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Keywords:

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Hydrometallurgy 95 (2009) 247253

Contents lists available at ScienceDirect

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l seto the high price of platinum and rhenium, several researches haveworked on the recovery of these precious metals from spent catalysts.Platinum recovery from spent reforming and isomerization catalystshas been studied by selective dissolution of base metals in 50% H2SO4and treating the insoluble residue with aqua regia from whichplatinum was recovered (Jeliyaskova et al., 1982). Chen and Huang(2006) examined PGM extraction from spent auto catalysts bycyanidation. However, at room temperature and pressure, the reactionbetween sodium cyanide and platinum group metals proceeds slowlydue to poor kinetics. As a result, the cyanide leaching of PGM must be

aqua regiamedium but is not soluble enough in singlemineral acid. des Pinheiro et al. (2004) considered the solubility of Pt in HF, NaF, HCl,HNO3, H2O2 and binary mixture of them. They concluded that, exceptHNO3/HCl mixture (aqua-regia), platinum solubility in such mixturesis low. In two recent works, platinum was recovered from spentcatalysts using aqua regia solution. In one work, the effects of somelimited factors on maximum Pt leaching values was investigated(Barakat and Mahmoud, 2004). Another group examined microwaveassisted leaching followed by chemical precipitation to recoverplatinum (Jafarifar et al., 2005). However, neither groups studied theperformed at elevated temperatures and preshigh cost of operation at high temperatuprocess employs cyanide which is highly toprecious metals producers and research grou

Corresponding author. Tel.: +98 21 6616 4577; fax:E-mail address: baghalha@sharif.edu (M. Baghalha).

0304-386X/$ see front matter 2008 Elsevier B.V. Adoi:10.1016/j.hydromet.2008.06.003aromatic hydrocarbons.ts to improve its activity,oy andMunk,1971). Due

such as sodium hydroxide, nitric acid, hydrochloric acid, sulphuricacid and aqua regia to extract platinum from FischerTropschcatalysts. They pointed out that platinum dissolves completely in anUsually rhenium is also added to these catalysselectivity and stability (Kluksdahl,1971;McCindustries for the catalytic reforming poctane naphtha is upgraded to higherrocess. In this process, the lowoctaneLeachingExtractionAqua-regia solutionReforming catalystPlatinumReaction ratePower-law

1. Introduction

Pt/alumina catalyst is widely usedliquid-to-solid mass ratio and the reaction temperature. Kinetic modeling using power-law rate equation forPt extraction revealed that increasing liquid-to-solid mass ratio increases the acid concentration, as a majorreactant. This quantitatively accounted for the increased Pt extraction rate. The effect of temperature on Ptextraction kinetics was studied using the Arrhenius equation. The activation energy for the platinum surfacedissolution reaction was calculated as 72.1 kJ/mol. This large value indicates that Pt extraction in aqua-regiasolution is controlled by surface chemical reaction. The reaction order was 1.5 for Pt concentration in solidand 1.3 for the hydrogen ion molarity in solution.

2008 Elsevier B.V. All rights reserved.

ning and petrochemical

ing alternate processes to replace the cyanide process. In our group,for instance, hypochlorite/NaCl solution as a strong alternate lixiviantsystem has already been tested for gold extraction from ores(Baghalha, 2007). Matjie et al. (2005) considered different lixiviantsKinetics extraction rate of platinum were then examined. Pt extraction rate was signicantly increased by increasingAvailable online 17 June 2008eliminated the internal anincluding HNO -to-HCl volume ratio, liquid-to-solid mass ratio, and the reaction temperature on theKinetics of platinum extraction from spenaqua-regia solutions

Morteza Baghalha a,, Homa Khosravian Gh. a, Hamida Department of Chemical and Petroleum Engineering, Sharif University of Technology, Azab Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran

A B S T R A C TA R T I C L E I N F O

Article history:Received 1 March 2008Received in revised form 7 June 2008Accepted 8 June 2008

Platinum content of two coatmospheric pressure and aparticle size, and impeller aresistances during Pt extra

j ourna l homepage: www.esures. In addition to theres and pressures, thisxic. In fact, many majorps are actively develop-

+98 21 6616 4391.

ll rights reserved.reforming catalysts in

eza Mortaheb b

t., Tehran, 11365-9465, Iran

ercial spent reforming catalysts were extracted in aqua-regia solutions undermperatures up to 100 C. Three factors, including presence of coke, catalysttion speed were rst tested to study the relative importance of mass-transfern reaction. Catalyst particle sizes b100 m and agitation speeds N700 rpmxternal mass-transfer resistances, respectively. The effect of other factors,

tallurgy

v ie r.com/ locate /hydrometextraction kinetics of platinum in aqua-regia solutions nor theyoffered a reaction kinetic model to quantitatively describe the effectsof various parameters.

