continuously, additional controls are needed, such as conductivity controls or timer rinse controls. For ﬁnal rinses following decorative chromium plating, a commonly accepted conductivity range is 100 to 300 µmho. Due to the lower bath temperature and higher dragout rates, most companies do not achieve closed loop rinsing (although two exceptions were identiﬁed by the benchmarking survey). As a result, companies frequently install recovery equipment, the most successful of which is the atmospheric evaporator.

One company that has closed-loop rinsing uses a drag-in tank and three ﬁnal rinses following plating. The tanks are organized in a counterﬂow arrangement. This company also has a separate "holding tank." Deionized water is fed to the last rinse and counterﬂows to the ﬁrst rinse. The solution in the ﬁrst rinse ﬂows to a holding tank. The plating solution is circulated continuously to the holding tank. The holding tank provides ﬂow equalization by balancing the incoming water ﬂow rate and the evaporation rate in the plating tank. Bath maintenance (porous pot) is also performed in the holding tank, which saves plating tank space. The drag-in solution is circulated to the ﬁrst rinse tank. A "bubble pump" (no moving parts) is used to circulate the drag-in tank solution (at 3.5 gpm). This arrangement is referred to as a drag-in/dragout recovery rinsing scheme. It contributes to closing the loop on rinsing.

Survey respondents listed additional P2 methods that relate to rinsing. These include using wetting agents to reduce viscosity, and spray rinsing parts over the process tank.

1.8.2 Bath Maintenance

Returning dragout to the process tank may result in a buildup of contaminants in the plating bath. This buildup is a concern especially with zero wastewater operations. Contaminants, contributed from various sources, can lead to slower plating rates and poor deposits. The ﬁrst line of defense against contaminant buildup is prevention. Examples of good practices include:

Thoroughly cleaning parts before plating
Minimizing corrosion of busing, racks, ﬁxtures and parts
Using deionized water for rinsing
Avoiding drag-in of sulfate and chlorides
Avoiding exclusively plating inner diameters
Retrieving parts or ﬁxtures that fall into the bath.

To deal with contaminant buildup, which occurs even when prevention measures are taken, companies can install bath maintenance technologies. Methods used include ﬁltering, porous pots, and more advanced technologies such as membrane electrolysis and ion exchange. Of the available technologies, ion exchange is the most effective and reliable. However, ion exchange creates a signiﬁcant waste stream as a result of resin regeneration. This stream is typically treated on-site using neutralization and precipitation, and results in the formation of hazardous sludge (i.e., F006). Therefore, the value of preventing bath contamination cannot be overemphasized.

Use of ion exchange for removing metal impurities is particularly well-established for hexavalent chromium plating baths. In these situations, cationic impurities (e.g., iron, copper, and nickel) are being separated from the desired anionic plating species (i.e., hexavalent chromium). This can be accomplished using a strong acid cationic resin. Ion exchange puriﬁcation using a trivalent chromium bath is a more signiﬁcant challenge because both the metal contaminants and the desired bath species are cationic. Trivalent baths are sensitive even to low concentrations of metal contaminants, including copper, zinc, and nickel (copper and zinc have been reported to cause discoloration and poor coverage at concentrations as low as 20 mg/l).⁴⁶ For trivalent chromium baths, puriﬁcation can be achieved by using a chelating ion-exchange resin having a very high afﬁnity for the cationic metal ion contaminants compared to the afﬁnity for trivalent chromium. Commercial systems for this application that operate on a continuous basis (i.e., the plating bath is not taken out of operation) are available.
⁴⁶ (Woodle, Roger and Hosea, J. Michael, Ion Exchange for Trivalent Chromium Plating Baths, Products Finishing Magazine, Nov. 1997.)

1.8.3 Air Quality

Decorative hexavalent chromium plating causes the evolution of chromic acid mist. The quantity of misting is generally much less than with hard chromium plating. Fume suppressants can be used to lower the surface tension of the bath and thereby reduce the generation of fumes. One benchmarking survey respondent suggested that maintaining the surface tension below 45 dynes/cm greatly reduces the amount of mist being generated and reduces dragout.

Well-designed and operated ventilation systems prevent the chromium fumes from entering the workplace. In turn, the ventilated air is processed by scrubbers and/or mist eliminators to prevent emissions of chromic acid to the atmosphere.

Additional information on air quality issues for chromic acid baths is found in the chromic acid anodizing section.

1.9 ELECTROLESS NICKEL PLATING

Compared with solutions used with electroplating processes, electroless nickel (EN) plating baths have a very limited life span. Because these baths are discarded regularly, electroless nickel plating generates a signiﬁcant quantity of waste.

Exhibit 1-35 summarizes the following discussion regarding pollution prevention strategies for electroless nickel plating.

1.9.1 Bath Maintenance

Operating environmentally efﬁcient electroless nickel plating processes begins with efforts to maintain the bath in a usable condition for an extended time. Minimizing outside contamination and maintaining the bath's constituents at proper concentrations can help.

In addition, technology is available to regenerate baths and keep them operating for an extended time. The overall electroless nickel deposition reaction can be generally written as:

3NaH₂PO₂ + 3H₂O + NiSO₄ = 3NaH₂PO₃ + H₂SO₄ + 2H₂ + Ni⁰.

Sodium hypophosphite (NaH₂PO₂) reduces nickel sulfate to metallic nickel, and is oxidized to sodium orthophosphite (NaH₂PO₃). As the deposition of nickel proceeds, the orthophosphite concentration increases. Nickel salts and other constituents are added to the bath to make up for the chemicals consumed by the plating process. However, the process cannot continue indeﬁnitely because of the buildup of by-products.

The quality of the nickel deposit diminishes over time and the nickel bath must usually be discarded after 7 to 12 metal turnovers (MTO). (One turnover is said to occur when the cumulative amount of nickel added is equivalent to the original quantity in the bath.) The deterioration of the bath is caused mainly by an excessive buildup of orthophosphite.

In recent years, electrodialysis (ED) has become a popular method to extend the life of electroless nickel baths. Originally, the ED process was used on a batch basis to rejuvenate the EN bath after it was partially contaminated, or after it had completely reached the end of its useful life.⁴⁷ Batch treatment, although relatively successful, had the drawbacks of having to remove the solution from production, and resulting in a "jig-saw" curve of bath contaminants, which leads to unpredictable performance results. Continuous processing was not originally used because of technical hurdles, including lack of available membranes that could tolerate the operating temperature of the EN bath. New developments have apparently overcome the technical issues. One promising approach is use of monopermselective ion exchange membranes and a modiﬁed arrangement of anionic and cationic ion exchange membranes in a system referred to as Electro-Dialysis of Electroless Nickel (EDEN).⁴⁸
⁴⁷ (The Merit Partnership, Extending Electroless Nickel Bath Life Using Electrodialysis, U.S. EPA Region 9, July 1998.)
⁴⁸ Longﬁeld, Peter and Orgill, Graham, The EDEN System for Electroless Nickel Bath Life Extension and Consistent Quality, AESF SUR/FIN 2005 Proceedings.)

A ﬂow diagram of continuously regenerated EN using the EDEN approach is shown in Exhibit 1-35. The electrodialysis stack is supplied with high volume pumps from two reservoir tanks for the diluate and the concentrate, respectively. The diluate tank is continuously supplied with a low volume ﬂow of approximately 10 gph (37.5 l/hr) from the electroless nickel-plating tank. The diluate is cooled by a counterﬂow heat exchanger to limit the solution temperature in the stack to maximum 104°F (40°C). The current density supplied to the electrodialysis stack is controlled by a speciﬁc gravity sensor in the diluate reservoir, and therefore adjusts the stack performance to the actual plating bath loading or operating time. The second solution circuit of the concentrate comprises an automatic pH-control and automatic water dosing to keep the concentrations of sodium, sulfate and orthophosphite in the optimum range. The typical bleed stream is approximately 4 gph (15 l/h). The regeneration unit also contains an automatic controller for the nickel content in the plating bath (photometric analysis of Ni) and the pH value to allow for fully automated addition of the replenishment chemicals to the electroless plating bath (Longﬁeld).

