Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 82:376–381 (2007)

Solvent extraction of lanthanidesand yttrium from nitrate medium withCYANEX 925 in heptaneWei Li,1,2 Xianglan Wang,1,2 Hui Zhang,1,2 Shulan Meng1 and Deqian Li1∗1Key laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Changchun 130022, China2Graduate School of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The extraction behavior of lanthanides and yttrium usinsg CYANEX 925 (mixture of branched chainalkylated phosphine oxides) in n-heptane from nitrate medium has been studied. The effects of aqueous phase ionicstrength, CYANEX 925 concentration in the organic phase, and temperature on Sm3+, Nd3+ and Y3+ extractionhave been investigated. The extractability of the lanthanides and yttrium increases with increasing nitrateconcentration, as well as with increasing CYANEX 925 concentration. An extraction mechanism is proposed basedon slope analysis. Furthermore, the infra-red spectra of CYANEX 925 saturated with lanthanides are employed toprovide evidence of the composition of the complex. The relationship between the logarithm of the distributionratio and lanthanide atomic number is also discussed which indicates that yttrium can be separated from lightlanthanides. In addition separation of the light and heavy lanthanide groups is also possible using CYANEX 925.From the temperature dependence data, the thermodynamic parameters values (�H, �S and �G) are calculated. 2007 Society of Chemical Industry

Keywords: lanthanides; yttrium; CYANEX 925; extraction mechanism; separation

INTRODUCTIONCYANEX 925 is a branched chain alkylated phos-phine oxide mixture whose structure is: R′

3P(O),R′

2RP(O), where R = [CH3(CH2)7], R′ = [CH3

C(CH3)2CH2CH(CH3)CH2 –]. The major compo-nent of the mixture is R′

2RP(O) representing approx-imately 85% of the mixture, while R′

3P(O) representsabout 8% of mixture. The physical properties ofCYANEX 925 are listed in Table 1.1 CYANEX 925has significant advantages as an extractant, such asgreater hydrolytic stability and lower aqueous sol-ubility than TBP;2 a liquid at room temperatureand available in a reasonably pure form, eliminatingthe need for tedious, reagent-consuming purificationprocedures.3 Because of the spatial steric hindranceeffect of the branched chain, the extraction selectivityof CYANEX 925 is superior to CYANEX 923 (con-sisting of mixtures of R3PO, R2R′PO, RR′

2PO andR′

3PO, R = [CH3(CH2)7], R′ = [CH3(CH2)5] andCYANEX 921 (R3PO, R = [CH3(CH2)7]).4

Extensive studies using CYANEX 925 have beencarried out; for example, it has been used to extractand separate scandium(III),2 gold,5–8 3d transitionelements,9,10 arsenic,11 uranium,3 lead,12 tin(II),13,14

zinc(II) and cadmium(II),15,16 antimony(III) andbismuth(III),17 tellurium(IV) and selenium(IV).18

Garcia-Valls et al. selectively separated the lan-thanides using supported liquid membranes con-taining CYANEX 925 as a carrier19 and Jia et al.investigated the synergistic extraction of La(III) using

it as synergistic component.20 Preston et al.21 showedthat the zinc ions introduced into rare earth liquorsby the use of zinc amalgam as a reductant could beremoved by solvent extraction into CYANEX 925 inxylene, without loss of rare earth values. The success-ful applications of CYANEX 925 make it worthwhileexploring its potential use in extraction of rare earths.To date, very few papers have reported the extractionof yttrium and rare earths with CYANEX 925, so wehave systematically investigated the solvent extractionof lanthanides and yttrium from nitrate medium withthis extractant in heptane solution.

EXPERIMENTALReagents and apparatusCYANEX 925, supplied by Cytec Canada Inc.,was purified by washing subsequently with 2%Na2CO3, 0.5 mol L−1 HNO3 and distilled water, thenthe concentration was determined by titration witha standard sodium hydroxide solution after beingacidified with a 3.0 mol L−1 nitric acid solution.22 Thepurified extractant was then diluted with heptane tothe required concentration. Stock solutions of eachlanthanide or yttrium were prepared by dissolving theiroxides with purity >99.9% in nitric acid and dilutingwith distilled water. All other reagents employed inthis work were of analytical grade.

