RARE EARTHSVolume 21

Handbook on the Physics andChemistry of Rare Earths, volume 21Elsevier, 1995

Edited by: Karl A. Gschneidner, Jr. and LeRoy EyringISBN: 978-0-444-82178-2

Handbook on the Physics and Chemistry ofRare EarthsVol. 21edited by K.A. Gschneidner, Jr. and L. Eyring 1995 Elsevier Science B. V. All rights reserved

PREFACE

Karl A. GSCHNEIDNER, Jr., and LeRoy EYRING

These elements perplex us in our rearches [sic], baffle us in our speculations, and hauntus in our very dreams. They stretch like an unknown sea before us - mocking, mystifying,and murmuring strange revelations and possibilities.

Sir William Crookes (February 16, 1887)

The contributions to this volume focus on selected chemical aspects of rare-earthmaterials. The topics covered range from a basic treatment of crystalline electric-field effects and chemical interactions in organic solvents, to separation processes,electrochemical behaviors which impact corrosion, oxidation resistance, chemical energystorage and sensor technology, and to analytical procedures.

Underlying the most subtle chemical and optical properties of these elements and theircompounds in the condensed state are the crystal field effects. Garcia and Faucher discussthis phenomenon in non-metallic compounds in Chapter 144.

Bautista begins the volume by reviewing important new solvent extraction proceduresas well as emerging alternative separation processes such as photochemical separation,precipitation stripping, and supercritical extraction, in Chapter 139. Scientific andindustrial procedures are illustrated.

In order to satisfy the most demanding needs for rare earths either for direct use oras components in advanced materials and devices it is necessary to obtain accurate traceanalysis. Bhagavathy, Prasada Rao and Damodaran (Chapter 146) examine eight majoranalytical techniques and tabulate and assess the most effective procedures of each.

Environmental corrosion and degradation of metals and alloys impose enormouslosses in modern industrial societies. In Chapter 140 Hinton considers a wide varietyof methods using rare-earth solutions and salts to modify advantageously the costlydeterioration of metals and alloys. This topic is expanded by Ryan in Chapter 141,giving particular attention to protection against high-temperature oxidation, sulfidizationand hot-salt corrosion.

The versatility of the rare earths in addressing current technical problems is reviewedin Chapters 142 and 143. In the former the use of rare-earth intermetallics, principallyLaNis-based materials to provide the skyrocketing need for environmentally friendly,usually portable, battery power is discussed by Sakai, Matsuoka and Iwakura. Adachi and

v

vi PREFACE

Imanaka present, in the latter chapter, an account of the use of the rare earths to satisfythe growing demand for chemical sensors for a wide variety of substances found inour environment that require monitoring. These include oxygen, fluorine, sulfur dioxide,carbon dioxide, moisture, alcohol, and nitric oxide.

Finally, in Chapter 145 Biinzli and Milicic-Tang review the solvation, interaction andcoordination of rare-earth salts in a variety of organic solvents including dimethylac-etamide, dimethylsulfoxide, various alcohols, acetonitrile, and propylenecarbonate understrict anhydrous conditions. They also contrast these interactions with those in whichwater is present with organic solvents.

CONTENTS

Preface v

Contents vii

Contents of Volumes 1-20 IX

139. R.G. BautistaSeparation chemistry 1

140. B.W. HintonCorrosion prevention and control 29

141. N.E. RyanHigh-temperature corrosion protection 93

142. T. Sakai, M. Matsuoka and e. IwakuraRare earth intermetallics for metal-hydrogen batteries 133

143. G.-y. Adachi and N. ImanakaChemical sensors 179

144. D. Garcia and M. FaucherCrystal field in non-metallic (rare earth) compounds 263

145. J.-e.G. Biinzli and A. Milicic-TangSolvation and anion interaction in organic solvents 305

146. V. Bhagavathy, T. Prasada Rao and A.D. DamodaranTrace determination of lanthanides in high-purity rare-earth oxides 367

Author Index 385

Subject Index 411

Vll

CONTENTS OF VOLUMES 1-20

VOLUME 1: Metals1978, 1st repro 1982, 2nd repro 1991; ISBN 0-444-85020-1

I. Z.B. Goldschmidt, Atomic properties (free atom) 12. B.1. Beaudry and K.A. Gschneidner Jr, Preparation and basic properties of the rare earth metals 1733. S.H. Liu, Electronic structure of rare earth metals 2334. D.C. Koskenmaki and K.A. Gschneidner Jr, Cerium 3375. L.J. Sundstrom, Low temperature heat capacity of the rare earth metals 3796. K.A. McEwen, Magnetic and transport properties of the rare earths 4117. S.K. Sinha, Magnetic structures and inelastic neutron scattering: metals, alloys and compounds 4898. T.E. Scott, Elastic and mechanical properties 5919. A. Jayaraman, High pressure studies: metals, alloys and compounds 707

10. C. Probst and 1. Wittig, Superconductivity: metals, alloys and compounds 749II. M.B. Maple, L.E. DeLong and B.C. Sales, Kondo effect: alloys and compounds 79712. M.P. Dariel, Diffusion in rare earth metals 847

Subject index 877

VOLUME 2: Alloys and intermetaJlics1979, 1st repr. 1982, 2nd repr. 1991; ISBN 0-444-85021-X

13. A. Iandelli and A. Palenzona, Crystal chemistry of intermetallic compounds I14. H.R. Kirchmayr and C.A. Poldy, Magnetic properties of intermetallic compounds of rare earth

metals 5515. A.E. Clark, Magnetostrictive RFe2 intermetallic compounds 23116. 11 Rhyne, Amorphous magnetic rare earth alloys 25917. P. Fulde, Crystal fields 29518. R.G. Barnes, NMR, EPR and Mossbauer effect: metals, alloys and compounds 38719. P. Wachter, Europium chalcogenides: EuO, EuS, EuSe and EuTe 50720. A. Jayaraman, Valence changes in compounds 575

Subject Index 613

VOLUME 3: Non-metallic compounds - I1979, 1st repro 1984; ISBN 0-444-85215-8

21. L.A. Haskin and T.P. Paster, Geochemistry and mineralogy of the rare earths22. lE. Powell, Separation chemistry 8123. C.K. Jl'Jrgensen, Theoretical chemistry of rare earths III24. W.T. Carnall, The absorption and fluorescence spectra of rare earth ions in solution 17125. L.C. Thompson, Complexes 20926. G.G. Libowitz and AJ. Maeland, Hydrides 29927. L. Eyring, The binary rare earth oxides 33728. D.1M. Bevan and E. Summerville, Mixed rare earth oxides 40129. C.P. Khattak and EEY. Wang, Perovskites and garnets 52530. L.H. Brixner, 1.R. Barkley and W. Jeitschko, Rare earth molybdates (VI) 609

Subject index 655

IX

x CONTENTS OF VOLUMES 1-20

VOLUME 4: Non-metallic compounds - II1979, 1st repro 1984; ISBN 0-444-85216-6

31. 1. Flahaut, Sulfides, selenides and tellurides32. 1.M. Haschke, Halides 8933. F Hulliger, Rare earth pnictides 15334. G. Blasse, Chemistry and physics ofR-activated phosphors 23735. M.J. Weber, Rare earth lasers 27536. FK. Fong, Nonradiative processes of rare-earth ions in crystals 31737A. 1.W O'Laughlin, Chemical spectrophotometric and polarographic methods 34137B. S.R. Taylor, Trace element analysis of rare earth elements by spark source mass spectroscopy 35937C. R.I. Conzemius, Analysis of rare earth matrices by spark source mass spectrometry 37737D. E.L. DeKalb and VA. Fassel, Optical atomic emission and absorption methods 40537E. A.P. D'Silva and VA. Fassel, X-ray excited optical luminescence of the rare earths 44137F FWV Boynton, Neutron activation analysis 45737G. S. Schuhmann and 1.A. Philpotts, Mass-spectrometric stable-isotope dilution analysis for lanthanides

in geochemical materials 47138. 1. Reuben and G.A. Elgavish, Shift reagents and NMR ofparamagnetic lanthanide complexes 48339. 1. Reuben, Bioinorganic chemistry: lanthanides as probes in systems ofbiological interest 51540. T.1. Haley, Toxicity 553

