ZG.15;6 qss F.04238

EFFECTIVE ELECTROLYTIC RECOVERY OF HEAVY METALS FROM DILUTE EFFLUENTS

Authors: J . P. Wiaux, J . J . D i e t r i c h

Over t h e p a s t severa l years , i t has become i n c r e a s i n g l y d i f f i c u l t and c o s t l y t o d ispose o f sludges t h a t c o n t a i n heavy meta ls . Therefore, t h e r e has been a major e f f o r t t o e i t h e r reduce o r e l i m i n a t e t h e sludge. c o n t r a s t t o p r e c i p i t a t i o n methods which produce l a r g 6 v o l umes o f t o x i c sludge, and ion-exchange systems which produce e l u t a n t s w i t h concentrated heavy meta ls e l e c t r o l y s i s a1 lows t h e e f f l u e n t t o be t r e a t e d a t t h e source t o recover t h e meta l i o n s as meta l .

I n

E lec t rochemica l meta l recovery techniques have been 1 i m i t ed by low e lec t rochemica l e f f i c i e n c i e s assoc ia ted w i t h r e c o v e r i n g a c t u a l i ons i n p a r t s p e r m i 1 1 i o n (ppm) concen t ra t i ons from s o l u t i o n s t r e a t e d . Progress i n e l e c t r o l y z e r des ign i n c o n j u n c t i o n w i t h new cathode m a t e r i a l s has dramat i c a l l y inc reased t h e e f f i c i ency o f e l e c t r o l y t i c meta l recovery ( 1 , 2 ) .

designed t o e f f i c i e n t l y remove heavy meta ls t o ve ry low ppm l e v e l s . sys tem consi s t s o f a1 t e r n a t i n g foam metal cathodes and prec ious metal ca ta l yzed t i t a n i u m anodes (F igu re 1 ) .

One o f these e l e c t r o l y z e r s , u t i l i z i n g r e t i c u l a t e meta l cathodes, was The

DSA" ANOOt- 1

CATHODE -\ BUS

RETICULATE - CATHODE

/"

, .-

R O W flLTER

ANODE BUS W / CONNECTOR CLIPS

CATHODE CONNECTOR -/

FIG. 1. RETEC-SO C E U

. ../._.*." ""- .-

This t i t a n i u m anode, which i s s t a b l e i n a c i d i c and b a s i c media, a l l ows o p e r a t i o n w i t h o u t contaminat ion w h i l e t h e r e t i c u l a t e cathode a1 lows e f f i c i e n t metal r e c t i m e s t h e geometr ic shown i n F i g u r e 2 .

ue t o the l a r g e surface area t h a t i s about 10 dow area. The expanded view o f t h e cathode i s

To better understand the critical electrolysis parameters, it i s necessary to look at some basic mathematics. from an industrial plating effluent, the following relationship appl ies (3):

For the recovery of a metal

C e o A x V Rf

where:

V = Vo t Pa t Pc t Ir,

Ce: A :

V: Cell voltaae:

and

Energy consumed expressed in KHhIkg; Theoretical quantity of current necessary to deposit 1 kg of metal in per kAh;

Vo: Pa: Anode polarization:

Minimum voitage required to deposit the metal;

Pc: Cathode polarization; Ir: Rf:

Figure 3 shows the key parameters.

Ohmic drop in the electrolyte; Faraday yield of the process;

'h

FIG. 3 KEY CELL PARAMETERS

A s a first approx imat ion, i t i s est imated t h a t f o r a g i v e n e f f l u e n t , t h e terms Vo, Pa and I r are reasonably cons tan t d u r i n g t h e course of t he e l e c t r o l y s i s and that t h e energy consumed can be w r i t t e n as:

The t e r m s Pc and R f va ry d u r i n g the course o f the e l e c t r o l y s i s . h i g h concent ra t ions , t he r e a c t i o n a t t he cathode i s governed by the d e p o s i t i o n o f meta l . concen t ra t i on decreases, t h e v o l t a g e increases and hydrogen e v o l u t i o n begins as shown i n the f o l l o w i n g two equat ions.