In this work, we experimentally studied the effects of variousparameters on Pt extraction kinetics and maximum platinumrecovery. Furthermore, aluminum extraction from the catalyst matrixwas also measured. Finally, a variety of reaction kinetic models wereconsidered for the obtained Pt extraction data.

2. Experimental

Two types of spent naphtha reforming catalysts were supplied by apetroleum renery. The chemical compositions of the catalysts wereobtained by X-ray uorescence (XRF) analysis. The platinum contentwas also veried by wet chemical analysis through long-term hotaqua-regia total dissolution and atomic absorption spectroscopy.Phase identication of the catalysts was performed by powder X-raydiffraction (XRD) analysis.

The two types of the catalysts were separately ground, dried andscreened into two different sizes, in order to examine the possibleeffect of particle size on the observed extraction kinetics. To investi-

does not hydrolyze into aluminum hydroxo species (Tagirov andSchott, 2001). The hydrolysis reactions may go forward only when theH+ concentration is low. Depending on such other factors as the agingtime, mono-nuclear hydrolytic Al species (Tagirov and Schott, 2001) orpoly-nuclear hydrolytic Al species (Bi et al., 2004) may then form atnear neutral pHs.

Table 2The experimental conditions of platinum leaching tests

Catalysttype

Test codename

Decoking Temp.C

Liquid/solidg/g

HNO3/HClmL/mL

Size m

Spherical S1 Yes 100 10 1/3 b50S2 No 100 10 1/3 b50S3 No 100 10 1/6 b50S4 No 100 10 1/9 b50

Cylindrical C1 No 80 10 1/3 50bsizeb100C2 No 80 10 1/3 b50C3 (800 rpm) No 80 10 1/3 b50C4 No 50 10 1/3 b50C5 No 100 10 1/3 b50C6 No 80 5 1/3 b50C7 No 80 15 1/3 b50

248 M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253gate the effect of decoking on Pt leachability, some of the ne-groundused catalyst sample was decoked by heating in a furnace at 600 Cfor 4 h. The coke contents of the as received and decoked catalystswere measured by Leco analysis.

Aqua regia was freshly prepared by mixing concentrated hydro-chloric acid (37%) and concentrated nitric acid (65%) at a volume ratioof 3 HCl:1 HNO3.

The batch reactor used in this work was a 1 liter glass containerequippedwith a thermometer, a mechanical turbine-type stirrer and areux condenser. The condenser was used to recover and send back tothe reactor as much vapor as possible. Any escaped gases (such as Cl2,NOCl, HCl, and NOx) at the exit of the condenser were scrubbed into acold water container. The reactor was maintained at a desirableconstant temperature by a controllable hot-plate heater. The reactortemperature was controlled with a precision of 2 C.

For each run, 600 cm3 of aqua regiawasrst charged into the reactor.Then, the systemwas quickly heated to the desired temperature undercontinuous stirring. When the temperature reached the pre-set valueand remained stable, a certain amount of catalysts powderwas added tothe reactor. Next, the mixture of the reactants was intensively stirred at700 rpm under atmospheric pressure. At appropriate time intervalsduring a run, approximately 15 mL of the sample was taken out andquickly centrifuged. The supernatant solution samples were then takenand analyzed by atomic absorption spectrometer (Perkin Elmer 1100 B)to determine its platinum and aluminum contents. A few verifying testsolutions with aqua-regia matrix and known contents of Pt and Al weremade and measured for those metals by Atomic absorption. The accu-racy of Pt and Al measurements were found to be within 2% and 3%,respectively. The obtained solution samples were also analyzed for H+

ion concentration through neutralization titration with 0.1 N NaOHstandard solutions.However, nomaskingagents suchas Fwereused forAl3+ ionsduringNaOHtitration.As a result, the consumedNaOHsolutionto reach a solution pH=6was due to both H+ ion neutralization and Al3+

ion precipitation to Al(OH)3. It will be shown later that the H+ ionconcentration can be calculated by knowing the aluminum concentra-tion in solution.

