If:! -,

:\'0: ,

",,' J"",,;,,.',,/'+,,7

,,',

'::Ji~r.,;:i;

Synopsis

Selective photore-duction, using thesemiconductor TiOzunder UV irradiation,has been investigatedas a possible recoveryand/or separationmethodfor the PGM'srhodium, palladium andplatinum. The dfect qfpH value and concen-tration qf the sacrljidalredudng agent(ethanol) on the rate qfreducdon qfRh(Ill),Pd(ll) and Pt(IV) ionshave been determinedseparatelY for these ionsas well asfor somemixtures. Conditions

favourable for thereducdon andadsorption (or predpi-tation) on TiOz havebeen established. It hasbeenfound that alreadydeposited Pt or Pd metalpromotes reduction qfRh(IlI) ions at a pHvalue where it wouldnot otherwise bereduced. A method forthe separation qf amixture comprisingRh(IlI), Pd(II) andPt (IV), based onphotoreduction andadsorption on the TiOz,is proposed.

.Department ifChemistry, University ifStellenbosch

@ The South Afn'can

Institute if Mining and

Metallurgy, 1994. SAISSN 0038-223X/3.00

+ 0.00. Paper receivedJul. 1994; revised paperreceived Nov. 1994.

Separation and recovery of someplatinum group metals (PGMs) by meansof selective photocatalytic reductionby R.J. Kriek, W.J. Engelbrecht, and J.J. Cruywagen *

Introduction

The economic importance of the platinum groupmetals (PGMs) for South Africa is well known.According to current estimates about 72 per centof the world's deposits (all PGMs) are found inSouth Africa and the country produces approxi-mately 60 per cent (286 tons per year) of theworld output of three of these metals, namelyplatinum, palladium and rhodium1,z. The PGMsare extracted from ores but, because of theirhigh value, are also recovered from industrialresidues of variable composition and quality.The development of new technology for theextraction, recovery and separation of thesemetals is therefore of special interest.

Important criteria in designing separationprocesses are, amongst other things, highseparation efficiency and minimum losses ofnoble metals. Ecological considerations are oftencrucial and can have a great effect on productioncosts. Current separation processes are quitecomplex and involve many steps, such as solventextraction, ion exchange, precipitation, crystal-lization and distillationz. The actual procedurefollowed, depends in most cases on thecomposition of the mixture. The solvents andother chemicals required in the separationtechniques are quite often costly. Considerationssuch as these have led us to investigate selectivephotocatalytic reduction as a possible separationand/or recovery method for the PGMs.

The Joumal of The South African Institute of Mining and Metallurgy

~---

The semiconductor TiOzwas used asphotocatalyst. Although TiOzis only a DVabsorber it is not expensive, insoluble undermost conditions, photostable and not toxic.Irradiation of TiOzresults in the production ofelectrons in the conduction band and positiveholes in the valence band which can bemanipulated to accomplish redox reactions. Inthe case of water purification, for example,organic pollutants are oxidized in the presence ofoxygen to form water, carbon dioxide andmineral acids3. The reduction of metal specieswith high redox potentials, on the other hand,can be accomplished in the presence of anysuitable oxidizable substance. The photodepo-sition of metals such as gold, silver, palladium,platinum, iridium and rhodium on TiOzduringDV irradiation has been reported by severalauthors4-7. The present paper deals with thephotocatalytic reduction of ions of three PGMs,namely rhodium (Ill), palladium(II) andplatinum(IV). Due to the exploratory nature ofthe work the systems investigated had to be keptas simple as possible. The characteristicphotochemical behaviour of each element wasfirst determined individually to establishconditions on which a separation procedure canbe based. However, when mixtures of the metalions were investigated, it was found that thereduction behaviours were different from thoseof the individual ions. For example, alreadydeposited platinum strongly promotes thereduction of rhodium (Ill), a factor whichimpedes selective reduction. The methodproposed for the recovery and separation ofplatinum (IV), rhodium (Ill), and palladium(II) isbased on selective reduction but it also makesuse of adsorption of species by TiOzfrom thesolution at a suitable pH value. The method wasconsidered to be particularly suitable forsolutions of very low concentrations and to havethe advantage of being simple and efficient.

