ELECTROLYTIC RECOVERY OF COPPER FROM CHELATED WASTE STREhMS

Paul L. Bishop Professor of Civil Engineering University of New Hampshire Durham, New Hampshire

Ronald A. Breton Environmental Engineer Environmental Compliance Services Manchester, New Hampshire

INTRODUCTION

The printed circuit board manufacturi R proce s creates many difficult wastewater treatment problems. Among these is the generation of dilute, chelated copper rinse waters re- sulting from the electroless deposition of copper on the circuit boards.

- -

Printed circuit boards are fabricated from nonconduc- tive board material such as plastic or fiberglass on which a circuit pattern of conductive metal, usually copper, has been deposited. conventional electroplating techniques cannot be used to deposit the copper in the circuit pattern. electroless copper plating process musf be used.

Because the boards are non-conductive,

Consequently, an

Electroless plating is a chemical reduction process which depends upon the catalytic reduction of a metallic ion in an aqueous solution containing a reducing agent without the use of electricity (1). Cahill (2) showed that the cupric ion could be reduced autocatalytically with formalde- hyde in an alkaline solution. Okinaka (3 ) established that the overall reaction of the catalytic copper plating process could be represented by:

2+ Cu -t 2HCHO + 40H- -t Cu + H, + 2H-0 + 2HC00- L L

The electroless copper bath must be highly alkaline (pH > 12.5) in order for the reducing agent to react properly. As a result, the copper ions must be complexed to prevent the precipitation of copper hydroxide. One method of

584

forming stable metal complexe! lating agents. Several chela1 most commonly used one being acid).

The unique significance bath formulation is its abilii solution, preventing indiscrii a wastewater treatment standp conventional copper waste tre copper precipitation ineffect tion of the complexed ion. A\ tion treatment schemes have b) or ferrous sulfate, these mec excessive quantities of sludg hauled away for disposal as a recent efforts have been dire copper recovery systems. The abatementlresource recovery s of the contaminant content o f regulatory standards, and 2) concentrated form suitable fa

Electrolytic extraction has been investigated in the intrinsic specificity for rem streams without adding additii recently, electrolytic system enough, though, to be able t o ] solutions at reasonable costs affecting the extraction of 11 the diffusion of the ionic sr electrolyte to the cathode, e: toward hydrodynamically reduc diffusion layer. One approac. corrosion-resistant, high sur: However, most of this work p. containing free copper ions; mentation has involved the U E ~ chelated metal ions.

The purpose of this res€ ance of a specific high-surf: treating dilute, chelated coi goals of this study were to 6

ness of the system in terms c and to optimize the operatior various chemical and operatic

C

? COPPER FROM CHELATED C R M S

:ing

rvices

anufacturing process creates ent problems. Among these is 2d copper rinse waters re- >sition of copper on the

iabricated from nonconduc- :ic or fiberglass on which a :al, usually copper, has ,ds are non-conductive, liques cannot be used to

s must be used. pattern. Consequently, an

mica1 reduction process reduction of a metallic ion

a reducing agent without 11 (2) showed that the atalytically with formalde- Lnaka (3) established that ytic copper plating process

, + 2H20 + 2HC00- nust be highly alkaline (pH agent to react properly. be complexed to prevent :$.de. One method of

forming stable metal complexes is by the addition of che- lating agents. Several chelating agents have been used, the most commonly used one being EDTA (ethylenediaminetetracetic acid) .

The unique significance of the chelated copper plating bath formulation is its ability to hold the copper metal in solution, preventing indiscriminate plating. However, from a wastewater treatment standpoint, this property renders conventional copper waste treatment techniques which rely on copper precipitation ineffective by preventing destabiliza- tion of the complexed ion. Although innovative precipita- tion treatment schemes have been developed using excess lime or ferrous sulfate, these methods are costly and generate excessive quantities of sludge which must be dewatered and hauled away for disposal as a hazardous waste. Therefore, recent efforts have been directed toward various in-line copper recovery systems. The design of these specific abatement/resource recovery systems assumes: 1) reduction of the contaminant content of an effluent to a level meeting regulatory standards, and 2) recovery of the contaminant in concentrated form suitable for reuse or sale.