Subject index 587

VOLUME 51982, 1st repr. 1984; ISBN 0-444-86375-3

41. M. Gasgnier, Rare earth alloys and compounds as thin films 142. E. Gratz and MJ. Zuckermann, Transport properties (electrical resitivity, thermoelectric power and

thermal conductivity) of rare earth intermetallic compounds 11743. FP. Netzer and E. Bertel, Adsorption and catalysis on rare earth suifaces 21744. C. Boulesteix, Defects and phase transformation near room temperature in rare earth sesquioxides 32145. O. Greis and 1.M. Haschke, Rare earth fluorides 38746. C.A. Morrison and R.P. Leavitt, Spectroscopic properties of triply ionized lanthanides in transparent

host crystals 461Subject index 693

VOLUME 61984; ISBN 0-444-86592-6

47. K.H.J. Buschow, Hydrogen absorption in intermetallic compounds 148. E. Parthe and B. Chabot, Crystal structures and crystal chemistry ofternary rare earth-transition metal

borides, silicides and homologues 11349. P. Rogl, Phase equilibria in ternary and higher order systems with rare earth elements and boron 33550. H.B. Kagan and 1.L. Namy, Preparation ofdivalent ytterbium and samarium derivatives and their use

in organic chemistry 525Subject index 567

VOLUME 71984; ISBN 0-444-86851-8

51. P. Rogl, Phase equilibria in ternary and higher order systems with rare earth elements and silicon52. K.HJ. Buschow, Amorphous alloys 26553. H. Schumann and W Genthe, Organometallic compounds of the rare earths 446

Subject index 573

VOLUME 81986; ISBN 0-444-86971-9

CONTENTS OF VOLUMES 1-20 xi

54. K.A. Gschneidner Jr and FW Calderwood, Intra rare earth binary alloys: phase relationships, latticeparameters and systematics I

55. X. Gao, Polarographic analysis of the rare earths 16356. M. Leskela and L. Niinisto, Inorganic complex compounds I 20357. J.R. Long, Implications in organic synthesis 335

Errata 375Subject index 379

VOLUME 91987; ISBN 0-444-87045-8

58. R. Reisfeld and C.K. Jmgensen, Excited state phenomena in vitreous materials59. L. Niiniste and M. Leskela, Inorganic complex compounds II 9160. J.-C.G. Biinzli, Complexes with synthetic ionophores 32161. Zhiquan Shen and Jun Ouyang, Rare earth coordination catalysis in stereospecific polymerization 395

Errata 429Subject index 431

VOLUME 10: High energy spectroscopy1988; ISBN 0-444-87063-6

62. Y. Baer and W-D. Schneider, High-energy spectroscopy of lanthanide materials - An overview I63. M. Campagna and F U. Hillebrecht, f-electron hybridization and dynamical screening of core holes in

intermetallic compounds 7564. O. Gunnarsson and K. Schenhammer, Many-body formulation ofspectra ofmixed valence systems 10365. A.J. Freeman, B.l. Min and M.R. Norman, Local density supercell theory ofphotoemission and inverse

photoemission spectra 16566. D.W Lynch and J.H. Weaver, Photoemission of Ce and its compounds 23167. S. Hiifner, Photoemission in chalcogenides 30168. J.E Herbst and J.W Wilkins, Calculation of4f excitation energies in the metals and relevance to mixed

valence systems 32169. B. Johansson and N. Martensson, Thermodynamic aspects of 4f levels in metals and compounds 36170. ED. HiIlebrecht and M. Campagna, Bremsstrahlung isochromat spectroscopy ofalloys and mixed valent

compounds 42571. J. Rohler, X-ray absorption and emission spectra 45372. EP. Netzer and J.A.D. Matthew, Inelastic electron scattering measurements 547

Subject index 60 I

VOLUME 11: Two-hundred-year impact of rare earths on science1988; ISBN 0-444-87080-6

H.I. Svec, Prologue 173. F. Szabadvary, The history of the discovery and separation of the rare earths 3374. B.R. Judd, Atomic theory and optical spectroscopy 8175. C.K. Jorgensen, Influence of rare earths on chemical understanding and classification 19776. U. Rhyne, Highlights from the exotic phenomena of lanthanide magnetism 29377. B. Bleaney, Magnetic resonance spectroscopy and hyperfine interactions 32378. K.A. Gschneidner Jr and A.H. Daane, Physical metallurgy 40979. S.R. Taylor and S.M. McLennan, The significance of the rare earths in geochemistry and

cosmochemistry 485Errata 579Subject index 581

xu

VOLUME 121989; ISBN 0-444-87105-5

CONTENTS OF VOLUMES 1-20

80. J.S. Abell, Preparation and crystal growth of rare earth elements and intermetallic compounds81. Z. Fisk and J.P. Remeika, Growth of single crystals from molten metal fluxes 5382. E. Burzo and H.R. Kirchmayr, Physical properties of R2FeI4B-based alloys 7183. A. Szytula and 1. Leciejewicz, Magnetic properties of ternary intermetallic compounds of the RT2X2

type 13384. H. Maletta and W Zinn, Spin glasses 21385. 1. van Zytveld, Liquid metals and alloys 35786. M.S. Chandrasekharaiah and K.A. Gingerich, Thermodynamic properties ofgaseous species 40987. WM. Yen, Laser spectroscopy 433

Subject index 479

VOLUME 131990; ISBN 0-444-88547-1

88. E.I. Gladyshevsky, 0.1. Bodak and VK. Pecharsky, Phase equilibria and crystal chemistry in ternaryrare earth systems with metallic elements I

89. A.A. Eliseev and G.M. Kuzmichyeva, Phase equilibrium and crystal chemistry in ternary rare earthsystems with chalcogenide elements 191

90. N. Kirnizuka, E. Takayama-Muromachi and K. Siratori, The systems R20rM203-M0 28391. R.S. Houk, Elemental analysis by atomic emission and mass spectrometry with inductively coupled

plasmas 38592. P.H. Brown, A.H. Rathjen, RoO. Graham and D.E. Tribe, Rare earth elements in biological systems 423

Errata 453Subject index 455

VOLUME 141991; ISBN 0-444-88743-1

93. R. Osborn, S.W Lovesey, A.D. Taylor and E. Balcar, Intermultiplet transitions using neutronspectroscopy 1

94. E. Donnann, NMR in intermetallic compounds 6395. E. Zirngiebl and G. Giintherodt, Light scattering in intermetallic compounds 16396. P. Thalmeier and B. LUthi, The electron~phonon interaction in intermetallic compounds 22597. N. Grewe and F. Steglich, Heavy fermions 343

Subject index 475

VOLUME 151991; ISBN 0-444-88966-3

98. IG. Sereni, Low-temperature behaviour ofcerium compounds I99. G.-y. Adachi, N. Imanaka and Zhang Fuzhong, Rare earth carbides 61

100. A. Simon, Hj. Mattausch, GJ. Miller, W Bauhofer and R.K. Kremer, Metal-rich halides 191101. R.M. Almeida, Fluoride glasses 287102. K.L. Nash and lC. Sullivan, Kinetics ofcomplexation and redox reactions ofthe lanthanides in aqueous

solutions 347103. E.N. Rizkalla and G.R. Choppin, Hydration and hydrolysis of lanthanides 393104. L.M. Vallarino, Macrocycle complexes of the lanthanide(III) yttrium(III) and dioxouranium(VI) ions

from metal-templated syntheses 443Errata 513Subject index 515

CONTENTS OF VOLUMES 1-20

MASTER INDEX, Vols. 1-151993; ISBN 0-444-89965-0

VOLUME 161993; ISBN 0-444-89782-8

105. M. Loewenhaupt and K.H. Fischer, Valence-fluctuation and heavy-jermion 4f systems106. LA. Smirnov and VS. Oskotski, Thermal conductivity of rare earth compounds 107107. M.A. Subramanian and A.W Sleight, Rare earths pyrochlores 225108. R. Miyawaki and 1. Nakai, Crystal structures of rare earth minerals 249109. D.R. Chopra, Appearance potential spectroscopy of lanthanides and their intermetallics 519

Author index 547Subject index 579

VOLUME 17: Lanthanides/Actinides: Physics - I1993; ISBN 0-444-81502-3

Xll1

110. M.R. Norman and D.D. Koelling, Electronic structure, Fermi surfaces, and superconductivity inf electron metals 1

I I I . S.H. Liu, Phenomenological approach to heavy-fermion systems 87112. B. Johansson and M.S.S. Brooks, Theory ofcohesion in rare earths and actinides 149113. U. Benedict and WB. Holzapfel, High-pressure studies - Structural aspects 245114. O. Vogt and K. Mattenberger, Magnetic measurements on rare earth and actinide monopnictides and

monochalcogenides 301115. J.M. Fournier and E. Gratz, Transport properties of rare earth and actinide intermetallics 409116. W. Potzel, G.M. Kalvius and J. Gal, Mossbauer studies on electronic structure of intermetallic

compounds 539I 17. G.H. Lander, Neutron elastic scattering from actinides and anomalous lanthanides 635