A 722 Model Grating Spectrophotometer (ShanghaiPrecision & Scientific Instrument Co. Ltd (SPSIC))

∗ Correspondence to: Deqian Li, Key laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Changchun 130022, ChinaE-mail: ldq@ciac.jl.cn(Received 15 October 2006; revised version received 4 December 2006; accepted 17 January 2007)DOI: 10.1002/jctb.1680

2007 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2007/$30.00

Extraction behavior of lanthanides and yttrium using CYANEX 925

Table 1. The physical properties of CYANEX 925

ExtractantContent

(%)Density(kg m−3)

Viscosity(kg m−1 s−1)

Average molecularweight (g mol−1)

Solubility in water(mg L−1)

CYANEX 925 93 880 11.7 386 2

was used for measurements of lanthanides and yttriumconcentration. Infrared spectra were detected using aBruker (Rheinstetten, Germany) Vertex 70 Fouriertransform infrared (FTIR) spectrometer.

Extraction proceduresFor the equilibrium experiments, equal volumes(4 cm3) of aqueous and organic phases were mixedand shaken for 30 min at 298 ± 1 K (except forthe temperature experiments), which was sufficientfor equilibrium attainment. After phase separation,the concentration of lanthanides and yttrium in theaqueous phase was estimated using a colorimetricmethod with Arsenazo III at a wavelength of 653 nmand the concentration in the organic phase wasdetermined by mass balance. These concentrationswere used to obtain the distribution ratio, D =[RE]o/[RE]a, where RE denotes lanthanides andyttrium, ‘a’ and ‘o’ denotes aqueous phase and organicphase, respectively.

Stripping experiments were carried out as forthe extraction experiment, except that the volumeratio of the organic and aqueous phase was 2:2(V/V). Y3+ concentration in the loaded organic was2 × 10−4 mol L−1 and the stripping acid was HNO3.

The same experimental conditions were usedfor each experiment: [CYANEX 925] = 0.12 molL−1, [Sm3+] = 4.01 × 10−4 mol L−1, [Y3+] = 4.13 ×10−4 mol L−1, [Er3+] = 4.03 × 10−4 mol L−1,[Nd3+] = 4.14 × 10−4 mol L−1, [NaNO3] = 1 molL−1, pH = 2.3; except where the effect of some factorwas determined, such as [Y3+] = 2–4 × 10−4 mol L−1;[CYANEX 925] = 0.05–0.24 mol L−1; [NO3

−] =0.9–2.0 mol L−1; T = 293–318 K; [HNO3]stripping =0.002–0.16 mol L−1.

RESULTS AND DISCUSSIONEffect of Y 3+ concentrationThe effect of yttrium ion concentration on theextraction of Y3+ with CYANEX 925 has beeninvestigated at constant aqueous phase concentrationof NO3

− and pH. The linearity observed in Fig. 1indicates that the extracted species does not changeover this range of metal ion concentration.

The effect of CYANEX 925 concentration onextractionThe effect of CYANEX 925 concentration on yttriumand lanthanides extraction (with Nd3+, Sm3+ and Er3+as representative elements) was studied at constantmetal concentration and constant pH. The resultsare shown in Fig. 2 where it can be seen that the

-3.75

-3.65

-3.55

-3.45

-3.35

-4.2 -4.1 -4 -3.9 -3.8 -3.7 -3.6

log [Y3+]alo

g [Y

3+] o

Figure 1. Effect of Y3+ concentration on extraction.[NaNO3] = 1 mol L−1, pH = 2.3, [Y3+] = 2–4 × 10−4 mol L−1,[CYANEX 925] = 0.12 mol L−1.

extractability of lanthanides and yttrium increaseswith increasing CYANEX 925 concentration. Fromthe slopes of the log D vs. log [CYANEX 925] plots,it is inferred that two molecules of the extractant areinvolved in the extracted complexes of lanthanides andyttrium. Reddy et al. have reported similar behaviourof yttrium extraction using CYANEX 923.23

yY = 1.8239x + 1.9792

R2 = 0.9938

ySm = 1.774x + 1.6772

R2 = 0.9873

yEr = 1.9136x + 2.1913

R2 = 0.993

yNd = 2.0694x + 1.7664

R2 = 0.9953

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

-1.5 -1 -0.5 0

log [Cyanex 925, mol L-1]

log

D

Sm

Y

Nd

Er

Figure 2. Effect of CYANEX 925 concentration on the extraction ofSm3+, Y3+, Er3+, Nd3+. [Sm3+] = 4.01 × 10−4 mol/L,[Y3+] = 4.13 × 10−4 mol L−1, [Er3+] = 4.03 × 10−4 mol L−1,[Nd3+] = 4.14 × 10−4 mol L−1, [NaNO3] = 1 mol L−1, pH = 2.3.[CYANEX 925] = 0.05–0.24 mol L−1.