Under production conditions, the EDEN system has proven to have a bath life of 2,150 MTO.

1.9.2 Disposal of Spent Baths

When baths deteriorate and require disposal, companies may process them on-site to recover the nickel (plating the metal out onto steel wool), or may send the baths off-site for recycling. Some chemical suppliers offer bath recycling. Recycling avoids treating the spent electroless nickel bath, which is a difﬁcult task due to the presence of complexing agents such as EDTA.

1.9.3 Dragout and Water Use Reduction

Benchmarking survey respondents employ common methods for dragout reduction such as good rack design and good racking practices, operator training, draining parts over the process tank (see draining/rinsing over the plating tank), and water use control methods such as ﬂow restrictors. Rinsing following electroless nickel plating typically consists of two to three counterﬂow rinses. Some companies use recovery rinsing, but there is concern that it may lead to faster deterioration of the bath. Also, some companies use spray rinsing over the bath.

On July 1, 2008 EPA published the National Emission Standards for Hazardous Air Pollutants for Plating and Polishing Operations.⁴⁹ This rule impacts many metal ﬁnishing operations (EPA estimates 2,900 facilities). It applies to facilities with plating, polishing or thermal spray processes that contain cadmium, nickel, lead, manganese and/or chromium. (The rule will not apply to chromium anodizing and electroplating, which are covered under a previous rule.)
⁴⁹ (National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Final Rule, Federal Register July 1, 2008.)

minimize bath agitation when removing tank objects
maximize dripping of bath solution back into tank by extending drip time when removing the tank objects and using drain boards (i.e., drip shields)
optimize the design of barrels, racks, and parts to minimize dragout of bath solution

1.9.4 Air Quality

Electroless nickel processes should be ventilated to avoid emissions within the workplace. One survey respondent indicated it uses a scrubber to remove contaminants from the ventilated air stream. Two survey respondents switched from using a plating solution containing ammonium hydroxide for pH control to one with potassium carbonate in order to reduce employee exposure.

1.9.5 RoHS & ELV Compliant Electroless Nickel

Over the past few years, a number of new environmental directives have come out of Europe and Asia, encompassing mainly the automotive and electronics industries. The purpose of these directives is to reduce the amount of toxic materials entering the environment by limiting the amount of certain toxic substances allowed in a manufactured product and providing for the recyclability of the manufactured product. The three major directives affecting the metal ﬁnishing industry are:

End of Life Vehicles (ELV, Directive 2000/53/EC) – Aims to reduce the amount of heavy metals contained in an automobile and provide for the recyclability of automobile components.
Restriction of the Use of certain Hazardous Substances in Electrical and Electronic Equipment (RoHS, Directive 2002/95/EC) -- Restricts use of hazardous substances in electrical and electronic equipment.
Waste Electrical and Electronic Equipment (WEEE, Directive 2002/96/EC) -- Imposes the responsibility for the disposal of waste electrical and electronic equipment (WEEE) on the manufacturers of such equipment.

The primary heavy metals addressed in these regulations are cadmium, lead, hexavalent chromium, and mercury. In electroless nickel plating, cadmium and lead are the major concerns; lead and cadmium are added to electroless nickel systems for stability and brightness. The ELV and RoHS Directives specify the limits for cadmium and lead in an electroless nickel deposit to less than 100 and 1,000 ppm, respectively. To meet this challenge, electroless nickel formulators have developed ELV and RoHS compliant baths. These systems can provide stable, simple-to-use processes capable of meeting any of the current electroless nickel speciﬁcations. The current downside to switching to these processes is that there will be a learning curve by both platers and suppliers as the new technology evolves and improves.⁵⁰⁵⁰ (Bellemare, Rich, RoHS and ELV Compliant Electroless Nickel, Products Finishing Magazine, Oct. 2005.)

1.10 HARD CHROMIUM PLATING

The key to environmental success with hard chromium plating is direct recovery. The hard chromium bath is operated at an elevated temperature (130 – 140°F). This results in surface evaporation rates of 0.05 to 0.07 gal/hr/ft². A 48-in. x 96-in. process tank operated at 140°F will evaporate 2.24 gal/hr. or about 54 gal over the course of 24 hours, which provides an opportunity to recover an equal volume of solution. As facilities approach 100% recovery of dragout, they must grapple with the buildup of bath impurities that will inevitably occur. Exhibit 1-37 summarizes the following discussion regarding pollution prevention strategies for hard chromium plating.

1.10.1 Dragout Recovery

Companies can maximize use of this recovery opportunity by returning concentrated solution to the process tank. Solution sources include

drips from parts exiting the process tank
solution caught by splash guards or trays
scrubber water or mesh pad wash-down
solution from rinsing

Because rinsing can generate a large volume of dilute chromium-bearing water, it is important to use a well-engineered, multiple-tank conﬁguration that concentrates the dragout into the smallest possible volume.

In most cases, a three-stage counterﬂow rinsing arrangement will generate a sufﬁciently low ﬂow that the entire volume of rinse water can be returned to the bath. In situations where there is insufﬁcient surface evaporation to achieve zero wastewater discharge, an atmospheric evaporator can be installed to increase the overall evaporation rate.⁵¹ Benchmarking survey respondents listed additional P2 methods that relate to rinsing. These include:

use of drag-in/dragout recovery rinsing
splash guards on all tanks
using wetting agents to reduce viscosity
suspending racks over the process tank to drain
spray rinsing parts over the process tank
unracking over the process tank

⁵¹ (Mason, Sam, Case Study: Zero Sewer Discharge for a Hard Chrome Plating Facility; 20th AESF/EPA Conference, February, 1999.)

1.10.2 Bath Maintenance

Returning dragout to the process tank may result in a buildup of contaminants in the plating bath. This buildup is of particular concern with zero wastewater operations. Contaminants, contributed from various sources, can lead to slower plating rates, poor deposits and higher energy bills. The initial line of defense against contaminant buildup is prevention. Examples of good practices include:

Installing a dedicated etch tank for reverse etching
Thoroughly cleaning parts before plating
Minimizing corrosion of busing, racks, ﬁxtures and parts
Using deionized water for rinsing
Avoiding drag-in of sulfate and chlorides
Avoiding exclusively plating inner diameters
Retrieving parts or ﬁxtures that fall into the bath

To deal with contaminant buildup, which occurs to a certain extent even when prevention is employed, companies can install bath maintenance technologies. Solution ﬁltering is an effective method of removing particulate contaminants from the bath; however, it has no effect on dissolved contaminants, such as iron and other tramp metals. Methods used for dissolved metal removal include porous pots, membrane electrolysis, and ion exchange.

Various technologies have been marketed for removal of dissolved tramp metals (e.g., Fe, Cu, Ni), including ion exchange, ion transfer (i.e., non-ion-speciﬁc membranes including ceramic porous pots and units with polyﬂuorocarbon membranes), and membrane electrolysis (i.e., ion-speciﬁc membrane). Each of these technologies is discussed in Section 5. Of these three technologies, only ion exchange is presently a viable method for removing dissolved metals at a rate that prevents a buildup in the plating bath. Membrane electrolysis does an effective job; however only one company produced a suitable unit and although they are still servicing existing units, they are no longer selling new equipment.⁵² Ion transfer has only a marginal effect on dissolved tramp metals and it cannot maintain a hard chrome plating bath below tolerable limits.
⁵² (Personal communication with Dan Vaughan of IonSep Corporation, Rockland, DE.)