A t

A s t h e e l e c t r o l y s i s proceeds and t h e metal

M+* + Ze' - M

2H20 + 2e- - Because o f t h e compe t i t i on between metal d e p o s i t i o n and hydrogen

H 2 + 20H-

e v o l u t i o n , i t i s c e r t a i n t h a t t he na tu re o f t he e f f l u e n t and the geometry o f t he e l e c t r o l y z e r p l a y an impor tan t r o l e i n t h e Farad ic e f f i c i e n c y o f the o v e r a l l e l e c t r o l y s i s . E l e c t r o l y z e r s w i t h p l a t e cathodes g i v e i n f e r i o r y i e l d s as t h e metal concen t ra t i on goes below 0.5 gm/ l . Th is i m p l i e s an unfavorable energy consumption and t h e n e c e s s i t y t o have a cathode m a t e r i a l w i t h a l a r g e su r face area t o s a t i s f y c o n d i t i o n s o f low metal concen t ra t i on .

A l i s t o f t h e prec ious and common meta ls t h a t can be recovered e l e c t r o c h e m i c a l l y i s shown i n Table 1 . Conspicuously m iss ing f rom t h i s l i s t a re i r o n and chromium. t h a t do n o t have separa tors between the anode and the cathode. I n a d d i t i o n , t h e presence o f i r o n and chromium can h i n d e r or prec lude the recovery o f t h e c,etals l i s t e d i n Table 1 .

These meta ls cannot be recovered i n sys tems

TABLE 1 : ADDllCatfOnS f o r t h e E l e c t r o l v t i c Recovery o f Me ta l s

Precious Metals

Au, Ag, Rh, Pd, P t . .. P l a t i n g baths and e tch baths Au-Cu, Au-N1 a1 loys Au from Co Separa t ion o f : Cu from Pd, Au from Co . . . Ag photographic f ix baths

Common Meta ls

c u

Cd

N i Sn Sn-Pb Zn Separa t ion

A c i d and ammoniacal etches o f H202 and (NH412S208 A c i d and Cyanide

Watts, Wood and sulfamate Ac id Ac id Cyanide Cd f rom N i and Co Cu from Cd and Zn

Because o f o x i d a t i o n a t t h e anode, some o f t he ba th c o n s t i t u e n t s may need replacement t o m a i n t a i n t h e des i red concen t ra t i ons . The o v e r a l l r e s u l t i s a l onger ba th l i f e t i m e and e f f i c i e n t recovery o f t he metal (Table 2 ) .

TABLE 2: Recovery o f A c i d i c Coppe r (CuSO4 - H2S04 - pH = 1 )

E l e c t r o l y s i s pa rs:

Voltage: 2.ov Curren t : 15 A E lec t rode Surface: 28 dm2 (3.1 f t 2 ) E lec t rode Type: E f f i c i e n c y

Vol ume 30 1 (7.9 g a l )

Ni-(20 pores pe r i n c h )

(3 .0 g r t o 0.8 ppm) 35%

Copper Concen t r a t i on

(Hours) (ma/ l ) T i me

0 31 00 1 2

2 300 1750

3 1240 4 92 5 80 6 06 7 320

T i me (Hours)

8 9 10 1 1 12 13 14 15

Copper Concent ra t ion

(mq/l)

160 82 26 12

6 1.2 1 .o 0.8

Recoverv o f Come r and D e s t r u c t i o n o f Cvanide: A l though n o t a g g r e s s i v e l y marketed f o r t r e a t 1 ng cyanide, a p rec ious meta l ca ta l yzed t i t a n i u m anode i n an ETS u n i t i s capable of e f f i c i e n t l y d e s t r o y i n g cyanide p resen t i n p l a t i n g s o l u t i o n s . Under the c o n d i t i o n s l i s t e d i n Table 3 , the e f f i c i e n c y shown i n Table 4 has been ob ta ined. The changing concen t ra t i on w i t h t ime has been p l o t t e d i n F igu re 6 . d e s t r u c t i o n p la teaus as t h e r e a c t i o n becomes ve ry i n e f f i c i e n t .

A t low concen t ra t i ons , cyanide

TABLE 3: E l e c t r o l v s i s Paramete rs f o r a Come r Cyanide Bath.

