platina elektroliza

Recovery o f Platinum f rom Concentrated

Sodium Chloride Brine by

Electrodeposition on Vitreous Carbon

J. E. Harrar and F. 8. Stephens

January, 1984

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

Contents

A b s t r a c t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Exper imental Apparatus and Procedures . . . . . . . . . . . . . . . . 3

Reagents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Apparatus and Procedures f o r Voltammetry . . . . . . . . . . . . . Apparatus and Procedures f o r P la t inum . . . . . . . . . . . . . . 4

4

Recovery Exper iments

R e s u l t s and D iscuss ion o f Vol tammetr ic Measurements . . . . . . . . . 8

R e s u l t s and D iscuss ion o f P la t inum . . . . . . . . . . . . . . . . . . 15

Recovery Exper iments

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Recovery of Platinum from Concentrated Sodium Chloride Brine

by Electrodeposition on Vitreous Carbon

J. E. Harrar and F. B. Stephens

Abstract

The voltammetric characteristics of Pt( 11) and Pt( IV) have been

examined at a vitreous carbon electrode i n slightly acidic 3 4 ( ~ 1 5

wt.%) NaCl solutions. Pt(1V) is reduced to Pt(l1) at %O V and Pt(I1)

is reduced to Pt(0) at -0.5 V vs. Ag-AgC1.

platinum metal at -0.5 V is very low on bare carbon, but increases as the

coverage o f platinum increases.

evolved in this medium is -0.85 V. A technique has been tested for

the removal o f sub-part-per-million levels of platinum from the

high-salinity brine by controlled-potential electrolysis using a

reticulated, vitreous-carbon, flow-through electrode. However, at

control potentials negative enough to begin to electrodeposit the

platinum at a significant rate, simultaneous reduction of hydrogen ion

reduces the current and energy efficiency t o an unacceptable level.

The rate of deposition of

The potential at which hydrogen is

Introduction

In a previous study (Harrar and Raber, 1983), in which a number of 6

natural brines were analyzed for precious and strategically-valuable

metals, it was found that one of the brines from the Salton Sea t

geothermal field contained approximately 50 pg/kg o f platinum.

Although this concentration i s very low, the large volumes of fluid that

are currently processed for energy extraction makes such a resource

-1 -

attractive if the precious metal could be economically extracted. The

brines of the Salton Sea geothermal field (or KGKA = Known Geothermal

Resource Area) are of extremely high salinity (15-20 wt % salts), and are

composed principally of sodium, potassium, and calcium chlorides.

The fluid that would most likely be processed for metals recovery

would be the relatively low temperature (<90°C) effluent brine from the

power plant facility, after the principal flash stages for energy

extraction.

high-temperature crystallizers for solids removal are currently being

tested, and valuable metals recovery is certainly possible at the higher

temperatures (>200°C) of the wellhead brine.

previous studies in geothermal mineral recovery can be found in our

However, different geothermal plant designs involving

A literature survey of

previous r e p o r t ( h a r r a r and Raber, 1983) and a paper by Maimoni (1983) .

To continue our investigations, two approaches have been examined for

removal of platinum from high-salinity geothermal brines. One involved

the use of various solid chemical media that were known from previous

studies in dilute waters to be effective in precious metal removal. Most

of these media had been tested previously only for non-precious-metal or

gold and silver removal (Acton, 1982) and in low-ionic-strength, low

temperature solutions. The results of this study are being reported

el sewhere (Raber, Thompson, and Gregg 1984).

The second technique examined for platinum recovery was electrolytic

For some of the elements, * I

deposition on a substrate of vitreous carbon.

electrodeposition of the metals on inert electrode materials is performed

for both metals recovery (Habashi, 1982; Ettel and Tilak, 1981; Coeuret,

1980), pollution control (Bennion and Newman, 1972; Kuhn, 1972) and for

trace metals analysis (Murthy, Holzbecher, and Ryan, 1982). However, not

only is there no knowledge of the conditions necessary for quantitative

-2-

electrodeposition of trace amounts of platinum from aqueous solutions in

general, but there is also no information on the electrochemistry of

p 1 at i num i n hi gh-chl ori de medi a.