Two types of ion transfer units were tested by the Navy on hard chrome baths; an external ion transfer unit and an in-tank porous pot.⁵³ The external unit is the same technology tested by the EPA ETV program for puriﬁcation of chromic acid anodizing solution (see anodizing section). Results of the testing showed that very little dissolved metals impurities are transferred through these membranes. The actual transport mechanism is the physical transfer of plating solution (bucketing or bailing) into the catholyte compartment. Once there, some "precipi-tation" of tramp metals occurs and they are removed as a sludge. Both systems are effective for conversion of trivalent chromium to hexavalent chromium. However, for this purpose, they are no more effective than simple "dummy plating," which can be accomplished using materials present in most chrome shops.
⁵³ (Koff, Jennie and Stencel, Nicholas, Evaluation of Performance Capabilities & Operational Requirements of Chromium Puriﬁcation Technologies, Naval Facilities Engineering Service Center ; 16th AESF/EPA Pollution Prevention and Control Conf. (Feb. 1995).)

A diagram of the ion transfer system is shown in Exhibit 1-38. Testing was performed by attempting to purify 90 gal. (341 liters) of plating solution (process tank). The initial catholyte solution was made by adding chrome plating solution to tap water until a pH of 1.8 was achieved. During the test, the pH of the catholyte solution increased and it was necessary to transfer 6.6 gal/day (25 l/day) of plating solution from the "process tank" to the catholyte tank in order to maintain a pH of 1.8 to 2.0. This physical transfer of anolyte is the mechanism for moving the dissolved tramp metals into the catholyte tank; electrotransport of metals was "minimal."

Testing of the porous pot showed similar results. The Navy concluded that "The performance of ion transfer systems appears to be quite low for tramp metal removal". Further, the Navy stated that "Relatively high O&M labor requirements were experienced with both ion transfer systems."

Although ion exchange is an effective technology for chromic acid anodizing bath maintenance, it has some limitations and drawbacks:

IX generates a signiﬁcant volume of chromium-bearing waste during resin regeneration, which is typically treated on-site using neutralization and precipitation, and results in production of a hazardous waste sludge (F006).
Since resin is a plastic material, it can degrade if left in contact with a full strength chrome bath for extended periods of time.
The concentrated nature of the chromic acid limits the exchange capacity that resin has for metals.

One commercial product for this application that overcomes most of the limitations is a reciprocating ﬂow ion exchange technology.⁵⁴ This system includes appropriately selected ion exchange resin and employs short cycle times that reduce the ion exchange resin/chromic acid contact time. In operation, a portion of the plating solution is transferred to a feed tank to allow cooling to minimize oxidative effects. The solution is then pumped through the short column of ﬁne mesh strong acid cation resin at 60% of its normal bath strength. The resulting puriﬁed solution, with up to 90% of the metallic impurities removed in a single pass, is collected in a product tank for return back to the plating bath. Extra volume created by the dilution effect is not a problem at most shops, as normal evaporation losses from the chrome bath itself creates a hydraulic balance (although it could reduce the amount of recovery accomplished by dragout recovery rinsing). Regeneration is undertaken by using diluted sulfuric acid. Typical process speciﬁcations for this technology (supplied by the manufacturer) are shown in Exhibit 1-39. This same technology is applicable to decorative chromium plating and chromic acid anodizing.
⁵⁴ (Pajunen, Paul, and Sheedy, Michael, Metal Removal and Recovery from Process Solutions Using Short Bed Ion Exchange, SM Technical Paper 181, Eco-Tec Corporation, Nov. 2006.)

Over the past 5 to 10 years there has been a ﬂurry of research activity in the U.S. and abroad aimed at developing a better method of chromium bath puriﬁcation. Among others, this includes:

Laboratory investigations of porous pot and membrane electrolysis⁵⁵
Laboratory testing of a high pH treatment chemical process⁵⁶
Research on a method using a ceramic microﬁlter⁵⁷
Research on modiﬁcations to membrane electrolysis using two and three-compartment cells⁵⁸
Laboratory testing of a fuel cell cathode ion exchange membrane process⁵⁹
Laboratory testing of membrane electrolysis using a sulfuric acid catholyte⁶⁰

⁵⁵ (Donepudi, V.S., Khalili, N.R., Kizilel R. and Selman, J.R., Puriﬁcation of Hard Chromium Plating Baths-Options and Challenges, Plating and Surface Finishing, April, 2002.)
⁵⁶ (Benaben, P. and Popakul, J., Puriﬁcation & Recycling Process For Hexavalent Chromium Solutions Using a Low-cost Chemical Method, Plating and Surface Finishing; December, 2000.)
⁵⁷ (Stewart, David, R. Thoen, Paul, and Grove, Tom, In Tank Removal of Hard Chromium Plating Contaminates, Aerospace/Airline Plating & Metal Finishing Forum, March 2001.)
⁵⁸ (Kruglikov, Sergey S., Turaev, Dmitri Yu, The Use of Anionic Membranes for the Removal of Cationic Impurities from Chromium Plating Solutions, SUR/FIN 2004, June 2004.)
⁵⁹ (Ahmed, M. I., Khalili, N. R., Donepudi, V. S., Smotkin, E. S., Selman, J. R. and Holsen, T.M., In-Process Contaminant Removal from Hard Chromium Plating Baths Using a Fuel Cell Electrode Ion-Exchange Membrane Process, 22nd AESF/EPA Conference; January, 2001.)
⁶⁰ (Miller, P.O., Taylor, E.J., Zhou, C.D., and Renz, R.P., In-process Puriﬁcation of Plating Baths, 21st AESF/EPA Conference; January, 2000.)

Although some of this research shows promise, as yet no commercial products have resulted from this work. As such, there remains a signiﬁcant opportunity for an effective chrome bath puriﬁcation technology that can
compete with ion exchange.

1.10.3 Air Quality

The operation of hard chrome plating is affected both by environmental and by safety and health regulations covering airborne emissions from the process:

National Emissions Standards for Hazardous Air Pollutants (NESHAP) (40 CFR. §63.6) for chromium regulates air emissions to the environment.
OSHA's chromium permissible exposure limit (PEL) regulates the amount of chromium in the air within the shop (29 CFR Parts 1910, 1915, et al.).

P2 practices can help meet these two regulations simultaneously. Chromium is released to the air from plating operations during the process, as gases created during electrolysis at the anode and cathode rise through the solution, break at the surface, and release an aerosol to the air above the tank. A common P2 practice involves the use of fume suppressants that lower surface tension and/or create a foam blanket that reduces the quantity of chromic acid mist released to the air.

There are two basic types of fume suppressants: wetting agents and foam blankets. The difference between foam blankets and wetting agents is the way in which they reduce emissions:⁶¹

A foam blanket fume suppressant generates a layer of foam across the surface of a solution when current is applied to that solution. The foam layer physically suppresses the mists produced on the surface of plating baths. Foam blankets do not prevent the formation of chromic acid mist – instead, they trap the mist under the blanket.
Wetting agents lower the surface chemistry of plating baths. This causes gases to escape at the surface of the solution with a diminished "bursting" effect, reducing mist formation.

⁶¹ (Riordan, Barrett J., Karamchandanl, Rohit T. Zitko, Larry J., and Cushnie George C., Capsule Report Hard Chrome Fume Suppressants and Control Technologies, United States Environmental Protection Agency Technology Transfer Ofﬁce of Research and Development, Washington, DC 20460, EPA/625/R-981002 December 1998.)

