9/21/2014 Metal Recovery and Wastewater Reduction Using Electrowinning : Products Finishing

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CLICK IMAGE TO ENLARGE (+)

2. All metals' cadmium

electroplating line

1. Cross-section of an

electrowinning unit.

3. Conductivity measurements in

rinse tank one.

Metal Recovery and Wastewater Reduction

Using Electrowinning

Article From: Products Finishing, Laura Bloch from EPA Region 9

Posted on: 1/1/2000

Merit Partnership pollution prevention project for finishers...

The Merit Partnership is a joint venture between

the 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. One of these industries is metal

finishing, which is represented by the Metal

Finishing Association of Southern California

(MFASC). This project involves implementing P2

techniques and technologies at metal finishing

facilities in southern California and documenting

and sharing results. Technical support for this

project is provided by Tetra Tech EM Inc.

(formerly PRC Environmental Management, Inc.).

Electrowinning is an electrolytic technology used

to recover metals from electroplating rinse

waters. Although electrowinning has traditionally

been used only for metal recovery, its application

in a well designed and controlled rinse system

can significantly reduce rinse water use,

wastewater generation and chemical discharge.

An electrowinning unit has three main

components: 1) electrolytic cell; 2) rectifier; and 3) pump. An electrolytic cell is a tank in

which cathodes and anodes are typically arranged in alternating order (Fig. 1). The

cathodes and anodes are attached to their respective bus bars, which supply electrical

potential to the unit. The electrolytic cell may include a flow disperser or air spargers to

improve rinse water circulation within the cell.

When operating, the electrical potential applied to the electrodes causes dissolved

metals and other positively charged ions to migrate toward and plate onto the cathodes.

As metals deposit on the cathodes, the metal buildup decreases the deposition rate. When the metal deposition rate is no longer sufficient, cathodes are removed from the

electrolytic cell for onsite or offsite metal recycling. In some cases, recovered metals are

pure enough to be reused in process baths. As metals are chemically reduced at the

cathodes, other rinse water components are oxidized at the anodes. If cyanide is

present, it is oxidized to cyanate and then to carbon dioxide and nitrogen.

Technology applications. Electrowinning is most commonly used to recover gold, silver,

copper, cadmium and zinc. Gold and silver are the most successfully recovered metals

because of their high electropotential. Chromium is the only common electroplating

metal that is not recoverable using electrowinning. Nickel recovery is possible, but the

pH must be maintained within a tight range for metal deposition to occur. Some

fluoroboratecontaining solutions, such as tin and tinlead solutions, can corrode certain

anode materials. Most etchant solutions dissolve metal off the cathodes as quickly as

they are deposited.

Design and implementation considerations. The design and implementation of an

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electrowinning unit depend on the configuration and control of the electroplating and

rinse operations. Electrowinning can eliminate the need for continuously flowing rinse

water if dragout reduction techniques and multiple rinse tanks are used.

Installing an electrowinning unit on a stagnant rinse tank after a dragout recovery tank is

the most common and costeffective application of electrowinning. To maintain a steadystate metal concentration in the stagnant rinse at or below the maximum contaminant

concentration, the rate of metal deposition onto the electrowinning cathodes must be

greater than or equal to the rate of dragin from the preceding tank. Thus, the metal

deposition rate is an essential design parameter that ultimately affects the capacity, size

and cost of the electrowinning unit.

The electrowinning unit should operate 24 hours per day to maximize metal recovery

and maintain the lowest possible metal and cyanide (if present) concentrations in the

stagnant rinse. Operating the unit during non-production hours allows it to recover

metals and destroy cyanide that accumulates during production.

Because metal and cyanide concentrations in the stagnant rinse are lowered,

subsequent rinses will be cleaner, allowing the flow rates in these rinses to be reduced

or turned off. In many cases, rinse water flow is reduced to a rate equal to the

evaporation rate from the stagnant rinse.

Maximizing and controlling metal concentration, current density, mixing and cathode

surface area will improve electrowinning performance.

