THE RECOVERY AND RECYCLING OF METAL UTILIZING ELECTROLYTIC TECHNIWES**

Author: 3. P. Wiaux*

* T i ta lyse, S A . , 8 Rue Bergere,

** Translated from French by J . J . D i e t r i c h . CH-1217 Meyrin/Geneva, Switzer land

1. ADDliCatiOn o f E l e c t r o l v s i s and the Treatment of Water

Because o f recent developments i n e l e c t r o l y t i c techniques, i t i s poss ib le t o f i n d a growing number of app l i ca t ions i n the area o f i n d u s t r i a l w a t e r p u r i f i c a t i o n .

I n con t ras t t o p r e c i p i t a t i o n methods which produce la rge volumes of t o x i c sludge, and i o n exchange sys tems which produce e lu tan ts w i t h concentrated heavy metals, e l e c t r o l y s i s a1 lows the e f f l u e n t t o be t rea ted a t the source o f the metal i o n and recovers i t as the metal.

Electrochemical metal recovery techniques have f o r a long t i m e been l i m i t e d by the low electrochemical e f f i c i enc ies associated w i t h recover ing l o w concentrat ions o f m e t a l l i c ions from the treatment so lu t ions . progress i n e l e c t r o l y z e r design i n conjunct ion w i t h new cathode mater ia ls i n the market have increased the e l e c t r o l y t i c metal recovery e f f i c i e n c y f o r concentrat ions i n the low ppm ( 1 , 2 ) .

The

1.1 Theoret ica l A s p e c t s i n the E l e c t r o l y s i s o f Low Concentrat ion E f f l u e n t s

E l e c t r o l y t i c systems u t i 1 i z e c e l l s w i t h two e lect rodes. The anode i s genera l l y made o f a non-consumable mater ia l such as p l a t i n i z e d t i tan ium, ru thenized t i tan ium, lead, g raph i te , e t c . The cathodes are adapted t o the nature of the operat ion conducted and t o the type o f e l e c t r o l y z e r u t i 1 ized. t o the q u a n t i t y o f cur ren t which flows through the e l e c t r o l y t i c c e l l . Two p r i n c i p l e fac to rs govern the effect iveness o f the operat ion:

-- The energy y i e l d which con t ro l s the cos t of the e l e c t r o l y s i s .

The quan t i t y o f metal deposi ted i s p ropor t iona l

-- The geometric dimensions o f the ,e lec t ro l ys i s sys tem which Inf luences the va lue o f the investment.

The energy y i e l d i s r e l a t e d d i r e c t l y t o the e l e c t r i c i t y consumed i n the process. Therefore, fo r the recovery of a metal i n an i n d u s t r i a l e f f l u e n t the f o l l o w i n g r e l a t i o n s h i p appl ies ( 3 ) :

C e = A x V R f

where:

V 0 Vo + Pa + Pc + I R

and

Ce:

A:

V:

vo:

Pa:

Pc:

I R :

R f :

Energy consumed expressed i n KWh/kg;

Theoret ica l q u a n t i t y of cur ren t necessary t o deposi t 1 kg of metal i n kAh/kg;

Voltage between the terminals o f the c e l l i n v o l t s ;

Minimum vol tage t o deposi t the metal;

Anode pol a r 1 z a t i on ;

Cathode p o l a r i z a t i o n ;

Ohmic drop i n the e l e c t r o l y t e ;

Faraday y i e l d of the process.

As a f i r s t approximation i t can be estimated t h a t f o r a g iven e f f l u e n t the terms Vo, Pa and I R are reasonably constant dur ing t h e course o f the e l e c t r o l y s i s , and t h a t the energy consumed can be w r i t t e n as :

C e = K & R f

The t e r m s Pc and R f w i l l go through impor tant mod i f i ca t ions d u r i n g the course o f the e l e c t r o l y s i s . necessary t o study the r e l a t i o n between i n t e n s i t y o f the cur ren t and the vo l tage o f the e l e c t r o l y s i s dur ing the course o f t i m e ( f i g u r e I).