Accordingly, our first objective was t o measure the current/electrode-

potential characteristics of platinum in solutions of NaCl at

concentrations simulating those of the high-salinity geothermal brines. a

Then, using these data, a series of experiments were carried out to

attempt to quantitatively electrodeposit platinum from a flowing stream

of brine. The electrode material selected was vitreous or "glassy"

carbon. This material was selected because of its relative inertness,

and because, for the flowing-stream experiments, it could also be

obtained in the form of reticulated-vitreous carbon (RVC) . This material

(see Wang, 1981) is commercially available and is configured as a rigid,

porous matrix that can be used advantageously in processes requiring

large electrode surface area. Both the current-potential (voltammetric)

and the f low-recovery measurements were performed using 3-electrode

controlled-potential techniques so that the applied cathode potentials

would be precisely known.

Experimental Apparatus and Procedures

Reagents. For the voltammetric measurements, the brine solutions

were prepared from J. T. Baker Ultrex-grade NaCl salt. It was found that

Mallinkrodt ACS reagent-grade NaCl contained sufficient heavy-metal *

impurities to raise the background current to an undesirably high level

in these concentrated solutions. For the recovery experiments, however, i

the larger volumes o f solution precluded use of the Ultrex-grade salt, so

the reagent-grade salt was used.

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Plat inum( 11) and p l a t i n u m ( I V ) s o l u t i o n s were prepared by d i s s o l v i n g ,

r e s p e c t i v e l y , (NH4)2PtC14 and (NH4)2PtC16 s a l t s i n water. These s a l t s

were ob ta ined f rom Engelhard I n d u s t

compounds. The concent ra t ions of p

assays o f t h e compounds as r e p o r t e d

p l a t i n u m recovery experiments, a so

0.002 - M HC1 was used.

t ies as n o m i n a l l y h i g h - p u r i t y

a t inum were c a l c u l a t e d f r o m t h e

by t h e manufacturer. For t h e

u t i o n c o n t a i n i n g 0.168 mg P t / m l and

Apparatus and Procedures f o r Voltammetry. To i n v e s t i g a t e t h e

vol tammetry o f p la t inum, equipment c o n s i s t e d o f an EG&G Pr ince ton A p p l i e d

Research Model 174A Po larograph ic Analyzer, Model 303 S t a t i c Mercury Drop

E l e c t r o d e Assembly, and a Houston X - Y recorder .

carbon e l e c t r o d e was s u b s t i t u d e d f o r t h e mercury c a p i l l a r y i n t h e c e l l

assembly.

p laced i n t h e c e l l , and 1 m l o f a s tock s o l u t i o n o f e i t h e r P t ( I 1 ) o r

Pt (1V) was p i p e t t e d i n t o t h e c e l l t o g i v e a p l a t i n u m c o n c e n t r a t i o n o f 2 x

A model GO173 g l a s s y

To prepare a s o l u t i o n f o r voltammetry, 5 m l o f b r i n e was

lo-' - M.

S o l u t i o n s were deoxygenated b y b u b b l i n g h i g h - p u r i t y n i t r o g e n through them

f o r a t l e a s t 4 min. p r i o r t o t h e vo l tammetr ic scans. N i t r o g e n was f lowed

over t h e t o p of t h e s o l u t i o n s d u r i n g t h e scans.

made a t an ambient temperature o f 22-25OC.

The pH o f t h e c e l l s o l u t i o n s was i n t h e range o f 4 t o 6.

A l l measurements were

Apparatus and Procedures f o r P la t inum Recovery Experiments. The f l o w

e l e c t r o l y s i s c e l l shown i n F i g u r e 1 was used f o r t h e recovery

experiments.

p rev ious designs ( S t r o h l and Curran, 1979; Wang and Dewald, 1983) a re t h e

dua l counter - and r e f e r e n c e e l e c t r o d e s and t h e use o f low-temperature

(140OC) heat -shr inkab le T e f z e l t o encase t h e RVC. The lower temperature

- 4-

Features o f t h i s f low- th rough R V C c e l l t h a t d i f f e r from

Solution Sol ut ion outlet inlet

Figure 1. Flow electrolysis cell for removal of platinum from simulated geothermal brine. 1. Reticulated vitreous carbon working electrode. 2. Heat-shrinkable Tefzel tubing. 3. Glassy carbon rod counter electrodes. 4. Ag-AgC1 reference electrodes. 5. Ace threaded-glass connectors. 6. Teflon plugs. 7. Viton O-Rings. 8. Zirconium electrical connection to working electrode.

would min imize chemical m o d i f i c a t i o n o f t h e R V C d u r i n g f a b r i c a t i o n o f t h e

assembly.