Electrolytic extraction of metal is one method which has been investigated in the last ten years because of its intrinsic specificity for removing contaminants from waste- streams without adding additional chemicals. recently, electrolytic system design has not been advanced enough, though, to be able to extract metal from dilute solutions at reasonable costs. Since the critical factor affecting the extraction of low concentrations of metals is the diffusion of the ionic species from the bulk of the electrolyte to the cathode, efforts have been directed toward hydrodynamically reducing the thickness of the diffusion layer. One approach was the design of porous corrosion-resistant, high surface area flow-through cathodes. However, most of this work has been done using wastes containing free copper ions; only a limited amount of experi- mentation has involved the use of wastewaters containing chelated metal ions.

Until very

The purpose of this research was to study the perform- ance of a specific high-surface area electrolysis system treating dilute, chelated copper rinsewaters. The specific goals of this study were to evaluate the overall effective- ness of the system in terms of copper removal efficiency, and to optimize the operation of the system by regulating various chemical and operational parameters.

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ELECTROLESS PLATING WASTE TREATMENT

The chelated waste stream consists of rinses following electroless deposition operations, In some cases, acid-dip rinses are also present. This particular wastestream usual- ly makes up 2 to 10% of the total discharge. Due to the presence of chelating agents in the electroless waste stream, the segregation of these wastes from all other streams is important. The combination of wastestreams will result in the chelation of other free metal ions originating from other processes, precluding effective use of relatively inexpensive metal hydroxide precipitation treatment schemes.

The current treatment technologies available to feasi- bly treat electroless process rinse water can be classified in two main categories: 1) advanced, chemical-aided pre- cipitation; and, 2) recovery.

Copper can be precipitated from chelated waste streams with massive amounts of lime or calcium sulfate at a pH of 11.6 to 12.5, but voluminous quantities of sludge result. In addition, the process is only effective with certain types of chelating agents ( 4 ) .

Other chemical treatment processes evaluated for che- lated metal waste treatment include the use of ferrous sul- fate ( 5 ) , insoluble starch xanthate, dithiocarbonates, sodium hydrosulfate and hydrazine ( 6 ) . techniques, including evaporative recovery, reverse osmosis, electrodialysis, ultrafiltration and ion exchange, have also been investigated but with limited success (4,7).

Several recovery

ELECTROLYTIC RECOVERY

Electrolytic metal recovery is radically different from the other methods of recovery because it is selective and removes only the metal, and thus decouples the production and recovery processes.

In the past, electrolytic metal recovery was studied as a means of recovering metals from spent plating baths. recent years, interest has grown in recovery of metals from dilute rinse solutions. significant problems, however, because of cathode polariza- tion. As plating proceeds, the layer of solution next to the cathode becomes depleted in metal ions. diffuse into and across the layer before they can plate out. In dilute solutions there are fewer ions present, so the rate of diffusion into and across the depleted layer is much

In

Dilute rinse waters have posed

The ions must

586

IF slower, and the layer becomes 4 One way to overcome this pro& 1; of the diffusion layer and/on +I area of the cathode.

ln response to the deman tant, high surface area cathu Corporation developed a meta; formed by depositing copper u The reticulate cathode presen almost 20 times its superfic:: the reactor vessel marketed 11 porous flow-through electrodu turbulent, tortuous path for

I$

The complete reactor con through cathodes and anodes ( 1

in a polyethylene electrode ((

nected in parallel to a direct stream to be treated is intrcc and removed through an overf::

The reticular electrochii cally for removal of metal i(( effluent streams. As with 0 1 1

the theory of operation is bii reaction whereby electrons a:: electrical source, reducing ' ' lyte to form elemental coppeii cathodic and anodic reaction:: follows :

Cathode: Cu2+ + 2e- - Anode: H20 - 2H+ -.

A critical competing cathodiii concentrations, is the evolu-

At low metal concentrations .. . reaction contributes to inef is not hydrogen evolution i t

' but rather bubble nucleation entrapped hydrogen gas (8). mized by increasing the spec

. such as by using a porous, h 1 i metal sponge cathode. For a

r

:NT

msis ts of rinses following i . In some cases, acid-dip irticular wastestream usual- ! discharge. Due to the :he electroless waste wastes from all other iation of wastestreams will free metal ions originating effective use of relatively .pitation treatment schemes.

dogies available to feasi- tse water can be classified inced, chemical-aided pre-

rom chelated waste streams alcium sulfate at a pH of .tities of sludge result. effective with certain

cesses evaluated for che- de the use of ferrous sul- te, dithiocarbonates,

( 6 ) . Several recovery recovery, reverse osmosis, nnd ion exchange, have also i success (4,7).