Two p r i n c i p l e react ions enter i n t o compet i t ion dur ing the e l e c t r o l y s i s o f a s o l u t i o n conta in ing a metal.

To understand these v a r i a t i o n s , i t i s

1 ) A t h i g h concentrat ions, the r e a c t i o n a t the cathode i s governed by the depos i t ion o f metal (curve l a ) , wh i le the e v o l u t i o n o f hydrogen produced by the e l e c t r o l y s i s o f water has l i t t l e chance o f occur r ing (curve 2 ) .

2) I n the course o f t ime, the reduct ion o f the metal concentrat ion causes a displacement o f the metal deposi t ion curve (curve l b , IC and I d ) towards the value necessary f o r the e v o l u t i o n o f hydrogen. e l e c t r o l y t i c metal deposi t ion y l e l d !s governed b y the r e l a t i o n :

A t t h a t moment the two reac t ions are concurrent and the

R f = l m l m + l h

where:

l m : C o n t r i b u t i o n of the e l e c t r o l y t i c cur ren t t o the depos i t ion o f metal ;

- 2 -

l h : Cont r ibu t ion o f the e l e c t r o l y t i c cu r ren t t o the evo lu t i on of hydrogen.

< W

< W 0

a

2 a 3 fn

ve VOLTAGE

FIG. 1. CHANGE IN CURRENT DENSITY AS A FUNCTION OF ELECTROLYSIS VOLTAGE

I t i s c e r t a i n t h a t the nature of the e f f l u e n t and the geometry o f the e l e c t r o l y z e r p l a y an important r o l e i n the f a r a d i c 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 as determined by the amount o f metal deposited versus the evo lu t i on o f hydrogen. I t i s genera l l y observed t h a t the 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 ve i n f e r i o r y i e l d s as the metal concentrat ion goes below 0.5 gm/l. This imp l ies an unfavorable energy consumption and the necess i ty t a have a cathode mate r ia l w i t h l a r g e r dimensions t o s a t i s f y low concentrat ion condi t ions.

- 3 -

1.2 Eneray Considerations

ELECTROLYSIS Pom YRAL ENERGY N" VOLTACE COHSUYPTION PRICE CWPOHEKf

, METAL CURRENT (Anode n) kwhr/kq FrS/kg AS A X OF pRlcE

Au 0.1 35 5.0 0.405 moo0 INF. 0.1

ENERGY COMPOSITION OF ELECTROLYTIC RECOVERY I

4 0.25 2.5 0.625 350 INF. 0.1

cu 0.84 2.0 1 .a 2 b4

la 0.48 2.5 1 20 8 1.5 1 I *" 0.82 3.0 2.46 0.8 30.0 --I

~-

MI 0.9 1 3.5 3.2 8 4.0

I & 0.26 3.0 0.78 0.6 14.0 I 1 % 0.45 3.0 1.35 10 1.4 1

TABLE 1. LIST OF METALS WHICH CAN BE RECOVERED ELECTROLYTICALLY FROM AQUEOUS SOLUTIONS

Table 1 g ives a l i s t o f the metals which can be ext racted from i n d u s t r i a l e f f l u e n t s by e l e c t r o l y t i c techniques. There i s a l s o i nd i ca ted the r e l a t i o n between the energy cost o f e l e c t r o l y t i c recovery a t h igh y i e l d versus the commercial value o f the metal. I t can a l s o be observed t h a t f o r most metals t h e energy cost represents o n l y a f r a c t i o n o f t he va lue o f the metal. Electrochemical recovery then presents i t s e l f as a pr imary a t t r a c t l v e method f o r t h e a p p l i c a t i o n under considerat ion. Other f a c t o r s t h a t can be added t o the l i s t o f advantages f o r e l e c t r o l y t i c recovery are:

* The p o s s i b i l i t y t o ob ta in the metal under var ious forms such as p la tes , f o i l , powder ...

* The s e l e c t i v i t y t o deposi t a pure metal from a composite waste stream u t i l i z i n g p o t e n t i a l con t ro l ... The p o s s i b i l i t y t o t r e a t e f f l u e n t s " i n l i n e " w i thout us ing a d d i t i o n a l water.