Author index 711Subject index 753

VOLUME 18: Lanthanides/Actinides: Chemistry1994; ISBN 0-444-81724-7

118. G.T. Seaborg, Origin of the actinide concept 1119. K. Balasubramanian, Relativistic effects and electronic structure of lanthanide and actinide

molecules 29120. J.V Beitz, Similarities and differences in trivalent lanthanide- and actinide-ion solution absorption

spectra and luminescence studies 159121. K.L. Nash, Separation chemistry for lanthanides and trivalent actinides 197122. L.R. Morss, Comparative thermochemical and oxidation-reduction properties of lanthanides and

actinides 239123. J.W Ward and J.M. Haschke, Comparison of 4f and 5f element hydride properties 293124. H.A. Eick, Lanthanide and actinide halides 365125. R.G. Haire and L. Eyring, Comparisons of the binary oxides 413126. S.A. Kinkead, K.D. Abney and T.A. O'DonneIl,j-element speciation in strongly acidic media: lanthanide

and mid-actinide metals, OXides, fluorides and oxide fluorides in superadds 507127. E.N. Rizkalla and G.R. Choppin, Lanthanides and actinides hydration and hydrolysis 529128. G.R. Choppin and E.N. RizkaIIa, Solution chemistry ofactinides and lanthanides 559129. J.R. Duffield, D.M. Taylor and D.R. Williams, The biochemistry of the j-elements 591

Author index 623Subject index 659

XIV CONTENTS OF VOLUMES 1-20

VOLUME 19: Lanthanides/Actinides: Physics ~ II1994; ISBN 0-444-82015-9

130. E. Holland-Moritz and G.H. Lander, Neutron inelastic scattering from actinides and anomalouslanthanides I

131. G. Aeppli and C. Broholm, Magnetic correlations in heavy-fermion systems: neutron scattering ji-omsingle crystals 123

132. P. Wachter, Intermediate valence and heavy fermions 177133. J.D. Thompson and J.M. Lawrence, High pressure studies - Physical properties of anomalous Ce, Yb

and U compounds 383134. C. Colinet and A. Pasture!, Thermodynamic properties ofmetallic systems 479

Author Index 649Subject Index 693

VOLUME 201995; ISBN 0-444-82014-0

135. Y. Onuki and A. Hasegawa, Fermi suifaces of intermetallic compounds 1136. M. Gasgnier, The intricate world of rare earth thin films: metals, alloys, intermetallics,

chemical compounds, . . . 105137. P. Vajda, Hydrogen in rare-earth metals, including RH2+r phases 207138. D. Gignoux and D. Schmitt, Magnetic properties of intermetallic compounds 293

Author Index 425Subject Index 457

Handbook on the Physics and Chemistry of Rare EarthsrfJl. 21edited by K.A. Gschneidner, fr. and L. Eyring 1995 Elsevier Science B. V All rights reserved

Chapter 139

SEPARATION CHEMISTRY

Renato G. BAUTISTADepartment of Chemical and Metallurgical Engineering, University ofNevada,Reno, NV 89557-0136, USA

Contents

List of symbols1. Introduction2. Solvent extraction

2.1. Carboxylic acids2.2. Tri-n-buty1 phosphate2.3. Di-2-ethy1hexyl phosphoric acid

2.3.1. Chloride medium2.3.2. Nitrate medium2.3.3. Sulfate medium

2.4. Amines2.5. Hydroxyoximes

List of symbols

1 2.6. Synergic extractants2 2.7. Macrocyc1ic extractants3 2.8. Other extractants4 3. Photochemical separation5 4. Precipitation stripping7 5. Ion-exchange7 6. Supercritical extraction8 7. Industrial producers9 8. Summary and conclusions9 References

12

]3]41618192021212323

Alamine mixture of 8- and 10-carbon straight Ll calixarene p-tert-butylcalix/6/arene336 chain tertiary amine hexacarboxylic acidAliquat 336 tricapryl monomethyl ammonium L2 calixarene p-tert-butylcalix/4/arene

chloride tetracarboxylic acidCalixarenes (1, n)-cyc1ophanes L3 calixarene [3,1,3, 1]-cyc1ophaneDACDA macrocyc1ic 1, I0-diaza-4, 7, 13, 16- LIX63 anti isomer of

tetraoxacyclooctadecane-N, N'-diacetic 5,8-diethyl-7-hydroxy-6-dodecanoneacid oxime

DAPDA macrocyclic I, 7-diaza-4, 10, 13- LIX70 2-hydroxy-3-chloro-5-nonyl-trioxacyc1opentadecane-N, N' -diacetic benzophenone oximeacid PC-88A 2-ethylhexyl phosphonic acid

DBTPA dibutylmonothiophosphoric acid mono-2-ethy1hexyl esterDDTPA didodecylmonothiophosphoric acid PHEN 1, 10-phenanthrolineDTPA diethylenetriaminepentaacetic acid RCOOH napthenic acidsEDTA ethylenediamine tetraacetic acid SME 529 2-hydroxy-5-nonyl-acetophenoneHDEHP di-2-ethy1hexy1 phosphoric acid oxime, anti-isomerHTTA thenoy1trifluoroacetone Socal 355L diluentK22DD an aza-crown ether: 4, 13-didecy1- TBP tri-n-butyl phosphate

1, 7, 10, 16-tetraoxa-4, 13- Versatic 10 2-ethyl-2-methy1heptanoic aciddiazacyc100ctadecane

21. Introduction

R.O. BAUTISTA

The separation chemistry and technology of rare earths have developed to a sufficientlyhigh degree of sophistication that very high purity products can generally be producedwhen required. The mineralogy (Haskin and Paster 1979), beneficiation (Bautista andWong 1989, Gupta and Krisnamurthy 1992), and extraction (Bautista and Wong 1989,Bautista 1992, Bautista and Jackson 1992, Gupta and Krisnamurthy 1992) of therare earths from resources such as monazite, bastnasite, xenotime, euxenite and as by-products of other metal resources processing are equally well developed and efficient.The rare earths present in these ores and other resources are readily leached into solutionas a nitrate, sulfate or chloride (Bril 1964, Bautista 1990, Gupta and Krisnamurthy1992).

The primary separation technique used in the first half of this century involved mainlyfractional precipitation to obtain satisfactory intermediate grade concentrates (Krumholzet al. 1958, Ryabchikov 1959, Healy and Kremers 1961, Topp 1965, Callow 1966, 1967).

The first successful ion-exchange separation of rare earths using 5% citric acid-ammonium citrate eluant at low pH carried out on either H+-state or NH.t-state resin bedswas reported in a collection of nine scientific papers in 1947 (Tompkins et al. 1947,Spedding et al. 1947a--e, Marinsky et al. 1947, Harris and Tompkins 1947, Ketelle andBoyd 1947, Boyd et al. 1947, Tompkins and Mayer 1947). This technique, however, wasuneconomical for moderate- or large-scale rare earth separation.

Spedding et al. (1954a,b) reported the use ofFe3+ and Cu2+ as the retaining ions to formsoluble ethylenediaminetetraacetic acid (EDTA) complexes, thus allowing redeposition ofthe rare earths on the resin bed and the transport of the chelating agent off the column insoluble form. The pilot plant for the ion-exchange separation of rare earths using EDTAis described by Powell and Spedding (1959a,b). Powell (1979) reviewed the separationchemistry of rare earths with primary emphasis on the ion-exchange process.

The chemistry of the rare earths is characterized by the similarity in the properties of thetrivalent ions and their compounds. Krumholz (1964) reviewed the structure, properties,solubilities and coordination chemistry of rare earth ions in solution. Moeller (1961)reviewed the electronic configurations, size relationships and various oxidation states ofrare earths. Carnall (1979) reviewed the literature on the absorption and fluorescencespectra of rare earth ions in solution. The complexes formed by rare earth ions have beenreviewed by Thompson (1979).

