Hydrometallurgy 100 (2010) 110–115

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Kinetics and mechanism of Re(VII) extraction with mixtures of tri-alkylamineand tri-n-butylphosphate

Ying Xiong ⁎, Zhenning Lou, Suang Yue, Junjun Song, Weijun Shan, Guangxi HanCollege of Chemistry, Liaoning University, Shenyang 110036, PR China

⁎ Corresponding author. Tel./fax: +86 24 62202006.E-mail address: xiongying_1977@hotmail.com (Y. X

0304-386X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.hydromet.2009.10.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 September 2009Received in revised form 20 October 2009Accepted 27 October 2009Available online 5 November 2009

Keywords:Interfacial extraction kineticsTri-alkylamineTri-n-butylphosphateRhenium(VII)Molybdenum(VI)

The extraction kinetics of rhenium(VII) with mixtures of tri-alkylamine (N235, R3N, R=C8−C10) and tri-n-butylphosphate (TBP) dissolved in heptane has been investigated by constant interfacial cell with laminarflow. The influence of stirring speed, temperature, specific interfacial area, extractant concentration andchloride concentration on the extraction rate has been studied. It is concluded that the extraction of Re(VII)takes place at the liquid–liquid interface, while the extraction regime belongs to kinetic control by chemicalreaction. The extraction rate equations and the rate-determining step have been obtained under theexperimental conditions, and the extraction rate constant is calculated. The results are also compared withthe system with N235 alone as extractant which shows that the Re(VII) extraction rate is enhanced and theactivation energy is decreased with the mixtures of N235 and TBP. The separation of Re(VII) from Mo(VI) bykinetics with the mixtures of N235 and TBP is similar to that with N235 alone. Nevertheless, the separation ofRe (VII) from Mo(VI) is better under kinetic conditions than under thermodynamic equilibrium conditions.

iong).

ll rights reserved.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Rhenium is usually recovered from molybdenite concentratesthrough roasting or direct reduction of the concentrates (Sutulov,1970). With the ever increasing demands of high purity rhenium andits compounds in the petrochemical industry, national defense andaviation and other specialized applications, manymethods are used topurify and separate rhenium and molybdenum, such as chemicaldeposition, ion exchange, capillary electrophoresis, liquid chroma-tography and solvent extraction. Among them, liquid–liquid extrac-tion provides an effective and simple separation method (Jordanovet al., 1968; Karagiozov and Vasilev, 1979; Almela et al., 1998).However, it is often complicated by multi-stage cycles, extractant lossand formation of stable emulsions etc. Thus, it is important to exploresome new extraction systems or separation methods superior to thecurrent extraction process which has been widely applied to therecovery of rhenium (Gerhardt et al., 2000; Cao et al., 2009).

It is well known that separation by extraction kinetics is possiblefor the quantitative separation of metal ions which cannot beseparated in the equilibrium state (Itabashi et al., 1997). Althoughthe thermodynamics of extraction are relatively well known, there is alack of comprehensive information on the kinetics of mass transfer inbiphasic solvent extraction systems (Chitra et al., 1995; Biswas et al.,1997; Corsi et al., 1998). Thus, kinetics studies on the extraction ofindividual element are necessary for the development of the

experimental procedure and to understand the mechanism andmass-transfer models.

The revised Lewis cell, called the constant interfacial-area cell withlaminar flow, developed by Zheng et al. (1998) was previously used inour work (Wang et al., 2002; Xiong et al., 2004). The operation iscarried out under laminar flow, which keeps the interface as stable aspossible, keeping no flow between two phases. But to date, littleresearch has been reported on the extraction kinetics of raremetals byconstant interfacial cell with laminar flow.