Theoretical considerations

The difference between metals, semiconductorsand insulators is wellknown (Figure1)8.

MARCH/APRIL 1995 75 ~

Valence band Valence band Valence band

Metal Semiconductor Insulator

Separation and recovery of some platinum group metals (PGMs)

Eb = Band gap energy

Conduction band

Eb= 0

Conduction band

] Eb= Small

Conduction band

EFigure 1-Band gap energies in metals, semiconductors and insulators

Acknowledgements

Financial support by MINTEK

and the University of

Stellenbosch is gratefully

acknowledged. The authors

thank Infruitec for the use of

their ICP instrument.

hv

~). < 383 nm

CB = Conduction bandVB = Valence band

>- In metals the electrons completely fIll thevalence band and there is no energy gapbetween the valence and conductionbands. At absolute zero the conductionband is empty. Above absolute zero,electrons at the top of the highest occupiedlevels can gain thermal energy and moveinto the low-lying empty levels of theconduction band.

TiO2

j

CB

Bandgap(3.23 eV)

VB

Electron in CB

Positive hole in VB

Figure 2-Excitation of an electron in TiO2 on absorption of a photon with an energy greater

than 3.23 eV

~ 76

--- -nn

-- ------

MARCH/APRIL 1995

>- In insulators and semiconductors thevalence band is completely filled and anenergy gap exists between this band and

the next higher energy band. If the energygap is large, there is little chance for the

electron to be excited into the emptyconduction band and the material is aninsulator. If there is only a small energygap the electron is rather easily excited

into the conduction band thereby alsocreating a positive hole in the valenceband as an additional carrier and thematerial can now conduct electricity and iscalled an intrinsic semiconductor.

Anatase, one of the crystal structures oftitaniumdioxide (Ti02), is a common semicon-ductor photocatalyst. Anatase has an energyband gap of 3.23 eV which corresponds towavelength 383 nm9. Therefore, when asuspension of Ti02 is illuminated by light of awavelength equal to or shorter than 383 nm,excitation of an electron from the valance bandto the conduction band will take place producinga positive hole in the valance band and anelectron in the conduction band (Figure 2). Theelectron and positive hole consequently migrateto the surface of the Ti02 particle.

The electrons can then be donated to speciesadsorbed on the TiOrsurface, for example to thechloro-complexes of the PGMs thereby causingreduction. The positive holes on the other handcan accept electrons thereby causing oxidationof, for example, organic pollutants in watef.lO.

By changing the energy of the photons

beyond what is needed to cause excitation across

the band gap and/or by changing conditions ofthe solution in contact with the semiconductorparticles, selectivity could be accomplished.

In order to accomplish either an oxidation orreduction the rate of recombination of theelectron-hole pairs has to be reduced to such anextent as to allow utilization of both the electronand positive hole.

>- To perform an oxidation by using thepositive holes, available in the valenceband, a sacrificial oxidizing agent (SOA) is

needed to take up the electrons in theconduction band. Oxygen (02) is generallyused as SOA and leads to the formation ofoxygen radical anions, which through a

series of further reactions form hydroxylradicals10.

Ti02(e-) +O2 --- Ti02 +O2-

O2- --- --- --- --- --- HO.

The hydroxyl radical, with a standardreduction potential of + 2.85 eV, is a very strongoxidizing agent and then assists in the oxidizingprocessl1.

The Joumal of The South African Institute of Mining and Metallurgy

,

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Separation and recovery of some platinum group metals (PGMs)

References

1. CoWLEY, A. Platinum1994. Johnson Matthey.

2. RENNER,H. Ullmann'sEncyclopedia ifIndustrial Chemistry.VCHPublishers. 1992.vo!.A21,p.75.

3. MILLS,A., DAVlES, R.H.,and WORSLEY,D. Waterpurification by semicon-ducror photocatalysis.Chem. Soc. Reviews.1993. vo!. 22, p. 417,and references therein.