Volume: F1 ow: Mode: Cur ren t : Vol tage: E lec t rodes : Anodes : Agi t a t i on :

1100 1 (291 g a l . ) 850 l / h (225 g a l / h r ) Recyc 1 e 600 A 3.0 V 50 Ni-(20 pores pe r i n c h ) 51 DSA A i r

TABLE 4: Treatment o f a CopDe r Cvanide Bath: A n a l v t i c a l Resu l t s ( *>

T i me i n Hours

0 1.75 5.33 5.83 7.58 9.25

13.25 20.25 29.25

Copper ( m o l l ) X E f f i c i e n c y

1900 -- 1470 19.0 800 16.0 724 i 5 . 6 434 14.9 228 13.9

69 10.8 5.4 7.2 0.2 5.0

Cyan i de (mall1 X E f f i c i e n c y

2200 1800 1000 900 570 350 120 70 40

-- 43.2 42.5 42.1 40.6 37.8 29.6 19.8 9.0

( * ) Fo r t h e two processes, an exchange o f one e l e c t r o n has been chosen i n making Faraday y i e l d c a l c u l a t i o n s .

2.2

2

1.8

1.6

+ CYANIDE

COPPER

TIME (HOURS) FIG. 6, SIS OF A COPPER CYANIDE

7 - *7- remove the l a s t traces

PLATING BATH

PRIMARY SECONDARY ION RINSE RINSE EXCHANGE

I ELECTROLYTIC 1 I REGENERANT I

.

I . _. . -

I

ELECTROLVIC / REGENERANT ( TREATMENT TANK

J

I TREATMENT r I TANK J

DISPOSAL

1 DISPOSAL

-

FIG. 7 TREATMENT OF ION LXCHANGE REGENERANT

1

Treatment of Ef f luent before Ion-Exchanue: concentrat ions o f metal and moderate volumes, i t i s des i r ab le t o u t i l i z e ETS t o e f f i c i e n t l y remove the metal t o the low ppm l eve l s and then t o po l i sh , i f necessary, w i t h ion-exchange (Figure 8). des i r ab le where the discharge regula t ions a r e extremely s t r i n g e n t o r recovery of a prec;ous metal i s involved. ion-exchange column can then be recycled through the ETS u n i t .

For so lu t ions w i t h h i g h

This method i s

The regenerant from the

PLATING BATH

PRIMARY SECONDARY ION

I RINSE .I * RINSE EXCHANGE -

FIG. 8 TREATMENT OF EFFLUENT BEFORE ION EXCHANGE

The economics o f e l e c t r o l y t i c metal recovery w i 1 1 v a r y depending on t h e scrap va lue o f t he metal recovered and the va r ious costs assoc ia ted w i t h sludge t rea tment , h a n d l i n g and d i sposa l . c a l c u l a t i o n s i n v o l v i n g n i c k e l and copper f o r a p l a t i n g f a c i l i t y i n Swi tzer land. numbers would have t o be changed.

Table 5 g ives t h e bas is for

The data a re summarized i n Table 6 . The methodology i s v a l i d f o r t he U n i t e d S ta tes b u t t he

TABLE 5: Economic Studies o f Metal Recovery by Electrolysis lNickel and C o m e r Cases)

Items

Capital :

Rectifier 10V-750A ' Total Depreciation over 5 years

RETEC-50

Metal recovery capacity Cathode costs Number to recover 4000 kg/of meta;

Total Annual Cost Cathode Capacity Electrolytic Copper Value Electrolytic Nickel Value Total Annual Recycle Annual Recycle Value

Operating Expense Electric Power

Cost in Frs/Year ($111.5 FrS)

34,000. - ($22 670) 11.500.- ( $7.670) 45,000. - ($30,000) 9.100.- ( $6,07O)/year

4,000 kg (8,800 lbs)/year 15.-FrS ($lO)/pi ece

1000 5 000. -( $1 0,000) year

4 kg (8.8 lbs)/cathode 1 .O FrS/kg ($0.30/lb) 10.0 FtS/kg ($3.03/1b) 4000 kg (8,800 lbs) Copper: Nickel :

Copper 750 A x 2.0 V x QQQ 8,333 x 0.15 ($0.10) = 1,250.-($833) 0.9 0.8

kWh x FrS/kWh

Nickel 750 A x 6.0 V x QQQ 25,000 x 0.15 ($0.10) = 3,750.- ($2,500) 0.9 0.8

kWh x FrS/kWh

Labor; 4 h/week (30 FrS ($20)/h times 50 weeks)

A t 10% metal content 250.- (8167)m ton For 4000 kg of metal, 40 tons of sludge disposed of annual ly 1Ol0O0.-($6,670)/year Precipitation, storage, s 1 udge hand1 i ng 100.-($67) /m ton 4,000.-($2,670) /year

S1 udge disposal cost 6,000. - ($4,000)

TABLE 6: Economic Summary o f the Electrolytic Recovery o f Metals (case of N i and o f Cu in FrS Der Year of ODeration).

