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Co-Extraction/Stripping of Mineral Acids and Iron(III) byTri-n-Butyl PhosphateDiptendu Dasa, V. A. Juvekara & R. Bhattacharyab

a Department of Chemical Engineering, Indian Institute of Technology, Bombay, Indiab Atomic Energy Regulatory Board, Mumbai, IndiaAccepted author version posted online: 20 Oct 2014.

To cite this article: Diptendu Das, V. A. Juvekar & R. Bhattacharya (2015) Co-Extraction/Stripping of Mineral Acids andIron(III) by Tri-n-Butyl Phosphate, Separation Science and Technology, 50:4, 545-553, DOI: 10.1080/01496395.2014.959133

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Separation Science and Technology, 50: 545–553, 2015Copyright © Taylor & Francis Group, LLCISSN: 0149-6395 print / 1520-5754 onlineDOI: 10.1080/01496395.2014.959133

Co-Extraction/Stripping of Mineral Acids and Iron(III) byTri-n-Butyl Phosphate

Diptendu Das,1 V. A. Juvekar,1 and R. Bhattacharya2

1Department of Chemical Engineering, Indian Institute of Technology, Bombay, India2Atomic Energy Regulatory Board, Mumbai, India

Fe(III) was extracted from mineral acids (HCl, HNO3, andH2SO4) into paraffin containing TBP and n-decanol (solvatingagent for Fe(III)-TBP complex), using a bulk liquid membraneextractor. Both the extraction equilibria and kinetics of extrac-tion were determined. Rate of extraction of Fe(III) by TBP wasfound to depend on the acid in the order HCl>HNO3�H2SO4.TBP was found to co-extract acid with the rate in order HNO3>H2SO4>HCl. The rate of coextraction of acid was significantlyenhanced in the presence of Fe(III). A kinetic model to predictconcentration of mineral acid and Fe(III) during extraction wasdeveloped and validated.

Keywords co-extraction; mineral acids and Iron(III) extraction;tri-n-butyl phosphate; n-decanol; bulk liquid membrane;liquid emulsion membranes

INTRODUCTIONIn recent years, Liquid Emulsion Membrane (LEM) pro-

cess has emerged as an important technique for the selectiverecovery of metals from dilute solutions. Liquid membrane isa homogeneous, nonporous thin film of water-immiscible liq-uid (oil/organic) which is interspersed between two aqueousphases. Using a complexing agent (carrier), the solute (metal)is extracted from the feed side of the membrane and releasedinto the strip phase. The greatest advantage of the LEM pro-cess is that it allows simultaneous extraction and stripping ofthe solute, thereby facilitating continuous regeneration of thecarrier. This results in a substantial increase in the driving forcefor extraction. LEM is therefore a cost-effective alternative toconventional extraction.

Tributylphosphate (TBP) is the most frequently used extrac-tant in solvent extraction of metals. It falls under the class

Received 01 July 2013; accepted 25 August 2014.Address correspondence to Diptendu Das, Department of Chemical

Engineering, Indian Institute of Technology, Bombay, India. E-mail:diptendudas@iitb.ac.in or diptendu49@gmail.com

Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/lsst.

of compounds containing P = O bond, which have the abil-ity to form complexes with metal. TBP extracts Fe(III) fromhigher acidity solutions (> 4N) and can release it into wateror low acidity solutions; therefore, it is important to maintainlow acidity in the stripping solution during extraction usingTBP. Researchers observed that TBP also extracts acids fromaqueous solutions (1–4). The acid extraction by TBP may notseriously affect the conventional solvent extraction process butit has a profound adverse effect on the efficiency of the LEMextraction process. In LEM, the internal strip phase volume ismuch smaller than the feed phase volume. Thus, even if a smallfraction of the acid from the feed phase is transferred to thestrip phase, there is a substantial increase in the concentrationof the acid in the strip phase. This can significantly reduce thedriving force for LEM extraction and may even result in rever-sal of driving force for extraction. The latter situation wouldcause leakage of the metal from the strip phase to the feedphase. It is therefore necessary to estimate the relative rates ofcoextraction of metal and acid by TBP. This is the focus of thepresent study. For the present study, Fe(III) has been chosenas the metal and the acids used are HCl, H2SO4, and HNO3.Since TBP sometimes forms membrane immiscible complexwith metals, a polar modifier is used in the conventional extrac-tion process to dissolve the complex. We have studied the effectof the commonly used modifier decanol on the co-extractionrates. We have also demonstrated leakage of metal from thestrip phase due to the reversal of the driving force.