In our earlier work, the kinetics of Re(VII) extraction andseparation from Mo(VI) with N235 was investigated (Lou et al.,2009), which indicates that the separation of Mo(VI) and Re(VII)could be easier to carry out by kinetics rather than equilibriumextraction. In the present work, research results on the extractionkinetics of Re(VII) with the mixtures of N235 and TBP dissolved inheptane using a constant interfacial cell with laminar flow arereported. The extraction controlling regime is carefully evaluated, andthe reaction zone is determined by considering different effects on theextraction process. The purpose is to provide useful informationtowards developing more efficient and economical hydrometallurgyprocess for rhenium separation and purification from molybdenum.

2. Experimental

2.1. Reagents

Tri-alkylamine (N235, R3N, R=C8−C10) was kindly supplied by or-ganic chemical factory Shanghai China. Tri-n-butylphosphate (TBP) was

Fig. 1. Dependence of distribution coefficient on the species concentration. [NH4ReO4]=5×10−4 M, [HCl]=0.01 M, T=298 K. ■, [N235]=1.5×10−2 M, [TBP]=4.5×10−2 M;●, [N235]=1.5×10−2 M, [Cl−]=0.1 M; and ▲, [TBP]=4.5×10−2 M, [Cl−]=0.1 M.

111Y. Xiong et al. / Hydrometallurgy 100 (2010) 110–115

kindly supplied by Tianjin Chemical Reagent No 1 plant. All the reagentswere used without further purification. Rhenium stock solutions wereprepared by dissolving NH4ReO4 (99.9%) in HCl and molybdenum stocksolutions were prepared by dissolving (NH4)6Mo7O24·4H2O in HCl. Allother reagents were of analytical grade.

2.2. Procedure

N235 was pre-equilibrated with the same volume of 3 M HClsolution in the absence of the metal ions. It was separated and washedwith distilled water until neutral pH for the following experiments. Theconcentration of the mixtures of N235 and TBP (at a ratio of 1:3) in theorganic phase was 0.06 mol L−1 and the chloride concentration inaqueous phase was kept at 0.1 mol L−1 except for the experiments oftheir effect of concentration on the extraction rate. The aqueous phasescontained 5.0×10−4 mol L−1 rhenate, together with 0.1 mol L−1 HCl.The interfacial areawas 1.94×10−3 m2 (19.4 cm2). Both the volumes ofaqueous and organic phase were 98 mL. The extraction kinetics wasinvestigated by using a constant interfacial area cell with laminardescribed previously (Zheng et al., 1998). The concentrations of Re(VII)andMo(VI) in the aqueous phaseweremeasured by using a 7230modelgrating spectrophotometer and PE-700 atomic absorption spectropho-tometer, respectively. The concentrations of metal ions in the organicphase were determined by difference.

The interfacial tension experiments were carried out by using JYZ-200 auto-tensiometer.

For the equilibrium experiments, equal volumes (5 mL each) ofaqueous and organic phases were mixed and shaken for 30 min at298±1 K (except for the temperature experiments). The distributioncoefficient (D) was taken as the ratio of the metal concentration inthe organic phase to the concentration in the aqueous phase.

2.3. Theoretical

Assuming that the mass-transfer process could be formally treatedas a pseudo-order reversible reaction with respect to the metal cation(Danesi and Vandergrift, 1981):

Mn+ðaÞ ⇔MðNÞðoÞ ð1Þ

The following rate equation can be obtained as described inEqs. (9) and (10):

RFð f Þ = −d½Mn+ �ðaÞ

dt= kf ½Mn+ �b½Cl−�c½Extractant�d ð2Þ

−d½Mn+ �ðaÞ

dt= −Q

Vðkoa½M �ðoÞ−kao½M �ðaÞÞ ð3Þ

ln 1−½M �ðoÞ½M �eðoÞ

!= −Q

Vð1+ KdÞkoat ð4Þ

ln 1−½M �ðoÞ½M �eðoÞ

!= −Q

V1+

1Kd

� �kaot ð5Þ

The slopes of the plots ln(1− [M](o)/[M]e(o) ) vs. t have been used toevaluate koa and kao. All plots are straight lines in the present work,indicating that above assumption is reasonable.