4. HERRMANN, J-M.,DISDlER, J., and I'1CHAT, P.

Photoassisted platinumdeposition on TiO, powderu1:ing various platinumcomplexes.f. PJrys. Chem.1986. vo!. 90, p. 6028,and references therein.

5. ISHITANI,0., IUSUE, C.,

SUZUKI,Y., and IBUSUKI,T.Photocatalytic reductionof carbon dioxide tomethane and acetic acidby an aqueous suspen-

sion of metal-depositedtitania. /. Photochem.Photobiol. A: Chem.1993. vo172, p. 269.

6. CURRAN, J.S., DOMENECH, J.JAFFREZIC-RENAULT, N., andPHILLlPE, R. Kinetics andmechanism of platinumdeposition by photo-electrolysis in illuminatedsuspensions of semicon-ducting titanium dioxide.

/. Phys. Chem. 1985.vo!. 89, p. 957.

7. BORGARELLO, E.SERPONE, N. Ewo, G.,HARRIS, R., PELlzzEm, E.,and MINERO, C. Light-induced reduction ofrhodium(III) and palla-dium(II) on titaniumdioxide dispersions andthe selective photochem-ical separation andrecovery of gold(III) ,platinum(IV) andrhodium(III) in chloridemedia.lnorg. Chem.1986. vo!. 25, p. 4499.

8. LAIDLER, K.J. andMEISER, J.H. PhysicalChemistry. The BenjaminlCummings PublishingCompany, Ine. 1982,p.732.

9. KAISE,M., KoNDOH,H.,NISHlHARA, c., NozoYE, H.,SHINDO,H., NIMURA, S.,and KIKUCHI,O. Photo-catalytic reactions ofacetic acid on platinum-loaded TiO : ESRevidence of radicalintermediates in thePhoto-Kolbe reaction.

f. Chem. Soc. Chem.Commun. 1993, p. 395.

10. AL-EKABI, H. andSERPONE, N. Kineticstudies in heterogeneousphotocatalysis. 1. Photo-catalytic degradation ofchlorinated phenols inaerated aqueous solu-tions over TiO, supportedon a glass matrix. /. PJrys.Chem. 1988. vo!. 92,p.5726.

~ In the same waya sacrificial reducingagent (SRA)is needed to supply electronsto the positive holes to allow the electronsin the conduction band to be used for thereductionof the PGMs. In our investi-gation ethanol and sucrose (cane sugar)were used as SRA's.

Experimental

Reagents and solutions

All solutions (stock and reaction mixtures) wereprepared with deionized water obtained from aMillipore Milli-Q system.

Stock solutions were prepared fromPt(metal), PdClz (Fluka Chemie AG and PGMChemicals (Pty) Ltd) and RhCl3,3HzO (Merckand PGM Chemicals (Pty) Ltd).

Platinum metal was dissolved in boiling aquaregia with an excess of hydrochloric acid to expelnitrogen compounds and then evaporated toform chloroplatinic acid, HzPtCI6,6HzO.

To HzPtC~, 6HzO, PdClzand RhCl3,3HzO acalculated amount of 34 per cent HClwas addedbefore the solution was made up with deionizedwater to prevent unwanted hydrolysis reactions.All stock solutions were 1 M in Cl-.

Degussa P25 TiOz(Anatase) was used asphotocatalyst, the amount of TiOzin suspensionbeing 2 g.dm-3. Sodium hydroxide solutionswere prepared using NaOH pellets (Merck) andwere standardized with potassium hydrogen-phthalate. HClwas standardized with NaOH andborax. Ethanol (Merck) and commercial canesugar were used as sacrificial reducing agents.

Apart from the PGMs which were added astheir chloride salts the reaction mixturescomprised TiOzand ethanol (or sugar) andwater. The pHvalue was adjusted by theaddition of either sodium hydroxide orhydrochloric acid.