Metal concentration. To achieve high recovery rates, electrowinning should be applied

to concentrated rinse waters; therefore, electrowinning is most effective on a stagnant

rinse.

Current density. Metal deposition occurs at faster rates with higher current densities.

However, if the current density is too high, the solution surrounding the cathodes can

become depleted of metals, which limits the metal deposition rate. The "excess" current

applied to the electrodes is wasted on converting water into hydrogen and oxygen.

Mixing. Mixing disrupts the metal depletion layer that would form in a stagnant solution,

allowing the electrowinning unit to be operated at a higher current density with a

corresponding higher deposition rate.

Cathode surface area. Metal deposition rate is proportional to cathode surface area.

Two main types of cathodes are flat-plate cathodes and reticulated cathodes. Flat-plate

cathodes are made of stainless steel, have an effective surface area equal to their

apparent area and are reusable. When deposited metal reaches a thickness of 3/16 to

1

/4 inch, flat-plate cathodes should be removed and cleaned. The advantages of flatplate cathodes are their reusability and the ability to recover metals onsite. Reticulated

cathodes are made of metalcoated carbon fibers and have an effective surface area that

is ten times their apparent area. The advantage of using reticulated cathodes is their

high deposition rate. Reticulated cathodes are not reusable; however, fully loaded

cathodes are and are sent off site for recycling.

Eventually, dissolved salts that are not removed or oxidized by the electrowinning unit

accumulate in the rinse water. If these accumulated salts start negatively impacting

rinsing quality, the rinse water tanks should be drained and filled with clean water. Spent

rinse water can be evaporated, treated or disposed.

Case study: Electrowinning at All Metals. All Metals Processing Co. is a job shop in

Burbank, CA, that performs cadmium, bronze and zinc electroplating and black oxide

coating for aerospace and other industrial customers. All Metals employs 15, and its

facility has about 8,000 sq ft of plating space.

In early 1996, All Metals set a goal to reduce water use and eliminate wastewater

discharge to the sewer. The shop was motivated by high city sewer fees and pressure

from the municipal wastewater treatment plant (POTW) to decrease the metal

concentrations in treated wastewater. In cooperation with the Merit Partnership, All

Metals agreed to pursue its goal in two phases. Efforts were focused on the cadmium

electroplating line because it was used most frequently. Dragout from this line

contributed the largest quantities of metals to the wastewater, and All Metals had

exceeded its cadmium wastewater discharge limits on several occasions.

Capital. Electrowinning units typically cost $5,000 to $15,000, depending on size, design

and type of cathodes used. Reusable, flat-plate cathodes cost about $200 each, and

"disposable" reticulated cathodes cost about $12 each. Labor, electrode replacement,

maintenance and energy costs are low.

Phase I involved evaluating overall process efficiency and control. The purpose of

Phase I was to reduce dragout and optimize rinse water use before selecting or
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purchasing recycling/recovery equipment. Phase I modifications resulted in a 50%

dragout and rinse water reduction, a 60% wastewater treatment chemical reduction,

improved rinsing quality and more efficient work flow. The Phase I payback period was

1.7 years.

By improving overall process efficiency first, All Metals was able to more cost effectively

apply a metal recovery technology and move toward eliminating cadmium wastewater

discharges to the POTW. For Phase II, electrowinning was determined to be the most

feasible technology for All Metals to reduce or eliminate wastewater discharges from the

cadmium electroplating operation.

Electrowinning unit installation and operation. All Metals purchased a Retec Model 6

electrowinning unit from U.S. Filter, Warrendale, PA. The unit holds six cathodes and

has a 100amp capacity. The electrolytic cell and rectifier were mounted on a shelf

above a stagnant rinse tank (rinse one) that followed a spray dragout recovery tank (Fig.

2). Reticulated cathodes were used in the electrowinning unit. Anticipating that the

electrowinning unit would reduce cadmium and cyanide concentrations in rinse one, All

Metals turned off the rinse water flow in the subsequent counterflow rinse and converted

it into a twostage stagnant rinse (rinses two and three). Rinse water was transferred

from rinse two to rinse one to make up for evaporative losses. Rinse water in rinse three

flows through a weir into rinse two when clean water is added to rinse three.