3. Results and discussion

3.1. Stoichiometry of the heterogeneous complex formation reactions forRe(VII)

The equilibrium equation corresponding to the extraction ofrhenium ion with the mixtures of N235 and TBP in heptane hasbeen investigated. As shown in Fig. 1, the plots of log D vs. log [Cl−] atfixed extractant concentrations give straight lines with the slope ofabout −1.0. Similarly, keeping the concentration of chloride ions andone of the extractants as constant, the plots of log D vs. log C are linearwith slopes of about 1.0 and 0.5 for N235 and TBP, respectively. Thus,the equilibrium equation of extraction reaction for Re(VII) can beproposed as follows:

ReO−4 + R3NHCl + 0:5TBP = R3NHReO4⋅0:5TBP + Cl− ð6Þ

Where R3NHReO4·0.5TBP is the organic complex.

3.2. Extraction regime

For extraction kinetics, the criterion generally used to identify theextraction regime is independence of the extraction rate on thestirring speed in constant interfacial area cell. Fig. 2 shows the effect ofthe stirring rate on kao for rhenium extraction with the mixtures ofN235 and TBP keeping the other conditions fixed. It shows an initiallinear dependence when the stirring speed is less than 250 r/min.because the thickness of the stagnant interfacial films is relativelylarge so that the process of diffusion is slow. When the chemicalreaction is slow enough to be competitive with the diffusionalprocesses, the progressive increase of the stirring rate (N250 r/min)reduces the thickness of the stationary diffusional film and a “plateauregion” appears indicating that the extraction rate is most likelykinetically controlled by chemical reaction. Nevertheless, a “plateauregion” can be also generated by other phenomena and it is necessaryto explore other approaches to identify the extraction regime.

Another criterion that distinguishes between diffusion control andkinetic control is the experimental determination of the activationenergy of the extraction process. The effect of the temperature on therhenium extraction rate is studied in the temperature range of 289 Kto 313 K. The apparent activation energy (Ea) for the extraction iscalculated from the slope of log kao vs. 1000/T, as shown in Fig. 3, andEa is calculated as 40.6 kJ/mol for rhenium extraction. In general, if therate is controlled by a chemical reaction, Ea is N40 kJ/mol but if the

Fig. 2. The effect of the stirring speed on the extraction rate. Q=19.4 cm2, V=98 mL,T=298 K, [N235]=2.5×10−2 M, [TBP]=4.5×10−2 M, [Cl−]=0.1 M, [HCl]=0.01 M,and [NH4ReO4]=5×10−4 M.

Fig. 4. The effect of the interfacial area on the extraction rate. V=98 mL, T=298 K,[N235]=2.5×10−2 M, [TBP]=4.5×10−2 M, [Cl−]=0.1 M, [HCl]=0.01 M, [NH4ReO4]=5×10−4 M, rpm=250 r/min.

112 Y. Xiong et al. / Hydrometallurgy 100 (2010) 110–115

rate is controlled by a diffusion process, Ea is b20 kJ/mol, whilst valuesbetween 40 kJ/mol and 20 kJ/mol indicate a mixed controlled regime(Kizim, 1992; Yu and Ji, 1992). The Ea of rhenium extraction in ourexperiments suggests a chemical reaction control regime in thetemperature range of 289 K to 313 K.

All further kinetic experiments were carried out at 250 r/min and298°K in order to maintain constant conditions.

3.3. Reaction zone

The effect of specific interfacial area on the extraction rate also canbe regarded as one of the important criteria to determine the rate-controlling step for a chemical reaction in kinetic regime. That is tosolve the problem: whether the rate-controlling step occurs in thebulk phase or at the interface. If in the bulk phase, the initial rate willbe independent of interfacial area. On the other hand, a reactionoccurring at the interface will show a direct proportionality betweenthe rate and the interfacial area.

The effect of the specific areas Q/V (interfacial area/phase volume)on the extraction rate (kao) was studied, and it shows a linearrelationship (Fig. 4), indicating that the reaction occurs in theinterfacial zone for Re(VII) extraction with the mixtures of N235and TBP.