Photoreactor

ru u-U"I I

I II II III II II II II II II II ILn u-'

Reservoir

" Peristaltic PumpMagnetic Stirrer

~I ~Blacklight Fluorescent Lamp

Figure 3-Diagram of the photoreactor

The Journal of The South African Institute of Mining and Metallurgy

------- --------

The concentrations of the metal ions insolution were determined by inductively coupledplasma (ICP) emission spectroscopy using aVarian Liberty 200 instrument.

Design of the photoreactor

A diagram of the reactor modePz chosen for ourlaboratory investigation is shown in Figure 3.

The photoreactor consists of a blacklightfluorescent lamp within a glass cylinder, with aninlet and outlet. The reactor is sealed at the endswith a-rings. The blacklight fluorescent lampemits light between 300 and 425 nm with a peakat 350 nm which is below 383 nm and thereforeefficient for electron excitation. A peristalticpump circulates the solution or suspension of500 ml from a reservoir through the photoreactorunder continuous illumination. The pumpingspeed throughout the investigation was kept at400 ml.min-1.

The suspension in the reservoir is contin-uously stirred by means of a magnetic stirrerand during a run samples are periodically takenfrom the reservoir for analysis.

Results and discussion

Effect of the concentration of the SRA(Ethanolj on the reduction rate of Rhodium(lII)

To investigate the effect of ethanol on thereduction rate of Rh (Ill) the pH value was keptconstant at 1.3 and the ethanol concentrationvaried from 0 to 2.1705 mol.dm-3. The results inFigure 4 show that the reduction rate of Rh (Ill)increases as the concentration of the SRA(ethanol) is increased.

Without ethanol no reduction of Rh(Ill) takesplace which demonstrates the necessity of a SRAto provide indirectly the electrons required forthe reduction process. It is seen that the rate ofrecombination of the electron-hole pair is sloweddown by increasing the concentration of theSRA, thereby giving the excited electron a longerlifetime which leads to a higher reduction rate ofRh (Ill).

An ethanol concentration of 0.13 mol.dm-3(curve no. 5) was chosen as standard for thesubsequent experiments.

Individual Photocatalytic Reduction of Rh(III),Pd(lI) and Pt(IV)and the Influence of pH Value

The conditions for these experiments were thesame as before. Only the pH value was varied byaddition of sodium hydroxide while the concen-tration of the SRA was kept constant at0.13 mol.dm-3.

MARCH/APRIL 1995 77 ...

60

0EtOH

........ 50 (mol.dm -3)

ECl

0 0Cl 40'-" 0.0012c0 2 0.0090

...... 30C 3 0.0219'-......

4 0.0436c 20IDU 5 0.1304c0 10 6 0.4344

U7 2.1705

00 15 30 90 105 120 135 150

Time (min)

60

,....... 50

EClCl 40

'-"C0

...... 300

'-......c 20 11ID(JC0 10U

Separation and recovery of some platinum group metals (PGMs)

Figure 4-Rate of photocatalytic reduction of Rh(lII) with variation of the ethanol

concentration

11. WOLD. A. Photocatalyticproperties of TiO,. Chem.

Mater. 1993. vol. 5.p.280.

12. MATIHEWS.RW. andMcEvoy. S.R. Acomparison of 254 nmand 350 nm excitation ofTiO, in simple photocat-alytic reactors.!

Photochem. Photobiol. A:Chem. 1992. vol. 66.p.355.

13. BAES. C.F. and MESMER.R.E. The HydrolYsis ifCations. WHeyInterscience. 1976.p.263.

Rhodium(llI)

The results of the reduction process are shownin Figure 5. It is seen that the rate of reductionreaches a maximum at pH value of 1.87 andthen decreases with increasing pH value. At pHvalue 3.1 Rh(III) shows resistance to reductionand stays in solution. Upon further increase ofthe pH value, Rh(III) is still not reduced butthere is a marked decrease in theRh(III)-concentration in solution before thesample has been illuminated. This is due toeither precipitation or adsorption of Rh(III) onto