Several studies are available on the extraction of either metalor mineral acids by TBP (5–15); we have not come acrossany study on co-extraction/ co-stripping of acid and metal.Although the leakage phenomenon in LEM is well studied, wehave not come across any study where the reversal of the driv-ing force due to accumulation of the acid in the strip phase isidentified as the mode of leakage.

EXPERIMENTALChemicals

The extractant n-TBP was used as supplied by the manu-facturer M/s Sigma Aldrich Chemicals Private Ltd, Mumbai.

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546 D. DAS, V. A. JUVEKAR, AND R. BHATTACHARYA

India, Abil EM90 supplied by M/s.Evonik, Germany and lightparaffin oil, supplied by M/s Merck, Mumbai, India, had kine-matic viscosity 30 cs and density 800 kg m−3. All other chemi-cals were of analytical grade and obtained from the companiesof repute. All chemicals were used without further purification.

Experimental ProcedureAll extraction and stripping experiments were conducted

at ambient temperature, i.e., 30◦C. Equilibrium studies werecarried out by shaking manually 10 mL of aqueous feed solu-tion containing iron(III), of predetermined concentration, with10 mL of organic phase containing 10% TBP and 40% n-decanol in a 30 mL stoppered test tube for ten minutes. Theaqueous and organic phases were separated using a separatingfunnel and the aqueous phase was analyzed for Fe(III). Theorganic phase was extracted with water and the extract wasanalyzed for the acid.

For kinetic studies, the extraction was carried out in bulk liq-uid membrane (BLM) apparatus shown in Fig. 1. The volume of

FIG. 1. Schematic diagram of BLM Extraction Apparatus M: Motor and GearAssembly, C: Controller and Tachometer. G: Top view of Gear Assembly, F:Feed phase (aqueous), O: Organic Phase, E: Extract/Strip Phase (aqueous), P:Partition/Physical barrier, S: Stirrer, J: Rotating Jack/sliding assembly.

each phase was equal to 100 cm3. The interfacial surface areawas 17 cm2. The phases were agitated by individual stirrers.All stirrers had equal speed of 100 rpm. Samples of the organicphase (0.5 cm3) were withdrawn at appropriate intervals oftime. These organic samples were extracted with 1 cm3 waterand analysed for the acid. 1 mL sample of strip was similarlyanalyzed for the acid. Concentration of acid in the feed phasewas estimated by mass balance, knowing its concentrations inthe organic and the strip phase.

Concentration of the acid in the water extract was estimatedby titration with standard (0.01 M) NaOH solution using phe-nolphthalein as indicator. The concentration of Fe(III) in thefeed and the strip phases were determined spectrophotometri-cally after forming complex with KSCN (16). The absorbanceof iron-thiocyanate complex was recorded at wavelength of480 nm. The concentration of Fe(III) was computed from thecalibration plot. Concentration of Fe(III) in the organic phasewas estimated by mass balance, knowing its concentrations inthe feed and the strip phase.

MATHEMATICAL MODELIt is hypothesised that co-extraction of Fe(III) and HX (X =

Cl, NO3, HSO4) by TBP occurs due to the following reaction:

FeX3 (a) + HX (a) + n1TBP (o) ↔ HFeX4.n1TBP (1)

where ‘a’ stands for the aqueous phase and ‘o’ for the organicphase. The equilibrium constant of the above complex forma-tion reaction,Keq,Fe is defined as

Keq,Fe = [HFeX4.n1TBP (o)]

[FeX3 (a)] [TBP (o)]n1 [HX (a)](2)

and the distribution coefficient of Fe(III), DFe is defined as

DFe = [Fe(III) (o)]

[Fe(III) (a)]= [HFeX4.n1TBP (o)]

[FeX3 (a)](3)

From Eqs. (2) and (3), we derive the following relationbetween the equilibrium constant and the distribution coeffi-cient,

DFe = Keq,Fe[TBP (o)]n1 [HX (a)] (4)

Extraction of HX by TBP in the absence of metal can berepresented as

n2TBP (o) + HX (a) ↔ n2TBP.HX (o) (5)

Corresponding equilibrium constant and distribution coefficientcan be respectively written as