I ~ V C and e f f e c t b e t t e r c o n t r o l o f i t s p o t e n t i a l . I n opera t ion , each

counter e l e c t r o d e and each r e f e r e n c e e l e c t r o d e i s connected e x t e r n a l l y t o

i t s t w i n .

The dual e l e c t r o d e s reduce t h e p o t e n t i a l g r a d i e n t th rough t h e

The work ing-e lect rode c y l i n d e r o f K V C was c u t b y means o f a c o r k

b o r e r f r o m p ieces o f R V C ob ta ined f r o m Energy Research and Generation,

Inc. Two grades o f R V C were t e s t e d : 45s ( s45 pores/ in . and 800 f t /ft

2 3 s u r f a c e area) and 100s ($100 pores/ in . and 2000 f t /ft sur face area) .

2 3

For t h e 45s-grade m a t e r i a l , our work ing e l e c t r o d e volume o f $4.5 cm

would have an e f f e c t i v e s u r f a c e area of 118 cm . By comparison, t h e

2 s u r f a c e area o f t h e vo l tammet r ic work ing e l e c t r o d e was 0.096 cm . 2

The counter e l e c t r o d e s were rods o f g l a s s y carbon ob ta ined f r o m

Polycarbon, Inc.

t h e t o p w i t h an i n s u l a t i n g f i l m o f N a r l i n e r s t r i p p a b l e f i l m , and

conver ted t o AgCl a t t h e t i p b y immersion i n aqua r e g i a .

bodies were f a b r i c a t e d f r o m Ace Glass Co. s i z e 15 threaded connectors,

and t h e R V C e l e c t r o d e was h e l d i n p l a c e b y T e f z e l tub ing , which, as

mentioned, c o u l d be shrunk a t t h e r e l a t i v e l y low temperature o f 14OOC.

The r e f e r e n c e e l e c t r o d e s were rods o f s i l v e r , coated a t

The g l a s s c e l l

A new R V C e l e c t r o d e was used f o r each exper iment; a f t e r reassembly b y

heat s h r i n k i n g t h e T e f z e l o n t o t h e RVC, and i n s e r t i n g t h e T e f l o n plugs, a

needle was used t o p i e r c e t h e T e f z e l and then a 0.5-mm-dia z i rcon ium w i r e

was i n s e r t e d as shown i n F ig . 1 t o make e l e c t r i c a l connect ion t o t h e

RVC.

sealant . A Technicon AutoAnalyzer p e r i s t a l t i c pump and a p p r o p r i a t e s i z e s

of Tygon t u b i n g were used t o pump b r i n e th rough t h e e l e c t r o l y s i s c e l l .

Connection t o t h e p o r t s of t h e c e l l were made v i a Cheminert f i t t i n g s .

The b r i n e was f lowed th rough an i n - l i n e , 1-pm Gelman f i l t e r b e f o r e

The w i r e was t h e n sealed a t t h e o u t s i d e w i t h Dow-Corning 3145 RTV

- 6-

B

entering the cell.

was pumped through the cell consisted of a 2-liter FEP Teflon bottle.

The cap of the bottle was fitted with a fritted-glass gas-dispersion tube

for deoxygenation of the solution, and a section of Teflon tubing

reaching to the bottom of the bottle for withdrawal of the solution.

The reservoir for the platinum-containing brine that

This tubing was connected to the pump tubing via a 3-way Teflon-plug

stopcock so that the cell could first be flushed with brine that did not

contain platinum. e

At the beginning of each experiment, exactly 1-liter of brine was

placed in the 2-liter reservoir, 1.000 ml of the stock Pt(I1) solution

containing 168 pg Pt was pipetted into the brine and mixed. The pH of

this test solution was 5.1.

purity nitrogen, and then, while the flow of nitrogen continued, brine

without platinum was pumped through the cell, most of the air bubbles

were removed by tapping the cell, and the working electrode was

The solution was deoxygenated with high-

preconditioned for a few minutes by polarization at the test reduction

potential. Then, without interrupting the electrolysis current, and

after at least 15-min. deoxygenation, the stopcock was turned to begin

the flow of Pt-containing brine to the cell.

The potentiostat was an EG&G Princeton Applied Research Model 173

with Model 176 Current Follower.