J Chem Technol Biotechnol 82:376–381 (2007) 377DOI: 10.1002/jctb

W Li et al.

Effect of nitrate concentrationThe effect of NO3

− concentration on the extractionof Y3+ and Nd3+ was investigated, keeping theaqueous phase pH, rare earth concentration andextractant concentration constant. The result shownin Fig. 3 indicates that the extraction of Y3+ and Nd3+increases with increasing NO3

− concentration. Thelog{D(1 + β1[NO3

−] + β2[NO3−]2)} vs. log[NO3

−]plot is a straight line with a slope close to 3, suggestingthe involvement of three nitrate ions in the formationof the extracted species. The subscripts β1 and β2

are the stability constants of Y3+ or Nd3+ withnitrate, respectively;24 log β1, Nd = 0.52, log β2, Nd =0.66; log β1, Y = −0.17, log β2, Y = −0.29.

Extraction mechanismBased on the above studies, the extraction mechanismof RE3+ from nitrate mediums with CYANEX 925was suggested as follows:

REa3+ + 3NO3

− + 2Bo ↔ RE(NO3)3.2Bo (1)

where REa represents the rare earths in the aqueousphase and Bo represents CYANEX 925 in the organicphase.

The equilibrium constant Kex, for the above reactioncan be expressed as:

Kex = [RE(NO3)3.2Bo]

[RE3+]a[NO−3 ]3

a[B]2o

=D(1 +

i∑

1

βι[NO−3 ]i)

[NO3]3[B]2o

(2)

The equilibrium constants (Kex) for the abovespecies were calculated and are shown in Table 2.Because only the values of the stability constant (βi) ofNd3+ and Y3+ were found in the literature24 and notthose of Sm3+ and Er3+, Kex values were calculated

yY = 2.743x + 0.7136 R2 = 0.9914

yNd = 3.0615x + 0.789 R2 = 0.9972

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

-0.1 0 0.1 0.2 0.3 0.4

log [NO3-, mol L-1]

log

{D(1

+β 1

[NO

3- ] +

β2

[NO

3- ]2 }

Y

Nd

Figure 3. Effect of NO3− on the extraction of Y3+ and Nd3+.

[Y3+] = 4.13 × 10−4 mol L−1, [Nd3+] = 4.14 × 10−4 mol L−1, pH = 2.3,[NO3

−] = 0.9–2.0 mol L−1, [CYANEX 925] = 0.12 mol L−1.

Table 2. Equilibrium constants and thermodynamic parameters for

the extraction of Y3+, Sm3+, Nd3+ and Er3+ with CYANEX 925

log Kex

�H(kJ mol−1)

�G(kJ mol−1)

�S(J mol−1 K−1)

Y3+ 1.19 −12.29 −18.78 ± 0.42 21.27 ± 0.51Sm3+ 0.98 −18.42 −9.84 ± 0.44 −28.11 ± 0.61Nd3+ 0.82 −13.21 −19.19 ± 0.29 19.59 ± 0.44Er3+ 1.32

using the first part of Eqn (2) to be of advantage tocomparing and keep consistent with Fig. 5.

Effect of temperature on extractionThe distribution ratios of Sm3+, Y3+ and Nd3+ byCYANEX 925 were studied at different temperatures.�H was obtained from the slope of the plot of log D vs.1000/T (Fig. 4) using the van’t Hoff equation shownbelow:

log K = − �H2.303R

1T

+ C (3)

where R is the gas constant and C is a constant fora solution of constant ionic strength. The free energychange (�G) and the entropy change (�S) of thesystem are defined as follows:

�G = −RT ln K (4)

�G = �H − T�S ⇒ �S = �H − �GT

(5)