3. Results and discussion

The main chemical compositions of the two different types ofspent catalysts, as determined by XRF andwet chemical analysis for Pt,

Table 1Chemical composition of two S and C catalysts determined by XRF

Component Spherical catalysts (S), wt.% Cylindrical catalysts (C), wt.%

Al2O3 90.9 93.2SiO2 0.29 0.27SO3 0.21 0.17Cl 0.70 0.80TiO2 0.03 0.17Fe2O3 0.13 0.17Re 0.39 0.30Pt 0.23 0.29LOI 7.00 4.30are shown in Table 1. As expected, these results show that the mainconstituent of these two catalysts is alumina. The crystalline structureof alumina in the catalysts was identied as gammaalumina throughpowder XRD analysis (Fig. 1).

The experimental conditions and the code name of the performedleaching tests are reported in Table 2. As seen from this table, the twocatalysts are distinguished by their original shape, i.e., Spherical andCylindrical.

It is believed (Massucci et. al., 1999) that HNO3 and HCl in aqua-regia undergo through the Reactions [I] and [II] to some small extent:

HNO3 3HClNOCl Cl2 2H2O INOCl H2OHNO2 HCl II

Dissolution of Pt from the catalyst is a redox reaction that under-goes according to Reaction [III]:

8H 8Cl 2NO3 PtPtCl26 4H2O 2NOCl IIIAs Reaction [IV] shows, aluminum extraction from the alumina

catalyst support is an acid-attack reaction.

Al2O3 6H2Al3 3H2O IVDue to high acidity in solution, the produced Al3+ in Reaction [IV]

Fig. 1. XRD pattern of the reforming catalysts C. The peaks marked with A representgammaalumina phase.

regia, in fact, depends on three factors; the metal being dissolved,aging of the prepared solution, and the ratio of HNO3 to HCl (Bonilla,1932). The effect of HNO3-to-HCl volume ratio on Pt extractionkinetics was investigated in three tests (S2, S3, and S4) on thespherical catalyst. As reported in Table 2, all these tests wereperformed at 100 C and at a constant liquid/solid ratio of 10. The

Fig. 2. The effects of de-coking (S1 and S2) and HNO3/HCl ratio (S2, S3, and S4) on A: Ptextraction; B: Al extraction. The smooth curves for Pt %leached are based on powerlawkinetics.

249M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253As mentioned in the Experimental section, the obtained solutionsamples were analyzed for H+ ion concentration through neutraliza-tion titration with 0.1 N NaOH standard solutions. However, nomasking agents such as F were used for Al3+ ions during NaOHtitration. Without masking agents, it is known that at solution pHsnear 6 and at temperatures lower than 100 C, Al3+ ions precipitatesinto gibbsite (Tagirov and Schott, 2001) according to Reaction [V].

Al3 3OHAlOH3s V

As a result, the consumed NaOH solution during titration to reach asolution pH=6 was due to both H+ ion neutralization and Al3+

precipitation to Al(OH)3; i.e., Consumed [OH]=[H+]+3[Al3+]. Themeasured [H+]+3[Al3+] molarities in the neutralization titration of thenal solution of all tests are reported in Table 3. The Al3+ concentra-tions of the nal solutions, as measured by Atomic Absorption, are alsoreported in Table 3. [H+] molarities were then calculated by thedifference of the column 3 and 3 times of the column 4. The values of[H+] molarities, as reported in Table 3, have the same order ofmagnitude accuracy as the measured aluminum in solution, i.e., ~3%.

The effects of six parameters, including state of coking, HNO3-to-HCl volume ratio, agitation speed, particle size of the ground catalysts,liquid-to-solid mass ratio, and temperature on Pt extraction kineticswere investigated in the tests that are reported in Tables 2 and 3.