1s radically different from iuse it is selective and lecouples the production

a1 recovery was studied as spent plating baths. In n recovery of metals from nse waters have posed ause of cathode polariza- yer of solution next to tal ions. The ions must before they can plate out . r ions present, BO the the depleted layer is much

slower, and the layer becomes thicker and more depleted (7). One way to overcome this problem is to reduce the thickness of the diffusion layer andlor increase the specific surface area of the cathode.

In response to the demand for a more corrosion-resis- tant, high surface area cathode media, Diamond Shamrock Corporation developed a metal "sponge" cathode which is formed by depositing copper onto a polyester foam material. The reticulate cathode presents an actual surface area almost 20 times its superficial geometric area when used in the reactor vessel marketed by MacDermid Incorporated. porous flow-through electrode, the cathode also provides a turbulent, tortuous path for electrolyte flow.

A s a

The complete reactor consists of a series of flow- through cathodes and anodes (titanium-coated steel) placed in a polyethylene electrode cellbox and electrically con- nected in parallel to a direct current rectifier. The waste stream to be treated is introduced at one end of the unit and removed through an overflow stand pipe at the other end.

The reticular electrochemical cell is designed specifi- cally for removal of metal ions from dilute process or effluent streams. As with other electrochemical devices, the theory of operation is basically an oxidation-reduction reaction whereby electrons are supplied by an external electrical source, reducing the cupric ions in the electro- lyte to form elemental copper at the cathode surface. The cathodic and anodic reactions for bivalent copper are as follows :

Cathode: Cuz+ + 2e- ---t Cuo Eo = +0 .34 volts

Anode: H20 - 2H + 1/202 + 2e E = -1.27 volts 3- - 0

A critical competing cathodic reaction, especially at low concentrations, is the evolution of hydrogen gas:

2H+ + 2e- - H2 Eo = 0 volts

At low metal concentrations and high current densities, this reaction contributes to inefficiencies of metal removal. It is not hydrogen evolution itself that causes the trouble, but rather bubble nucleation which "blinds" the cathode with entrapped hydrogen gas (8). These inefficiencies are mini- mized by increasing the specific surface area cf the cathode, such as by using a porous, high surface area, reticulated metal sponge cathode. For a given geometric current den-

58 7

sity, the actual current density on the surface of the cathode will be significantly lower. This permits the operation of the cathode at low current densities while maintaining cathode potentials below the evolution of hydrogen gas until very low metal concentrations are reached, In addition, the complex flow-through path promotes turbu- lence which increases the transfer of metal ions to the cathode surface.

METHODS

All experimentation was conducted on site at a printed circuit board manufacturing facility located in Merrimack, New Hampshire. The electrolytic recovery device was pro- vided by MacDermid Incorporated, Waterbury, Connecticut. An advanced design model is currently marketed under the trade-

mark of Retecm cell. study consists of a polyethylene tank with a hydraulic capacity of 48 gallons in which 25 copper-plated polyester "sponge" cathodes alternated by 26 titanium-coated anodes were mounted transverse to the direction of flow. Each anode and cathode measured 45 cm by 38 cm and was spaced 0.64 cm apart in the unit. D.C. current for electrolysis was provided by a solid state, 12-volt, 500 ampere rectifier All studies were conducted using 7.5 volts. Two one-inch diameter compressed air lines were situated at the bottom of the tank to provide mixing.

The actual unit utilized in this

In order to study unit performance under controlled operational conditions, synthetic waste was formulated utilizing the solution from an electroless chemical bath, tap water, concentrated sulfuric acid and sodium hydroxide.

The principle experimentation was conducted in four distinct phases:

1. Phase I - Variable influent pH 2. Phase I1 - Variable influent copper concen-

trat ion 3. Phase 111 - Variable influent flow rate 4. Phase IV - Variable effluent recirculation

ratio

Phase I

A series of ten runs were performed in order to evalu- ate the effect of influent pH on chelated copper removal. Duplicate test runs at pH 3,5,7,9 and 11 were conducted,

588

with a flow rate of 2.0 gpmi tion of 60-80 mg/L. Three run were procured at eight (influent; cells 1, 5, 10, three operating time interv- for each pH condition.