(bel) Model 731 Uigital Integrator. All experiments were carried out at

Currents were integrated using an ECO

b an ambient temperature of 22-25OC.

After the electrolysis, the Tefzel tubing surrounding the RVC

electrode was cut, and the KVC was flushed thoroughly and quickly with

water.

vial and dried in a desiccator.

i'

The KVC sample was then placed in a HN03-cleaned polyethylene

The samples were analyzed for platinum

by neutron activation analysis by General Activation Analysis, Inc. (San

-7-

Diego) u s i n g t h e f o l l o w i n g procedure. The samp

standards were i r r a d i a t e d f o r 30 min. i n a T R I G

a t a f l u x o f 1.8 x lo1‘ n/cm -sec. The element 2

es p l u s comparator

L Mark I n u c l e a r r e a c t o r

p l a t i n u m produces gold-199

w i t h a h a l f - l i f e o f 3.15 days.

were counted on a Ge(L i ) d e t e c t o r coupled t o a mul t i channe l gamma-ray

spectrometer.

0.5 p g .

A f t e r a decay o f n i n e days, t h e samples

The l i m i t o f d e t e c t i o n o f p l a t i n u m i n t h e R V C samples was

Resu l ts and D iscuss ion o f Vol tammetr ic Experiments

The predominant species o f p l a t i n u m t h a t would be present i n t h e 2- 2- a r e PtC16 o r P t C I 4 , depending i n

o r +2 o x i d a t i o n s t a t e . Reduct ion

concent ra ted-ha l ide geothermal b r i n e s

whether p l a t i n u m i s present as t h e +4

o f t h e these species a t a cathoae wou d i n v o l v e t h e r e a c t i o n s :

P t C l 6 ’ - + 4e- - P t meta l + 6 C1- ( 3 )

React ions ( 1 ) and ( 2 ) would r e p r e s e n t t h e e l e c t r o d e p o s i t i o n process if i t

occur red as a s tepwise r e d u c t i o n of t h e P t ( I V ) th rough P t ( I 1 ) t o P t ( 0 ) ;

o r React ion ( 3 ) would represent t h e process i f P t ( I V ) were reduced

d i r e c t l y t o t h e metal .

P t ( 1 I ) t o t h e metal.

React ion ( 2 ) corresponds t o t h e r e d u c t i o n o f

There i s some disagreement i n t h e l i t e r a t u r e as t o t h e exac t values

of t h e s tandard p o t e n t i a l s of React ions 1 t o 3 ( M i l a z z o and C a r o l i , 1978;

L l o p i s and Colom, 1976; Lingane, 1958). However, a l l o f t h e values f o r

c h l o r i d e media l i e between +0.5 and +0.75 V v s . t h e s tandard hydrogen

e l e c t r o d e (SHE), and i t m i g h t be expected t h a t t h e e l e c t r o d e p o s i t i o n o f

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platinum could be effected at quite positive, or at least relatively low

negative potentials. Unfortunately, however, both polarographic and

voltammetric studies (Llopis and Colom, 1976, et seq.) have revealed that

a rather large overpotential is associated with these reactions at

practical current densities. Although there are no reported

investigations of the voltammetry of platinum using carbon electrodes, or

in strong halide media, several studies of the reduction of platinum

chlorocomplexes at platinum metal electrodes (Llopis and Colom, 1976) b

have shown that reactions (l), (Z), and ( 3 ) take place in the vicinity of

-0.2 to -0.4 V vs. SHE. Accordingly, we anticipated in our studies with

vitreous carbon that we would also be dealing with this electrode

potential region. A practical difficulty with this characteristic is

that hydrogen ion is also reduced (to H2 gas) in this potential region,

and this process would tend to obscure and interfere with the platinum

electrodeposition to a greater extent as the solution pH is lowered. Our

hope, therefore, was to demonstrate that platinum could be quantitatively

deposited from solutions under typical hypersaline geothermal brine

conditions, without consumption of a large fraction of the current by

hydrogen ion reduction.