3.1. The effect of state of coking

The effect of decoking on Pt extraction kinetics was examined intests S1 and S2 (Table 2). The coke contents of the as received and

Table 3The results of NaOH titration (=[H+]+3[Al3+]), Al3+ measurement and calculated [H+]molarities

Catalysttype

Test codename

[H+]+3[Al3+],molar

Measured[Al3+],molar

Calculated[H+],molar

kobs R2,

goodnessof t

ln(k)

Spherical S1 7.30 2.25 0.55 0.0698 0.95 S2 7.62 2.25 0.87 0.0520 0.93 S3 8.05 1.86 2.47 0.0048 0.96 S4 8.00 2.03 1.91 0.0043 0.89

Cylindrical C1 7.25 0.62 5.39 0.0148 0.96 C2 7.20 0.7 5.11 0.0154 0.96 6.30C3(800 rpm)

7.10 0.53 5.51 0.0160 0.95

C4 7.68 0.155 7.21 0.0023 0.95 8.84C5 6.24 1.185 2.68 0.0229 0.99 4.92C6 6.96 1.067 3.76 0.0103 0.93 6.30C7 7.44 0.50 5.94 0.0187 0.99 6.30

The tted values for kobs in Eq. (6) are also reported.decoked catalyst samples, as measured by Leco analysis, were 1.4%and 0.3%, respectively. The results of the S1 and S2 tests are comparedin Fig. 2. The smooth curves in this gure are based on power-lawkinetic modeling, as it will be discussed later. As seen in this gure, theextraction of as received and decoked samples are rather the sameduring the rst 15 minutes of the leaching tests. From then, Ptextraction of as received sample is visibly lower than the decokedsample. The nal extraction of platinum in decoked sample wasabout 3% higher than that in the other sample. Since, de-coking of thespent catalysts requires large amounts of energy for heating thecatalyst to 600 C, the 3% higher Pt recovery may not justify the addedcost of de-coking. As a result, the catalysts were tested as received(i.e., decoking was not performed) for the next leaching tests.

3.2. The effect of HNO3-to-HCl volume ratio

Although the xed 1/3 volume ratio of HNO3-to-HCl is oftenapplied in commercial labs for total dissolution of metals andminerals, it has been reported that the dissolution power of aqua-results of these tests are also shown in Fig. 2A for Pt and Fig. 2B for Al.For platinum extraction, the HNO3-to-HCl volume ratio of 1/3produced the best results in terms of reactions rates and maximumrecovery. The ORP (oxidation reduction potential) of the solutionsamples produced at the end of these tests were measured by aplatinum electrode vs. the saturated Ag/AgCl reference electrode, asreported in Table 4. The data in this table shows that the solutionsample containing HNO3-to-HCl volume ratio of 1/3 has the highestoxidation potential. The highest oxidation potential of the solution

Table 4Obtained results of ORP electrode vs. the saturated Ag/AgCl reference electrode fordifferent values of HNO3/HCl ratio

Test code name HNO3/HCl mL/Ml ORP (mV)

S2 1/3 951S3 1/6 939S4 1/9 922

corresponding to the HNO3-to-HCl ratio of 1/3 may, in fact,corresponds to the stoichiometry of Reaction [I] for maximumconcentration of Cl2 (a very active oxidant). According to Reaction[III], higher redox potential of solution in test S2, in fact, explains the

better performance of this test in Fig. 2A. As a result, in considering theeffects of other parameters on Pt extraction kinetics, the HNO3-to-HClvolume ratio was xed at 1/3.

For the aluminum extraction results in Fig. 2B, there is nosignicant differences between these tests. As Reaction [IV] shows,aluminum extraction from the alumina catalyst support is an acid-attack reaction. Hence, the Al extraction kinetics is only affected by theacid level which is rather the same (Table 3) in the three tests.

3.3. The effect of agitation speed

To achieve faster kinetics and smaller reactor sizes, solid leachingprocesses require severe agitation to suspend solid particles and topreferentially eliminate the external mass-transfer resistances. Asreported in Table 2, the conditions of tests C2 and C3 were exactly thesame except that the agitation speed of test C3 was 800 rpm (for thetest C2 it was the normal speed of 700 rpm). The obtainedexperimental Pt extraction results from these tests were virtuallythe same, Fig. 3. Hence, it was concluded that 700 rpm agitation wasenough to eliminate the external particle mass-transfer resistances.

3.4. The effect of particle size of the ground catalysts

The factor that impacts the internal particle mass-transferresistances is the catalyst particle size. Two different catalysts sizes:

Fig. 3. The effects of particle size and agitation speed on Pt extraction. The smoothcurves for Pt %leached are based on powerlaw kinetics.