Phase I1

Three different influe: 190-210 mg/L, 120-140 mg/L, different pH values (pH=9.01 mental runs at each pH were! gpm was used in all the tes;

Phase I11

Four different flow rai were experimentally tested. at each flow rate, while thi the influent copper concent:

Phase IV

The effect of effluent: moval was evaluated using r. and 8 . 0 , with influent pH hL 2.0 gpm and copper concentr'

RESULTS

Preliminary studies wi vice indicated an interesti:, tration did not occur as ex: instead occurred between ce, concentrations progressivel:: effluent port in almost all per removal profiles with d copper concentrations with

Since the charge distr directly affects the rate o theorized that a non-unif or exist over the surface of t semi-conductor. A1 though t behave as a perfect conduct

t y on the surface of the Lower. This permits the J current densities while below the evolution of Ea1 concentrations are reached chrough path promotes turbu- ;fer of metal ions to the

mducted on site at a printed iility located in Merrimack, ic recovery device was pro- 1, Waterbury, Connecticut. An itly marketed under the trade-

tal unit utilized in this Le tank with a hydraulic 1 25 copper-plated polyester r 26 titanium-coated anodes direction of flow. Each :m by 38 cm and was spaced :. current for electrolysis 12-volt, 500 ampere rectifier Ig 7.5 volts. Two one-inch 'ere situated at the bottom of

formance under controlled IC waste was formulated electroless chemical bath, c acid and sodium hydroxide.

ion was conducted in four

Le influent pH Le influent copper concen-

Le influent flow rate Le effluent recirculation

I

lerformed in order to evalu- . chelated copper removal. 9 and 11 were conducted,

$8

with a flow rate of 2.0 gpm and an influent copper concentra- tion of 60-80 mg/L. run were procured at eight locations within the 25 cell unit (influent; cells 1, 5, 10, 15, 20, 23; effluent) for each of three operating time intervals (90, 95 and 100 minutes) and for each pH condition.

Three sets of samples per experimental

Phase I1

Three different influent copper concentration ranges of 190-210 mg/L, 120-140 mg/L, and 8-12 mg/L were tested at two different pH values (pH19.0, pH~3.0). Duplicate experi- mental runs at each pH were performed. gpm was used in all the tests.

A €low rate of 2.0

Phase I11

Four different flow rates (0.5, 1.0, 1.5, and 2.5 gpm) were experimentally tested. Duplicate runs were performed at each flow rate, while the influent pH was held at 9.0 and the influent copper concentration at 90-110 mg/L.

Phase IV

The effect of effluent recirculation on ultimate re- moval was evaluated using recirculation ratios of 1.0, 2.0, and 8.0, with influent pH held at 9.0, influent flow rate at 2.0 gpm and copper concentration at 110-120 mg/L.

RESULTS

Preliminary studies with the electrolytic recovery de- vice indicated an interesting trend. Minimum copper concen- tration did not occur as expected at the effluent port, but instead occurred between cells 15 and 23. In fact, copper concentrations progressively increased from cell 23 to the effluent port in almost all cases. An investigation of cop- per removal profiles with depth showed progressively higher copper concentrations with depth in all instances.

Since the charge distribution over the cathode surface directly affects the rate of electrodeposition, it was theorized that a non-uniform distribution of charge m y exist over the surface of the cathode, a characteristic of a semi-conductor. behave as a perfect conductor, exhibiting uniform charge

Although the cathode media was designed to

58 9

I f i c a n t e f f e c t of i n f luen t over t h e e n t i r e su r face , i t w a s found t h a t e l e c t r i c a l resis- tance increased s i g n i f i c a n t l y wi th depth. Therefore, metal removal e f f i c i e n c y w a s a func t ion of depth i n the t reatment u n i t . A t t h e end of t he device (beyond ce l l 23) , wastes from the bottom of the u n i t were migrat ing toward the e f f l u e n t po r t a t the top and thus adding higher concentra- t i o n s of metal t o the upper waters.

Ef fec t of In f luen t pH on Copper Removal

The e f f e c t of i n f l u e n t pH on removal e f f i c i ency is shown i n Figure 1. It is apparent t h a t t he b e s t removals occurred a t t he high (pH = 11) and low (pH = 3) ranges, wi th a s l i g h t decrease i n e f f i c i e n c y i n t h e n e u t r a l range. d a t a w a s analyzed s t a t i s t i c a l l y f o r var iance t o determine the s ign i f i cance of pH on copper removal; only a s l i g h t dependency was evident .

