E AP Project Summary

United States

Environmental Protection

Agency

Alkaline Noncyanide Zinc

Plating and Reuse of

Recovered Chemicals

Jacqueline M. Peden

Risk Reduction

Engineering Laboratory

Cincinnati, OH 45268

Research and Development

duced treatment and disposal costs for

the company. The recycling of the recovered water and the zinc hydroxide

further reduced the costs for treatment

and disposal. The replacement of this

single CN line with an ANC line resulted in an annual savings to P&H

Plating of $14,000 from the elimination

of the need to pretreat the plating line

rinsewater to oxidize cyanide. The addition of the recovery/recycle system

increased the company’s savings to

$62,000/yr. The reuse of 30% of the

recovered zinc hydroxide and 70% of

the treated rinsewater reduced annual

water usage and wastewater discharge

by 841,911 gal and reduced the amount

of sludge disposed annually by 14 yd3.

The payback period for the recovery/

recycle system is slightly less than 18

mo. Installation and use of this system

for other ANC plating operations would

result in reductions in wastes and increased economic benefits similar to

those experienced by P&H Plating Co.

This Project Summary was developed

by EPA’s Risk Reduction Engineering

Laboratory, Cincinnati, OH, to announce

key findings of the WRITE program

demonstration that is fully documented

in a separate report of the same title

(see Project Report ordering information at back).

Introduction

This cooperative effort between the U.S.

Environmental Protection Agency (EPA)

and the Illinois Hazardous Waste Research

EPA/600/SR-94/148 September 1994

A metal finishing process can create

environmental problems because it

uses chemicals that are not only toxic

but also resistant to degradation or decomposition. A study was undertaken

at a zinc electroplating operation to

achieve zero discharge of wastewater

and total recycle of recovered precipitates. The first step in this project was

to change an existing zinc cyanide (CN)

plating line to one that used an alkaline

noncyanide (ANC) zinc bath. The

project then investigated a closed-loop

system to treat plating rinsewater from

the ANC zinc plating line so the plating

chemicals were recovered and the water purified. The goal was to return both

the recovered zinc hydroxide and the

clean water to the plating line for continued use. The system that was designed and installed, at P&H Plating

Co., a Chicago area operation used precipitation by pH adjustment to remove

the zinc from the rinsewater. The precipitated zinc hydroxide was collected

on filters, dewatered using a filter press,

and stored for reuse in the plating line

as needed. Once filtered, the water was

recirculated to the rinsing portion of

the plating line. The recovery/recycle

system successfully purified the

rinsewater and facilitated the recycling

of the cleaned water and the precipitated zinc hydroxide. Eliminating cyanide from the plating process meant

the line workers were dealing with a

less toxic plating bath, made compliance with regulations easier, and re-
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and Information Center (HWRIC) was undertaken to evaluate the feasibility of using an innovative, closed-loop rinsewater

treatment system to precipitate plating

chemicals for recovery and reuse and to

produce purified water for recirculation to

the rinsing tanks and sprayers. The goal

of this study was to achieve zero discharge of the wastewater and total recycle of the recovered precipitate thereby

reducing the amount and toxicity of the

waste from an electroplating shop’s zinc

plating operation. The CN plating line was

converted to one that used an ANC plating bath. A recovery/recycle (R/R) unit

was then designed, installed, and tested

to determine how completely the goals of

the project were being met. The effectiveness of the R/R system in reducing the

process wastes was evaluated by: quantifying the effectiveness of the removal of

the zinc through precipitation by pH adjustment (the basis of the recovery system); determining the quality of the

precipitate and the treated water that were

recovered; comparing the plating quality

of the CN-based operation to that of the

ANC-based processes and analyzing the

costs associated with the change in the

process and the installation of the R/R

system. The R/R unit was designed, installed, and tested by engineers from the

Center for Neighborhood Technology

(CNT). Their initial analysis that the system would be effective in reducing wastes

and facilitating recycling of the bath components was confirmed by HWRIC engineers who again tested the system after

more than 1 yr of operation. A slightly

modified unit that accommodates two plating lines, rather than one, is still in operation at P&H Plating Co. after more than

3 yr.

Industry Partner

P&H Plating Co. is one of 174 plating

operations in Cook County, Illinois. It is a

large job shop that operates 16 hr/day, 6

days/wk. It uses barrels, hoists, and racks

to move parts through the plating process. Although this project concentrated

on zinc plating, the shop is capable of and

does plate nickel, brass, copper, and cadmium on a variety of surfaces. The facility

contains a waste treatment area that receives effluent at an average rate of 150

gal/min. For this project, the company

made two changes to one of its barrel

plating lines. It replaced a CN plating line

with one that used an ANC bath and installed the CNT-designed R/R system. An

evaluation of the effectiveness of these

changes continued over 2 yr.