TiOz' At a pH value of 11.75 Rh(III) is almostcompletely precipitated and/or adsorbed ontoTiOzbut on further increase in pH value it goesinto solution again.

pH

1.32

1.872

3

8'--7 2.17

2.434

5 2.92

3.105

.6

7

8

3.24

3.35

7.50

11.75

9

1010

0 110 15 30 45 60 75 90 105 120 135 150

12.82

Time (min)

Figure ~ate of photocatalytic reduction of Rh(III) with variation of pH

~ 78 MARCH/APRIL 1995

u------- ----

-- ------

From these results it follows that separationof Rh(III) might be obtained at two differentpH values, namely (i) pH =3.1, where Rh(III)stays in solution resisting reduction, and (ii)pH = 11.75, where Rh(III) is almost completelyreduced and precipitated or adsorbed onto TiOzas a reduced species.

Palladium(lI)

If the results of the reduction of Pd(Il) shown inFigure 6 are compared with those of Rh(III)(Figure 5), it is seen that at a pH value of 3.1,where Rh(III) remains in solution, Pd(II) isentirely present in solution and upon illumi-nation it is completely reduced to pd metal. As inthe case of Rh(III) , all Pd(II) is adsorbed ontoTiOzat a pH value of 11.8.

These results indicate that separation ofRh(III) and Pd(II) may be accomplished at a pHvalue of 3.1 by means of selective photocatalyticreduction in that Rh(III) is kept in solution whilePd(II) is reduced.

Platinum(lV)

From the results pertaining to zero illuminationtime (Figure 7) it is seen that Pt(IV) remains insolution over the whole pH range since noadsorption and/or precipitation occurs.Separation of Pt(IV) from Pd(II) and Rh(III) maytherefore be obtained at a pH value of 11.8 bymeans of selective adsorption onto TiOz' Pt(IV)stays in solution while Rh(III) and Pd(Il) arecompletely adsorbed onto TiOz'

Possible schemes for separation

From the previous experiments it is evident thatseparation of Pt (IV), Pd(Il) , and Rh(III) can beattempted via two different routes. Both routesmake use of selective adsorption and selectivephotocatalytic reduction and differ only in theorder in which the two processes should becarried out.

Scheme 1

The Joumal of The South African Institute of Mining and Metallurgy

40Ea.a.

.........

30c:

0.....

020

~.....

c:Q),uc: 100U

60

,......, 50Ea.a. 40

.........

c:0..... 30

0~.....

c: 20Q)

uc:0

10U

00 15 30

60

50..--.

Ea.a. 40

c::0

+'30

0

~+-'c::Q)

20uc::0U

10

pH

1 1.60

2 3.15

3 4.00

4 5.73

5 7.05

6 11.84

Separation and recovery of some platinum group metals (PGMs)

50

00 15 30 45 60 75 90 105 120 135 150

Time (min)

Figure6-Rate of photocatalytic reduction of Pd(lI)with variation of pH

:~\".

'" "'."., .

45 60 75 90 105 120 135 150

Time (min)

Figure 7-Rate of photocatalytic reduction of Pt(lV) with variation of pH

00 30 60 90 120150180210240270300

Time (min)

2

3

0.58

1.90

3.32

7.104

5

6

11.95

12.85

Figure B-Rate of photocatalytic reduction of a mixture of Rh(III),Pd(lI)and Pt(IV) at pH=

3.9

The Journal of The South African Institute of Mining and Metallurgy

According to this method Rh (Ill) is separatedfrom Pt(IV) and Pd(II) by means of selectivephotocatalytic reduction at a pH value of 3.1followed by separation of Pt (IV) and Pd(II) bymeans of selective adsorption at a pH value of11.9.

However. actual separation of Rh (Ill) fromPt(IV) and Pd(II) by means of selective photocat-alytic reduction at a pH value of 3.1 did notsucceed because Rh(IlI) did not resist reductionas was inferred from previous experiments(Figure 8).