Our examination of the voltammetry of platinum was limited in the

3 - M ($15 wt % ) present work to one specific set of brine conditions:

WaCl, a pH range of 4 to 6, and ambient ( 2 2 - 2 5 O C ; ) temperatures. Higher

temperatures such as those typical o f real geothermal fluids would not be

expected to change the relevant electrode potentials or reaction

characteristics significantly, thus it was felt that ambient temperature ("

tests would be sufficient to determine whether the electrolytic approach

was promising. Atmospheric-flashed hypersaline brine has a pH of 5 to 6,

thus the acidity conditions of these experiments are typical of those

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found in the field.

chlorocomplexes, any chloride concentration in the range of 0.2 to 4 - M

probably would yield about the same characteristics for the platinum

species electrochemistry. What is not definitely known is the oxidation

state of the platinum in the real geothermal brine, but because the brine

is anoxic, contains H2S, and is generally mildly reducing even after

loss of the H2S through flashing, the platinum is probably present as

Pt(I1).

briefly investigated to learn the effect of the platinum speciation on

the electrodeposition process.

Because of the stability of the platinum

However, the characteristics of both Pt(1V) and Pt(I1) were

Figure 2 shows the results of a series of voltammetric scans of a

solution of Pt(I1) as PtClt- in 3 - M NaC1. Note that this solution of

platinum corresponds to 400 ppm Pt vs. the 50 ppb Pt or less that might

be present in a real geothermal brine.

higher concentrations of platinum to study its electrochemical

characteristics because the typical background currents in voltammetry

would completely obscure the platinum reduction at concentrations below

1 ppm.

starting with a clean, bare, glassy-carbon electrode.

completed, the potentiostat was disconnected and the solution stirred

briefly.

Very little reduction of the Pt(I1) is evident at first, but as platinum

metal accumulates on the electrode (in effect making it a platinum

electrode), it becomes easier to reduce the Pt(I1) species, and the

height of the current peak seen in Fig. 2 increases.

of reduction of Pt(I1) on a pure glassy carbon surface is very low, thus

it would be necessary in an efficient recovery process to first

predeposit platinum on the electrode to ensure rapid subsequent

deposition of the platinum to be recovered.

It is necessary to use these

Figure 2 shows a series of successive scans of potential,

As each scan was

Then the succeeding scan was initiated with a quiet solution.

Apparently the rate

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?

a

i

T 100 p A

I

i

I I 1 I 1 I 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

Potential, V vs. Ag-ASCI

F i g u r e 2. RedGction o f P t ( I 1 ) t o P t ( 0 ) on a g lassy carbon e l e c t r o d e i n 3 - M (s15 wt.%) NaC1.

P t ( I r ) = 2 x 10-3 - M Scan Rate = 100 mV/sec

-11-

The p o t e n t i a l r e g i o n where P t ( I 1 ) i s reduced i s i n general agreement

w i t h p rev ious work u s i n g p l a t i n u m e l e c t r o d e s as no ted above.

Ag-AgC1 re fe rence e l e c t r o d e i s +0.2 V vs. t h e SHE).

r i s e i n c u r r e n t a t -0.85 V (F ig . 2) i s due t o t h e r e d u c t i o n o f hydrogen

i o n t o hydrogen gas, thus i t i s r a t h e r c l o s e t o t h e p o t e n t i a l r e q u i r e d t o

reduce t h e P t ( 11).

(The

The n e a r l y v e r t i c a l

The p l a t i n u m meta l t h a t has been depos i ted on t h e g l a s s y carbon

e l e c t r o d e can be removed b y e l e c t r o l y t i c s t r i p p i n g as shown b y t h e curves

o f F i g . 3. I n these experiments, t h e p l a t i n u m was depos i ted f o r a g i v e n

l e n g t h o f t i m e b y h o l d i n g t h e e l e c t r o d e a t -0.7 V w i t h a g i t a t i o n o f t h e

s o l u t i o n b y t h e b u b b l i n g o f n i t r o g e n .

rep laced by 3 - M b r i n e n o t c o n t a i n i n g P t ( I I ) , and deoxygenation was

c a r r i e d o u t f o r 4 min. w h i l e t h e p o t e n t i a l o f -0.7 V was mainta ined on

t h e e lec t rode. Then t h e q u i e t , deoxygenated s o l u t i o n s were scanned f r o m

-0.7 t o +0.5 V w i t h t h e r e s u l t s shown i n F ig . 3.

p l a t i n u m can be removed i n t h i s manner migh t be u s e f u l i n an e l e c t r o l y t i c

recovery process because t h e carbon e l e c t r o d e m a t e r i a l would n o t have t o

be dest royed and c o u l d be reused.