Another approach to preventing chromium losses to the air is to use tank covers that enclose the top of the anodizing tank. These are discussed in the chromic acid anodizing section.

Mesh pad mist eliminators can be used instead of standard wet scrubber systems to recover chromic acid that is exhausted from the surface of a tank. A well designed system includes an automatic spray wash down device that keeps the mesh pads clean while returning the chromium rinse back to the plating tank. The return of the wash-down solution will compete with recovery rising for available tank space generated by evaporation, and in some cases will require that supplemental evaporation be provided (e.g., installing an atmospheric evaporator, or keeping chrome tanks at operating temperature during idle hours).

1.10.4 Energy Conservation

Electricity is a signiﬁcant operating cost for hard chrome plating, as shown by the Benchmarking Survey results. Fortunately, a number of P2 methods exist that can signiﬁcantly reduce these energy costs:

Keep total metallic impurities (e.g., Fe, Cu) below 3,500 mg/l. A conventional chromium plating solution containing 7,500 mg/l (1 oz/gal.) of total metallic impurities has a potential plating efﬁciency of 9 to 10%, while the same solution containing 3,500 mg/l (0.5 oz/gal) has an efﬁciency of 12 to 14% efﬁciency. This may translate into a 20 to 40% reduction in energy consumption and a corresponding reduction in mist generation. Maintain trivalent chromium concentration below 3.5% (3,500 mg/l).⁶²
Reduce anode to cathode distance by using conforming anodes instead of stick anodes. Stick anodes are inefﬁcient because they cannot be placed sufﬁciently close to the part. Much of the electricity used in the process ends up heating the solution rather than depositing chromium. Conforming anodes can use 40%less electricity than stick anodes placed at ﬁve inches or more from the cathode.⁶³
Use eductor systems for bath agitation instead of compressed air.
Install and maintain an energy efﬁcient ventilation system. Check ventilation rates on a regular basis to be certain they are within the accepted standards (i.e., ANSI Z91).
Insulate plating tank walls to prevent heat loss. Tank wall insulation will reduce solution heating costs by 80% or more.⁶⁴ Cover tanks when not in use.
Use enclosed chrome plating tanks to eliminate tank ventilation.⁶⁵
Purchase and maintain energy efﬁcient air compressors. Compressed air accounts for up to 15% of the total energy used in a hard chrome shop.⁶⁶
Select rectiﬁers based on workload. Rectiﬁers are most efﬁcient when they are used as close to maximum output voltage as possible. Convection-cooled and water-cooled rectiﬁers use less energy than forced air units for removing excess heat. Maintain rectiﬁers so that they are at peak performance.

⁶² (Altmayer, Frank, Energy Conservation In Hard Chromium Plating, Plating and Surface Finishing, January, 2003.)
⁶³ (Advanced Tooling Corporation, Hard Chrome Plating Improvement Implementation No-Mask Anode Fixtures for Warner Robins AFB, Final Report, March 2005.)
⁶⁴ (Altmayer.)
⁶⁵ (Graves, Beverly, Encapsulating Hard-Chrome Plating Tanks, Products Finishing Magazine, Aug., 1999.)
⁶⁶ (Altmayer.)

1.10.5 Hard Chrome Plating Alternatives

Considerable effort has been made over the past 20 years to develop alternative processes that can match the characteristics, performance, and economics of hard chromium plating. However, no single alternative has been accepted as a replacement, and the hard chromium process is not likely to be superseded in the foreseeable future for many critical applications. Examples of alternative technologies that have been investigated and/or demonstrated include:

thermal spray
electroless nickel
trivalent chromium
electrodeposited nanocrystalline coatings
plated nickel-cobalt alloys
vacuum techniques

The most promising of the alternatives are thermal metal sprays, including the high-velocity oxegen fuel (HOVF) process. However, HVOF and other alternatives cannot provide a complete hard chromium replacement (e.g., HVOF is limited to line-of-sight applications), which leads to installation of substitute technologies on top of continued use of the hard chrome process. This situation makes it difﬁcult for any substitute to be economically competitive, since the cost of O&M for multiple processes, including hard chrome, would have to be compared to the baseline of hard chrome alone. In addition, research has shown that hard chromium deposits have a number of unique properties that have not been duplicated by substitute processes; e.g., recent research shows that hard chrome contains ribbon-like inclusions (i.e., microribbons) which are unique to this process and positively inﬂuence its corrosion and wear resistance properties.⁶⁷ Further, the substitutes have their own operational disadvantages (e.g., HVOF: extreme noise levels, use of pressurized gases, difﬁculty of stripping deposits, hazardous waste, and toxic air emissions). Some DoD activities, including the Corpus Christi Army Depot, are hesitant to switch from hard chromium plating to HVOF or other alternatives.⁶⁸
⁶⁷ (Jones, Allen and Bishop, Craig, Microribbons in Hard Chromium Deposits, Journal of Applied Surface Finishing, 1(3), 249-262 (2006).)
⁶⁸ (National Center for Manufacturing Sciences (NCMS), Implementation of Inovative Hard Chromium Plating Tooling at Corpus Christi Army Depot Phase II, Commercial Technologies for Maintenance Activities (CTMA), June 2007.)

Some hard chrome substitute researchers have compared their technologies to outdated hard chrome methods and data in order to promote their substitutes. For example, one website⁶⁹ uses hard chrome plating rates from the 1930's for comparison, and appears to conclude that HVOF is a competitive technology. It is more realistic to use modern hard chrome plating methods, including use of conforming anodes and no-mask tooling to make such comparisons. Modern methods can compete successfully with, or even outperform HVOF in many respects, including production rates (based on parts per square foot of operating space, parts per unit cost, or other measurements of production rate). Coupled with pollution prevention methods such as covered tanks, closed-loop rinsing, and dedicated etch tanks, the hard chrome process can be operated with acceptable environmental, health and safety impacts.
⁶⁹ (Hard Chrome Alternatives Team (www.hcat.org).)

1.11 NICKEL PLATING

Nickel plating pollution prevention practices include a wide range of activities and technologies relating to dragout prevention and recovery, rinse water minimization, plating bath maintenance, and in-process recovery.

Exhibit 1-40 summarizes the following discussion regarding pollution prevention strategies for nickel plating.

1.11.1 Dragout and Water Use Reduction

Rinsing following nickel electroplating should include dragout recovery and counterﬂow rinsing. Some survey respondents have achieved zero water discharge using three to four recovery tanks connected in a counterﬂow arrangement. Nickel tanks that run at a low temperature may not evaporate sufﬁciently for the dragout to be completely fed back. In this case there are several methods (e.g., reverse osmosis, and evaporation) that can concentrate the rinse water before return. Recovery rinsing will accelerate the buildup of impurities, which must be dealt with (see bath maintenance); otherwise, bath disposal or recovery will be necessary. A drag-in/dragout conﬁguration can also be employed; note however, an important warning when considering this approach. Dilute nickel solutions have been known to cause passivity of components, particularly when stray DC current is possible. To avoid this problem, use a non-metallic drag-in tank.⁷⁰
⁷⁰ (Fisher, Bryan, Avoid nickel plating losses – Protect the environment and improve proﬁtability (extended version), Products Finishing Magazine, Sept. 2005.)

When workloads are continuous, water use control can be achieved simply with ﬂow restrictors and by closing water valves when plating lines are idle. However, for nickel lines that are operated intermittently, additional controls are needed, such as conductivity controls or timer rinse controls. For ﬁnal rinses following nickel plating, a commonly accepted conductivity range is 300 to 600 µmho.