The analytical chemistry of rare earths has been reviewed by Banks and Klingman(1961), Loriers (1964), Ryabchikov (1959), and Ryabchikov and Ryabukhin (1964).Fassel (1961) reviewed the analytical spectroscopy of rare earth elements. In volume 4of this Handbook chapters can be found on the chemical spectrophotometric andpolarographic methods (O'Laughlin 1979), spark source mass spectrometry (Conzemius1979, Taylor 1979), optical atomic emission and absorption (DeKalb and FasseI1979), x-ray excited optical luminescence (D'Silva and Fassel 1979), neutron activation (Boynton1979), mass spectrometric stable isotope dilution analysis (Schuhmann and Philpotts1979), and shift reagents and NMR (Reuben and Elgavish 1979).

SEPARATION CHEMISTRY 3

Table 1The rare earths: chronology, atomic number and ionic radii

Year Element Symbol Atomic Ionic radius Discoverernumber [R3+ ion, CN = 6]

(A)

1794 Yttrium Y 39 0.900 Gado1in1814 Cerium Ce 58 1.010 Berzelius1839 Lanthanum La 57 1.045 Mosander1843 Erbium Er 68 0.890 Mosander1878 Terbium Tb 65 0.923 Mosander1878 Ytterbium Yb 70 0.868 Marignac1879 Samarium Sm 62 0.958 de Boisbaudran1879 Scandium Sc 21 0.745 Nilson1879 Holmium Ho 67 0.901 Cleve1879 Thulium Tm 69 0.88 Cleve1880 Gadolinium Gd 64 0.938 Marignac1885 Praseodymium Pr 59 0.997 Auer von We1sbach1885 Neodymium Nd 60 0.983 Auer von Welsbach1886 Dysprosium Dy 66 0.912 de Boisbaudran1896 Europium Eu 63 0.947 Demar~ay1907 Lutetium Lu 71 0.861 Urbain1945 Promethium Pm 61 0.97 Glendenin/Marinsky

Rare earths have been known for over two hundred years since the discovery of yttriumin 1794 by Gadolin (Weeks 1956, Szabadvary 1980). Table 1 lists the chronology of theirdiscovery (Habashi 1990), their atomic number and ionic radii (Gschneidner 1993).

2. Solvent extraction

The earlier work on the solvent extraction of rare earths has been reviewed by Peppard(1961, 1964) and Weaver (1964, 1968, 1974).

The conventional separation scheme is to leach the primary ore or concentrates and usethe resulting solution containing the rare earth mixtures as the feedstock to the solventextraction plant. Solvent extraction of the rare earth mixture in the leached solutionseparates them into bulk concentrates of light (La, Ce, Pr, Nd, etc.), middle (Sm, Eu, Gd,etc.) and heavy (Tb, Dy, Ho, Er, Tm, Vb, Lu, Y) rare earths. A typical solvent extractionof rare earths in a HCl medium is with di-2-ethylhexyl phosphoric acid, HDEHP, in akerosene diluent. The individual rare earth is separated from the bulk light, middle, andheavy rare earth solution mixtures by additional individual rare earth solvent extractionstreams. The number of stages for solvent extraction cascades or batteries increaseswith the increase in purity of each individual rare earth produced. Further purification

4 R.O. BAUTISTA

of individual rare earths into the high ninety-nines purity are usually carried out byion exchange and/or chromatographic techniques. The concentrated, stripped rare earthsolution from the extraction is precipitated as an oxalate from which the rare earth oxideis produced after calcination.

The primary industrial extractants in use for the separation of rare earths by solventextraction are di-2-ethylhexyl phosphoric acid (HDEHP), tributyl phosphate (TBP),carboxylic acids, and amines. Many other extractants have been examined and reportedin the literature in the search for improving the extraction and separation of individualrare earths. This chapter discusses several of these extractants for their interestingchemistry and potential future development, in addition to the available industrialextractants now in use and proposed for the separation of rare earths.

The solvent extraction reaction chemistry in specific medium, extractants, pH, diluents,and in synergistic systems are discussed in relation to the transfer of the rare earthextractable complex from the aqueous phase to the organic phase.

2.1. Carboxylic acids

Carboxylic acids are commercially available and are relatively inexpensive. Naphthenicacids have the general formula RCOOH, where R is a radical derived predominantly fromcyclopentane or a homolog of cyclopentane.

Bauer and Lindstrom (1964) reported moderate success in the use of naphthenic acidas an extractant for rare earth sulfates with diethylether or n-hexanol as a diluent. Theextraction required 6 mols of naphthenic acid to 1mol of rare earth oxide at pH ~ 7.6.Rare earth extraction was dependent on pH, naphthenic acid concentration, and the molratio of naphthenic acid to rare earth.

Addition of an aqueous phase chelating reagent such as EDTA enhanced the separationfactors to 2.2 for adjacent yttrium group elements. Addition of DTPA enhanced theseparation factor to 3.5 for adjacent cerium group elements.

Preston (1985) described the solvent extraction behavior of a large number of metalcations including rare earth nitrates in solutions of Versatic 10 (2-ethyl-2-methylheptanoicacid), naphthenic, 2-bromodecanoic and 3, 5-diisopropylsalicylic acids in xylene. The lasttwo acids extract metal cations under more acidic conditions, pH~ 1-2. For Versatic 10the order of extraction of yttrium and lanthanides is La < Ce < Nd < Gd < Y < Ho < Yband for naphthenic acids it is La < Ce < Y < Nd < Gd:::::l Ho ~ Yb. The lanthanides tend toform complexes of predominantly ionic nature. In the case of Versatic 10, the stability ofthe complexes increases uniformly with atomic number due to the increase in electrostaticenergy as a result of the decrease in ionic radius. The primary branched naphthenic acidallows the formation of complexes with high coordination number, nine for La to Nd,eight and eventually six as the metal ionic radius decreases. In general, the extraction ofa metal ion M n+ by a carboxylic acid H2A2 can be represented by the reaction

SEpARATION CHEMISTRY 5

A fundamental study of the extraction of trivalent lanthanides and yttrium nitrates bycarboxylic acids in xylene diluent was reported by duPreez and Preston (1992). The stericparameter E~ of the substituent alkyl group, representing the steric bulk of the carboxylicacid molecule, shows a definite relationship to the extractabilities of the lanthanidesdefined by their atomic numbers. The extraction reaction of a trivalent lanthanide Ln3+by a carboxylic acid dimer is

where j is the degree of oligomerization of the extracted complex and x is the numberof carboxylic ligands per metal ion extracted.

The extraction of trivalent rare earths by sterically hindered carboxylic acids, -E~ > 2,such as 2-ethylhexanoic, I-methyl-cyclohexanecarboxylic, 2,2 ,3-trimethylbutanoic, Ver-satic 10 and 2-butyl-2-ethylhexanoic can be represented by the reaction

The pH values at which 50% extraction occurs, pHo.5, decrease from lanthanum through tolutetium, in the same order as the decrease in ionic radii with increase in atomic number.Yttrium behaves more like the middle lanthanides, such as gadolinium or terbium.

With the straight-chain and less sterically hindered carboxylic acids, -E~ < I, such asn-hexanoic, n-octanoic, 3-cyclohexylpropanoic, iso-nonanoic and cyclohexanecarboxylicacids, the pHo.5 values go from lanthanum through a minimum to the middle rare earthsand then gradually increase through to lutetium. The yttrium behavior in this case is moresimilar to the lighter lanthanides such as cerium and praseodymium.

The extraction of the light and middle rare earths by the straight-chain and lesssterically hindered carboxylic acid, -E~ < 1, may be represented by the reaction

For the heaviest rare earths, the extraction reaction may be represented by

2.2. Tri-n-butyl phosphate

Warf (1949) was the first to report the use of tributyl phosphate (TBP) as an extractantfor the preferential extraction of Ce(IV) with respect to La(III) in 8-10 F RN03 . Theextractability of rare earths from an aqueous HCI phase and from an aqueous phase8 to 15.6 M RN03 by diluted and pure TBP was studied by Peppard et aI. (1953, 1957)and was shown to increase with increasing atomic number.