Fig. 3. The effect of the temperature on the extraction rate. Q=19.4 cm2, V=98mL,[N235]=2.5×10−2 M, [TBP]=4.5×10−2 M, [Cl−]=0.1 M, [HCl]=0.01M, [NH4ReO4]=5×10−4 M, and rpm=250 r/min.

The chemical reaction occurring in interfacial zone is alsosupported by the studies of interfacial tension of extractant. Mostsolvent extractant systems are interfacially absorbed and decrease theaqueous–organic diluent interfacial tension. Fig. 5 shows the plots ofthe interfacial tension and interfacial excess vs. extractant concen-tration on the basis of Gibbs and Szyszkowsi isotherm equations(Szymanowski and Prochaska, 1987). The equation is as follows:

Γ = − 12:303RT

⋅ dγdðLgCÞ ð7Þ

γ = γ0 1−B LnCA

+ 1� �� �� �

ð8Þ

Γ =Bγ0

RT⋅ CC + A

ð9Þ

Γm =Bγ0

RTð10Þ

The values of Γm, Ai and Cmin are obtained as follows: Γm=7.67×10−7 mol cm−2, Ai=2.16 nm2 and Cmin=5.0×10−4 mol L−1, whichshows high interfacial activity of extractant. Therefore, the strong

Fig. 5. The effect of extractant concentration on interfacial tension. pH=2 andT=298 K.

113Y. Xiong et al. / Hydrometallurgy 100 (2010) 110–115

surfacial activity of N235 at the heptane–water interface makes theliquid–liquid interface the probable location for the chemicalreactions.

3.4. Extraction rate equation for Re(VII)

The influence of the extraction rate on the concentration ofextractant and chloride ion is shown in Fig. 6(a). It can be concludedthat the extraction rate does not change with varying concentration ofchloride ion, which suggests that the dissociation between R3NH+ andCl− is faster than the rate-controlling reaction. The relationshipbetween the extraction rate and extractant concentration remainslinear. On the other hand, as shown in Fig. 6(b), whether for N235 orTBP, the reverse extraction rate does not change with varying con-centration of extractant. Thus, from the slopes of log kao and log koavs. log [Cl−], it can be determined that the orders of chloride ionconcentration are 0 and 1, respectively. In the same way, the orders ofN235 concentration are 1.07 and 0 for log kao and log koa, respectively;while for TBP, they are 0.53 and 0, respectively.

Moreover, using the values of the intercepts in Fig. 6(a) and (b),the extraction rate constants of the forward reaction as well as thereverse one, kf and kb are calculated to be 101.06 mol1.5 L−1.5 s−1 and10−1.36 mol L−1 s−1, respectively. Accordingly, the rate equation forextraction at 298 K can be written as:

RFð f Þ = 101:06½ReO−4 �⋅½R3NHClðoÞ�1:07½TBPðoÞ�0:53 ð11Þ

RFðbÞ = 10−1:36½R3NHReO4⋅0:5TBPðoÞ�⋅½Cl−�0:86 ð12Þ

RF = RFðf Þ−RFðbÞ = RFðf Þ = 101:06½ReO−4 �⋅½R3NHCl�1:07½TBP�0:53 ð13Þ

3.5. Mechanism and kinetic model for Re(VII)

From above, the chemical reaction for the extraction of Re(VII) bythe mixtures of N235 and TBP occurs at the liquid–liquid interface.Referring to the interfacial reaction model proposed by Danesi andChiarizia (1980), the following reactions will be considered.

R3NHClðoÞ↔K1 R3NHClðiÞ ð14Þ

Fig. 6. The effect of the species concentration on the extraction rate. Q=19.4 cm2,V=98 mL, T=298 K, [HCl]=0.01 M, [NH4ReO4]=5×10−4 M, rpm=250 r/min.■, [TBP]=4.5×10−2 M, [Cl−]=0.1 M; ○, [N235]=2.5×10−2 M, [Cl−]=0.1 M;▲, [N235]=2.5×10−2 M, and [TBP]=4.5×10−2 M.