Keq,HX = [n2TBP.HX (o)]

[TBP (o)]n2 [HX (a)](6)

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CO-EXTRACTION/STRIPPING OF MINERAL ACIDS AND IRON(III) BY TRI-N-BUTYL PHOSPHATE 547

DHX = [HX (o)]

[HX (a)]= [n2TBP.HX (o)]

[HX (a)](7)

From which we obtain

DHX = Keq,HX[TBP]n2 (8)

In a bulk liquid membrane (BLM), the solute [acid/Fe(III)]is extracted by the carrier (TBP) from the feed-phase on oneside and released into the strip phase on the other side throughorganic phase as shown in Fig. 1. This type of facilitation isalso called carrier-facilitated transport. The rate of transport ofthe metal ions can be described in the following set of threedifferential equations:feed phase:

−Vfd[Fe (f )

]dt

= kFe,foA fo

([Fe (f )

]− [Fe (o)]

DFe,fo

)(9)

organic phase:

Vod [Fe (o)]

dt= kFe,foA fo

([Fe (f )

]− [Fe (o)]

DFe,fo

)

− kFe,osAos([Fe (o)] − DFe,os [Fe (s)]

) (10)

strip phase:

Vfd [Fe (s)]

dt= kFe,osAos

([Fe (o)] − DFe,os [Fe (s)]

)(11)

Here “f ”, “o”, and “e” denote feed, organic, and strip phases,respectively. Subscripts “fo” and “os”, respectively, representthe feed-organic interface and the organic-strip interface. kFe,fo

represents mass transfer coefficient of Fe(III) across the feed-organic interface and kFe,os represents mass transfer coefficientof Fe(III) across the organic-extract interface. A is the interfa-cial area, V is the phase volume, and t, the time.

Using Eq. (4), we can replace the distribution coefficient DFe

with equilibrium constant Keq,Fe to getfeed phase:

−Vfd[Fe (f )

]dt

= kFe,foA fo

([Fe (f )

]− [Fe (o)]

Keq,Fe[TBP (o)]n1[HX (f )

])

(12)

organic phase:

Vod [Fe (o)]

dt= kFe,foA fo

([Fe (f )

]− [Fe (o)]

Keq,Fe[TBP (o)]n1[HX (f )

])

− kFe,osAos

([Fe (o)] − Keq,Fe[TBP (o)]n1 [HX (s)] [Fe (s)]

)(13)

strip phase:

Vsd [Fe (s)]

dt= kFe,osAos ([Fe (o)]

−Keq,Fe[TBP (o)]n1 [HX (s)] [Fe (s)]) (14)

Similarly, concentration of the acid in the absence of iron isdescribed by the following equations:feed phase:

−Vfd[HX (f )

]dt

= kHX,foA fo

([HX (f )

]− [HX (o)]

Keq,HX[TBP (o)]n2

)(15)

organic phase:

Vod [HX (o)]

dt= kHX,foA fo

([HX (f )

]− [HX (o)]

Keq,HX[TBP (o)]n2

)

− kHX,osAos([HX (o)] − Keq,HX[TBP (o)]n2 [HX (s)]

)(16)

strip phase:

Vfd [HX (s)]

dt= kHX,osAos ([HX (o)]

−Keq,HX[TBP (o)]n2 [HX (s)]) (17)

The following initial conditions are used

[Fe(f )

]t=0 = [

Fe(f )]0

, [Fe (o)]t=0 = 0, [Fe(s)]t=0 = 0,

[HX(f )

]t=0 = [

HX(f )]0

,[HX(f )

]t=0 = 0, [HX(s)]t=0 = 0

(18)

where[Fe(f )

]0and

[HX(f )

]0are the initial concentrations of

Fe(III) and HX in the feed, respectively.The above equations are coupled ordinary differential equa-

tions and were solved numerically.