The thermodynamic values over the temperaturerange 293–318 K were obtained using Eqns (3)–(5)and are shown in Table 2. �H is negative, so thereaction is exothermic. It is probable that CYANEX925 coordinates with the metal ions and forming

yNd = 0.6901x - 2.4861 R2 = 0.9672

ySm = 0.9619x - 3.1756 R2 = 0.961

yY = 0.6418x - 1.8539 R2 = 0.9907

-0.9

-0.7

-0.5

-0.3

-0.1

0.1

0.3

0.5

3.05 3.25 3.45

1/T × 1000, K

log

D

Sm

Y

Nd

Figure 4. The effect of temperature on the distribution ratio of Sm3+,Y3+ and Nd3+. [Sm3+] = 4.01 × 10−4 mol L−1,[Y3+] = 4.13 × 10−4 mol L−1, [Nd3+] = 4.14 × 10−4 mol L−1,[NaNO3] = 1 mol L−1, pH = 2.3, [CYANEX 925] = 0.12 mol L−1,T = 293–318 K.

378 J Chem Technol Biotechnol 82:376–381 (2007)DOI: 10.1002/jctb

Extraction behavior of lanthanides and yttrium using CYANEX 925

strong metal coordination bonds. All values of �Gare negative, so the reactions of rare earths andyttrium extraction are all spontaneous and favorablefor complexation. The negative value of �S showsthat more order is introduced in the system uponSm3+ extraction, so that the disorder caused by metalion dissociation is more than compensated for thereduction of the number of particles brought about bythe formation of the complex.

Relationship between the logarithm distributionratios (log D) and lanthanide atomic number (Z)Relationship between log D and Z is shown inFig. 5. The distribution ratio increases with increasinglanthanide atomic number and demonstrates the‘tetrad-effect’. The increase of distribution ratioamong the light lanthanides (La–Gd) is greater thanthat among heavy lanthanides (Tb–Lu), and theextractability of yttrium is between Gd and Tb,which is different from that reported by Chu Deqinget al. that using CYANEX 923 the extractabilityof yttrium was between Ce and Pr.25 However,the extractability of yttrium was between Ho andEr with 2-ethylhexylphosphonic acid mono- (2-ethylhexyl) ester (P507, the Chinese version of PC-88A (Daihachi) and Ionquest 801 (Rhodia)).26 Theseparation factors (SF) of yttrium and lanthanideswere also calculated form Dmetal 2/Dmetal 1 and areshown in Table 3. It is observed that SF betweenyttrium and light lanthanides are large with thesmallest (1.58) between yttrium and gadolinium,which indicates that CYANEX 925 may be usedto separate yttrium from light lanthanides. At thesame time, the separation factors between the lightand heavy lanthanides are large enough for them tobe separated into two groups using CYANEX 925.Therefore, CYANEX 925 may be used to separatelight lanthanides from yttrium, as well as the light andheavy lanthanide groups.

Stripping studiesThe loaded organic phase was stripped by differentconcentrations of HNO3 (Fig. 6). The percentage

56 58 60 62 64 66 68 70 72-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Y

log

D

atomic number

Figure 5. Relationship between the logarithm of distribution ratios(log D) and lanthanide atomic number (Z). [RE] = 4.0 × 10−4 mol L−1,[NaNO3] = 1 mol L−1, pH = 2.3, [CYANEX 925] = 0.12 mol L−1.

stripping of yttrium increases with increasing acidityand reaches nearly 100% when the initial pH ofstripping nitric acid solution is less than 2, whichindicates that yttrium is easy stripped from the loadedCYANEX 925. The organic phase after stripping canbe easily regenerated by washing it with water untilthe washings are neutral. It has been shown that2-ethylhexylphosphonic acid mono- (2-ethylhexyl)ester (P507), is a good reagent for the extractionand separation of lanthanides,27 and has been usedindustrially,28 but it requires high acidity for strippingespecially with yttrium and the heavy lanthanides.27

In contrast the easy stripping of loaded CYANEX 925suggests it will have potential for practical application.Therefore, because CYANEX 925 is superior to P507in its stripping capability it may be used to separateyttrium from light lanthanides, as well as separatingthe light and heavy lanthanide groups.