*

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2.

goal of increasing the effectiveness of the electrolysis at competitive prices. i n order to achieve the required ppm (mg/l) of the metal ions in the treated effluent.

The Dif ferent Electrolvtic Syste ms and Their Areas of ADD^ i cation

Electrolytic systems are appearing in the marketplace which have the

This i s being done by reducing the polarization of the electrodes

The general approach i s to modify the active part of the electrodes and

This last operation can be

to avoid a high polarization by reducing the current density on the electrode. electrode and agitating the solution around it. done in many ways: veloci ty of the electrolyte in the vicinity of the electrode, or by a combination of the two operations. As a rule, agitation or circulation at high speed reduces the thickness of the diffusion layer from 0.5 mm (natural convection) to about 0.005" (turbulent flow). The material transfer coefficient is therefore increased by a factor of 100.

This is generally done by increasing the surface area of the

by agitating the electrode itself, by increasing the

The increase in the surface area per unit volume (S/V) i s represented by:

S/V - A active V cellule

where:

S/V: Surface per unit volume.

A (Active): Active surface of the electrode.

V (cell): Volume of the cell containing the electrode

It can be obtained in an electrolyzer with sheet cathodes by using the most compact configuration possible of the active surfaces and the diaphragms. This results in a filter press type electrolyzer. An electrolyzer containing this electrode configuration gives an increasing yield due to the significant increase o f surface area per unit volume (figure 11).

- 5 -

3

t 1,Ooo

4 \ c 7 - w ( K w s E L E ~

(nnw Y. JAccAuo (4) )

d 10 I a I I I

0.05 0.5 SO DmANcE BRwED( ELEcTRooEs (em)

FIG. I I RELATION BETWEEN SURFACE AREA PER UNIT VOLUME AND DISTANCE BETWEEN ELECTRODES IN ELECTROLYSIS CELL

When concentrated solutions a re t rea ted , the c e l l can be operated under h i g h current dens i t ies (greater than 10,000 A / m Z ) , i n s p i t e f he modest

same i s not the case when d i l u t e solutions must be t rea ted whlch require extremely weak current dens i t ies (10 t o 100 A / m 2 ) . conditions, a conventional e lec t ro lyzer operates a t only a f rac t ion of i t s capacity. This can be t ranslated as an unjust i f ied use o f the technology based on i t s cost . Dispersed electrodes provide an important improvement because the flow o f the solution acrass the porous s t r u c t u r e r e s u i t s I n a diffusion layer o f reduced thickness i n contact w i t h the sol id conductor. The porous matrix a l s o plays a ro le i n promoting turbulence. Therefore, the ut i 1 i za t ion of electrodes w i t h h i g h dispersion per u n i t volume a1 lows a t the same time, the advantages of surface area per u n i t volume and of material t r a n s f e r coef f ic ien t .

surface area per u n i t volume of the electrolyzer ( 3 t o 100 m S Z / m ). The

Under these

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2.1 The Different Tvoes o f Elect rol vters

Table 2 sunnrarizes the different types of el ectrolyzers w h i c h have e f fect ive ly found industrial applications. A brief description o f these systems will allow a better understanding of their operating character1 s t t c s .

UCTROLYZERS USED I N WATER T R F A m

D o e of E 1 ec t ro l vzer Companv

The e lect ro lyzers wi th sized cathodes.

- Ret lcu la te Electrodes

- Packed Bed - S w i s s Ro l l

Folded Metal

ELTECH Meprelor-Charbonni e r & Le Carbone ELTECH b Chemelec

The e lect ro lyzers wi th moving cathodes.

- Rotatlng Cathode T i t r a l Loran - F l u i d Bed Sorapec & Chemelec - Rotatlng Ret lcu la te Cathode T l ta lyse - Dispersed bed Sorapec & PBC

* The e lect ro lyzers wi th membranes.

- E lec t rod ia lys is - E ld la - Bipolar

Leight EIF Aquatec Ion1 cs

Table 2 L i s t o f the companies comnerciallzing e lect ro lyzers for water treatment.