The s o l u t i o n i n t h e c e l l was then

The f a c t t h a t t h e

2- F i g u r e 4 shows t h e behav io r of Pt (1V) as PtC16 i n t h e h i g h - s a l i n i t y

b r i n e . No background c u r r e n t c u r v e i s shown; i t i s t h e same as t h a t i n

F ig . 2. The curves f o r p l a t i n u m r e d u c t i o n were ob ta ined i n a s e r i e s of

s t a r t i n g w i t h a bare carbon sur face, and again show t h e

r e d u c t i o n o f P t ( I 1 ) as p l a t i n u m meta l b u i l d s up on t h e

r s t peak i n F ig . 4, which i s due t o t h e r e d u c t i o n of

i s a p p a r e n t l y n o t s e n s i t i v e t o t h e surface composi t ion

No f u r t h e r work was done t o unravel t h e c o n t r i b u t i o n

o f t h e two consecut ive r e a c t i o n s t o t h e c u r r e n t - p o t e n t i a l c h a r a c t e r i s t i c s ;

i t i s e v i d e n t t h a t t h e c o n s t a n t - p o t e n t i a l e l e c t r o d e p o s i t i o n c o n d i t i o n s

-1 2-

consecut ive scans

increased r a t e o f

e lec t rode. The f

Pt(1V) t o P t ( I I ) ,

o f t h e e lec t rode.

P

‘a

i

0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 .o Potential, V vs. Ag-ASCI

Figure 3. Anodic stripping of platinum from a glassy carbon electrode in 3 - M (c15 wt.%) NaC1.

Scan Rate = TOO mV/sec

Peak 1:

Peak 2:

After 4 min deposition from Pt(I1) solution

After 20 min deposition from Pt(I1) solution

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0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2

Potential, V vs. Ag-AgCI

F i g u r e 4. Reduct ion o f P t ( I V ) t o P t ( I 1 ) and P t ( 0 ) on a g l a s s y carbon e l e c t r o d e i n 3 - M (*15 wt .%) NaC1.

P t ( 1 v ) = 2 x 10-3 - M

Peak 1: P t ( I V ) + P t ( I 1 ) Peak 2: ( P t ( I 1 ) .+ P t ( 0 )

Scan Rate = 100 mV/sec

- 14-

must be selected to ensure that Pt(I1) is reduced, whether the geothermal

platinum is present as either Pt(I1) or Pt(1V).

Results and Discussion of Platinum Recovery Experiments

Based on the results of the voltammetric measurements, it is obvious *

that the control potential would have to be at least as negative as

i -0.6 V vs. Ag-AgC1 for complete electrodeposition of the platinum. The

two complicating factors are the need to predeposit platinum t o

"sensitize" the carbon electrode and the proximity of the platinum

reduction wave to that of hydrogen ion. A series of electrolytic

recovery experiments were performed, first at -0.60 V to examine the

effect of RVC porosity and flow rate, and then at more negative

potentials. The results are presented in Table 1.

These results show that the recovery of platinum was not very

sensitive to the porosity (surface area) of the RVC, but was influenced

significantly by the flow rate. The lowest flow rate, 1 ml/min, was

comparable to the flow rate found by Strohl and Curran (1979) to be

necessary for quantitative electrolysis of several other species using

t h e same porosity and similar size electrode o f RVC. No preplating o f

platinum on the electrode was performed before the experimental runs,

thus some of the lower recoveries found for the faster flow rates was due

t o this. At the lowest flow rate, ~ 1 7 h was required for a run; this

was believed to include enough time for.sensitization of the electrode

'I without loss of a significant fraction o f the platinum.

The data in Table 1 also show that, even at the most negative

potential, -0.80 V, the % recovery of the platinum was still relatively

poor.

reference electrodes, the actual potential on the interior of the RVC

It is known that, because of the positioning of the counter and

-1 5-

Table 1. Recovery o f P la t inum f rom NaCl B r i n e by E l e c t r o l y t i c Reduction.