250 M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253Fig. 4. The effect of Liquid/Solid ratio on A: Pt extraction; B: Al extraction. The smoothcurves for Pt %leached are based on powerlaw kinetics.Fig. 5. The effect of reaction temperature on A: Pt extraction; B: Al extraction. The

smooth curves for Pt %leached are based on powerlaw kinetics.

smaller than 50 m (test C2) and between 50 and 100 m (test C1)were leached in otherwise exactly the same conditions. No measur-able differences were observed between the Pt extraction kinetics ofthese two tests, as also shown in Fig. 3. Hence, it was concluded that ifthe catalyst particles are ground below 100 m sizes, no internalparticle mass-transfer resistance would remain. This conclusion wasconsistent with the ndings of Barakat and Mahmoud (2004), whoreported a critical size of 106 m below which the kinetics was notaffected by the catalyst particle size.

3.5. The effect of liquid-to-solid mass ratio

The liquid to solid mass ratio was the other major factor that wasinvestigated. Three levels of 5, 10, and 15 liquid/solid ratios wereexamined in tests C6, C2, and C7, respectively. The temperature of allthese three tests was constant at 80 C (see Table 2). The obtainedextraction results for these experiments are presented in Fig. 4A for Ptand in Fig. 4B for Al. In both cases of Pt and Al, as the liquid/solid ratioincreases, the extraction kinetics becomes faster. The data in Table 3for tests C6, C2, and C7 reveals that as the liquid/solid ratio increases,[H+] molarity also increases. As Reactions [III] and [IV] for the

Fig. 7. EhpH diagram of NClPt system at 100 C, drawn using HSC chemistry software(Outokompou Research, Finland): Chlorine species.

Fig. 8. EhpH diagram of NClPt system at 100 C, drawn using HSC chemistry

251M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253extractions of Pt and Al suggest, H+ ion is a major reactant for thosereactions. As a result, it is expected that higher acidity due to higherliquid/solid ratio produces faster kinetics. This is clearly observed inboth cases of Pt and Al in Fig. 4A and B, respectively.

3.6. The effect of temperature

The Pt and Al extraction kinetics were nally studied at threetemperature levels of 50, 80, and 100 C in tests C4, C2, and C5,respectively. The other experimental conditions, such as liquid/solidratio and HNO3/HCl ratio were the same for these three tests (seeTable 2). The results of these tests for the extraction of Pt and Al arepresented in Fig. 5A and B, respectively. These gures clearly showthat increasing temperature greatly enhances the extraction kineticsand maximum recovery of both Pt and Al. This large temperatureimpact must be due to relatively high activation energies for theextraction reactions.

4. Kinetics modeling

To predict themajor N, Cl, and Pt species in concentrated HNO3/HClsolution (with the volume ratio of 1/3), HSC chemistry software(OutokumpuResearch, Finland)was implemented. The Eh-pHdiagram

Fig. 6. EhpH diagram of NClPt system at 100 C, drawn using HSC chemistrysoftware (Outokompou Research, Finland): Nitrogen species.of this system at 100 C were produced and drawn in Figs. 68. Thedistribution of nitrogen species in Fig. 6 shows that at low pHs and inthe presence of NO3 in solution, the redox potential of solutionmust beclose to 1.0 V (vs. the StandardHydrogen Electrode). For the Eh value of1.0 V and pHs close to zero, Fig. 7 shows that themajor chlorine speciesis Cl and HCl. For the same conditions, Fig. 8 shows that the majorplatinum species are H2PtCl6 and PtCl62. No NOCl was identied by thesoftware as the major nitrogen/chlorine species.

In heterogeneous soliduid reactions, at least, the following threesequential steps occur: (a) diffusion of reactants through the uid lmsurrounding the solid, (b) diffusion of reactants through the solidmatrix, and (c) chemical reaction on the internal areas of the solid. Asexplained in the previous section, the level of slurry agitation and thesolid particle sizes were selected in such a way that diffusions ofreactants in the uid lm and inside the particles were not thelimiting steps (as conrmed by the leaching tests). Hence, it isassumed that the observed Pt extraction kinetics is solely governed bythe surface chemical reactions.