- 7 -

1 . Fluidized and DisDersed Beds

FlG.lll

DISPERSED - CATHODE

i I CoNDUCToR OAR 7 I --

I I CURRENT LEAD a Urr

2 ANODE

POROOS PART"

. . 1

SCHEMATIC REPRESENTATION OF AN ELECTROLYZER WITH 3-D OR DISPERSED ELECTRODE

Figure 111 i l l u s t r a t e s one way of representing t h i s type of c e l l , The e l e c t r o l y t e i s c i rculated through a cylinder by means of a porous p l a t e i n the bottom of the cylinder. A c i rcu la t ing e lec t ro ly te keeps the bed of cathodic par t i cl es 1 n suspension whi 1 e a porous diaphragm separates the cathodic p a r t i c l e s from the anode. The e l e c t r i c a l connection t o the bed can be made by meta l l ic screens o r rods which can be carefu l ly placed i n a manner t o minimize the IR drop. metal that i s being recovered, neutral par t ic les such as graphite, o r i n general , by any s t a b l e conductive p a r t i c l e which maintains the recovered metal as d i s c r e t e p a r t i c l e s . dens i t ies o f p a r t i c l e s by varying the speed of the f lu id iza t ion over a large range.

The p a r t i c l e s can cons is t o f the same

This technique can handle d i f f e r e n t s izes and

In a c e l l w i t h a dispersed electrode, the par t ic les a r e pumped along w i t h the e l e c t r o l y t e ; the suspended p a r t i c l e s a re p u t i n contact w i t h a conductive s t r u c t u r e which serves t o d i s t r i b u t e the current.

- 8 -

2.

The reticulate electrode is composed of a polymeric substrate with a The Reticulate E 1 ec t rode

high surface area which is metallized in order to render it conductive (figure I V ) .

FIG. IV. SCHEMATIC REPRESENTATION OF A RETICULATE ELECTRODE

The active surface of this three dimensional electrode is five to six times that o f the geometric surface projection (shadow area). In addition, the path taken by a solution which goes through this electrode is much longer than the linear path based on the thickness of the material. ELTECH Systems Corporation has commercialized an electrolyzer based on a cathode with a high electrochemical ly active surface area (2-5). el ectrolyzers present the fol lowing advantages:

These

-- The reduction of polarization due to the nature of the electrode and the fast circulation of the electrolyte.

-- The reduction i n polarization dire to the increased active surface and the resulting low current densities.

-- The possibility to increase the yield of the electrolysis with decreased size of the electrolyzer.

- 9 -

For the same cathode surface, a fluidized bed cell has an average volume 3 to 4 times l ess than a cell with sheet electrodes. A cell with reticulate cathodes whose surface projection 1 s 1 sq. meter in reality presents an active surface which is 5 to 6 sq. meters.

Interest In thls new technology i s understandable since it: * Reduces the consumption o f electric1 ty, *

* Permits a smaller sized investment,

Reduces the cost of the treatment, * Enables electrochemical systems to be competitive in the recovery of

metals from dilute solutions.

An electrolyzer based on reticulate electrodes is shown in figure V .

inill ELECTRODE

CELL COVER

FLOW FILTER

RETICULATE

AIR SPARGER

FIG. V RETEC-50 CELL

- 10 -

2.2 The Areas o f A p ~ l i cat1 on

Appl i cat ions f o r e l ectrochemi ca l techniques are numerous. The Surface treatment shops are obv ious ly the f i r s t p r i o r i t y :

are discharged t o the waste treatment sys tem . I n p r i n t e d c i r c u i t board shops the etch bath o r the copper p l a t i n g bath can e a s i l y be t rea ted by e l e c t r o l y s i s t o avoid the surcharges associated w i t h e f f l uen ts conta in ing h i g h l y concentrated heavy metal s. El e c t r o l y s i s can a1 so be u t i 1 i zed t o t r e a t s t a t i c r i n s e baths i n o rder t o increase t h e i r use fu l l i f e .

The treatment o f r i n s e baths decreases the amount of heavy metals which

The recovery of go ld or o the r precious metals and t h e i r a l l o y s on an e lec t rode reduces the t i m e fo r recover ing the metal value.