RESULTS AND DISCUSSIONExtraction Equilibria

The reaction stoichiometry for both acid extractions withmetal and free acid extraction by TBP was investigated usingthe classical slope analysis technique. It is seen from Eq. (19)below (obtained from Eq. 8) that for a fixed concentration of theacid in the aqueous phase, plot of ln (DHX) versus ln[TBP (o)]should be a straight line with the slope equal to n2 and theintercept on ln (DHX) axis equal to ln

(Keq,HX

),

ln DHX = n2 ln[TBP] + ln Keq,HX (19)

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from which the equilibrium constant Keq,HX can be determined.Figure 2 presents the plots of ln DHX vs. ln[TBP] for

4 M and 8 M concentration of different mineral acid (HCl,HNO3, H2SO4) with varying concentration of TBP in paraf-fin containing 40% n-Decanol as described below the figure.In 8 M H2SO4, the formation of the third phase was observedeven in the presence of 40% n-decanol; hence equilibrium datafor this concentration has not been included.

As seen in Fig. 2, these plots are straight lines for all theacids. The values n2 and Keq,HX obtained from these plots arelisted in Table 1.

From the values of the equilibrium constants, we concludethat the affinity of TBP for acid extraction is in the order HNO3

>H2SO4>HCl. It is also seen from values of n2 that one mole

FIG. 2. Effect of TBP concentration on extraction of different mineralacid extraction from various aqueous feed solution 1) 8M HNO3 in feed(slope = 0.57, intercept = −1.42) 2) 8M HCl in feed (slope = 0.53, inter-cept = −2.00) 3) 4M HNO3 in feed (slope = 0.48, intercept = −2.83 4)4M H2SO4 in feed(slope = 1.02, intercept = −4.26) 5) 4M HCl in feed(slope = 0.50, intercept = −4.53).

TABLE 1Equilibrium constant Keq,HX and n2 for acid extraction reaction

by 10% TBP −40% n-Decanol

Feedcomposition

n2, Molaritiesof TBP

Equilibriumconstant,Keq,HX

Ln2 .mol−n2

4M HCl 0.50 0.0118M HCl 0.53 0.1354M HNO3 0.48 0.0598M HNO3 0.57 0.2424M H2SO4 1.02 0.014

FIG. 3. Effect of TBP concentration on extraction of HCl extraction withand without iron(III) in aqueous feed solution 1) 10 g.L−1 of Fe(III) in 8MHCl aqueous feed (slope = 0.53, intercept = −1.69) 2) only 8M HCl in feed(slope = 0.53, intercept = −2.01).

of TBP forms a complex with two moles of HCl and HNO3, butonly one mole of H2SO4.

In order to investigate whether the presence of Fe(III) in thefeed phase affects the extraction of the mineral acid by TBP,10 g.L−1 of Fe(III) was added to the feed phase containing 8 MHCl. The plots of ln (DHX) versus ln[TBP (o)], in the presenceand the absence of Fe(III), are compared in Fig. 3. A significanteffect of Fe(III) of the extraction equilibrium is evident fromthis figure.

The two lines in Fig. 3 are parallel to each other, indicatingthe same value of n2. Hence the presence of Fe(III) does notalter the stoichiometry of TBP.HX complex. However, the val-ues of the equilibrium constant in the absence and the presenceand of Fe(III) are 0.135 and 0.184, respectively. This increase issignificant. The probable reason for this increase is that the TBPFe(III) complex also extracts HCl into the organic phase. Bothfree TBP and Fe(III).TBP complex have the same stoichiometryto combine with HCl, but the affinity of Fe(III).TBP for HCl ismuch higher than the affinity of free TBP. Hence the net resultis greater extraction of HCl.

Equilibria for extraction of Fe(III) from HCl and HNO3

using TBP were also estimated (no measurable extractionFe(III) from H2SO4 was observed). The equilibrium distribu-tion coefficient of Fe(III) is expressed by Eq (4). This equationcan be written in the following form

ln DFe = ln(Keq,Fe [HX (a)]

)+ n1 ln[TBP (o)] (23)

Equation (23) shows that for a fixed concentration of HX, theplot of ln DFe versus ln[TBP (o)] should be a straight line having

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CO-EXTRACTION/STRIPPING OF MINERAL ACIDS AND IRON(III) BY TRI-N-BUTYL PHOSPHATE 549

slope n1 and the intercept on ln DFe axis of ln(Keq,Fe [HX (a)]

).

These plots are shown in Fig. 4.The values of the n1 and Keq,Fe, obtained from this analy-

sis are listed in Table 2. From the values of the equilibriumconstants, we conclude that the affinity of TBP for Fe(III)extraction varies with different acids according to the followingorder HCl > HNO3 with no extraction from H2SO4.