Annual ExDense CoDDer Nickel

Depreciation 9,100- -( $6,067) * 9,100 -($6,067 1 Cathodes 15,000. -( $1 0,000) 1 5,000. -( $10,000) El ectri ci ty 1,250.-($833) 3,750. -($2,500) Labor 6,000. -( $4,000) 6.000. -( $4,000)

TOTAL 31 ,350. -( $20,900) 33,850. -($22,567)

Operating Expense: FrS/Kg ( $ / l b ) 7.85 ($2.38) 8.50 ($2.58) Annual Savi nas

Recovered Metal 4,000. -( $2,667) 40,000. -( $26,667 Sludge Treatment 4,000. -( $2,667) 4,000 -( $2,667) D i sposal 10.000. -($6,666) 10.000.-($6,666)

TOTAL 18,000. -( $1 2,000) 54,000. -( $36,000)

Savings: FrS/kg ( $ / l b ) 4.50 ($1.36) 13.50 ($4.09)

$1 =I 1.5 FrS.

It can be clearly seen in this particular case that electrolytic recovery i s the economic choice for nickel but sludge disposal is more favorable for copper. A s can be seen from the tables, the numbers can be dramatically a1 tered depending on the recovery value of the metal and the disposal cost of the sludge. Utilizing the Swiss data, it i s possible to draw the chart shown i n Figure 9. converts from Swiss Francs per kilogram into dollars per pound.

The validity of the chart does not change when one

FIG. 9. ELECTROLYTIC RECOVERY OF METAL: PRICE OF METAL AND ELECTROLYSIS COST

Developments i n t h e s t r u c t u r e o f t he e lec t rodes and i n t h e des ign of e l e c t r o l y z e r s have l e d t o t h e appearance i n t h e marke tp lace o f a number of grow'lng a p p l i c a t i o n s i n t h e area o f d i l u t e e f f l u e n t t rea tments . A s sludge d isposa l becomes more d i f f i c u l t and more c o s t l y , t h e r e i s i n c r e a s i n g i n c e n t i v e t o employ e l e c t r o l y t i c recove y sys tems. r e d u c t i o n o r e l i m i n a t i o n o f sludge, t h e e l e c t r o l y t i c systems a l l o w the u s e r t o recover t h e waste as metal and, thus e l i m i n a t e t h e ongoing l i a b i l i t y assoc ia ted w i t h sludge. I t i s i n t e r e s t ng t o n o t e t h a t t h e r e i s no one s i m p l i s t i c s o l u t i o n t o waste t rea tment . The waste t rea tment problem can b e s t be so lved by a j u d i c i o u s cho ice of t rea tment s y s t e m s which m e e t the env i ronmenta l r e s t r a i n t s and g i v e the user the b e s t economic r e t u r n . u l t i m a t e goal i s t o o b t a i n t e c h n i c a l , p o l i t i c a l , economic and environmental harmony,

I n a d d i t i o n t o the

The

4091 P

B i b l iocrraphy

( 1 ) Sioda. e t a l . Flowthrough Porous E lec t rodes . Chemical Engineer ing. Feb 21 pp 57-67. (1983)

( 2 ) Konicek M.G., P l a t e k G. R e t i c u l a ted E lec t rodes C e l l Removes Heavy Meta l s f rom R i nse Waters. New Mater1 a1 s and Processes.

Procede E lec t roch imique de Recuperation. No. 166. Fev. (1979). b > Coeuret F .e t S to rck A . Elements de Genie E lec t roch imique. Ed. L a v o i s l e r Tech. & Doc. P a r i s 1984.

V o l . 2. pp 232-235 (1983) (3) a) Don ia t D.

T r a i tements de Sur face .

I S B N 2-85206-243-7