Hesford et al. (1959) determined the dependence of the distribution ratio K for ninedifferent trivalent lanthanides in 15.6 M RN03 vs. TBP diluted with 1-5% kerosene. On

6 R.O. BAUTISTA

the basis of the slope of log K vs. log % TBp, the extractable species was determinedto be M(N03)3(TBP). The TBP concentration vs. log K was found to be independent ofthe acidity. The mechanism of extraction of a trivalent rare earth was represented by theextraction reaction

Yoshida (1964) investigated the solvent extraction behavior oflanthanides in the systemM3+-HCI04- TBP and reported the distribution ratio of the lanthanides to be third-powerdependent upon the concentration of perchlorate ions in the aqueous phase and sixth-power dependent upon the concentration of TBP in the organic phase. The lanthanideextraction reaction can be represented by

The lanthanide distribution ratio was observed to increase with atomic number.The nature of the solvent-solute complex formation in NdN03- TBP-H20 was

reported by Bostian and Smutz (1964) using infrared spectra of the organic phase datacomposition and weight measurements made from equilibrium extraction experiments. Itwas determined that the complexing takes place at the P=O bond on the solvent molecules.The dependence of the extraction on the solvent structure variation suggests that thecomplex is formed by weak intermolecular attractions depending on dipole effects. Thecomplex formation reaction over the entire concentration range can be represented by

The temperature effect on the extraction of lanthanides in the TBP-HN03 systemwas measured by Fidelis (1970) at 10, 17, 25 and 40C. The separation factors foradjacent lanthanides and their enthalpy, relative free energy, and entropy associated withthe extraction process, were calculated.

The extraction of mineral acids such as HCI, HN03, H2S04, HCI04 and HF fromaqueous solutions by concentrated and diluted TBP has been reported by Hesford andMcKay (1960). HCI04 was found to be a strong electrolyte in TBP. The first ionizationconstants of HCl, HN03 and H2S04 were all determined to be equal to about 9x 10-5 .The monosolvates HClTBP, HN03TBP, H2S04TBp, and HCI04TBP were formed withHCI04 also yielding higher solvates. With the exception of HN03, the water content ofthe TBP phase increased with acid concentration.

A critical examination of the large amount of data on the extraction of mineral acidsfrom aqueous solutions by TBP has been made by Davis et al. (1966) and Hardy (1970).The activities of TBP in equilibrium with water, with aqueous nitric acid and withhydrochloric acid have been calculated by means of the Gibbs-Duhem equation andcompared to the distribution data for the TBP-HN03-H20 and TBP-H20 systems andthe TBP-HCI-H20 and TBP-H20 systems. The activity of TBP decreases more sharply

SEPARATION CHEMISTRY 7

at high RN03 concentration than at high HCI concentration in the equilibrated TBP phase.This is due to RN03 being more strongly extracted by TBP and displacing water fromthe TBP phase (strong bond between TBP and RN03), whereas HCI and H20 are co-extracted by TBP (TBP weakly bonded to hydrated HCI).

Hoh and Wang (1980) reported the rate of extraction of nitrate (denitration) by asolution of 75% tributyl phosphate diluted with kerosene and the rate of stripping ofnitrate (acid recovery) from nitrate-loaded TBP with water. The experiments carried out ina mixing vessel indicate an optimum rpm for extraction. Their experimental observationscan be explained by the results ofSheka and Kriss (1959) where HN03"TBP predominatesat nitric acid concentration up to 4 M and 3RN03"TBP is the dominant species up to 9 Mnitric acid concentration. The first extraction period is the rate controlling step.

2.3. Di-2-ethylhexyl phosphoric acid

2.3.1. Chloride mediumPeppard et al. (1958) proposed the extraction mechanism for lanthanides by HDEHP basedon tracer concentration studies as

where (HG)z is the dimeric form of the HDEHP in the organic phase and M3+is the trivalent lanthanide ion. This reaction indicates that the extraction is stronglypH dependent.

Lenz and Smutz (1966) confirmed that three H+ ions are liberated to the aqueousphase for every rare earth ion extracted into the organic phase Jor concentrated lanthanidesolutions. Dimerized HDEHP is a more efficient extractant than monomerized HDEHPup to 1M SmCh.

Harada et al. (1971) defined the conditions under which rare earth polymers ofHDEHPare formed to be a function of the initial HDEHP concentration, final acidity, and finalaqueous concentration of the metal. When the initial HDEHP concentration is less than1.5 M, the polymers form at a ratio of HDEHP to metal concentration of less than 6.5. Theminimum HDEHP concentration for polymer formation is different for each lanthanide ataqueous lanthanide concentrations of more than 0.2M. The degree of polymerization n forx: Ho, etc. is 4000, 600 for La and Yb, and 3 for Fe(III). The polymer is soluble in amixture of benzene and cyclohexane, but decomposes in concentrated HCI, RN03 , andH2S04 at room temperature and in 2 M HCI at its boiling point.

Sato (1989) investigated the extraction of all the rare earths by HDEHP in kerosene.The extraction efficiency increases with increase in atomic number in the orderof La

8 R.O. BAUTISTA

Table 2Separation factor' of rare-earth elements in the extraction systems Ln(III)-HCI-HDEHP

Ln) Ln2Ce Pr Nd Sm Eu Od Tb Dy Ho Er Tm Yb Lu

La 2.14 2.28 2.43 11.8 26.3 44.6 71.1 101 125 212 319 414 425Ce 1.07 1.14 5.2 12.3 20.9 33.3 47.2 58.4 99.1 149 193 199Pr 1.06 5.16 11.5 19.5 31.1 41.1 54.7 92.7 139 181 186Nd 4.86 10.8 18.3 29.2 41.5 51.3 87.1 131 170 175Sm 2.23 3.75 6.02 8.55 10.6 17.9 27.0 35.1 36.0Eu 1.69 2.70 3.83 4.74 8.04 12.0 15.7 16.2Od 1.60 2.26 2.80 4.75 7.15 9.30 9.55Tb 1.42 1.76 2.98 4.48 5.83 5.90Dy 1.24 2.10 3.16 4.11 4.22Ho 1.70 2.55 3.31 3.41Er 1.50 1.90 2.01Tm 1.30 1.34Yb 1.03

a a = E~ (Ln])/E~ (Ln2); E~ (Ln1) and E~(Ln2) represent the distribution coefficients of Lnl and Ln2, respectively,in the extraction from 0.1 mol dm-3 hydrochloric acid solution with 0.2 mol dm-3 DEHPA in kerosene.

2.3.2. Nitrate mediumThe complex fonnations and reaction stoichiometry of the equilibrium extraction oflanthanum nitrate by purified-water equilibrated HDEHP was reported by Kosinski andBostian (1969). The transfer of water molecules from the lanthanum-enriched organicphase to the aqueous phase was verified, indicating that it should be included in theextraction mechanism. The three predominant extraction mechanisms involving purelanthanum ions and partially nitrated lanthanum ions are

[La3+]AQ + [3 [(HDEHPh . H20lloRG +=! [La [H(HDEHPhh]ORG+ [3H20]AQ + [3H+]AQ'

[LaNO~+] AQ + [2 [(HDEHP)2H20 ]]ORG +=! [La(N03) [H(HDEHPhh] ORG+ [2H20]AQ + [2H+]AQ'

[La(N03)i] AQ + [(HDEHP)2H20 ]ORG +=! [La(N03)2H(HDEHPh]ORG+ [H20]AQ + [H+]AQ'

The lanthanum ion replaces the hydrogen ions and interacts with the P=O structure ofthe solvent. The climer structure is maintained throughout the extraction since only onehydrogen molecule and one water molecule are displaced per climer consumed.

SEPARATION CHEMISTRY 9

Fidelis (1971) measured the distribution coefficients and separation factors of selectedlanthanides in the HDEHP-HN03 system over the temperature range 1O--50C. Thedistribution coefficients decreased with increase in temperature.

2.3.3. Sulfate mediumThe extraction of Ce(IV) from sulfuric acid solution by HDEHP dissolved in keroseneas a function of sulfate and HDEHP concentration and pH was determined by Tedescoet al. (1967). The first extraction reaction represents tracer concentration and the secondrepresents saturation concentration:

[Ce4+JAQ + [4(HX)2]ORG ~ [CcH4XS]ORG + [4H+]AQ'[nCe4+]AQ + [2n(HXh]ORG ~ [(CeX4)n]ORG + [4nH+]AQ"

The above two reactions represent the mechanism of extraction of Ce(IV) at pH ~ 1.0 orlower. At higher pH, the reactions are

[(CeO)2+]AQ + [2(HX)2]ORG ~ [CeOH2X4]ORG + [2H+]AQ'[n(CeOi+] AQ + [n(HX)2]ORG ~ [(CeOX2)n]ORG + [2nH+] AQ

The above reactions are dominant at pH ~ 1.7 or higher.The presence of sulfate and bisulfate ions in the extraction of europium by HDEHP

results in the formation of complexes of europium in the aqueous phase. Cassidy andBurkin (1971) quantitatively analyzed the extraction of Eu(H!) from sulfate-perchloratesolutions by correcting for the value of [Eu3+]AQ, or of its activity in the equilibriumconstant.