Problem Statement

Achieving waste reduction in the electroplating industry is important because

the wastes can create many environmental problems. The chemicals used are not

only toxic but also resistant to degradation or decomposition. Many opportunities

are open to electroplaters to reduce their

wastes. Simple housekeeping techniques

such as slowing down the movement of

parts through the operation to increase

drain time can significantly reduce the

amount of plating bath chemicals carried

over into the wastewater. Other options

include recovering bath chemicals, cleaning and recycling rinsewater, using less

toxic chemicals when possible, and using

technologies such as ion exchange to

clean and maintain plating baths. Adopting these types of process modifications

are generally not only environmentally advantageous but frequently result in economic benefits.

Federal and state regulations set limits

on the amounts of toxic plating line constituents that can be discharged. The costs

to comply with these regulations can be

considerable. Cyanide is a particularly difficult contaminant to treat; it readily combines with metals, particularly iron, to form

stable complexes that may not be destroyed by standard treatment methods. It

poses substantial health and environmental risks if not handled correctly or treated

completely. Enforcement efforts on both

the state and federal levels are well coordinated, and prosecution of violators has

increased. Electroplaters must develop an

integrated approach to waste management

that meets compliance standards and includes waste reduction as a vital component.

The CNT Recovery/Recycle

(R/R) System

The relatively simple principles involved

in the design of the CNT R/R system are

similar to those for a standard wastewater

flocculation treatment to remove metals

before disposal of water and sludge

wastes. Ideally, 100% of the zinc in the

CNT R/R system’s rinsewater would be

recovered and returned to the plating bath.

Additionally, all rinsewater would be recycled. The projected result would be substantial savings for the company in plating

chemicals and water from this rinsewater

purification that both recovers and reuses

as well as treats.

In the plating process at P&H, there is

the usual parts pretreatment or cleaning,

followed by the plating process, a spray

rinse, and finally, submersion into two

counterflow rinse tanks. Although cleaning requirements for ANC plating are generally more stringent than those for CN

plating, no change was required to the

pretreatment portion of the line at P&H.

The company had already installed a very

stringent cleaning component to their plating lines to ensure good parts cleaning

and, presumably, better plate quality. The

CNT R/R system was plumbed from the

spray rinse tank into which the counter

flow tanks ultimately overflowed. The

rinsewater flows into a tank where the pH

is measured and automatically adjusted

to a pH between 10 and 10.5.

This pH monitoring and control tank is a

continuous flow stirred reactor (CFSR). It

is designed to stimulate precipitation by

sparging with compressed air entering from

the bottom of the tank. The flow rate

through the CFSR is set at 10 gal/min.

The precipitate/water slurry next flows into

a flat-bottomed clarifying tank. To facilitate the settling process, the tank is baffled.

A recirculating pump is used to pull water

from the clarifying tank through the dual

filtering system to remove suspended hydroxide. The filtered water then either flows

to a storage tank for reuse or to the waste

treatment area for disposal. The precipitate that has settled in the clarifying tank

is removed and combined with that collected on the filters. This composite hydroxide is placed in a filter press to remove

as much water as possible. The water

that is removed is returned to the precipitation reactor and the dewatered hydroxide is analyzed and stored for future use

or disposed if not needed. Figure 1 shows

the system components and the path of

the water and the solids as they flow

through the system.

R/R System Performance

Although the system had problems, it

proved to be a highly efficient and economically advantageous addition to the

plating operation. The quality of the zinc

hydroxide precipitate and the purified water were sufficient for recycling. Careful
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Product

Pretreatment Rinse Four-part plating process meter

(adjust pH)

Rectifier

cooling

water

Counter flow rinses

Holding tank

Treatment

Rinsewater recycle

Filtering system Precipitate

Clarifying

tank

Settled

precipitate

slurry

Continuous

stirred

reactor

stir

Filter press

Water

From

filter

press

Metal

solids

Precipitate collection

Treatment

or

disposal

Can be returned to plating bath

Water

Figure 1. P&H Plating alkaline noncyanide plating line with CNT designed recovery/recycle system.

and regular analysis of the essential bath

chemicals is recommended to ensure contaminants are not introduced through these

recycling efforts.

The CNT R/R system has been in continuous operation for over 3 yr. Maintenance consists of proper care of the

mechanical parts and periodic replacement

and cleaning of the filters. Zinc hydroxide

is removed from the rinsewater with an

efficiency averaging 84%. The amount of

precipitate recycled to the bath depends

largely on the number of jobs requiring

use of that line. The company estimates

that it recycles 30% of the zinc hydroxide

it recovers. The quality of the purified water is generally acceptable for recycling or

for discharge. Approximately 70% is used

in the rinsing operation or to replenish the

plating bath. The system works well providing both environmental and economic

benefits to P&H.