Infrared spectraThe organic solution obtained from the extraction ofSm(NO3)3 with CYANEX 925 in heptane was ana-lyzed by infrared (IR) spectroscopy (Fig. 7). Table 4

Table 3. Separation factors (SF) of lanthanides and yttrium in nitrate medium with CYANEX 925

SF Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y

La 1.54 2.60 2.52 5.00 4.68 5.40 8.98 10.82 10.48 10.54 10.55 14.01 13.09 8.50Ce 1.69 1.64 3.25 3.04 3.51 5.84 7.04 6.82 6.85 6.85 9.11 8.52 5.53Pr 0.97 1.92 1.80 2.08 3.45 4.16 4.03 4.05 4.05 5.39 5.03 3.27Nd 1.98 1.86 2.14 3.56 4.29 4.16 4.18 4.19 5.56 5.19 3.37Sm 0.94 1.08 1.79 2.16 2.10 2.11 2.11 2.80 2.62 1.70Eu 1.15 1.92 2.31 2.24 2.25 2.25 2.99 2.80 1.82Gd 1.66 2.00 1.94 1.95 1.95 2.59 2.43 1.58Tb 1.21 1.17 1.17 1.17 1.56 1.46 0.95Dy 0.97 0.97 0.97 1.29 1.21 0.79Ho 1.01 1.01 1.34 1.25 0.81Er 1.00 1.33 1.24 0.81Tm 1.33 1.24 0.81Yb 0.93 0.61Lu 0.65

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W Li et al.

0

20

40

60

80

100

120

0 2 4 5 6pH

% Y

3+ s

trip

ping

1 3

Figure 6. Effect of pH on yttrium stripping from loaded CYANEX 925.Loaded organic phase containing [Y3+] = 2.03 × 10−4 mol L−1,[CYANEX 925] = 0.12 mol L−1, [HNO3]stripping = 0.002–0.16 mol L−1.

gives the frequencies and probable band assign-ments for the CYANEX 925–Sm(NO3)3 extractionsystem together with those for an unloaded phaseof CYANEX 925. The following bands are iden-tified: P=O yields absorption peaks at 1173 cm−1

for CYANEX 925 and at 1123 cm−1 for its com-plex indicating that coordination takes place betweenthe reagent P=O and the metal ion. Strong peaks at1302 cm−1 and 1468 cm−1 are assigned to the symmet-rical and asymmetrical stretching vibration of NO3

−

group, respectively.25 The difference of 166 cm−1

between these bands indicates that the NO3− can be

regarded as occupying a bidentate coordination.26 Thebands at 1030 cm−1 are also due to the symmetricalstretching vibration of NO3

− and these results demon-strate that NO3

− takes part in the coordination. So theIR spectra support the proposed extraction of RE3+

from nitrate media with CYANEX 925.

-5005001500250035004500Wavenumber, cm-1

Tra

nsm

ittan

ce

a

b

Figure 7. Infrared spectra of CYANEX 925 and complexes ofCYANEX 925 loaded with Sm(NO3)3: (a) CYANEX 925; (b) CYANEX925–Sm(NO3)3.

Table 4. Characteristic IR spectral data for CYANEX 925 solution in

heptane loaded with Sm(NO3)3

CYANEX925

Sm(NO3)3 –CYANEX 925

Probableassignment

2960–2860 2960–2860 νsC–H, νas

C–H1477 1479 –CH3, δas1467 1468 NO3

−(ν1)

1394 1396 δsCH3

1377 1380 δsCH

1365 1365 νsNO2

1286 1302 NO3−(ν4)

1237 1241 NO3−(ν3)

1173 1123(1200) stretching P=O1117,1075,1047,1101 1030 NO3

−(ν2)

968 967 γCH931 931 γCH911 911 γCH820 818 stretching P–C723 737 ρCH2

CONCLUSIONSThe extraction of lanthanides and yttrium usingCYANEX 925 in nitrate medium has been studied andindicate their extraction as RE(NO3)3 • 2B species.The thermodynamic parameters of the extractionprocess have been determined and show that thereaction is exothermic. The distribution ratio increaseswith increasing lanthanide atomic number anddemonstrates the ‘tetrad effect’. The extractabilityof Y is between those of Gd and Tb, indicatingthat yttrium can be separated from light lanthanideswith CYANEX 925. The separation factors indicatethat the extractant can also be used to separate thelight and heavy lanthanides. Loaded CYANEX 925is easily stripped with nitric acid solution when thepH of stripping solution is less than 2. Therefore,CYANEX 925 has potential practical applicationsin the extraction and separation of lanthanides andyttrium.

ACKNOWLEDGEMENTSThe authors wish to thank Cytec Canada Inc. forsupplying CYANEX 925. The project was supportedby the State Key Project of the Foundation Research(2004CB719506) and National Natural ScienceFoundation of China (20371046).

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