The

a w

% 60 a/ 501 1 I

8 I I 2 4 6 0 10 I2

PH

Figure 1. Ef fec t of i n f l u e n t pH on copper removal.

In c o n t r a s t t o t hese experimental r e s u l t s , research performed by o t h e r s on o t h e r e l e c t r o l y t i c recovery devices t r e a t i n g uncomplexed metal waste s t reams show a very s ign i -

590

p le , Williams e t al. (9) s; i ences wi th decreasing pH. from an “ idea l” , uncomplex

However, t h i s was due t o t: t ion . A t lower pH values , and e x h i b i t s higher mass-t: cathode sur face . I n c o n t r che la ted so lu t ions are ins;

a t pH = 3 and above. Ther I s t a b l e wi th in t h e t e s t ed pt

One e f f e c t of removini reduct ion on t h e cathode i

, ions may occur as a r e s u l t ’ anode. As a r e s u l t , s i g n i

throughout t he ce l l u n i t , t a t i o n of t h e experimental changing pH throughout the , trochemical research and i. t h i s research.

I n summary, due t o cot] i n f l u e n t pH on removal e f f ings suggest t h a t the i n s t # # systems p r i o r t o e lec t ro lyn

E

Effec t of In f luen t Copper ((

The e f f e c t of varying : removal e f f i c i e n c i e s a r e d::

e f f i c i e n c y i s e s s e n t i a l l y :: concent ra t ion above 50 mg/ll

t r a p i d l y b e l w t h i s concentii

The m i n i m u m in f luen t 11

removal e f f i c i e n c y is affecc ‘ s i o n of t h e copper ions f r a

) cathode sur face . Thus, t ha i. l y in f luence t h e l imi t ing n 1 example, a s l i g h t increase

very e f f e c t i v e i n reducing l a y e r and, consequently, ra

6 l i m i t i n g i n f l u e n t concentrct i: I c ! Effec t of Flow Rate on Copgi

The funct iona l dependtt

Eound that electrical resis- th depth. Therefore, metal 2 of depth in the treatment (beyond cell 2 3 ) , wastes

s adding higher concentra- rs.

migrating toward the

Removal

n removal efficiency is nt that the best removals nd low (pH = 3) ranges, with in the neutral range. The for variance to determine

I removal; only a slight

--*--A-

I

R i / !

I 1 ficant effect of influent pH on the removal rate. For exam- ple, Williams et al. (9) showed increasing removal effic- iences with decreasing pH. Optimum pH for copper removal from an '*ideal", uncomplexed CuSO waste stream was pH = 3 . 4

' I ,

However, this was due to the effect of pH on copper specia- tion. At lower pH values, the free cupric ion predominates and exhibits higher mass-transfer characteristics to the cathode surface. In contrast, the speciation of highly chelated solutions are insignificantly affected by pH changes

2+ at pH = 3 and above. Therefore, the Cu - EDTA complex is stable within the tested pH range of 3 to 11.

One effect of removing copper ions from solution by reduction on the cathode is that a net increase in hydrogen ions may occur as a result of the evolution of oxygen at the anode. As a result, significant pH fluctuations may occur throughout the cell unit, precluding a simplified interpre- tation of the experimental data. The "dynamic" effect of changing pH throughout the system requires extensive elec- trochemical research and is beyond the intended scope of this research.

I In summary, due to copper complexation the effect of influent pH on removal efficiency is minimal. These find- ings suggest that the installation of costly pH adjustment systems prior to electrolytic recovery is not justified.

!

Effect of Influent Copper Concentration on Copper Removal

The effect of varying influent copper concentrations on removal efficiencies are displayed in Figure 2 . Removal efficiency is essentially independent of influent copper concentration above 50 mg/L, but efficiency decreases ! 1 rapidly below this concentration.

I

The minimum influent copper concentration at which 1 removal efficiency is affected may be linked to the diffu- sion of the copper ions from the bulk solution to the cathode surface. Thus, the design of the system may direct- - ' ly influence the limiting influent concentration. For

8 IO 12 ! example, a slight increase in solution turbulence may be very effective in reducing the thickness of the diffusion layer and, consequently, result in a significantly lower limiting influent concentration.