The se lec t i ve separat ion of metals such as cadmium from coba l t and n i c k e l i s poss ib le , and i s being done w i t h the sludges produced i n shops whi ch fab r i ca te n i cke l /cadmium b a t t e r i e s ( 6 ) .

S i l v e r from i ndus tri a1 photographi c f i x i ng baths i s a1 so recovered by

An incomplete l i s t o f a p p l i c a t i o n p o s s i b i l i t i e s f o r e l e c t r o l y t i c

e l e c t r o l y s i s ( 7 ) .

treatment o f e f f l uen ts i s given i n t a b l e 3.

APPLICATIONS OF ELFC TROLYTIC SYSTEMS TO WATER TREATMENT

Precious Metals

- Au, Ag, Rh, Pd, P t ... - Au-Cu, Au-Ni a l l oys . - Separation of : Au from Cu, Cu from Pd... - Ag from photographed baths.

Common Metals

- Cu: ac id , ammoniacal and cyanide. - Cd: ac id and cyanide - N i : Watts, sulphamate and ammoniacal - Sn: a c i d - .Sn-Pb: ac id - Zn: cyanide and acid. - Separation of : Cd from N I and Co, Cu from Cd and Zn

* Membrane Processes

- Concentration o f metal i n e f f l uen ts . - Acidjbase regeneration - Chromic a c i d recyc le

Table 3 L i s t o f E lec t ro l yze r App l ica t ions I n Ef f luent Treatment

- 11 -

3.

concept o f u t i l i z i n g the waste heavy metals. Economic mot iva t ion i s the d r i v i n g fo rce mast o f t e n behind the recyc le concept.

The ConceDt o f Recvcl inq

I t I s inc reas ing ly des i rab le t o consider the idea of recyc le i n the

3.1

composite i s at tacked by an o x i d i z i n g and complex so lu t ion . Most common etch so lu t ions are composed of f e r r i c c h l o r i d e and cupr ic c h l o r i d e o r complex ammoniacal so lu t ions. Gradually, as p a r t s a r e t reated, the bath becomes enriched i n copper which i s etched from un-masked areas. When the copper concentrat ion i n the bath reaches about 100 t o 160 g / l , the etch ing s o l u t i o n becomes i n e f f i c i e n t , and i t i s necessary t o dispose of the bath. An e l e c t r o l y t i c c e l l can be i n s t a l l e d i n the etch ing s y s t e m t o remove the copper accumulated dur ing the opera t i on.

Etc h i n q Solut ions from Pr in ted C i r c u i t S

One can g ive here examples of app l i ca t ions where the p l a t e d copper

Given a h igh copper concentrat ion i n an etch bath, a c e l l w i t h p a r a l l e l p la tes i s s u f f i c i e n t t o t r e a t the bath. The d i f f i c u l t y posed by the e l e c t r o l y t i c c e l l i s the c o n t r o l o f the chemical q u a l l t y o f the etch bath which changes w i t h time. order t o mainta in the o r i g i n a l composition.

The e tch bath must be adjusted i n

3.2 S i l v e r From PhotosraDhic F i x a t i o n Baths

The procedure t o recover s i l v e r from used b lack and whi te photographic f i x baths has been i n general use fo r some time. These so lu t ions , which conta in metal concentrat ions i n the neighborhood o f 3 t o 4 911, are e lec t ro lyzed on a s t a i n l e s s s t e e l cathode. Usual ly , the cathode i s r o t a t e d t o permi t an increase i n the recovery e f f i c iency . S ta in less s t e e l i s the prefer red cathode mater ia l because the s i l v e r deposi ts on i t but does no t adhere t o i t . The metal can be removed as very h igh p u r i t y i r r e g u l a r p a r t i c l e s by h i t t i n g the e lec t rode wi th a t o o l .