Since the strip phase contains low concentration of acid,in order to understand equilibria between the organic-stripphases, we must extend extraction equilibria of both HX andFe(III) to lower concentrations of HX. As an illustration, weplot in Fig. 5 the logarithm of the distribution coefficientsof HCl and Fe(III) versus logarithm of the concentration ofHCl. The concentration of HCl ranges from 2 M to 8 M.It is observed that the distribution coefficient of HCl showsthe same trend over the entire concentration range. On theother hand, the distribution coefficient of Fe(III) show twodifferent trends. Below 4 M concentration, the distributioncoefficient is very low. Above 4 M concentration, it increases

FIG. 4. Effect of TBP concentration on extraction of Fe(III) extraction fromaqueous from various aqueous feed solution 1) 8M HCl in feed (slope = 1.30,intercept = 6.20) 2) 8M HNO3 in feed (slope = 0.33, intercept = −1.22).

TABLE 2Equilibria for extraction of Fe(III) by 10% TBP in Paraffin

with 40% n-Decanol

Feed compositionn1, Molarities

of TBP

Equilibriumconstant,Keq,Fe

Ln1 .mol−n1

8M HNO3

+1g.L−1 Fe(III)0.29 0.037

8M HCl+10 g.L−1

Fe(III)1.27 61.6

FIG. 5. Effect of HCl concentration on the distribution coefficient of Fe(III)and HCl (Organic phase is paraffin containing 10%TBP and 40% n-decanol.Filled circles: Extraction of Fe(III) (slope = 0.42 up to 4M HCl in feed;slope = 10.16 4M-8M HCl in feed) Filled Squares Extraction of HCl(slope = 1.35, intercept = −5.56).

rapidly. From this we expect that the stripping of Fe(III)from the organic phases will be efficient as long as the con-centration of HCl in the strip phase does not exceed 4 M.

Study of Kinetics of Mineral Acid Extraction by TBPModified with n-Decanol

These studies were conducted in BLM setup shown in Fig. 1.The concentration-time data for extraction of HX in the absenceof Fe(III) are plotted in Fig. 6. In all cases 10% (v/v) TBPand 40% (v/v) n-Decanol in paraffin diluent were used forextraction. The concentrations of the acid in the organic phaseand the strip phase were monitored as a function of time andthe concentration in the feed phase was estimated by materialbalance.

We see from these plots that the rate of extraction/strippingis in the order HNO3>HCl>H2SO4. This trend is the same asthat obtained from the extraction equilibria, which is expected.If we examine the concentration profiles, we find that concen-tration of acid in the organic phase goes through a maximum.In the beginning, the rate of extraction is higher than the rateof stripping; hence there is accumulation of the acid in theorganic phase. At a certain point in time, the rate of extrac-tion exactly matches with the rate of stripping. This is thepoint at which the profile shows a maximum. Beyond thispoint, the rate of stripping dominates and hence concentrationdecreases. Using Eq. (16), we derive the following conditionunder which the concentration of the acid in the organic phase ismaximum:

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FIG. 6. Concentration –time profile of mineral acids in three phases of BLMextraction by TBP without Fe(III) in feed aqueous solution[Feed: HCl (�)(red), HNO3(�) (black),H2SO4 (�) (blue); Strip: HCl(�), HNO3(+),H2SO4(•);Organic: HCl (◦), HNO3(�),H2SO4 (×) phase concentration].

kHX,foA fo

([HX (f )

]m − [HX (o)]m

Keq,HX[TBP (o)]n2

)

= kHX,osAos([HX (o)]m − Keq,HX[TBP (o)]n2 [HX (s)]m

)(24)

where the subscript m corresponds to the maximum.The model Eqs. (15) to (18) are fitted to the data presented

in Fig. 6 in order to estimate the two unknown parameters, i.e., mass transfer coefficients kHX,fo, kHX,os. The best fit valuesof the mass transfer coefficients for the three acids are listedin Table 3. The simulated concentration profiles based on thebest-fit values of the parameter are plotted in Fig. 6 as solidlines (HCl- red line, HNO3- black line, and H2SO4- blue line).A good agreement between the experiments and the model isseen.