P t Taken: 168 pg as PtC14 2- NaCl : 3 - M (%15 w t % ) P t Concentrat ion: 168 pg/ l i t e r pH: 5.1

Working E lec t rode

P o t e n t i a l , V vs Ag-AgCl

none (b lank) -0.60

none (b lank ) -0.60

none (b lank ) -0.60

none (b lank ) -0.60

non (b lank ) -0.60

none (b lank ) -0.60

-0.75 -0.80

R VC

P o r o s i t y

Grade I

4 5s 4 5s

100s 1 00s

4 5s 4 5s

100s 100s

4 5s 4 5s

100s 100s

100s 100s

Flow

Rate

m l / m i n

11.5 11.5

11.5 11.5

3.9 3.9

3.9 3.9

1 .o 1 .o

1 .o 1 .o 1 .o 1 .o

P t Recovered

<o. 5 2.0, 1.8, 1.9

<o. 5 1.6, 1.3

(0.5 5.2,4.9

<O. 5 7.4. 7.8

(0.5 19,11

(0.5 12912

22 29,33

%

Recov e ry

1.1

0.8

3.1

4.5

9

7

13 18

\

'6

-1 6-

matrix is less negative than the control potential at the exterior edges

(Fleischmann and Oldfield, 1971).

employing a significantly more negative control potential. However, in

this instance we must try to establish as negative a potential as

possible for complete platinum reduction on the interior surfaces of the

RVC, while minimizing hydrogen ion reduction and gas evolution on the

edges. At the pH of this brine, 5 to 6, and conceivable, practical

electrolysis cell designs, this would be very difficult to achieve.

In general, this can be compensated by

A control potential still more negative than -0.80 V could be used,

but then additional electrolysis current would be wasted. In our

experiments, initial currents were 4-8 mA at -0.60 V and 15-30 mA at

-0.80 V. In every experiment, the current decayed to ~ 0 . 5 mA at the

end of the electrolysis.

"desensitization" is not known; however, it should be noted that many of

the impurities in the NaCl that was used to prepare the brine also would

be codeposited with platinum under these conditions.

measured total charge that was passed, at the flow rate of 1.0 ml/min,

the average current was ~ 0 . 7 mA at -0.60 V and ~2 mA at -0.80 V.

comparison, only -2 PA would be consumed in quantitatively depositing

163 ug of platinum, thus the current efficiency of the process here was

very low. Real geothermal brines containing additional trace metals

would require still more current; however, if the recovery were carried

out using controlled-potential techniques, only Pb( 11), which is found at

-50-100 mg/l concentrations in hypersaliie geothermal brines, would be

important. Copper, gold, silver, and cadmium, if present, also would

codeposit; but manganese and zinc, which are present n high

concentrations in some brines, woula reqJire potentia s more negative

than -1.0 V vs. Ag-AgC1 (Kolthoff and Liligane, '1952).

The reason for this decay or apparent

Based on the

By

-1 7-

Conclusions

Our objective in these experiments was to determine whether platinum,

as the chlorocomplex in high-salinity brine, could be deposited

electrolytically as the metal on a relatively inexpensive substrate with

reasonable current efficiency. Reticulated vitreous carbon was found to

be a convenient material to use as a working electrode, and

electrodeposition o f the platinum was clearly observed at millimolar

concentrations (a few hundred ppm). However, the process was found to

require a significant overpotential, especially at the uncoated carbon

surface. For the slightly acidic solutions typical of the hypersaline

geothermal brine, the required control potential is close to that o f the

reduction of hydrogen ion.

process were thus unacceptably low for the very low concentration level

(<1 ppm) found in the geothermal brine.

The current and energy efficiency of the

If the pH of geothermal brine that could be used as the input to the

recovery process were higher, e.g., neutral or slightly alkaline, or if

the concentrations of platinum in the brines were higher, then the

electrodeposition technique would become much more favorable.

negative control potential could then be used to obtain quantitative

deposition of the platinum, and the interference by hydrogen-ion

reduction would be lessened.

brine in a processing plant by the addition of alkali, merely to

facilitate platinum recovery, would not, however, prove to be economical,

because of the amount of alkali that would be required.

electrolytic removal o f platinum had been successful, it is probable that

the platinum could also be stripped from the carbon electrolytically by

changing the control potential, thus avoiding destruction o f the

deposition substrate.

A more

Deliberately raising the pH of a geothermal

I f the

-1 8-

c

It appears that the activated charcoal extraction technique

investigated by Raber, Thompson, and Gregg (1984) should be the method

o f choice for precious metal recovery.

Acknowledgement 4

H. C. Crampton assisted in the fabrication o f the flow-through

electrolysis cell. L

0

-7 9-

References

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Lingane, J.J., 1958. Electroanalytical Chemistry, 2nd Ed., Interscience,

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b

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-21 -

Strohl, A.N., and Curran, D. J., 1979. Reticulated Vitreous Carbon

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