Two classical uidsolid reaction models, namely, homogeneousdiffusion model (HDM) and shrinking core model (SCM) (Zhou et al.,2005; Lee et al., 2005; Levenspiel, 1999) and a newer adsorptionkinetics model, namely, Elovich equation (Juang and Chen, 1997) didnot produce meaningful results.software (Outokompou Research, Finland): Platinum species.

assumed that Wsolids remains constant and was canceled from bothsides of Eq. (2). Further, replacing Eq. (1) into Eq. (2) results in

d wPt =dt kwn1Ptmn2H 3

Since, platinum extraction data are conveniently reported in termsof percent extracted, Eq. (3) should be modied accordingly, using therelationship

wPt w0Pt 1X 4

Where wPt0 is the platinum weight fraction in the ground catalystfeed and X is the fraction of platinum extracted. Thus, Eq. (3) reducesto:

k w0Pt n11

mn2H 1X n1 d 1X =dt 5

The total concentration of H+ was measured in each test during the120 min of the leaching time. For all the tests, it was observed that H+

concentration in solution drops sharply during the rst 5 min of thetest. Thereafter, it remained relatively constant until the end of eachtest. Therefore, by assuming mH+ to be approximately constant afterthe rst 5 minute of each test, Eq. (5) can be integrated from 5 min to

252 M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253The empirical powerlaw equation (Baghalha and Papangelakis,1998a) has been successfully implemented to model the nickelextraction kinetics from solid particles in aqueous acid solutions.Assuming that the powerlaw rate equation can also predict theplatinum extraction data at constant oxidizing power, i.e., constantHNO3/HCl ratio, the results from the C tests in Tables 2 and 3 wereconsidered for the kinetics modeling. The powerlaw rate Eq. (1)describes the Pt extraction rate in terms of the concentrations of thereactants of Reaction [III].

rPt kwn1Ptmn2H 1

Where rPt is the observed rate of platinum extraction per unitmass of solids in the reactor, wPt is the weight fraction of platinum insolids,mH+ is the molarity of the hydrogen ion in solution, and n1 andn2 are the reaction orders. The parameter k is the overall reaction rateconstant which is temperature dependent. This kind of rate equation,which involves solid concentrations of reactants, has beenwidely usedfor non-catalytic solidgas reactions (Froment and Bischoff, 1979;Doraiswamy and Kulkarni, 1987).

In Reaction [III] for Pt extraction, the concentrations of the othertwo reactants, namely Cl and NO3 are constant in all tests. As a result,their contribution in the powerlaw rate Eq. (1) is, in fact, included inthe parameter k, i.e., the overall reaction rate constant.

Fig. 9. Linear behavior of ln(kobs) vs. ln(mH+) for tests C2, C6, and C7 at constanttemperature of 80 C.For highly non ideal systems such as concentrated electrolytesolutions, rate equations must be in terms of activities rather thanconcentrations (Carberry, 1976). Hence, in Eq. (1) the activitycoefcient of H+ is needed to change its molarity to activity. Activitycoefcients, however, are mostly inuenced by the total ionicstrength, and the interaction parameters of the major species(Baghalha and Papangelakis, 1998b), which, in this case, are H+, Cl

and NO3 (refer to Figs. 6 and 7). These ions remained within the sameorder of magnitude in each test performed in the present work.Furthermore, due to the high initial concentration of the electrolytesin solution, the ionic strength also remains relatively constant. As aresult, the hydrogen ion activity coefcient may be assumed to be aconstant value in all the performed tests. Hence, its net effect on therate is absorbed in parameter k of Eq. (1). Therefore, Eq. (1) wasmaintained in terms of H+ ion molarity.

For a batch operation, a mass balance for platinum yields:

rPtWSolids d wPtWSolids =dt 2

Where, Wsolids is the total weight of the solids inside the reactor.Figs. 4B and 5B show that the extraction of aluminum for the aluminamatrix remains between 5 and 45% during the tests. Hence, it wasany time (t).

1Xt 1n1 1X5 1n1 h i

kobs t5 6

Where,

kobs k n11 w0Pt n11

mn2H 7

For each leaching test from t=5 min to the end of the extraction,kobs is constant. To obtain the optimum value for n1, different valueswere tried to t platinum extraction data (from 5 to 120 min) toEq. (6). After repeating this procedure for all the 11 sets ofexperimental data in Table 3, the optimum value for n1 was found tobe 1.5. The values of kobs and the correlation coefcients for each testwhen n1=1.5 are reported in Table 3. The average goodness of t forthe S and C catalyst runs was 0.93 and 0.96, respectively. Theassumption of constant weight of solid is more valid for the Ccatalysts; hence, as expected, a better t for these catalysts wasobtained. Although, this assumption is not quite valid for the S seriestests, perhaps some unknown factors cancel out this error in such away that Eq. (6) still represents a good mathematical t for those data.

Fig. 10. The Arrhenius effect of temperature on reaction rate constant k for tests C2, C4,

and C5.

indicates that Pt extraction in aqua-regia solution is controlled by thesurface chemical reaction of platinum dissolution.