Plating Quality Comparison

Although the reasons for plating are

sometimes purely ornamental, more frequently the plate is for protection. Finishes may be bright or dull, and it may not

always be possible to achieve the desired

luster with the ANC system. Advances in

the last decade have, however, provided

less toxic bath alternatives that produce

parts more like the bright, shiny objects

that result from a CN-based operation.

Ultimately, whether the plate is satisfactory or not is up to the customer, but two

commonly used standard tests can be performed to evaluate quality—thickness and

corrosion resistance. When parts plated

on both the CN and ANC lines were compared for these standards, both processes

produced parts with the desired thickness

and acceptable corrosion protection.

Toxicity Comparison

 Had there been no economic benefit

from the change to ANC plating, the reduction in health and environmental risk

resulting from the elimination of cyanide

from the process would have been sufficient to warrant its adoption. CN plating

requires extensive treatment before disposal and uses chemicals hazardous to

human health. Chemical substitution to

Return filtrate to rinse
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achieve source reduction, as was done

for this project, not only reduced process

costs but also the company’s liability because of the reduced toxicity of the chemicals being handled and disposed.

The exposure of shop workers to toxic

chemicals presents the most serious

health and safety problems for the electroplating industry. Although no occupational illness has been documented for

electroplating operators, they are routinely

exposed to hazardous substances known

to cause serious health problems. The

use of cyanide, generally considered the

most potentially dangerous of the electroplating chemicals, is carefully monitored

and employees are trained to use it properly. Combining employee education with

substitution of less toxic chemicals may

provide the least costly and most productive control of workplace hazards. Replacing cyanide plating solutions with

noncyanide baths is strongly recommended.

Economic Comparison

When the R/R system is installed on

the ANC line, the economic benefits become significant. An economic comparison for the change to ANC and then for

that change plus the addition of the R/R

unit is provided in Table 1. An annual cost

savings of $14,000 is achieved by switching from a CN to an ANC process with a

savings of $62,000 when the R/R system

is also added to the line; this savings

takes into account the annual cost of operation of the R/R system which is

$10,900.

Making the switch from CN to ANC requires disposal of the existing 1,800 gal

CN bath at a cost of $20/gal for a total

cost of $36,000. This must be calculated

as part of the capital investment for this

project. Adding the R/R system increases

the capital investment by $51,822 for a

total of $87,822. These capital investments

and the operational expenses listed in

Table 1 were entered into a spreadsheet

program that calculates a number of economic indices. The calculations were

based on 1992 costs. They use an inflation rate of 4%, a discount rate of 7.7%, a

depreciation schedule of 7 yr, and a project

life of 10 yr. It was assumed for all calculations that the line operates 8 hr/day, 5

days/wk for 50 wk/yr. Table 2 contains the

results of the spreadsheet calculations.

This analysis shows the recycling option

provides the greater economic benefit, the

shorter payback period, and the larger

return on investment.

Discussion

As a normal part of a plating line, a

rectifier is used to convert alternating curProcess Operation CN Costs ($) ANC Costs ($) ANC + R/R* Costs ($)

Bath makeup 1771 1860 1860

Bath maintenance 22325 21225 19425

Water usage

1. Use @ $7.56/7480 gal 1213 1213 364

2. Sewering @ $5.59/7480 gal 897 897 269

Wastewater treatment

1. Cyanide oxidation 14000 0 0

2. Metal precipitation 69000 69000 20700

3. Labor @ $15/hr 7500 7500 2250

Sludge disposal @ $209/yd3 2600 2600 1820

Total 119306 104295 46688

* Assumes 70% water and 30% zinc hydroxide recycled

Table 1. Comparison of Annual Operational Costs for CN Process, ANC Process without R/R Unit, and ANC Process with R/R Unit at P&H Plating

Co.

Table 2. Comparison of Economic Indices for the Alkaline Noncyanide Plating Process with

and without the Recovery/Recycle System

Index Option

ANC ANC + R/R

Capital Investment $36,000 $87,822

Payback Period 3 yr 1.5 yr

Net Present Value $57,500 $281,122

Implied Rate of Return 27.0% 71.9%
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rent to the direct current needed for the

plating process. Some type of cooling is

needed to prevent the rectifier parts from

overheating and thus damaging or destroying the entire unit. The rectifier on

the test plating line is cooled by water

although air-cooled models are used on

other zinc plating lines at P&H. Tap water

used in this noncontact cooling system is

not discarded but is rather added to the

counter flow rinsewater tanks on the plating line. Because of this continuous addition of fresh water to the plating line, a

holding tank to collect the R/R treated

water was installed. Water in this tank can

be used in the rinsing operation and as

makeup water for replenishing the plating

bath. The tank is designed with an overflow valve that opens and sends excess

water to P&H’s wastewater treatment area

for further treatment before discharge. Although this means that the objective of

zero discharge has not been met and that

this precipitation process is partially serving as a pretreatment step, the company

estimates that the amount of water from

this line that needs treatment has been

reduced by more than 60%. Additionally,

the cost of the precipitation pretreatment

is less than would be the cost of pretreating the cyanide containing rinsewaters.