It p~ on copper removal.

bental results, research pctrolytic recovery devices

I I Effect of Flow Rate on Copper Removal

t

I I

100 200 INFLUENT COPPER (MG/L 1

Figure 2. Effect of influent copper concentration on copper removal.

rate (superficial velocity) is shown in Figure 3 . Statis- tical analysis of the data indicates a strong correlation ' between superficial velocity and copper removal. Approxi- mately a 15 percent increase in removal (from 77 to 92 percent) resulted when the flow rate was decreased from 2.5 to 0.5 gpm.

Analysis of variance performed on the experimental data indicates that superficial velocity is the best single predictor of treatment unit performance and is the most important parameter for determining unit size (cathodic specific area). Based on flow rate alone, a simply nomo- graphic represeatstior, may b2 developed for the design of actual installations.

Effect of Recirculation on Copper Removal

Attempts to improve performance by recirculating efflu- ent proved unsuccessful. efficiency was independent of recirculation ratio; removal improved only 2 percent, from 78 to 80 percent, with recir- culation ratios from zero to as much as eight times the influent flow rate.

Statistically, copper removal

592

a 70 a 0

Figure 3. Effect of

The results of the e x effect of influent copper efficiency may offer an ex: since it was shown that di. (below 50 mg/L) resulted i. removal efficiency. In ef'

i incoming copper concentrat provement. Therefore, rec beneficial when treating d

i;. i

f - Time - Dependent Phenomena c The rate of increase ..

time the cathodes may be k

them thus inhibiting strea

Because the metal dep cathode area increases) vi

i in theory, improve with ea I statistical analyses indic

was independent of operati of this study. Apparently

function of copper accumul

pores of the cathode becoml

1.043 +0.011 C.) l.997

-I

> 0 I w a a a a

*

a

w

0 0

200

znt copper concentration 11.

shown in Figure 3 . Statis- tcates a strong correlation Id copper removal. Approxi- I removal (from 77 to 92 J rate was decreased from 2.5

mned on the experimental data )city is the best single rformance and is the most ining unit size (cathodic rate alone, a simply nomo- leveloped for the design of

)er Removal

m n c e by recirculating efflu- tstically, copper removal :ecirculation ratio; removal r8 to 80 percent, with recir-

90 - 80 -

70 -

60 -

I

%R = 85.9-12.14LNQ r 2 ~ 0 . 9 3 7

1 1 I I 2 3

FLOWRATE (GPM) ao:,

Figure 3. Effect of flow rate on copper removal.

The results of the experimentation concerned with the effect of influent copper concentration on copper removal efficiency may offer an explanation to this phenomenon, since it was shown that dilute influent concentrations (below 50 mg/L) resulted in significant reductions in copper temoval efficiency. In effect, recirculation "dilutes" the incoming copper concentration precluding an "overall" im- provement. Therefore, recirculation of effluent is not beneficial when treating dilute waste streams.

Time - Dependent Phenomena The rate of increase of flow-through resistance as a

function of copper accumulation determines the fraction of time the cathodes may be kept on stream. Specifically, the pores of the cathode become smaller as copper plates out on them thus inhibiting stream flow.

Because the metal deposit morphology changes (specific cathode area increases) with time, removal efficiency should, in theory, improve with each experimental run. statistical analyses indicated that copper removal efficiency was independent of operating time, at least for the duration of this study.

However,

Apparently, the rate of copper build up was

192 593

not great enough to significantly increase the specific cathodic surface area. Visual inspection of the cathodes subsequent to runs 18 and 36 failed to reveal significant pore size reduction. loading, in excess of the quantities deposited during the 50 experimental runs performed here, are apparently required to affect the removal rate.

Significantly more influent copper

Electrolytic Copper Recovery Model

A simplified mathematical model can be developed based on physical principles which influence the performance of most electrochemical devices. The following model repre- sents the mass balance analysis of a typical plug-flow reactor in which a first-order chemical reaction occurs:

(-ka l/v) R = l - e

where, R is the fractional removal of copper, k is the mass- transfer coefficent (cmfsec), a is the specific cathode area

(cm ), 1 is the length of the recovery unit (cm), and v is the superficial fluid velocity (cm/sec). The model predicts that copper concentration should decrease exponentially along the length of the device.

-1

Figure 4 represents the fit of experimental data to the 2 model. Based on a regression analysis, which yielded an R

value of 89.5% (120 degrees of freedom), the mass-transfer coefficient, k, for this recovery device was determined to be 0.0072 cm/sec.