To find the effect of the presence of Fe(III) on the rate ofextraction/stripping of acids, experiments were conducted inBLM by using 1 g.L−1 Fe(III) in the feed phase. All other

FIG. 7. Concentration –time profile of mineral acids in three phases of BLMextraction by TBP with 1000ppm Fe(III) in feed aqueous solution[Feed:HCl (�)(red), HNO3(�)(black), H2SO4 (�)(blue); Strip: HCl(�), HNO3(+),H2SO4(•); Organic: HCl (◦), HNO3(�),H2SO4 (×) phase concentration].

conditions were identical to those employed in the previousexperiment. The resulting plots are as shown in Fig. 7.

A comparison between Figs. 6 and 7 reveals that the rateof acid extraction/stripping is enhanced in the presence ofFe(III). This enhancement in the rate is expected to be causedby increase in the value of Keq,HX . To check this point, modelEqs. (15) to (18) were simulated using the values of Keq,HX inthe presence of Fe(III), as listed in Table 1. The best fit values ofthe mass transfer coefficients are listed in Table 2. These valuesare close to those estimated in the absence of the metal. Thisis expected, since the concentration of Fe(III) is too small toaffect the hydrodynamics of the system. The best fit profiles areshown by solid line in Fig. 7. They are also in good agreementwith the experimental data.

The values of Mass transfer coefficients of acids kfo, kos werecalculated using least square method, were collected in Table 3.

Table 3 results shows that in the presence of metal [Fe(III)],the mass transfer coefficient of acid extraction is enhanced. Thisobservation is similar to the equilibrium study. It was observed

TABLE 3Mass transfer coefficients of acids kfo, kos of acid extraction by 10% TBP −40% n-Decanol

Stirring rate (rpm) Initial Composition Feed phase kHX,fo (m/s) kHX,os (m/s)

100 8(M) HCl without Fe(III) 7.8 × 10−7 8.8 × 10−7

100 8(M) HCl 1000ppm Fe(III) 9.1 × 10−7 9.2 × 10−7

100 8(M) HNO3 without Fe(III) 2.2 × 10−7 2.9 × 10−7

100 8(M) HNO3 with 1000 ppm Fe(III) 3.1 × 10−7 3.2 × 10−7

100 8(M) H2SO4 without Fe(III) 1.24 × 10−7 1.9 × 10−7

100 8(M)∗ H2SO4 with 1000 ppm Fe(III) 2.25 × 10−7 2.55 × 10−7

‘∗’ Third phase was formed.

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CO-EXTRACTION/STRIPPING OF MINERAL ACIDS AND IRON(III) BY TRI-N-BUTYL PHOSPHATE 551

that aqueous phase 8(M) H2SO4 with organic phase 10% TBP,40% n-decanol third phase was formed. Figure 7 shows thatH2SO4 extraction by TBP-nDecanol system is higher comparedto HCl extraction by TBP. Third phase promotes H2SO4 acidextraction due to adduct formation (H2SO4.TBP).

Study of Kinetics of Extraction of Fe(III) by TBP-nDecanol System

This study was limited to HCl. In these experiments, the con-centrations of the Fe(III) in the feed phase and the strip phasewere monitored as a function of time and the concentration inthe organic phase was estimated by material balance.

The resulting concentration-time profile of Fe(III) is shownin Fig. 8. A maximum in the concentration profile of Fe(III)in the organic phase is seen from the figure. The followingcondition is satisfied at the point of maximum.

kFe,foA fo

([Fe (f )

]m

− [Fe (o)]m

Keq,Fe[TBP (o)]n1[HX (f )

]m

)

= kFe,osAos([Fe (o)]m − Keq,Fe[TBP (o)]n1 [HX (s)]m[Fe (s)]m

)(25)

Mass transfer coefficient of Fe(III) between the feed andorganic phase, kFe,fo and Organic to Strip phase, kFe,os wasdetermined by fitting the developed model with experimentalconcentration- time data of Fe(III) for all the three phases (Feed,Organic, and Strip) as shown in Fig. 8 and the result obtainedkFe,fo =7.51×10−7 (m/s) and kFe,os =7.43×10−7(m/s).

FIG. 8. Experimental conc. profile of Fe(III) all three phase[Feed phase(red),strip phase(green), organic phase (blue)] (organic phase: 10%TBP,40% n-decanol in paraffin, Feed phase: 8MHCl +1000 ppm Fe(III), Strip phase: plainwater stirrer rpm = 100, Initial Feed 8(M) HCl, [Fe(III)] = 1000 ppm.