TBPðoÞ↔K2 TBPðiÞ ð15Þ

R3NHClðiÞ + ReO−4⇌

k1

k−1

R3NHReO4ðiÞ + Cl− ð16Þ

R3NHReO4ðiÞ + 0:5TBPðiÞ⇌k2

k−2

R3NHReO4⋅0:5TBPðiÞ ð17Þ

R3NHReO4⋅0:5TBPðiÞ↔K3 R3NHReO4⋅0:5TBPðoÞ ð18Þ

Considering Eqs. (16) and (17) as the rate-controlling steps, onecan write the following equations.

K1 =½R3NHClðiÞ�½R3NHClðoÞ�

ð19Þ

K2 =½TBPðiÞ�½TBPðoÞ�

ð20Þ

K3 =½R3NHReO4⋅0:5TBPðoÞ�½R3NHReO4⋅0:5TBPðiÞ�

ð21Þ

Based on Eqs. (19) and (20), the forward initial rate of extractioncan be written as follows:

RFð f Þ = k1k2½ReO−4 �½R3NHClðiÞ�½TBPðiÞ�0:5 ð22Þ

RFð f Þ = k1k2K1K2½ReO−4 �½R3NHClðoÞ�½TBPðoÞ�0:5 ð23Þ

Based on Eq. (21), the reverse initial rate of extraction can bededuced as follows:

RFðbÞ = k−1k−2½R3NHReO4⋅0:5TBPðiÞ�½Cl−� ð24Þ

RFðbÞ = k−1k−2K3½R3NHReO4⋅0:5TBPðoÞ�½Cl−� ð25Þ

Eqs. (23) and (25) can be simplified as follows:

RFð f Þ = kf ½ReO−4 �½R3NHClðoÞ�½TBPðoÞ�0:5 ð26Þ

RFðbÞ = kb½R3NHReO4⋅0:5TBPðoÞ�½Cl−� ð27Þ

Where kf=k1k2K1K2, kb=k−1k−2K3.The above mechanism is consistent with the rate Eqs. (11) and

(12) obtained from experimental results. The equations derived frominterfacial-reaction models have been found to be in good agreementwith the ones obtained from experimental data, confirming that thebasic assumption is reasonable. That means the chemical reaction islocated at the liquid–liquid interface and the extraction rate iscontrolled by two-step (Eqs. (16) and (17)) interfacial chemicalreactions.

3.6. Separation of Re(VII) or Mo(VI) with the mixtures of N235 and TBP

The extraction kinetics of Mo(VI) with the mixtures of N235 andTBP have been studied under the same experimental conditions of Re(VII) extraction. The data for the extraction of Mo(VI) or Re(VII) aresummarized in Table 1 (Song and Xiong, 2009; Lou et al., 2009). Itshows that combining N235 with TBP accelerates the extraction rateof Re(VII). However, the extraction rate also gives a slight increasewhen the mixtures of N235 and TBP are used as extractants for Mo(VI). Therefore, it can be concluded that using the mixtures of N235and TBP is not better than using N235 alone for the separation of Re

Table 1The values of extraction rate constant (kF) and separation factor (β) for the extraction ofMo(VI) and Re(VII).

kF(Re) kF(Mo) △kF=kF(Re)/kF(Mo)

Kinetics result N235 101.03 10−0.77 63.03N235-TBP 101.06 10-0.71 58.87

D(Re) D(Mo) β(DMo/DRe)

Thermodynamic result N235 9.30 131.14 14.08N235-TBP 10.23 157.22 15.36

Condition of thermodynamic: [N235]=0.015 M, [TBP]=0.045 M, [Cl−]=0.10 M,[metal ions]=5×10−4 M, [HCl]=0.01 M, and T=298 K.