2.4. Amines

Amines extract rare earths in reversed order compared to organophosphates. Aliphaticamines form salts with acids in aqueous solution and precipitate the hydroxides ofmetals. The solubility of amines in water decreases with increase in molecular weight.Primary amines extract from sulfate solutions while the tertiary amines extract fromnitrate solutions. The lighter rare earths are preferentially extracted by amines and whenused with aqueous soluble aminocarboxylic acid, chelating reagents preferentially extractthe heavy rare earths.

Coleman (1963) reviewed the application of amines as an extractant in the separation ofmetals, with particular attention to amine extraction mechanism, separation and recovery.Amines extract metals by an anion exchange mechanism, given by the reaction

[mR3NH+X-] ORG + [MXZ:+n)]AQ +=i [(R3NH)mMX(m+nl]oRG + [mX-]AQ'or by adduct formation:

[mR3NH+X-]ORG + [MXn]AQ ~ [(R3NH)mMX(m+n)]oRG'where R is an alkyl group, M is a lanthanide ion and X is an inorganic anion.

10 R.O. BAUTISTA

The separation of light rare earths (cerium group) sulfates obtained from bastnasiteby primary amine with or without an aqueous phase chelating reagent was studied byBauer et al. (1968). Kerosene was used as the diluent. The pH of the lanthanide-aminesystem affects the molecular species present in both the aqueous and organic phases. Theextraction of rare earths at high acid concentrations is inhibited due to the formation ofstable amine bisulfate

The lanthanides at high pH values form nonextraetable hydrolysis products with aminesalt reverting to free-base form

The lanthanides are extracted in the intermediate pH range by formation of anextractable neutral salt

where R is an alkyl group and M is the lanthanide ion. A pH value greater than 2was requir~d for the aqueous phase in the presence of aminocarboxylic acid chelatingreagents. Diethylenetriaminepentaacetic acid (DTPA) enhanced the separation factors tovalues greater than 2. Ethylenediaminetetraacetic acid (EDTA) was not as effective.

Gruzensky and Engel (I959) reported the separation of yttrium and rare earth nitrateswith a tertiary amine, 10% by volume of tri-n-butylamine diluted in a ketone, 3-methyl-2-butanone. The maximum extraction was obtained when sufficient amine was present toneutralize the aqueous phase to the point of precipitation. Extraction increased with metalconcentration and the separation factors were found to be affected by the concentrationof amine.

Bauer (1966) studied the extraction of lanthanides in tertiary amines with the additionof aqueous-phase chelating reagents. EDTA and DTPA were used as the chelatingreagents. Certain aqueous-phase chelate compounds solubility-limited the range of thelanthanide concentrations to between 15 and 45 gmll. The salting out effect for lanthanidenitrate was enhanced with the addition of lithium nitrate. Alamine 336, a mixture of 8- and10- carbon straight chain tertiary amines, was dissolved in an inert organic diluent. Nitricacid was equilibrated with the organic phase to minimize pH changes during extraction.Rapid phase disengagement was obtained with 33% amine concentration at pH between3 and 4. Stripping of the lanthanides from the organic phase was carried out with 1.2 Mammonium chloride solution.

The quaternary ammonium compounds which have amine-type structure in thepresence of a chelating reagent were successfully used by Bauer and Lindstrom (1968,1971) to separate La-Pr, Pr-Nd, and Nd-Sm. Aliquat 336, tricaprylyl monomethylammonium chloride diluted with Socal 355L and containing the chelating reagent DTPA,

SEPARATION CHEMISTRY II

was found to effectively separate the light rare earth group. Optimum separationwas obtained by closely controlling pH, nitrate ion concentration and extractantconcentration.

With all the rare earths chelated with DTPA, a rare earth displacement method forreleasing one rare earth from the DTPA was used. Copper, which is not extractable bythe quaternary ammonium compound and which has a high stability constant with DTPArelative to the light rare earth-DTPA stability constants, was used as the displacementmetal.

High purity separation of Pr and Nd (99.9%) was carried out by Hsu et al. (1980)using quaternary alkylammonium nitrate R3CH3N+N03 (0.65 M) in xylene as diluent.The extraction reaction was determined to be

where M3+ represents the trivalent rare earth ions, L- the nitrate ion, (~N+L~)2 thequaternary ammonium nitrate, and ML3~N+L- the extractable species.

Efficient extraction of light rare earths by tertiary amines and quaternary aminecompounds can be carried out only at high inorganic nitrate concentration. The practicalsupporting electrolyte used is ammonium nitrate, which is less efficient than lithiumnitrate. Formation of undissociated lanthanide nitrates which can form extractablecomplexes requires the presence of a high concentration ofN03 ions. Cerna et al. (1992)reported the enhanced extraction of light rare earths by Aliquat 336 in aromatic andaliphatic dilucnts by increasing the NH4N03 concentration to 4 or 8M.

The observed decrease in separation factor with increased extractant loading wasattributed to three different extraction reaction mechanisms as a result of the presenceof three extractable complexes in the organic phase.

At small lanthanide concentrations, the equilibrium extraction reaction between thequaternary ammonium salt and Ln(N03h is

The stoichiometric coefficient n was found to decrease with extractant loading, andcomplexes with a low amine to metal ratio were formed in the region of high lanthanideconcentrations.

The formation of the anionic species Ln(N03)4 is given by the reaction

The cation of the dissociated amine nitrate in the aqueous phase reacts with Ln(N03)4'The resulting complex is transferred into the organic phase according to the extractionreaction

12 R.G. BAUTISTA

Further reaction of this 1: 1 complex formed by the above anion exchange reactionwith another molecule of Ln(N03)3 results in the formation of the 1:2 amine to metalcomplex:

The Ln(N03)3 in the above reactions is in equilibrium with Ln3+ and NO; ions:

The supporting electrolyte, NH4N03, increases the NO.3 concentration by the equilibriumreaction

and shifts all the equilibrium reactions to the right.

2.5. Hydroxyoximes

The equilibrium extraction of Ce(IlI) and La(IlI) from sodium chloride solutions by thecommercial extractant SME 529, 2-hydroxy-5-nonyl-acetophenone oxime, in n-heptanediluent was found by Urbanski et al. (1992) to be highly dependent on the pH andextractant concentration. SME 529 contains mainly the anti-form (> 99%) which is amuch better extractant than the .syn-isomer. The equilibration time was around 2 minutesand the disengagement time was found to depend on the extractant concentration andpH. Phase disengagement is very slow and lanthanum hydroxide precipitate appearsat the interface at pH values where extraction and precipitation occur simultaneously.Precipitation generally occurs above pH:::::: 6.5. The extraction of cerium is more efficientbelow pH:::::: 6.5 compared to lanthanum and thus less cerium is precipitated.

The cerium extraction reaction into SME 529 solution in n-heptane is

Hydrolyzed lanthanum species is extracted according to the reaction

The extraction of La(III) and Ce(IlI) from aqueous NaCI solutions with LIX 70 inkerosene or n-heptane diluent have been reported by Abbruzzese et al. (1992). LIX 70is 2-hydroxy-3-chloro-5-nonylbenzophenone oxime with an anti- to syn-isomer ratio ofabout 7.3 and contains about 40% of a hydrocarbon diluent and about 1% v/v ofLIX63.

The equilibration time is about 2 minutes and no lanthanum hydroxide precipitates areformed at pH above 6.5. Both Ce and La can be extracted quantitatively with LIX 70,

SEPARATION CHEMISTRY 13

with Ce more extractable at lower pH. The extraction of both metals depends on pHand extractant concentration. Ce and La complexes with LIX 70 in the organic phase areformed according to the extraction reaction

with the extractant HR as a monomeric species.