Extraction of Fe(III) using Liquid Emulsion Membranes(LEM)

LEM studies of Fe(III) extraction were conducted in 500 mLglass vessel provided with a 3-blade propeller as agitator. Thefeed phase was 400 mL of 8M aqueous HCl solution. The ini-tial concentration of Fe(III) in the feed phase was 1 g.L−1.The membrane phase was light liquid paraffin containing 10%(v/v) TBP and 40% (v/v) n-decanol as the polymeric surfac-tant. The strip phase consisted of 0.1 M HCl. The surfactantsystem consisted of 2% (w/w) Abil EM 90 (based on paraf-fin) and 10 g.L−1 PVP (MW ca 60000). The ratio of feed toemulsion phase was 10:1 (v/v), and the ratio of the feed to stripphase was 20:1 (v/v).

Emulsion was prepared using Ultra-Turrax T-25 homoge-nizer (IKA, Germany). The emulsification speed of 9000 rpmand emulsification time of 30 min was used. Pertraction wasconducted at stirring speed of 400 rpm. The samples of the feedphase were withdrawn at regular intervals of time and tested forthe concentration of Fe(III). The concentration–time of Fe(III)in the feed phase is shown in Fig. 9(a).

It is seen that the concentration of Fe(III) falls sharply dur-ing the initial period, but passes through a minimum, and thenbegins to rise. The concentration during the initial period can berelated to time by the following equation as seen from Fig. 9(b).

ln

( [Fe (f )

][Fe (f )

]0

)= kt (26)

where[Fe (f )

]0is the initial concentration of Fe(III) in the feed

and k is a rate constant. The value of the rate constant was esti-mated from the plot, is 0.97 min−1 or 0.0162 s−1. Since in thissystem we expect the mass transfer coefficient kL of the orderof 10−5 m.s−1 and the interfacial area a of 1000 m2.m−3, therate constant appears to have the same order of magnitude asthe mass transfer coefficient. Hence it is reasonable to assumethat rate is controlled by mass transfer.

It was initially believed that the increase in the concentrationof Fe(III) in the feed after 5 minutes was due to leakage of thestrip phase into the feed phase. To test this possibility, experi-ments were conducted to test leakage of a dye added into thestrip phase. This study is described in the next section.

Leakage StudyA standard Methylene blue marker solution from Merck was

used as the strip phase in this study. The feed solution was 8MHCl without Fe(III). The rest of the parameters were identicalto those used in pertraction studies. Samples of the feed werewithdrawn at regular intervals and analysed for the concentra-tion of methylene blue (λmax = 668nm). The leakage (%) ofthe strip phase from strip to feed phase was computed by thefollowing equation:

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552 D. DAS, V. A. JUVEKAR, AND R. BHATTACHARYA

FIG. 9. Back extraction of iron(III) from strip phase to feed phase duringLEM pertraction after 5 min (Organic phase: 10%TBP, 40% n-decanol, 2%Abil EM90, in paraffin, Initial Feed: 8(M) HCl, [Fe(f)]◦ = 1000 ppm), Stripphase: 0.1 M HCl, 10 mg/mL PVP).

%leakage(t) = Vf Cf (t)

VsiCsi× 100 (27)

where Vf is the volume of the feed phase, Cf (t) is the con-centration of the marker in the feed at time t, Vsi is the initialvolume of the strip phase, and Csi is the initial concentrationof the marker in the strip phase. In Fig. 9, we have plotted thepercent leakage versus time at different concentrations of TBPand the polymeric surfactant.

FIG. 10. %leakage of marker (methylene blue) from internal strip phase toexternal feed aqueous phase during LEM pertraction (External aqueous feedphase: acidity 8(M) HCl; Organic phase: 10%TBP, 4% Abil EM90 in Paraffin,Strip phase: methylene blue marker primary emulsion 50%W/O, Treat Ratio,TR = 1:10, pertraction stirrer speed 300 rpm.

Figure 10 show that percent leakage increases linearly withtime. It is also seen that the leakage increases with increase inthe concentration of TBP, but decreases with concentration ofAbilEM90. Higher concentration of TBP makes more unsta-ble emulsion as leakage is more. This is caused by competitiveadsorption of TBP with the polymeric surfactant AbilEM90 atwater-oil interface. Although leakage was observed by LEMstabilized by polymeric surfactant Abil EM90 but the rateof leakage is very low. The time of the experiment in

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CO-EXTRACTION/STRIPPING OF MINERAL ACIDS AND IRON(III) BY TRI-N-BUTYL PHOSPHATE 553

the pertraction (LEM extraction), was less than 15 minutes.Negligible leakage occurs during this short time and hence thetrend in Fig. 9(a) cannot be explained on the basis of leakage.