One benchmarking survey company that has closed-loop rinsing employs three ﬁnal rinses following nickel plating that are organized in a counterﬂow arrangement. This company also has a separate holding tank. Deionized water is fed to the last rinse and counterﬂows to the ﬁrst rinse. The solution in the ﬁrst rinse ﬂows to a holding tank. The holding tank provides ﬂow equalization for the deionized water ﬂow rate and the evaporation rate in the plating tank. During the work week, the quantity of solution in the holding tank increases. Over the weekend, the idle nickel plating tank is kept at its normal operating temperature so that evaporation continues. During this time, a signiﬁcant percentage of the solution in the holding tank is transferred to the plating tank. The return ﬂow is pumped through a carbon ﬁlter, thus providing solution maintenance.

On July 1, 2008 EPA published the National Emission Standards for Hazardous Air Pollutants for Plating and Polishing Operations.⁷¹ This rule impacts many metal ﬁnishing operations (EPA estimates 2,900 facilities). It applies to facilities with plating, polishing or thermal spray processes that contain cadmium, nickel, lead, manganese and/or chromium. (The rule will not apply to chromium anodizing and electroplating, which are covered under a previous rule.)
⁷¹ (National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Final Rule, Federal Register July 1, 2008.)

minimize bath agitation when removing tank objects
maximize dripping of bath solution back into tank by extending drip time when removing the tank objects and using drain boards (i.e., drip shields)
optimize the design of barrels, racks, and parts to minimize dragout of bath solution
use a wetting agent or fume suppressant in the nickel electroplating bath

1.11.2 Chemical Recovery

One benchmarking survey respondent uses an ion exchange system for recovering nickel. This company's plating tank is followed by three rinses, connected in a counterﬂow conﬁguration. A conductivity probe is present in the ﬁrst rinse, nearest the plating tank (the most concentrated rinse). When the conductivity reaches a set point, solution in the ﬁrst rinse is transferred to the ion exchange holding tank (220 gal). This ﬂow is balanced by the addition of deionized water to the last rinse. The solution in the ion exchange holding tank is pumped through dual ﬁlters and the ion exchange column, which contains a cation bed that retains the nickel. The ion exchange unit is regenerated with sulfuric acid, and nickel sulfate is eluted. A small volume of hydrochloric acid is added to the recovered solution to destroy organics. The resulting nickel sulfate is tested using a Hull cell, and is subsequently used to replenish the plating bath. The water processed through the ion exchange unit is treated by a second ion exchange step to remove any residual nickel, and is then discharged to sewer, without the need for conventional treatment. Regenerant from the second ion exchange unit is recycled off-site.

In-process recovery (i.e., connected to the rinse system) is often used with nickel electroplating. The most common technology employed is the atmospheric evaporator. This technology is typically used in conjunction with recovery rinsing. Other applicable technologies include vacuum evaporators, ion exchange, electrowinning, reverse osmosis, and electrodialysis. Each of these technologies has both advantages and disadvantages:

Evaporation. Atmospheric evaporations are relatively inexpensive and simple to operate. They are capable of recovering over 90% of nickel dragout and concentrating the nickel to 40 g/l Ni or more. Evaporators do not generate a waste stream like most other recovery technologies. One drawback is that all dissolved materials (including degraded brighteners) are recovered, which increases the need for bath maintenance. In addition, vacuum evaporators are expensive and are relatively complicated to operate and service.
Ion Exchange. IX is capable of 97% recovery of nickel and it yields a relatively high concentration product stream (up to 40 g/l Ni). The IX resins must be periodically regenerated with acid (sulfuric or hydrochloric, depending on nickel bath chemistry), which creates a wastestream that must be treated. Some areas of the country have IX services that will regenerate columns off-site.
Electrowinning. EW is relatively easy to operate and maintain. This technology has been applied to nickel, although it is better suited for recovery of other metals such as copper and precious metals. EW is not capable of removing nickel to low concentration levels, and the recovered nickel is difﬁcult to harvest and reuse.
Reverse osmosis. RO is capable of recovering over 90% of nickel while rejecting almost all dissolved materials, including brighteners (both usable and degraded). RO produces a recovery stream containing 10 to 20 g/l of nickel. The process could require an evaporation step if there is insufﬁcient tank evaporation. The main weaknesses of RO are limited membrane life, the generation of a signiﬁcant reject stream that contains some nickel and must be treated, and the loss of expensive brightener chemistry.
Electrodialysis. ED can achieve 95% recovery of nickel. It concentrates the nickel salts sufﬁciently that in most cases the product stream can be returned directly to the plating tank. It has approximately the same cost as RO, and membrane replacement is a concern.

1.11.3 Additives

Anti-pitting agents are used to lessen pitting in nickel electroplating. The agent promotes the release of gas bubbles from the surface of the part. Anti-pitting agents also reduce surface tension of the solution, which, in turn, will help solution drain from the part, minimizing dragout.

1.11.4 Bath Maintenance

The key aspects of P2 for nickel electroplating operations are preventing bath contamination and maintaining the bath. Some important considerations and alternatives are discussed below.

During electroplating, the nickel anode dissolution efﬁciency is 100%. The cathode efﬁciency of nickel plating is typically 95–98%. When a moderate to high concentration of bath contaminants is present, the cathode efﬁciency can drop below 90%. This situation may cause the nickel content of a bath to increase over time, especially when recovery rinsing is used. To prevent the wasteful buildup of nickel metal, it is important to eliminate all sources of contaminants in the nickel plating bath. Even when contaminants are kept to a minimum, closed-loop nickel systems will generate an increase in nickel concentration. The holding tank concept used by one respondent (described under Water Use Reduction) may help in dealing with this problem. Copper anode bus bars can be a source of copper contamination, which is quite detrimental in nickel plating. A number of options are available that reduce the introduction of copper contamination in nickel plating:

Wrap bus bars with a plating grade of nonadhesive vinyl tape, except for contact areas
Plate copper bars with nickel
Install PVC drain boards over the anode bars.

Parts fallen into the tank are a signiﬁcant source of contamination and should be removed quickly. An inexpensive tank magnet can be used for retrieving ferrous parts. A siphon hose or makeshift shovel can be used to remove non-ferrous materials.

During electroplating, the organic additives of nickel baths produce decomposition products that, if allowed to accumulate, can result in unacceptable plating. To help reduce the occurrence of these problems, ﬁnishers treat the solution using activated carbon, alone or in combination with hydrogen peroxide or potassium permanganate. If excessive carbon treatment is needed on a regular basis, the plating procedures should be reviewed to identify sources of organic contamination (e.g., drag-in or poor cleaning), and these should be eliminated.

Not all activated carbon products are suitable for use with nickel plating solutions. Suppliers of proprietary additives can recommend a suitable grade. The quantity of carbon required will depend on the degree of contamination.⁷²
⁷² (Bless, Diana R., A Review of Nickel Bath Life Extension, Nickel Recovery and Copper Recovery from Nickel Baths, Plating and Surface Finishing, April 2000.)

Although effective, carbon treatment increases downtime, creates liquid and solid waste, and increases employee contact with toxic materials. In addition, the treatment process can result in solution losses up to 20%.⁷³ There are alternatives to using activated carbon for organics removal. At least one supplier offers a polymeric resin system that provides continuous nickel solution maintenance.⁷⁴ The resin is regenerated with mild alkaline solution and is reused. Another alternative technology recently investigated is use of catalyzed hydrogen peroxide.⁷⁵ During long-term testing of this process, it signiﬁcantly outperformed conventional hydrogen peroxide/carbon treatment, especially with regard to detrimental organic breakdown products.⁷⁶⁷³ (Fisher, Bryan, Avoid nickel plating losses, Guidelines to help prevent the loss of nickel and its salts from the ﬁnishing shop, Products Finishing Magazine, Sept. 2005.)
⁷⁴ (Atotech, Nikotect Adsorber System: Advanced Maintenance and Recycling Technology for Bright Nickel Plating Processes, www.atotech.com.)
⁷⁵ (Forth, Peter, Vibert, Art, Busch, Madeline and Stevenson, Sarah, Treatment and Control of Nickel Electroplating Baths Using Catalyzed Hydrogen Peroxide, 23nd AESF/EPA Conference, Jan. 2002.)
⁷⁶ (Forth, Peter, No More Activated Carbon Treatments Required, Products Finishing Magazine, May, 2005.)