2.6. Synergic extractants

Cox and Davis (1973) studied the extraction of Dy, Ho, Tm and Cm with thenoyltri-fluoroacetone (HTTA)/TBP/dilute HN03/kerosene. One molecule of TBP was found toassociate with one molecule of HTTAH20 to form a metal complex, M3+(TTAh(N03)-(TBPh. Their experimental data indicate only two HTTA molecules are valence bondedto the metal ion, since one ion of H+ is released to the aqueous phase from each HTTAmolecule thus bonded. This also indicates that one NO)" group is present in the complexto balance the charge on the trivalent metal ion. Additional work by Hayden et al. (1974)on Dy indicates that dysprosium is present as Dy(N03)! in the aqueous phase, furthersupporting the conclusion that the extractable complex in the organic phase is of the formDy(N0 3)(TTAh (TBPh

Synergistic extraction of Sm and Gd using a mixture of tributyl phosphate andAliquat 336 and the influence of diluents, salting out agents and acidity was studied byHuang and Bautista (1983). This is one of the best systems for the separation of Sm andGd, with a separation factor greater than 3. N~N03 was extracted by pure TBP or pureAliquat 336, but was not extracted by a mixture of TBP and Aliquat 336.

The synergistic extraction of Am(lIl), Cm(llI), Eu(Ill), CeCIl!), and Pm(Hl) from anaqueous acetate buffer system at pH~ 4.8 into thenoyltrifluoroacetone (HTTA)/4, 13-didecyl-I,7, 10, 16-tetraoxa-4, 13-diazacyclooctadecane (K22DD)/chloroform phase wasreported by Ensor et al. (1988). The extraction reaction for trivalent metal ions by HTTAalone is

In the presence of a synergistic agent, the overall extraction reaction is

The synergism between HTTA and K22DD significantly increased the distributioncoefficients of these metal ions by factors of 104-105, compared to no extraction usingHTTA alone. The extraction increased rapidly with increasing K22DD concentrationbefore it levels off at the highest concentration used. Comparison of the organic phasestability constants with other macrocyclic agents showed the importance of the nitrogendonor groups in the synergistic activity. There was no significant effect of the cavity sizeon the extractability.

14 R.G. BAUTISTA

The synergistic extraction of europium from acetate media with didodecylmonothio-phosphoric acid (DDTPA) and 1,10-phenanthroline (pHEN) in a toluene diluent wascompared to that of dibutylmonothiophosphoric acid (DBTPA) and PHEN by Kondo et al.(1992). The extraction reaction of ionic Eu by DDTPA (HR) and PHEN can be representedby

[Eu3+] AQ + [3HR]ORG + [PHEN]oRG ~ [EuR3 . PHEN]ORG + [3H+] AQ .The synergistic effect of the DBTPA system was found to be superior to that of theDDTPA system.

An extensive study of the synergistic enhancement by TBP of the extraction oflanthanides by trioctylmethylammonium nitrate (Aliquat 336) was reported by Majdanand Kolarik (1993). The assessment of the separation potential of this synergistic systemwas carried out with the extraction of nine Ln(III) and Y(III). Extraction of adjacentLn(III), effect of diluents, a wider range of extractant concentrations, compositions ofthe extracted complexes, absence or presence of TBP and the effect on the separationfactors were among the variables taken into account in the experimental work.

The synergistic effect was found to be dependent on the diluent. It is pronouncedwith a mixed dodecane/xylene diluent, and is weak with mixed heptane/xylene, hex-ane/xylene, and pure xylene diluents. The organic phase was found to contain simulta-neously two or three synergistic complexes, with the compositions (A+)(Ln(N03)42B-),(A+)(Ln(N03)43B-), and (A+)2(ln(N03)sB2-), where Ln is a lanthanide (III), A+ is atrioctylmethylamrnonium cation and B is a tributyl phosphate molecule. The synergisticcomplex stability was found to decrease with increase in atomic number. Increasing thetributyl phosphate concentration gradually suppressed the separation factors for Pr-Ndand Eu-Gd pairs.

Kubota et al. (1993) compared the extraction equilibria and extraction mechanism ofthe lanthanides with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) inn-heptane diluent and HDEHP and in the presence of diethylenetriamine-pentaacetic acid(DTPA). The extraction equilibrium of PC-88A in n-heptane can be expressed as

where HR is PC-88A. The separation factors for adjacent lanthanides were the same forPC-8SA and HDEHP. The presence of DTPA in the aqueous phase at high pH (pH> 1.5)decreases the distribution coefficient of each lanthanide. The separation factors betweenY and Ho and Er are increased by the addition of DTPA.

2.7. Macrocyclic extractants

The results of using a crown ether carboxylic acid (sym-dibenzo-16-crown-5-oxyaceticacid) as the extractant in the solvent extraction of lanthanides was reported by Tangand Wai (1986). High extraction efficiencies and significant selectivities were observed.

SEPARATION CHEMISTRY 15

Crown ether carboxylic acid was dissolved in a 80:20 chloroform-heptanol mixtureand was contacted with the aqueous solution of the lanthanides. The solubility ofsym-dibenzo-16-crown-5-oxyacetic acid in water at different pH was measured by itsultraviolet absorption.

The solvent extraction of 177Lu3+ in the pH range 6-7 is quantitative. Only severalminutes of shaking is required to obtain 98% extraction without addition of a counterion.177Lu3+ can be back extracted into water at pH below 3 after a few minutes of shaking. Thepresence of chloride, nitrate, sulfate and acetate (1 x 10-2 M) did not decrease extractionefficiency. The extraction of 1x 10-5 M lanthanides (La3+, p2+, Sm3+, Eu3+, Tb3+, E2+,and Yb3+) with an excess of the chelating agent (3 x 10-3 M) are also nearly quantitative(> 98%) around pH:=::: 6.5. The stoichiometry of the extracted lanthanide-sym-dibenzo-16-crown-5-oxyacetate complex is 1:2 with a net charge of + 1. The lanthanide cationsare found to fit the cavity of the crown ether carboxylate without any problem.

The nature of the extractable species in the solvent extraction of aqueous complexes ofLa(III), Eu(HI) and Lu(III) with macrocyclic ligands I, 7-diaza-4, 10, 13-trioxacyclopenta-decane-N, N'-diacetic acid (DAPDA) and 1, I0-diaza-4, 7, 13, 16-tetraoxacyclooetadecane-N, N'-diacetic acid (DACDA) using thenoyltrifluoroacetone (HTTA) as the extractant inchloroform, nitrobenzene and benzene, has been compared by Manchanda et al. (1988).The extraction reaction is

where HA is protonated HTTA and [HAJORG is the concentration of HTTA inthe organic phase at equilibrium. The highest extraction equilibrium constant is ob-tained with nitrobenzene and the extraction for both ligands is in the order ofnitrobenzene> benzene> chloroform. The favored extractable complexes in the highdielectric constant diluent nitrobenzene are ternary species ion pairs of the type[Lu(DAPDAtTTA-J which are not readily formed in nonpolar benzene.

The solvent extraction of trivalent lanthanides (La, Nd, Eu, Er, and Yb) in threecalixarene-type cyclophanes was described by Ludwig et al. (1993). Calixarenes are (1, n)-cyclophanes with a cavity formed by bridged phenyl units and various derivatives areformed with the introduction of substituents onto the skeleton. The three calixarenesstudied are p-tert-butylcalix/6/arene hexacarboxylic acid (L1), p-tert-butylcalixl4/arenetetracarboxylic acid (L2) and [3, 1,3, IJ-cyclophane (L3). L1, L2 and L3 have differentcavity sizes and contain carboxylic acid groups at the "lower" rim to achieve a highcoordination number for Ln3+ and to prevent the phase transfer of counter anions.

The extractability of the lanthanides from the aqueous phase at pH:=::: 2-3.5 withL1 into chloroform is Nd, Eu > La> Er > Yb. A cation exchange mechanism with a1:2 metal:ligand complex is indicated. With an excess of Na+ in the aqueous phase,the extractability of the lanthanides decreases and the lanthanides are extracted as1: 1 complexes at low extractant concentration.

The smaller cavity in L2 dissolved in chloroform or toluene resulted in lower distri-bution coefficients compared to L1. The order of extraction of the 1:2 metal:extraetant

16 R.O. BAUTISTA

complexes for the water-toluene system is Eu > Nd > Yb > Er > La. Extractability andselectivity increased with addition of excess Na+.

Heavy lanthanides were better extracted than the light and medium ones by the thirdcyclophane, L3, with a cavity size similar to Ll containing four carboxylic acid groups.The order of extractability is Yb > Er, Eu > Nd > La at pH ~ 3.0.