The p r i n c i p l e l i m i t a t i o n o f these e l e c t r o l y z e r s i s the d i f f i c u l t y o f removing s i l v e r a t concentrat ions below 200 ppm. problem i n two cases:

This can be a

1. S a t i s f y i n g the normal l e g a l d isposal l i m i t s .

2. T rea t ing la rge volumes o f so lu t ion .

By u t i l i z i n g r e t i c u l a t e e lect rodes it i s poss ib le t o reduce the s i l v e r concentrat ion t o 10 t o 20 ppm i n reasonable e l e c t r o l y s i s times. The recyc le o f p u r i f i e d f i x baths can a l s o be considered, bu t the d i f f i c u l t y t o mainta in a reproduclb le s u l f i t e composition and mod i f i ca t ions produced by the s t rong o x i d a t i o n condi t ions l i m i t the scope o f appl i cat ions o f t h i s procedure.

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3.3 Recoverv o f Gold from S t r i m e r S o l u t i o ns

Gold s t r i p p e r so lu t ions are p a r t i c u l a r l y aggressive and d i f f i c u l t t o t r e a t f o r metal recovery by conventional e lectrochemical methods. Cyanide baths which conta in organic agents such as sul fonated nitrobenzene are e lec t ro lyzed a t h igh cur ren t dens i t y on r e t i c u l a t e cathodes and t h e i r y i e l d i n precious metal can be maintained i n the lower acceptable l i m i t s . I n e f fec t , as i n the case o f the etch baths fo r p r i n t c i r c u i t s , the s t r i p p e r looses i t s o x i d i z i n g power as the metal concentrat ion i n the so lu t i on increases. Regular ly removing the metal permits one t o prolong the l i f e o f the s t r i p p e r bath.

3.4 R ecvcle o f Hard Chrome Bat h s

CATHODE ANODE

FIG. VI CHROME BATH REGENERATION ..

- 13 -

The electrolysis proceeds in a two-compartment el ectrolyzer separated by a cation-exchange membrane. The principle of operation is described in figure VI, In the anode compartment, which contains lead el ectrodes , the tri Val ent chrome 1 s 0x1 di zed to hexaval ent chrome. The impurities in the hard chrome bath, which circulate through the anode compartment, are eliminated by passing through a semi-permeable cation selective membrane toward the cathodic compartment. Elements such as iron, copper, nickle, zinc and cadmium are thus eliminated in the cathodic department where a concentrated oxalic or sulfuric acid solution i s circulated.

Elimination of the Impurities from the solution permits the regeneration of the hard chrome bath. In a country such as Switzerland, the disposal of a concentrated chrome bath costs more than FRS. 2,000 per cu. meter (approximately $5.00/gal). One can understand the motivation of industry to find a recycle solution to the problem of hard chrome bath disposal.

4. Purification of Water bv Electrolvsir

The efficient recovery of precious metals such as gold from plating line rinse baths i s generally considered more important than recovery time and investment requirements.

In the case of the common metals the sltuation i s considerably the low value of the metal does not cover the operating different:

cost of the recovery. lndustry i s forced to adopt the most efficient procedure since the costs are directly dependent on the efficiency of recovery.

4.1 Purification o f Nickel

Nickel has always been utilized to protect certain steels and certain pieces of brass provided by the machine industry. of this metal from the rinse bath remains a major concern o f the el ectropl ati ng industry.

One example of the application of an electrolzyer with reticulate electrodes Is the removal of nickel from the rinse bath. The problem can be viewed as follows: i f the treatment system cannot absorb the excess metal carried because of an increase in production it i s n ec e s s ary to :

The removal

1. Double the size of the treatment system; this requires

2. Install an ion-exchange column in the rinse circuit. 3.

Increased i nvestrnent and addl tfoiial space costs.

Electrolyze the rinse solution in a closed circuit or in an inline system.

- 14 - ,

V M : 5ooL

CURREW. 500 Amp.

VOLTAGE: 3.5 V

pH: 9 (NH3)

I I I * I I 1 1 - 1

I

0 20 40 60 80 100 120 110 TlME (MIN)

FIG. VI1 TREATMENT OF RINSE WATER NICKEL: C LO S E D C I R C U IT E LE CTRO LY S I S

I n the example we are discussing, the f low t o consider i s o f the order o f 100 1 per hour fo r 12 hours per day, and the average n i c k e l concentrat ion i s 300 t o 400 mg/l . maximum o f 480 g. Over a per iod o f 30 days continuous running a t o t a l o f 14.4 kg o f metal i s t o be removed.