Electrolytic treatment (dummying) can be effectively used to remove copper, zinc, iron, and excesses of certain organic brightening agents. A corrugated cathode is used in this treatment, and the bath is electrolyzed at 2 to 5 amps per square foot (asf). One survey respondent indicated that their facility has implemented continuous electrolytic treatment by adding a holding tank with a separate rectiﬁer. Solution from the bath is transferred continuously to the holding tank and returned to the bath. The circulation loop includes carbon ﬁltration.

Metal precipitation or "high pH" treatment can be used for removal of trivalent chromium, aluminum, copper, iron, and silicates. Removal is accomplished by transferring the solution to a batch treatment tank, raising the temperature (145–150°F), and adding a slurry of plating-grade nickel carbonate (raises pH to 5.2). Hydrogen peroxide can be added to help oxidize the iron. The solution is ﬁltered and returned to the plating tank. The solution may then be electrolyzed at low current density until deposit quality is acceptable.⁷⁷ One survey respondent pointed out that high pH treatment results in solution loss, sludge creation, and holding tank cleaning. Instead of performing this type of bath maintenance, this company periodically decants a portion of the bath and reuses it on-site or recycles it off-site.
⁷⁷ (Dibari, George A., Nickel Plating, Elsevier Metal Finishing Guidebook and Directory, 2008.)

1.11.5 Additional P2 Methods

Benchmarking survey respondents listed additional P2 methods for nickel electroplating. These include:

Converting from a nitric acid strip to a less hazardous and regenerable sulfuric acid/ peroxide process
Replacing cyanide stripper with non-cyanide stripper
Employing equipment rotators to improve drainage of dragout
Using a closed-loop chiller system to control bath temperature

1.11.6 Air Emissions

A cold, still, nickel plating bath most likely does not generate any detectable nickel-bearing mist. Also, given that the deposition efﬁciency of a nickel electrolyte is in the order of 96-98%, (and therefore there is very little gassing from electrolysis), the application of direct current for deposition is unlikely, by itself, to increase the presence of nickel in the immediate atmosphere by any signiﬁcant quantity (Fisher). However, heating the solution and applying air agitation can cause signiﬁcant nickel losses to the atmosphere. To reduce losses, consider operating the bath at the lowest practical temperature, using mechanical agitation (i.e., solution pumping or cathode bar movement). In addition, although not speciﬁcally used for nickel plating as yet, tank covers could be used to assure compliance with any new air pollution control rule (see the anodizing section for discussion of EED covers).

1.11.7 Nickel Strip Acid Recovery

The most common nickel stripping solution is nitric acid. During use, acid is consumed and the rate of stripping declines. Eventually the solution must be removed from service and disposed of (off-site disposal can run as high as $5.00 per gallon). At this point, some active nitric acid is still present, although it is diluted by the unusable portion. One method of maximizing the life of the nitric acid strip is to employ diffusion dialysis, a membrane separation process commonly used for maintaining sulfuric acid anodizing baths.

When recovering acids using diffusion dialysis, an anion exchange membrane acts as a semi-permeable barrier placed between a ﬂowing water stream and a ﬂowing acid solution containing dissolved metals. The anion exchange membrane has ﬁxed positive charges located on its surface. These positive charge centers attract the negatively charged anions in solution that come into close contact with the anion exchange membrane surface.

With nitric acid baths, the overwhelmingly predominant anion is the nitrate ion (NO₃^-1). As the nitrate ions in the acid solution are attracted to the membrane, they are also driven by the concentration difference to diffuse across the membrane to the water side. Simultaneously, the Law of Electroneutrality (in solution total charge must balance to zero) requires that the transfer of every nitrate ion (negative) into solution is accompanied by the transfer of one unit of positive charge. Positively charged ions such as Ni⁺² or other metal ions, are strongly inhibited from crossing the positively charged membrane because of the repulsion between like charges. The hydrogen ion, present in the acid solution as H₃O⁺ ions, is also positively charged, but is able to cross the membrane with very little hindrance. This is due to the high concentration of hydrogen ions in the acid solution, the small size of the hydrogen ion, and the ability of the hydrogen ion to transfer rapidly between one water molecule and another. The net effect is that both components of the nitric acid molecule, the hydrogen ion and the nitrate ion, diffuse across the membrane faster than the dissolved nickel. Typically, 80-90% of the acid is recovered, with 70-90% of the dissolved metals removed. ⁷⁸⁷⁸ (Capalbo, Ralph, New Method for Recycling Acids at New Method Plating, Products Finishing Magazine, May 2004.)

1.12 TIN AND TIN–LEAD PLATING

There are several pollution prevention opportunities for tin and tin–lead plating operations. The most common focus on alternative processes for tin–lead, reducing water use, plating solution control and maintenance, and recovery. Exhibit 1-41 summarizes the following discussion regarding pollution prevention strategies for tin and tin–lead plating.

1.12.1 Alternative Processes

Tin–lead is commonly used by the printed wiring board (PWB) industry and for other electronics applications. In PWB manufacturing, tin is essentially just as effective as tin–lead as an etch resist and can replace tin–lead plating when solder-mask-over-bare-copper (SMOBC) is employed. Tin-only baths can be used as a substitute in many other cases, although not for all current tin–lead applications. Emerging substitute technologies include tin–bismuth alloys, conductive adhesives, and nickel–gold or nickel–palladium electroplating.

1.12.2 Dragout and Water Use Reduction

Metal ﬁnishers can achieve water use reduction in tin and tin–lead plating by minimizing dragout, using multiple rinse tanks, and controlling water use. To reduce dragout, efforts should focus on rack design, part orientation, and dwell times. With continuous processes, such as reel-to-reel plating for electronics, an air knife can be used to further reduce dragout losses.

To reduce water use while still meeting rinsing needs following tin or tin–lead plating, a two-stage counterﬂow rinsing conﬁguration will typically sufﬁce. One survey respondent indicated that they use three-stage rinsing after tin plating. A dragout tank for recovery can be used with baths operated above 100°F (e.g., potassium stannate or sodium stannate).

On continuous production lines, where rinse water needs are relatively stable, ﬂow can be controlled easily by periodically monitoring rinse water conductivity and manually adjusting ﬂow, using a valve. If workﬂow is sporadic, a conductivity control or timer rinse control should be considered. These devices help to match rinse water use and production ﬂow. For ﬁnal rinses following tin and tin-lead plating, commonly accepted conductivity ranges are 70 to 500 µmho (ﬁnal tin alkaline rinses should be operated below 340 µmho).