253M. Baghalha et al. / Hydrometallurgy 95 (2009) 247253ln(mH+), as Eq. (7) can be reduced to

ln kobs ln k4 n2ln mH 8

Where

k4 k n11 w0Pt n11 9

The values of mH+ are reported in Table 3. As Eq. (9) shows, k isproportional to k, the overall reaction rate constant, which, in turn, is afunction of temperature. To obtain a linear plot of ln(kobs) against ln(mH+), k must be constant; i.e., according to Eq. (9), using tests withthe same constant temperature. To obtain the optimum value for n2,Fig. 9 was constructed for tests C2, C6, and C7 at constant temperatureof 80 C. The optimum value for n2, as reported in the linearcorrelation in Fig. 9 was found to be 1.3.

Tests C2, C4, and C5 have been performed at different tempera-tures. Hence, temperature-dependent parameters may be obtainedthrough these experimental data. The modied reaction rate constant,k, maybe expressed according to Arrhenius Eq. (10)

k4 A exp E=RT 10

Where A is the frequency factor, E is the surface-reaction activationenergy in kJ/mol, R is the universal gas constant (8.3145103 kJ/Kmol) and T is the reaction temperature in K. The values of ln(k),calculated using Eq. (8) are also reported in Table 3. By plotting ln(k)vs.1/T for tests C2, C4, and C5 in Fig.10, the parameters of Eq. (10) wereobtained as follows:

E=R 8670 oKZE 72:1 kJ=mol 11

ln A 18:217 12

In general, the activation energyof a physical process, e.g. diffusion isless than 20 kJ/mol while that of a chemical process exceeds 40 kJ/mol(Lee et al., 2005). For instance, the activation energies of chromiumextraction from sludge (Lee et al., 2005) andmetallic copper dissolutionin nitric acid (Demir et al., 2004) have been reported as 47 and48 kJ/mol,respectively. As a result, the calculated value of 72.1 kJ/mol for theactivation energy of platinumdissolution in aqua-regia suggests that theoverall extraction rate must be controlled by the surface chemicalreaction (Levenspiel, 1999).

To verify the kinetic modeling work, Eq. (6) was used to predict theplatinumextraction from the tested spent catalyst. In Figs. 2A, 3, 4A, and5A, the predicted values are compared with the experimental data. Ingeneral, there is a good agreement between the predictions and theexperimental data. Test S2 in Fig. 2A and test C6 in Fig. 4A show somesmall differences between the predictions and the experimental data.Thismay be due to some small variation in the acidity during those tests,in contrast to the simplifying assumption of constancy ofmH+ in Eq. (7).

5. Conclusion

Decoking did not enhance platinum extraction from the spentreforming catalyst signicantly. Catalyst particle sizes b100 m andagitation speeds N700 rpm eliminated the internal and external mass-transfer resistances, respectively. Pt extraction rate was signicantlyincreased by increasing liquid-to-solid mass ratio and the reactiontemperature. Through a kinetics modeling using powerlaw rateequation for Pt extraction, the reaction order was calculated as 1.5 forPt concentration in solid and 1.3 for the hydrogen ion molarity insolution. The effect of temperature on Pt extraction kinetics wasmodeled using Arrhenius equation. The activation energy for theplatinum extraction was obtained as 72.1 kJ/mol. This large valueNotationsA Frequency factor of reaction rate constantE Surfacereaction activation energy, kJ/molk Overall reaction rate constantkobs The observed reaction rate constantk The modied reaction rate constant as dened by Eq. (10)mH+ The molarity of the hydrogen ion in solution, mol/Ln1 and n2 The reaction ordersrPt The observed rate of platinum extraction per unit mass of

solids in the reactor, mol/s kgR The universal gas constant (8.3145103 kJ/K mol)T Reaction temperature, KwPt Weight fraction of platinum in solidswPt0 The initial platinum weight fraction in the ground catalyst

feedWsolids Total weight of the solids inside the reactor, kgX Fraction of platinum extracted

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Kinetics of platinum extraction from spent reforming catalysts in aqua-regia solutionsIntroductionExperimentalResults and discussionThe effect of state of cokingThe effect of HNO3-to-HCl volume ratioThe effect of agitation speedThe effect of particle size of the ground catalystsThe effect of liquid-to-solid mass ratioThe effect of temperature

Kinetics modelingConclusionNotationsReferences