The d a i l y entrainment o f n i c k e l i s a

The e l e c t r o l y s i s op t i on was first studied i n c lose c i r c u i t t reatment fo l lowed by i n - l i n e treatment. o f so lu t i on i n a closed loop are presented i n f i g u r e V I I . A f t e r ad jus t i ng the pH w i t h ammonia, the e l e c t r o l y t i c system was able t o reduce the n i c k e l concentrat ion t o a f e w ppm i n l e s s than two hours. A t the t ime o f the i n - l i n e treatment, ne i nod i f l ca t ion o f the pH was made, b u t the f l ow was c l o s e l y c o n t r o l l e d a t 100 l / h w i t h an average n i c k e l concentrat ion i n the e f f l u e n t o f 300 ppm. cond i t ions the concentrat ion o f n i c k e l leav ing the e l e c t r o l y z e r was reduced, on the average, t o on l y 10 ppm ( f i gu re VIII). fo r the e l e c t r o l y s i s sys tem i s FRS. 35,000.- ($23,000.00) and the consumption o f cathodes for one month o f operat ion i s FRS. 225.-

The r e s u l t s i n t r e a t i n g 500 1

Under these

The investment

- 15 -

400

350 - 300 - 250 -

2 200-

150 - 100 - 5 0 -

0

($150.00). figure 1x1 permitted a writeoff of the existing treatment system because of the favorably lower investment and maintenance costs as comoared to the conventional systems.

The adoption of this solution (according to the outline in

8

8 ENTER R-50 + EXIT R - 5 0

8 8 8 8

8

t + + + + + + I I I

0 2

95% EmClENCT

t

PRECJPITATK)N

95% RECOVERY - x G r - -

I I I I

t 0.5 kg Of HYDROXIDE WOCE

I I

FIG. IX RECOVERY OF METAL BY IN LINE ELECTROLYSIS OF RINSE BATHS

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4.2 E l im ina t ion o f Come r from Rinse Baths

I t i s p a r t i c u l a r l y e f f e c t i v e t o i n s t a l l cathodes on the s t a t i c r i n s e tanks which f o l the e tch ins tanks f o r Dr in ted c l r c u i t s . The

c e l l s w i t h r e t c u l a t e ow the p l a t i n g baths o r o b j e c t i s t o s g n i f i c a n t l y

A shop which d a i l y reduce the-volume o f co per hydroxide sludge

average 3 kg (6.6 1bs.I o f metal i n the r i n s e baths. This metal i s recovered i n the waste treatment system as 30 t o 50 kg (66 t o 110 l b s ) o f hydroxide sludge. recovery o f 3 kg (6.6 l b s ) copper a t the source of emission i n a very dense form.

t r e a t s 300 m2 (3,228 ft s I o f p r i n t e d c i r c u i t s produces on the

E l e c t r o l y s i s a t the s t a t i c r i n s e tank permits the

The savings inc lude:

Reduction o f the labor necessary f o r t r e a t i n g the r i n s e baths.

Reduction of the labor a t waste t reatment sys tem and a l s o less wear and t e a r on the equipment.

Reduction i n sludge disposal costs.

Reduction o f the amount o f chemicals needed t o p r e c i p i t a t e the metal hydroxi des.

Reduction o f water consumption t o the r i n s e baths which corresponds t o an increase i n the l i f e of the s t a t i c r i n s e baths.

An increase i n the q u a l i t y o f the r i n s e per u n i t volume o f water consumed.

Other ApDl icat ions

I t i s o f i n t e r e s t t o consider o ther a o o l i c a t i o n s where e l e c t r o l y s i s provides a s o l u t i o n t o the problem o f waste management:

* Contro l of the concentrat ion of z i n c i n a cyanide bath. e l e c t r o l y z e r i s mounted i n a c lose c i r c u i t manner on a f o u r cubic m e t e r bath. The e l e c t r o l y z e r i s fed a constant c u r r e n t t o remove the q u a n t i t y o f z i n c corresponding t o the increase i n z i n c i n the p l a t i n g bath due t o the f a c t t h a t the d i s s o l u t i o n of the z inc anode i s more r a p i d than t h e q u a n t i t y needed t o p l a t e the pieces being t reated.