On July 1, 2008 EPA published the National Emission Standards for Hazardous Air Pollutants for Plating and Polishing Operations.⁷⁹ This rule impacts many metal ﬁnishing operations (EPA estimates 2,900 facilities). It applies to facilities with plating, polishing or thermal spray processes that contain cadmium, nickel, lead, manganese and/or chromium. (The rule will not apply to chromium anodizing and electroplating, which are covered under a previous rule.)
⁷⁹ (National Emission Standards for Hazardous Air Pollutants: Area Source Standards for Plating and Polishing Operations; Final Rule, Federal Register July 1, 2008.)

minimize bath agitation when removing tank objects
maximize dripping of bath solution back into tank by extending drip time when removing the tank objects and using drain boards (i.e., drip shields)
optimize the design of barrels, racks, and parts to minimize dragout of bath solution
use a wetting agent or fume suppressant in the tin/nickel electroplating bath

1.12.3 Bath Maintenance

Bath maintenance can reduce the need to discard process solutions. The primary source of tin and tin–lead bath contamination is metal impurities from anodes. These contaminants can be partially avoided by purchasing high-quality, low-impurity anodes.

Dissolved metal impurities can be removed using low current density electrolysis (dummying). Stannic ion is an unavoidable contaminant that must be maintained at relatively low levels by performing continuous ﬁltration and chemical treatment. Fluoborate baths are particularly subject to buildups of sludge. Therefore, continuous in-tank or external ﬁltration during operation is recommended with these solutions. Organic bath additives are decomposed during plating and will concentrate in the bath. A carbon treatment can be performed on tin–acid and tin–lead baths to eliminate all organics. Fresh organic additive is then reintroduced.

Stannous tin (stannite) is a contaminant in a stannate solution. It can break down to produce sludge by forming colloidal tin. The presence of stannite may also produce rough, spongy deposits. To convert stannite back to stannate, dilute a small amount of hydrogen peroxide with water to make a 10% solution. Add this to the plating tank with a lot of agitation. After addition of the peroxide, the plating solution will need to be dummied until the excess peroxide is converted to water.^{80

80}(Altmayer, Frank, Advice and Council: Tin Basics, Plating and Surface Finishing, May 2005. )

Process control for methanesulfonic acid systems using insoluble anodes, where tin is not automatically replaced, often involves bath bailouts to maintain a sufﬁciently high tin concentration (due to the fact that all of the tin that is plated comes from the tin methanesulfonic acid). The bailouts result in high production of F006 wastewater treatment sludge. A novel approach to the problem using a membrane technology was developed at a reel-to-reel shop that uses a tin replacement cell hydraulically connected to the plating tank. The system is shown in Figure 12. In the plating tank, oxygen is liberated at the insoluble anode and tin is plated on the cathode, resulting in a lower tin concentration in solution and an increase in the free acid concentration. The solution is continuously re-circulated from the plating cell to the anolyte chamber of the tin replenishment cell via the lower plating sump (not shown). In the tin replenishment cell, tin is electrochemically dissolved into the anolyte at the soluble tin anode, and it combines with the free acid produced in the plating cell, liberating hydrogen ions. The hydrogen ions pass through the membrane into the catholyte chamber, which is ﬁlled with methanesulfonic acid at approximately the same concentration as the plating bath. The ions replace those lost to the evolution of hydrogen gas at the tin replacement cell cathode. Some tin ions pass through the membrane (approximately 5%), and are deposited at the tin replacement cell. Bath bailouts were eliminated and the process control of the solution was simpliﬁed. Only small adds of tin methanesulfonic acid were required to make up for drag out losses. Waste treatment and sludge disposal costs were reduced, due to elimination of bail-outs. The net annual savings after energy and membrane replacement costs was in excess of $40,000. The capital cost of the equipment was less than $30,000.⁸¹⁸¹ (William Copping; SUR/FIN 2003; 2003 Jun Novel Technique for Replenishing Tin & Controlling Tin Baths)

1.12.4 Chemical Recovery

Various chemical recovery technologies have been used with tin and tin–lead electroplating. The most frequently used technologies are ion exchange, electrowinning and evaporative recovery.

1.12.5 Disposal of Spent Baths and Sludge

Tin and tin–lead sludges do not have signiﬁcant economic value and are by themselves usually not recycled off-site. However, one survey respondent indicated that their facility treats tin-bearing rinse waters and spent baths, and sends the resultant hydroxide sludge to a reclamation site.

1.12.6 Tin Strip Recovery

Off-site disposal or on-site treatment of spent nitric acid tin stripping solutions can be expensive. Diffusion dialysis has been used for these baths, but with limited success.⁸² A novel approach to this problem was developed where a caustic stripper was substituted for the conventional nitric bath and electrowinning was used to capture the metal.⁸³ Using this process, tin concentrations were reduced from about 150 g/l to 30 g/l.
⁸² (Ford, Christopher and Coletta, Ted, Reclamation of Nitric Acid from Solder Strip, The Massachusetts Toxic Use Reduction Institute, 1995.)
⁸³ (McKesson, Doug, Miracles in Tin Stripping, PC Fab Magazine, June 1999.)

1.13 ZINC PLATING

Operating environmentally efﬁcient zinc electroplating processes begins with the selection of the plating bath. Twenty years ago, most zinc plating was carried out with cyanide baths. Today, low-cyanide and non-cyanide plating solutions are available that meet most metal ﬁnishers' needs.

Many of the survey respondents identiﬁed conversion from cyanide to non-cyanide as a major P2 improvement. Several of the respondents indicated that complete conversion to non-cyanide was not possible due to special work requirements. In such cases, companies have converted one or more tanks to non-cyanide and maintain a single zinc cyanide tank, or have switched to a low-cyanide chemistry.

Zinc cyanide plating baths are signiﬁcantly less affected by poor cleaning than alkaline non-cyanide baths. One respondent noted that it is possible to operate a zinc cyanide bath with ﬂoating oil. In contrast, alkaline non-cyanide zinc plating requires extensive cleaning, similar to that needed for nickel plating directly onto steel. Exhibit 1-42 summarizes the following discussion regarding pollution prevention strategies for zinc plating.

1.13.1 Bath Maintenance

Even with non-cyanide baths, solution chemistry control must be exercised to minimize pollution. Several survey respondents indicated that zinc plating bath control was an integral part of their P2 approach. Several survey respondents indicated that the use of automatic chemical bath additions improves control. One survey respondent controls the use of brightener by storing the chemical in totes, which platers cannot access without permission. This restriction has reduced brightener usage by 30%. Important aspects of solution control include controlling metal buildup, making proper additions of bath components, and contamination control. Metal buildup can occur when zinc anodes dissolve during idle times. This situation can be avoided by removing the anodes from the solution when it is not in use. This practice is more effective for alkaline baths than for zinc chloride baths.

Zinc metal concentration can also be controlled in cyanide and alkaline non-cyanide plating solutions by using inert anodes (i.e., steel or carbon). Using deionized water for evaporative makeup, retrieving fallen parts or racks, ﬁltration, and removing metallic impurities can improve contamination control. One respondent uses permanent media ﬁlters rather than disposable ﬁlters to reduce the quantity of waste being discarded. Electrolysis is used to remove lead and cadmium. Various chemical treatments followed by ﬁltration are also used to remove metallic impurities, including:

Sulﬁde reagents (lead, cadmium removal)
Sodium hydrosulﬁte (chromate removal)
Zinc dust (cadmium and lead removal)
Hydrogen peroxide (iron removal).

Activated carbon treatment is employed to remove organic impurities (cleaner components, plasticizers, and decomposed additives). One survey respondent completely removes plating solution from their zinc plating tank when it is not in production mode. This company indicated that this practice prevents buildup of organics on the anode baskets, extends the service life of anode baskets, prevents zinc metal buildup in the plating bath, and provides a safeguard against tank failure.