114 Y. Xiong et al. / Hydrometallurgy 100 (2010) 110–115

(VII) from Mo(VI) as far as kinetics is concerned. This is confirmed bycalculating the value of ≥kF (defined as ≥kF=kF(Re)/kF(Mo)), whichdecreases from about 63.0 to 58.9. Moreover, the admixture of N235and TBP decreases the activation energy for Re(VII) and Mo(VI) to thesame extent (shown in Table 2), which is the reason why it canaccelerate their extraction rate but not improve the separation effectbetween Re(VII) and Mo(VI).

On the other hand, according to their separation factor (β(DMo/DRe=15.36, shown in Table 1), the higher value≥kF indicates that theseparation of Mo(VI) and Re(VII) could be easier by kinetics methodthan thermodynamic equilibrium at the same concentration of speciesand at the temperature of 298 K.

4. Conclusions

The kinetics of extraction of Re(VII) with the mixtures of N235 andTBP in heptane is a chemical reaction controlled process occurring atthe interface. The pseudo-first order extraction rate constants of theforward as well as the reverse reactions are calculated to be101.06 mol1.5 L−1.5 s−1 and 10−1.36 mol L−1 s−1, respectively. Theextraction rate for Re(VII) with the extractant mixture is higher thanthat with N235 because of a decrease of the activation energy. Thedependence of the extraction rate on species concentration and therate equations are deduced as follows:

RFð f Þ = 101:06½ReO−4 �⋅½R3NHClðoÞ�1:07½TBPðoÞ�0:53

RFðbÞ = 10−1:36½R3NHReO4⋅0:5TBPðoÞ�⋅½Cl−�0:86

The extraction rate is controlled by a two-step interfacial reaction,as follows:

R3NHClðiÞ + ReO−4⇌

k1

k−1

R3NHReO4ðiÞ + Cl−

R3NHReO4ðiÞ + 0:5TBPðiÞ⇌k2

k−2

R3NHReO4⋅0:5TBPðiÞ

Mixtures of N235 and TBP also give a slight increase in the Mo(VII)extraction rate, and offer no advantage in the separation of Re(VII)from Mo(VI) by the kinetics method. Nevertheless, the separation ofRe (VII) from Mo(VI) is better under kinetic conditions than bythermodynamic equilibration.

Table 2Apparent activation energy (Ea) for the extraction of Mo(VI) and Re(VII).

Ea(Re) Ea(Mo) ΔEa

N235 42.0 27.5 14.5N235-TBP 40.6 25.0 15.6

Acknowledgements

This project is supported by National Natural Science Foundationof China (20701017).

Appendix A

Derivation of Eq. (4):

−d½M�ðaÞ

dt= − dno

Vdt=

QVðkoa½M�ðoÞ−kao½M�ðaÞÞ

∵ ½M�ðaÞ = ½M�iðaÞ−½M�ðoÞ

½M�e = ½M�iðaÞ−½M�eðoÞ

∴ Kd =½M�eðoÞ½M�eðaÞ

=kaokoa

kao =koa:½M�eðoÞ

½M�iðaÞ−½M�eðoÞ

∴ The extraction rate equation can be obtained:

−d½M�ðaÞdt

=QV

koa½M�ðoÞ−koa½M�eðoÞ

½M�iðaÞ−½M�eðoÞ:ð½M�iðaÞ−½M�ðoÞÞ

( )

=QV

koa½M�ðoÞ−koa½M�eðoÞ

ð½M�eðaÞ + ½M�eðoÞÞ−½M�eðoÞð½M�eðaÞ + ½M�eðoÞ−½M�ðoÞÞ

( )

=QVkoa

½M�eðaÞ + ½M�eðoÞ½M�eðaÞ

!:ð½M�ðoÞ−½M�eðoÞÞ

=QVkoað1 + KdÞ⋅ð½M�ðoÞ−½M�eðoÞÞ

At 0∼ t, [M](o)=0∼[M](o) by integral representation aboveequation:

ln 1−½M�ðoÞ½M�eðoÞ

!= −Q

Vð1 + KdÞkoat

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