Scheme 2

pH

According to this method selective adsorption ofPd(II) and Rh (Ill) at a pH value of 11.9 is carriedout as the first step, thereby separating theseions from Pt(IV). Pd(II) and Rh(IlI) are thenseparated at a pH value of 3.1 by means ofselective photocatalytic reduction. Application ofthe principle shows that selective adsorption ofPd(II) and Rh(IlI) at a pH value of 11.9 shouldbe successful as can be seen from the results ofan experiment where a 500 ml solutioncontaining 50 ppm of each of Pt (IV), Pd(II) andRh(IlI) was stirred for five hours in the presenceof 19 TiO2.Pt(IV) remains in solution whilePd(II) and Rh(IlI) are completely precipitatedand/or adsorbed onto TiO2 (Figure 9).

The selective reduction of Pd(II) in thepresence of Rh (Ill) at a pH value of 3.1 alsoproved to be successful (Figure 10). The pHvalue was 3.3 when illumination was started anddecreased to 2.8 at the end of the experiment. Ithas been shown in previous experiments thatthe reduction of Rh (Ill) occurs at a pH valuelower than 3.1. The slight decrease in theRh(IlI)-concentration during the reduction ofPd(II) can thus be attributed to a fall in pH belowthe critical value of 3.1.

MARCH/APRIL 1995 79 ....

-- -----

60

Pt50

r-...Ea. 40a.

'-'I::0 30.....c'-.....I::

20Q)

UI::0

U 10 Rh

60

'""'50 Rh

Ea.a. 40

'-Jc:0 30......0'-......c: 20Cl)uc:0 10

U

00 30 60 90 120 150 180 210 HO 270 300

Time (min)

50

40 ."r-...Ea.a.

'-' 30I::0

.....c'- 20.....I::Q)UI::0 10U

Separation and recovery of some platinum group metals (PGMs)

Pd

00 30 60 90 120 150 180 210 240 270 300

Time (min)

Figure 9--Selective adsorption of Pd(lI) and Rh(lII) onto TiO. at pH = 11.9

Figure 1o-selective photocatalytic reduction of Pd(lI) in the presence of Rh(lII) at pH =3.3

00 30 60 90120150180210240270300

Time (min)

Figure 11-Photocatalytic reduction of Pd(lI) and Rh(lII) at pH =3.3 after adsorption atpH =11.9

~ 80 MARCH/APRIL 1995

---"--" "

The two experiments described previously,Le., the selective adsorption of Pd(II) and Rh(III)and the selective reduction of Pd(II) have beencarried out independently. Subsequentlyselective adsorption and selective photocatalyticreduction were combined in that a solution ofPd(II) and Rh(III) was first stirred for two hoursin the presence of 19 TiOzat a pH value of 11.9simulating the conditions for the selectiveadsorption of Pd(II) and Rh(III) before the pHvalue was lowered to 3.3 for the selectivereduction of Pd(II) by illumination in thepresence of 0.13 mol.dm-3 ethanol.

The Pd(II) and Rh(III) , however, did notseparate as expected. Reduction of Rh(III)started almost as soon as all the Pd(II) wasdeposited on the TiOz.Again there was a drop inpH value from 3.3 to 2.7 (Figure 11).

This dramatic change in the reductionpattern for Rh(III) , shown in Figure 11, can beascribed to the presence of other Rh(III)-speciesformed at high pH values.

~ At the outset, at low pH values, the Rh(III)solution has a red-pink colour due tocomplexes of the type [Rh(HzO)6-nClnp-n,e.g. [Rh(HzO)4Clzr.

~ Raising the pH value to 11.9, in order toperform selective adsorption, gives rise toa yellow solution, indicating the formationof a different complex ion, most likely the[Rh(HzO)sOH]z+complex ion13.

~ When the pH value is lowered again, to3.1, to carry out selective reduction, thesolution keeps its yellow colour. The factthat the Rh(III) solution does not return toits original colour shows the kineticstability of the complex formed at high pHvalues.