1.13.2 Dragout and Water Use Reduction

Rinsing following zinc plating presents challenges to the metal ﬁnisher. Due to the relatively low temperature of the zinc bath, there is limited opportunity for use of drag-out recovery rinsing. However, some proprietary zinc chloride baths can be operated as high as 130°F, which is sufﬁciently high to make recovery rinsing feasible and effective. One respondent uses an atmospheric evaporator to increase evaporation and recover plating solution. Although several survey respondents indicated they employ recovery rinsing, most rely more on several other aspects of rinsing: dragout minimization, counter-ﬂow rinsing, and water use control. Survey respondents have implemented dragout reduction by:

Increasing dwell time
Rotating racked parts for better drainage
Redesigning barrels to reduce the volume of trapped solution
Rotating barrels above the plating tank after they exit the solution (see draining/rinsing over the plating tank)
Training operators

Operating the zinc plating bath at the lowest practical metal level or using excess wetting agent will also decrease dragout losses. Numerous respondents cited water use control as a key aspect of pollution prevention on the zinc plating line. The equipment employed includes ﬂow restrictors, conductivity controls, shut-off valves, and timer rinse controls. For ﬁnal rinses following zinc plating, commonly accepted conductivity ranges are less than 630 µmho for zinc acid and 300 to 1,200 µmho for cyanide zinc. Although the traditional dragout recovery rinse is not used by many of the benchmarking survey respondents, at least two companies employ drag-in/ dragout recovery rinsing tanks to recover a portion of the dragout.

1.13.3 Chemical Recovery

Various chemical recovery technologies have been used with zinc plating, although there are fewer total applications than with most other plating metals. The most frequently used technologies are evaporative recovery and electrowinning. Ion exchange has also been used, but with less success than the other two recovery technologies.

Zinc is the least recovered of the common electroplated metals due to:

technology issues -- recovery technologies used by metal ﬁnishers are less efﬁcient and effective with zinc than other metal applications
lack of incentive – the relatively low value of zinc and the relatively high zinc efﬂuent discharge standards (especially in comparison with local standards for metals such as chromium, cadmium, and lead).

Off-site recovery, although not within the true deﬁnition of pollution prevention, is highly applicable to zinc wastewater treatment sludges, particularly when they are not combined with wastes of other metals. There are two reasons that make off-site recycle an attractive option:

When wastewater treatment sludge is generatedfrom segregated zinc cleaning, plating (on carbon steel) and/or zinc stripping processes, that sludge is exempt from the scope of RCRA hazardous waste F006 (40 CFR 261.31). To obtain this status, any chromating wastes, if present, would have to be handled on a segregated basis. Therefore, unless the sludge is a characteristic hazardous waste (e.g. corrosive), then it is non-hazardous, and the rules that apply to management, transportation and disposal are relaxed.
There are a number of licensed facilities around the U.S and Mexico that recycle zinc wastes. Zinc sludges can be easily recycled into usable industrial products, including construction material.

Recycling of zinc wastes is carried out by large zinc waste processing facilities in the U.S. and Mexico that employ rotary kilns or other technology. The most common waste processed by these facilities is electric arc furnace dust generated by steel making/recycling processes. Zinc electroplating sludges are an ideal secondary feedstock for the rotary kilns. The kilns utilize a high temperature metal recovery (HTMR) process for zinc recycle. At temperatures over 2000 °F, valuable metals are volatilized and captured for further processing into reﬁned metal products, which are sold for industrial uses. The slag from the kiln produces an iron-rich material (IRM) which can be used as construction material or as filter media in wastewater treatment applications.

1.14 OTHER METAL FINISHING PROCESSES

This section contains benchmarking survey respondents comments related to processes other than those speciﬁcally covered in previous sections. Readers may ﬁnd some of these comments useful when evaluating their own practices and identifying new opportunities for pollution prevention. The comments are essentially unedited except for spelling and other minor corrections to improve readability. Square brackets denote paraphrasing of respondent comments, or editorial comments.

Readers should fully investigate and evaluate any options before implementing them.

1.14.1 Phosphating

The following comments are pollution prevention methods benchmarking survey respondents used to address phosphating.

Automatic additions to phosphate baths— resulting in less human exposure.
Changed to a chemistry that generates only one quarter of the sludge previously produced. Iron control system used on zinc phosphate bath.
Installed motorcontrolled hoists to reduce dragout. Use counter ﬂow rinsing. Conducted operator training. Installed bulk storage system and eliminated use of waste drums.
Filter phosphate solution so we don't have to decant periodically. [One respondent indicated that this is not a very effective practice.]
Converted existing rinse tanks to counterﬂow.
Use water restrictors. Increased dwell time. Installed drip pans. Use vacuum evaporator for recovery. Installed oil skimmer and microﬁlter on cleaner. Reduced concentration of chromium in sealers (magnesium phosphate) and use non-chromium sealers (zinc phosphate).
We have tried non-chromium sealers and will move into one shortly.
Increased analysis of phosphate solution.
Added acid ﬁlter. Increased analysis and bath maintenance.
Use counterﬂow rinses. Use acid wetters.
Use manual titration for prep and phosphating baths. Substituted water-soluble oil whenever customer requirements permit. Implemented operator training to minimize dragout and water use and to extend bath life.
Use counterﬂow rinsing.
Installed counterﬂow rinses to reduce rinse water use, installed ﬂow meters at all rinse tanks to reduce water use, installed oil separator on the cleaner system. Installed heat exchangers to heat zinc phosphating bath in place of live steam, saving zinc phosphate bath from displacement overflow.

1.14.2 Passivation

The following comments are pollution prevention methods survey respondents used to address passivation.

We are pilot testing of an acid sorption unit.
Extended dwell time over process tank.
Use deionized water for bath makeup. Deionized water used throughout passivation line.
Maintain racking orientation to achieve best drainage; allow sufﬁcient drainage over process tanks.
Replaced nitric acid process with a proprietary citric-acid-based process. The new process is less toxic and dangerous; however it is more expensive to operate. [Another survey respondent is testing a citric acid process.]
Installed oil skip process to reduce the oil ﬂowing into the system, thereby reducing chemical usage.
Changed acids and reduced the quantity of acid on site.

1.14.3 Black Oxide

The following comments are pollution prevention methods survey respondents used to address black oxide.

Use dead rinses. Reduced acid concentration in pickle. Use ﬂow restrictors.
Use deionized water for rinsing and replenishment.
Use counterﬂow rinsing.
More efﬁcient burners purchased (natural gas reduction).
Flow restrictors on water rinses. Plumbed existing rinse tanks to counterﬂow.
Use oil recovery drip trays and recover oil from spin dryers.
Water restrictors, dwell time, drip pans, caustic cleaner skimming and microﬁltration, elimination of solvent-based cleaners, lower-VOC water-based oils, cold evaporation of water from waste production oils (a 65% reduction).
Automatic additions to the bath resulting in less human exposure. Better method for disposing of the sludge generated in the bath through waste treatment.
New line installed—less hydroxide loss and better process control.
Basic P2 methods. Equipment rotators for better drainage, cleaner recycling.

1.14.4 Electropolish

The following comments are pollution prevention methods survey respondents used to address electropolishing.

Save rinse; closed-looped rinse system with evaporator used to maintain temperature of process tanks.
Found end users for used electropolishing solution. Closed manufacturing loop to purify acidic rinse water. Sludge from clariﬁer is Class V non-hazardous.
Rinse water is reused for different rinses up the line.
Cover tanks when not in use to prevent adsorption of moisture from the air.

Continue to Section 2

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You should consult with legal counsel and appropriate authorities before interpreting any regulations or undertaking any specific course of action.

Please note that many of the regulatory discussions on STERC refer to federal regulations. In many cases, states or local governments have promulgated relevant rules and standards
that are different and/or more stringent than the federal regulations. Therefore, to assure full compliance, you should investigate and comply with all applicable federal, state and local regulations.