CONCLUSIONS

The results of this study indicate that use of high- cathodic surface electrolysis technology can effectively remove copper from dilute, chelated rinse waters, with cop- per removal efficiency ranging from 80 to 92 percent. In- fluent pH had relatively little effect on copper removal over the pH range of 3 to 11 and consequently the installa- tion of pH control equipment prior to electrolytic recovery is not necessary. rate (superficial velocity) exerted the strongest influences on unit performance. influent copper concentration above 50 mg/L, but removal efficiency decreased significantly below this concentration. Flow rate (superficial velocity) proved to be the best pre- dictor of unit performance. Recirculation of recovery unit

Influent copper concentration and flow

Removal efficiency was independent of

594

1.0 r J

L- i, a 6 LL

R = l

II

0.0- 0 200 400,

Figure 4 . Electrolyt::

effluent had no effect on 0-7

mathematical model was deve:: using experimental data, ancc parameters for similar elecll lated rinse waters.

REFERENCES

1.

' 2. !

3.

; 4.

' 5 . t i

6.

Bellis, H., Electrolesii Electroplaters Society Cahill, A., Proceeding:: Society, 4 6 , 130, 1957 Okinaka, Y., presentat:: Society Meeting, Detro:: U.S.E.P.A., Development! Pretreatment Standards-

Wing, R., Rayford, W., Electroless Copper Plaii

EPA 440 / 1-79/ 003, 1979 ..

and Metal Finishing, Vcc Whalen, B., "New Develii Wastes", Printed Circu., 1981.

tly increase the specific inspection of the cathodes ailed to reveal significant antly more influent copper tities deposited during the 50 re, are apparently required to

ode1

model can be developed based .nfluence the performance of The following model repre-

.s of a typical plug-f low

. chemical reaction occurs:

./VI

ioval of copper, k is the mass- a i s the specific cathode area

! recovery unit (cm), and v is (cm/sec). The model predicts

ild decrease exponentially -. lit of experimental data to the

2 analysis, which yielded an R i freedom), the mass-transfer rery device was determined to

' indicate that use of high- technology can effectively dated rinse waters, with cop- ; from 80 to 92 percent. .e effect on copper removal md consequently the ins talla- lrior to electrolytic recovery lpper concentration and flow :erted the strongest influences efficiency w a s independent of above 50 mg/L, but removal ntly below this concentration. . y) proved to be the best pre- ncirculation of recovery unit

In-

I .o J

0 s 3 0.0 a a 2 0.6 a. 0 0

0.4 a o_ 0 0.2 a

z c 4

L

0 .a

(- kal /v)

r 2 =0.895

R = 1 - 0

I I 1 I 1

200 400 600 800 1000

I /v

Figure 4. Electrolytic copper recovery model.

effluent had no effect on overall treatment efficiency. mathematical model was developed and tested for its validity using experimental data, and may be used to estimate design parameters for similar electrolytic devices treating che- lated rinse waters.

A

REFERENCES

1. Bellis, H., Electroless Plating of Metals, American Electroplaters Society, Winter Park, FL, 1975.

2 . Cahill, A., Proceedings of American Electroplater's Society, 46, 130, 1957.

3. Okinaka, Y., presentation at the Electrochemical Society Meeting, Detroit, 1969.

4. U.S.E.P.A., Development Document for Existing Source Pretreatment Standards for the Electroplating Industry, EPA 44011-791003, 1979.

5. Wing, R., Rayford, W., and Doane, B . , "Treatment of Electroless Copper Plating Rinse Waters", Plating -, Vol. 64, No. 57, 1977.

6 . Whalen, B . , "New Developments in the Treatment of P.C. Wastes", Printed Circuit Fabrication, Vol. 9, No. 9, 1981.

595

7. Swank, C., "Electrolytic Recovery from Rinse Waters", Printed Circuit Fabrication, Vol. 5, No. 5, 1982.

8. Bennion, D., "Electrochemical Removal of Copper from -. Very Dilute Solutions", Journal of Applied Electrochem- istry, 2, 1972.

9. William, J. and M. Olson, Extended Surface Electro- lysis for Trace Metal Removal: Scale System, E . I . duPont de Nemours, Inc., Wilmington, DE, 1976.

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TOXIC AND HAZARDOUS WASTE Proceedings of the Fifteenth M id-Atlantic Industrial Waste Conference

edited by

Michael D. LaGrega Linda K. Hendrian

BUlTERWORTH PUBLISHERS Boston London Sydney Wellington Durban Toronto An Ann Arbor Science Book