In order to separate a mixture of Pd(II) andRh(III) successfully the reduction of Rh(III) mustbe prevented. The pH value of the solution, as afirst step, was kept constant between 3.1 and3.4. This had a dramatic effect in preventing thetotal reduction of Rh(III) (Figure 12). It can beseen that Rh(III) was reduced only after Pd(II)had been almost fully deposited onto TiOz.

As a second measure, to prevent reduction ofRh(III), the photocatalytic activity of the TiOzcanbe reduced by lowering the concentration of theSRA (ethanol) and thereby reducing theavailability of the exicited electrons.

Such an experiment was carried out in whichthe ethanol concentration was lowered from 0.13to 0.087 mol.dm-3. This had the desired effect inthat the reduction of Rh(III) was prevented andthe separation of Pd(II) and Rh(III) wassuccessfully accomplished (Figure 13).

The Journal of The South African Institute of Mining and Metallurgy

50

Rh40

Ea.a.

'-' 30I:0

......

0L. 20......I:Cl)0I:0 10U

00 30 60 90120150180210240270300

Time (min)

50

Rh

40~E

a.a.

'-' 30I:0......CL. 20......I:Cl)0c:0 10U

Separation and recovery of some platinum group metals (PGMs)

Figure 12-Photocatalytic reduction of Rh(lII) and Pd(lI) at 'constant' pH(3.1-3.4) preceded

by adsorption

00 30 60 90 120 150 180 210 240 270 300

Time (min)

Figure 13-Photocatalytic reduction of Rh(lII)and Pd(lI)at 'constant' pH(3.1-3.4) with a

decrease in the SRA-concentration preceded by adsorption

50

, ,Ea.a.

'-'

40 Rh

30c:0

......CL.

......c:Cl)uc:0

U

20

10

00 30 60 90 120 150 180 210 240 270 300

Time (min)

Figure 14-Selective photocatalytic reduction of Pd(IO and Rh(lII) with table sugar as

SRA(O.OOO6 M). The pH was kept between 3.1 and 3.35

The Joumal of The South African Institute of Mining and Metallurgy

Sucrose as SRA

Having demonstrated that Rh(III) and Pd(II) canbe separated satisfactorily with ethanol as SRA,the use of another SRA was investigated, namelysucrose (table sugar).

Sucrose can be expected to be a muchstronger SRA than ethanol because it has moresites per molecule where oxidation can takeplace. This expectation was confirmed by thesuccessful separation of Rh(III) and Pd(II) usingsugar as a SRA at a concentration of0.0006 mol.dm-3 or 0.lg/500 ml (Figure 14).

Conclusion

We have shown that Pt(IV) can be separatedfrom Pd(II) and Rh(III) by means of selectiveadsorption of the latter onto TiO2at a pH valueof 11.9. Subsequent separation of Pd(II) andRh(III) by means of selective photocatalyticreduction at a pH value of 3.1 can then beaccomplished.

Both ethanol and sugar are suitable assacrificial reducing agents, but various othercompounds can be considered depending on therequirements of the system.

Although the blacklight fluorescent lampproved to be a sufficient energy source,especially for a laboratory investigation, sunlightcan be utilized in the recovery of the PGMs;separation of the PGMs by using sunlightwarrants further investigation. For the systeminvestigated, the process of photocatalyticreduction can be easily manipulated by eitherchanging the pH value or the concentration ofthe SRA.

By using the photocatalytic reduction processdescribed in the paper it is possible to recoververy low concentrationsof the metals, especiallyof Rh, which is the most valuable cbmponent.Only one stage has been used in the procedureas described. If implemented on an industrialscale more than one stage may be required. Forexample, if as a result of a rather high Rh:Pdratio the reduced Pd would contain a smallamount of Rh, the metals can be brought intosolution again and separated by a secondreduction.

The results obtained for the chosen modelsystem, comprising Rh(III), Pd(II), and Pt (IV),illustrates the potential of photoreduction as arecovery and/or separation method for PGMs. .

MARCH/APRIL 1995 <Ill81

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~ 82 MARCH/APRIL 1995 The Journal of The South African Institute of Mining and Metallurgy

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