The other possibility is the back extraction of Fe(III) byTBP. We have seen from the equilibrium studies that TBP hasa strong tendency to form complex with HCl. In the presenceof Fe(III) this tendency is enhanced. When this TBP. HCl orFe(III).TBP.HCl complex undergoes decomposition, HCl isreleased in the strip phase. As a result, the strip phase getsgradually enriched with respect to HCl. High concentrationHCl is less effective in decomposing the TBP.Fe(III) complex.The driving force is further reduced due to enrichment of thestrip phase by Fe(III). Reversal of the driving force occurswhen both the concentrations of the acid and Fe(III) reachcritical values in the strip phase. This explains increase in theconcentration of Fe(III) in the feed phase during pertraction.

To find the change in the concentration of HCl in thestrip phase during pertraction, the strip phase was analyzedafter demulsifying the emulsion by heating at 80◦C. Theconcentration of HCl in the strip phase was found to reach4 M within 15 seconds and 7 M within 5 min of pertraction.This shows that after 5 minutes the strip phase will not onlybe ineffective in decomposing TBP.Fe(III) complex. Thus after5 min of pertraction, at this high concentration acid (7 M)in the strip phase, Fe(III) extraction can take place from thestrip phase to the feed phase. This causes ferric ion transportin opposite direction which we term as back extraction. Thusconcentration of Fe(III) in the feed phase began to rise after5 minutes of pertraction.

CONCLUSIONSThe results of this study revealed that TBP (modified by n-

decanol) has a high propensity for extracting mineral acid intohydrocarbon phase. The affinity is in the order HNO3> H2SO4.>HCl. TBP forms complex with the acid of the type TBP.n2HX.The values n2 are approximately 0.5 for HCl and HNO3 andn2 = 1 for H2SO4. It is possible that more than one type ofcomplex may be formed between acid and TBP. Hence the(TBP.HX) complex cannot be expressed using integer valuesof stoichiometric coefficients.

It was also found that the rate of extraction of mineral acidswas significantly enhanced in the presence of the metal. Thiseffect is significant in the case of HCl extraction.

TBP (modified by n-decanol) extracts Fe(III) into hydrocar-bon phase. The affinity is in the order HCl > HNO3 with noextraction from H2SO4. TBP forms complex with the metalof the type FeCl3 · n1TBP. The values n1 are 0.29, 1.27 forHNO3 and HCl, respectively. It is possible that more than onetype of complex may be formed between Fe(III). Therefore,[Fe(III).TBP.HCl] complex cannot be expressed using integervalues of stoichiometric coefficients.

The rate of extraction of Fe(III) using bulk liquid membranewas found to be fastest from HCl, since the equilibrium constantfor the extraction of HCl by TBP is slowest, whereas, that for

Fe(III) extraction is highest. The model fitted well and masstransfer coefficient of HCl is estimated 7.8×10−7 m/s.

The mass transfer of Fe(III) from HCl using LEM was veryfast but the concentration of Fe(III) in the feed began to riseafter a certain time. Negligible leakage of the strip phase wasobserved from the independent studies. Hence it was concludedthat the reversal of the concentration-time profile is caused byback extraction of Fe(III) from the strip phase to the feed phase.This reversal of the driving force is caused by extraction of HClfrom the feed to the strip phase.

FUNDINGThe author wishes to thank the Board of Research in Nuclear

Sciences (BRNS), India for funding the experimental work.

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9. Hirato, T.; Wu, Z-C.; Yamada, Y.; Majima, H. Improvement of thestripping characteristics of Fe (III) utilizing a mixture of di (2-ethylhexylphosphoric acid and tri-n-butyl phosphate. Hydrometallurgy,28: 81–93.

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13. Mishra, R. K.; Rout, P. C.; Sarangi, K.; Nathsarma, K. C. (2010)Comparative study on extraction of Fe(III) from chloride leach liquor usingTBP, Cyanex 921and Cyanex 923. Hydrometallurgy, 104: 298–303.

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