Ideally, a heater should be installed on the rinse tank connected to the electrowinning

unit. This concentrates the influent to the electrowinning unit, increasing the metal

deposition rate. It also increases the countercurrent flow of rinse water from subsequent

rinses.

In addition, All Metals installed an electric heater on rinse two to increase evaporation,

thereby increasing the countercurrent flow of clean water from rinse three. No heater

was installed on rinse one because it was a plastic tank. All Metals also installed an intank filtration system on rinse two to remove suspended solids from the rinse water.

Results. The electrowinning unit operates 24 hours a day, 7 days a week. The cathodes

are replaced every three months on average. Based on the start and finish weights of

the cathodes, about 2 kg (4.4 lb) of cadmium were recovered from the six cathodes after

two months of operation. Spent cathodes are picked up by a scrap metal dealer for

recycling.

Before the electrowinning unit was installed, rinse water flowed through the counterflow

rinse tank at a rate of 0.5 gpm. Since the electrowinning unit was installed, all the rinse

tanks have been operated in a stagnant mode, and the temperature of the rinse water in

rinse two has been maintained at about 115F, resulting in the evaporative loss of about

15 gpd. Consequently, 15 gpd of clean water per day is added to rinse three in order to

compensate for the water transferred into rinse two to make up for evaporative losses.

Based on comparison of water use on the cadmium electroplating line before and after

installation of the electrowinning unit, rinse water use has been reduced by 94% and

wastewater is no longer generated from the rinses.

Because not all dissolved solids are removed by the electrowinning unit, conductivity is

monitored in rinses one and three (Fig. 3). Rinse three is drained to dispose of dissolved

solids and replenished with clean rinse water every six weeks on average. The drained

rinse water is evaporated onsite. In addition, cyanide concentrations are periodically

measured in rinse one. These concentrations have been significantly reduced by the

electrowinning unit.

Lessons learned. Cathode installation and maintenance is critical to the electrowinning

unit's performance. All Metals experienced two incidents in which the cathodes

dissolved into the circulating rinse water when the electrical connection between the bus

bar and cathodes was disrupted. The first incident occurred when three cathodes were

improperly placed in the electrowinning unit. The second incident was caused by reuse

of cathode connectors. As the connectors gradually became corroded, the electrical

connection between the cathodes and bus bar was lost. All Metals now replaces the

cathode connectors every time the cathodes are replaced. It also periodically checks the

cathodes to assure proper connection to the bus bar.

After installing the electrowinning unit, no increase in the number of reject parts

occurred, and no adverse impacts on production were observed. Motivated by the

success of the electrowinning unit for the cadmium operation, All Metals has installed

another electrowinning unit on a copper electroplating stagnant rinse.

The estimated 8.7-year payback period is relatively high because it considers only direct

costs and savings. Other beneficial outcomes may lower the payback period. For

example, the electrowinning unit takes All Metals onestep closer to its goal of zero

discharge to the sewer, which will eliminate the $2,860 annual wastewater discharge
9/21/2014 Metal Recovery and Wastewater Reduction Using Electrowinning : Products Finishing

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fee. Also, All Metals is no longer susceptible to cadmium discharge violations, which can

result in $1,000 fines.

Cost Savings

TABLE I—Cost Savings

Water Use - Cd

Sewer Discharge

Wastewater Treatment

Sys/

Operation and

Maintenance

Filter Cake Disposal

Annual Savings =

$1,564/yr*

Total Equip Costs =

$9,010

O&M Costs = $530/yr

Payback Period = 8.7 yrs

Before

240 gpd

240 gpd

$11,080/yr

760 lb/mo

After

15 gpd

0 gpd

$10,220/yr

760 lb/mo

Savings

$110/yr

$480/yr

$860/yr

$114/yr

*Annual savings does not include potential reclaim value

for recovered metals.

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