The

This p r i n c i p l e was adopted because i t e l im ina ted the disposal o f the concentrated bath a f t e r p l a t i n g by keeping i t a t the optimal working concentrat ion.

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Small e l e c t r o l y s i s c e l l s can be i n s t a l l e d i n c losed c i r c u i t i n s t a t i c r i n s e baths a t go ld and s i l v e r p l a t i n g shops. The precious metal i s cont inuously recovered t o reduce the turnaround t i m e of the metal and permits b e t t e r metal management.

Se lec t ive deposi t ion i s i l l u s t r a t e d by the separation, a t c o n t r o l l e d p o t e n t i a l , o f 30 mg/l o f copper which i s poisoning a bath conta in ing 30 g / l of paladlum. By main ta in ing a c o n t r o l l e d p o t e n t i a l between the electrodes a s e l e c t i v e deposi t l o n of copper i s obtained which i n d e f i n i t e l y prolongs the l i f e of the paladium bath ( f i g u r e X I .

*

Cu / Pd AMMONIATED SOLUTlONS ;; j g

1 .o 2.0 3.0 4.0

VOLTS

FIG. X SELECTIVE RECOVERY OF COPPER FROM A PALLADIUM BATH

..

* The p o s s i b i l i t y of coupl ing the recovery o f metal w i t h a usefu l anode reac t ion f o r water treatment has n o t been discussed i n t h i s a r t i c l e . I t i s known, however, t h a t c e r t a i n electrodes such as ruthenized t i t a n i u m o r p l a t i n i z e d t i t a n i u m have an o x i d i z i n g a c t i o n on anions such as cyanide and c o n t r i b u t e t o t h e i r d e s t r u c t i o n dur ing the e lec t rodepos i t ion of metal which takes p lace a t the cathode (5).

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,

5. Go nclusionf

The application of electrochemical techniques in the area of metal The recovery and recycling of metals has been known for a long time.

recent developments in the structure of the electrodes and the design of the electrolyzers has lead to the appearance of a number of growing applications in the area of dilute effluent treatments. Many factors have come together which favor the utilization o f electrochemical systems:

-- A quantification of the legal limits of metal waste which are also expressed as the total concentration of metal in a given volume of waste materi a1 .

-- A growing difficulty of disposing o f heavy metal sludges.

-- A necessity to manage surface treatment shops by recycling water and waste products.

The electrochemical techniques allow one to bring original solutions which are on a level wl th state-of-the-art water treatment methods and which offer balanced economic solutions to the treatment problems.

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B i bl i OuraDhY

( 1 ) Sioda. et al. Flowthrough Porous Electrodes. Chemical Engineering. Feb 21 pp 57-67. (1983)

( 2 ) Konicek M.G. , Platek G. Reticulated Electrodes Cell Removes Heavy Metals from Rinse Waters. New Materials and Processes.

Procede Electrochimique de Recuperation. Trai tements de Surface. No. 166. Fev. (1979). b) Coeuret F.et Storck A . Elements de Genie Electrochimique. Ed. Lavoisier Tech. & Doc. Paris 1984.

Electrodes volumiques dispersees. Revue generale des electriciens Tome 88, no 12. Dec. 1979 pp. 959-967.

(5) Branchick K.J. et al. US Patent No 811286551. Device for Waste Water Treatment.

(6) Baring N . E . Nickel-Cadmium Batteries Recycling. Proc. Fourth. Int. Cad. Conference. Munich 1983. Cadmi um A s s . London, 1 983, pp 58-60.

The performances of Electrochemical Reactors removal of silver from Photographic Process Liquors. IChemE Symposium Series No. 98.

Vol. 2. pp 232-235 (1983) ( 3 ) a) Doniat D.

ISBN 2-85206-243-7 (4 ) Jaccaud M.

(7) Wal sch C. F.

4075P

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