The compliance criteria that need to be

met by the final treatment are only those

for zinc, since there is no longer cyanide

in this rinsewater. The maximum discharge

levels for metals are generally easier to

achieve than are those for cyanide. This

results in reduced treatment costs and

fewer compliance problems.

The recovery efficiency for zinc hydroxide from the CNT system averages 84%;

however, because of the variability in the

numbers and types of plating jobs and the

general fluctuations in business (all of

which play a role in the amount of zinc

hydroxide needed for the line), the company does not recycle all of the precipitate that is recovered. Additionally, the

recovered zinc hydroxide is not a totally

suitable substitute for the zinc ingots traditionally used to add zinc to the plating

bath. All of the precipitate produced is

passed through the filter press, which

greatly reduces its volume. A portion,

which varies depending on production

needs, but generally averaging 30%, is

returned to the plating bath. The remaining 70% is stored for later use or disposed as a hazardous waste. While it

would be possible to petition to delist this

waste, the amount being produced is less

than 5% of the metal waste that the company produces and must routinely dispose. Since much of the other metal waste

was from cyanide-based lines and probably contained residual amounts of cyanide, even after treatment, it was disposed

as a hazardous waste. Currently, it is more

economically advantageous to simply add

the zinc hydroxide to that waste and dispose of it as hazardous rather than separating it, storing it, and applying for

delisting.

The company has recently converted

all of its plating lines from CN-based to

ANC-based. This change will eliminate the

need for cyanide destruction in the

company’s treatment operations, will eliminate one potential compliance problem,

will eliminate the potential health risk associated with cyanide exposure, and will

ultimately save the company money. The

change was completed more quickly than

originally anticipated. Now that it is complete, it may become more economically

inviting to pursue the delisting process for

the zinc hydroxide that isn’t being recycled.

Also, the elimination of the cyanide lines

will allow use of the precipitated zinc hydroxide to replenish other zinc plating

baths, which is an option currently under

consideration at P&H.

Conclusions And

Recommendations

The CNT-designed R/R system proved

quite successful in meeting the project

objectives. By converting from a CN-based

to an ANC-based plating bath, the company eliminated one step from the treatment cycle, i.e., the destruction of cyanide.

The removal of cyanide from the system

also reduced the risk to employees by

eliminating a highly toxic substance from

their work environment. The purification of

the wastewater proved so successful that

nearly all of the recovered water could be

recycled as well as much of the precipitated zinc hydroxide. As a result of installing this recovery unit, annual operational

costs (including treatment and disposal

costs, raw material purchases, and water

usage fees) were reduced by $62,000.

The costs to design and install a R/R

system like the one in use at P&H Plating

is recovered during the first year and a

half of operations. Additionally, recycling

of the recovered materials reduced water

usage and discharge by 841,911 gal and

reduced the sludge disposed by 14 yd3/yr.

These savings apply to a single line, but

the test unit could easily accommodate a

second line’s waste, which would double

these waste reduction figures. This system is simple and functions well. It is in

use at P&H Plating today, and its use in

similar electroplating operations would result in economic benefits comparable to

those described in this project summary.

Finally, there are still many pollution

prevention opportunities for the electroplating industry left to explore. Documented

case studies of process modifications and

technology evaluations that lead to source

reduction are available, but more are

needed. Distribution of this information to

the appropriate audience can be achieved

with the assistance of trade groups for the

electroplating industry. Continued association with these industry organizations is

essential to identify new pollution prevention options and to promote adoption of

those that have proven successful. This

continued interaction can only benefit all

parties involved. Economic benefits could

be substantial to companies willing to work

toward source reduction. The environmental benefits that could result from the reduction in toxicity and volume of this

industry’s wastes would be significant and

would be reason enough to continue to

support research that will bring about those

reductions.

The full report was submitted in partial

fulfillment of Cooperative Agreement

#CR815829 by the Hazardous Waste Research and Information Center under the

sponsorship of the U.S. Environmental Protection Agency.
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Jacqueline M. Peden is with Hazardous Waste Research and Information

Center, Champaign, IL, 61820.

Paul M. Randall is the EPA Project Officer (see below).

The complete report, entitled "Alkaline Noncyanide Zinc Plating and Reuse of

Recovered Chemicals," (Order No. PB94-205549; Cost: $19.50, subject to

change) will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

Risk Reduction Engineering Laboratory

U.S. Environmental Protection Agency

Cincinnati, OH 45268

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United States

Environmental Protection Agency

Center for Environmental Research Information

Cincinnati, OH 45268

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EPA/600/SR-94/148