2.8. Other extractants

Freiser (1988) summarized the results of a systematic investigation of the equilibriumextraction behavior of selected trivalent lanthanides with various chelating reagentfamilies. For a monomeric weak: acid extractant, HL, the formation of both simple (a = 0)and self adduct (a > 0) chelates are given by the reaction

The following reaction applies in the presence of a neutral auxiliary ligand, oradductant, B:

The reaction system involving ion pair extraction of anionic chelates is given by

[Ln3+JAQ + [(4 + a)HLloRG + [Q+, X-JAQ ~ [Q+LnL4 aHLfJORG+ [4H+JAQ + [X-lAQ

where Q+, X- is the ion pair.The formation of a cationic intermediate chelate, either coordinated or ion paired with

thiocyanate is represented by

[Ln3+JAQ + [2HLlORG + [CNS-lAQ ~ [L~ . CNSloRG + [2H+JAQ .The extraction reaction for the acidic phosphorus ligands which exist predominantly

as dimers in the organic phase is

In the presence of an adductant this becomes

[Ln3+JAQ + [2(HL)ZlORG + [BloRG ~ [Ln(HLz)Lz . BloRG + [3H+JAQ .The extraction of the lanthanide chlorides by 2-ethylhexyl-2-ethylhexylphosphonic acid

(EHEHPA) in kerosene has been reported by Sato (1989). The extraction efficiency inEHEHPA also increases with atomic number, but lower in comparison with HDEHP

SEPARATION CHEMISTRY 17

Table 3Separation factor a of rare-earth elements in the extraction systems Ln(III)-HCI-EHEHPA

Ln, LnzCe Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La 1.30 1.42 1.67 3.33 6.52 9.52 22.5 36.4 93.9 117 156 175 199Ce 1.09 1.28 2.57 5.02 7.36 17.3 28.0 72.3 90.5 120 135 152Pr 1.17 2.35 4.59 6.72 15.8 64.2 66.0 82.7 110 123 140Nd 2.00 3.94 5.74 13.5 21.8 56.3 70.5 93.7 105 119Sm 1.96 2.87 6.74 10.9 28.2 35.3 46.8 52.6 59.5Eu 1.46 3.45 6.39 14.4 18.0 24.0 26.9 30.4Gd 2.35 3.81 9.82 12.3 16.3 18.3 20.7Tb 1.62 4.18 5.23 6.95 7.81 8.83Dy 2.58 3.23 4.29 4.82 5.45Ho 1.25 1.66 1.87 2.11Er 1.33 1.49 1.69Tm 1.12 1.26Yb 1.13

a a = E~(Lnl )IE~(Ln2); E~(Lnl) and E~(Ln2) represent the distribution coefficients of Ln, and Lnz, respectively,in the extraction from 0.1 mol dm-J hydrochloric acid solution with 0.2 mol dm-J EHEHPA in kerosene.

under the same conditions. The separation factors for the extraction of rare earths from0.1 mol/dm3 hydrochloric acid solution with 0.2 mol/dm3 EHEHPA in the Ln(III)-HCI-EHEHPA-kerosene system are given in table 3. EHEHPA is more selective for theseparation of heavy rare earths from light rare earths in this system.

The extraction mechanisms of Nd(SCN)3' Nd(CI04)3 and Nd(N03)3 with di(l-methyl-heptyl) methyl phosphonate (P-350) in kerosene or hexane have been characterized byHuang et al. (1989). The extraction reactions for the salting out agents can be representedby

where A-represents SCN-, CI04 or NO),. In this extraction system, the M3+ and theanion A-are extracted simultaneously into the organic phase to form the extractedcomplex such as MA3nL, where L is the extractant. In the aqueous phase, hydrationwater of the metal ion, M3+, or anion, A-, is removed.

The extraction equilibrium of Ce(Il!) and La(III) in NaCl solutions with 7-(4-ethyl-l-methyloctyl)-8-hydroxyquinoline (KELEX 100) in n-heptane with the addition of 10% v/vn-decanol has been reported by Urbanski et al. (1990) to be dependent on the pH aswell as on the extractant, metal and cWoride concentrations. The lanthanide-KELEXcomplex, LnR, is formed in the organic phase by the cation exchange reaction

18 R.O. BAUTISTA

where HR is the extractant KELEX 100. The possibility also exists that the hydrolyzedspecies LaOH2+ is extracted according to the reaction

3. Photochemical separation

Donohue (1977) described the photochemical separation of Eu from other rare earthsusing a low pressure mercury lamp with no filter at 185 nm and with a Vycor filter at254nm, and an ArF excimer laser operating at 193 nm. Equimolar mixtures (0.01 M)of binary or ternary lanthanide perchlorates and 0.05 M K2 S04 in 10% isopropanol wereused in the experiments. The separation factors for the binary mixture EulLn varied from 1for EulPr to > 200 for EuiTm.

The irradiation of Eu3+ in its charge transfer band is

The isopropanol scavenged the radicals formed in the above primary process

The Eu2+ are precipitated with sulfate, forming the insoluble EuS04. The presence of thesulfate ions makes possible the shift of the charge-transfer band to the longer wavelengthlight at 240 nm. The precipitate is formed homogeneously during the photolysis. Furtherreduction of Eu3+ by organic radicals is by the reaction

Separation of Eu from a solution mixture of SmCh, EuCh, and GdCh in a rare earthsaturated ethanol-isopropanol system by photoreduction of Eu with a high pressuremercury lamp has been carried out by Qiu et al. (1991). The yield of Eu(H) was 95%and the purity of the precipitated Eu was 92%. EuCh was produced by photoreductionand precipitated from the alcohol mixture. The photochemical separation process is asfollows:

Step 1: dissolution and complexation.(Sm, Eu, Gd)Ch . nH20 + 3EtOH -+ (Sm, Eu, Gd)Ch . 3EtOH + nH20,(Sm, Eu, Gd)CI3 . nH20 + 3(isopropanol) -+ (Sm, Eu, Gd)Ch . 3(isopropanol)

+nH20.

SEPARATION CHEMISTRY

Step 2: photoexcitation.hv(Sm, Eu, Gd)Ch . 3EtOH ------t (Sm, Eu, Gd) Ch . 3EtOH,

(Sm, Eu, Gd)Ch . 3(isopropanol)~ (Sm, Eu, Gd)Ch . 3(isopropanol).

19

Step 3: electron transition.Eu Ch . 3EtOH ----1 EuC12 + 2EtOH + organic oxidation products,Eu Ch . 3(isopropanol) ----1 EuCh + 2(isopropanol) + organic oxidation products.

The reduction potential of Sm(IlI) and Gd(III) is more negative than that ofEu(III)/Eu(II).This only makes the reduction of Eu(III)-Eu(II) possible, although Sm(IlI) and Gd(III)are also excited during the photoreduction.

Step 4: precipitation.

EuC12 was precipitated and separated from the other rare earths since the solubility ofEuCh in an EtOH-isopropanol mixturc is low. Thc watcr content of the alcohol mixturemust be kept low in order to precipitate EuCh.

4. Precipitation stripping

Recovery ofrare earths from very dilute leach solutions by conventional solvent extractiontechnique can be a problem due to the high acid concentration that is required for thestripping of low rare earth concentrations. The stripped solution containing very lowrare earth concentrations needs to be neutralized prior to the precipitation of the rare earthsas an oxalate. The very high consumption of the neutralizing reagent can create technical,environmental and/or economic problems.

The rare earths form slightly soluble double sulfates with ammonium and alkalimetal sulfates. Addition of an appropriate salt to the stripping solution can facilitate theremoval of the very low concentration rare earth ions from the aqueous phase withoutneutralization. When sulfuric acid is used as the acidic reagent in the stripping solution,the precipitation step can be combined with stripping in the same stage. Success of thisprocedure depends on the stripping behavior of the lanthanide ions and on the nature ofthe precipitated solid.

Zielinski et al. (1991) reported the precipitation stripping of lanthanum and neodymiumfrom an equimolar mixture of monomeric and dimeric di-2-ethylhexyl phosphoric acid ina kerosene diluent. The sulfuric acid strip solution contained potassium sulfate, sodiumsulfate or ammonium sulfate as the precipitating reagent. The precipitation efficiency forlanthanum and neodymium reached up to 98% with the addition of potassium or sodiumsulfate, compared to 80% for lanthanum, and 70% for neodymium with the addition ofammonium sulfate.

20 R.G. BAUTISTA

Both lanthanum and neodymium double sulfates are precipitated. The general formulaof the double sulfates is xMe2S04yLn2(S04)3nH20. The co-precipitated salts arepotassium hydrogen sulfate, sodium sulfate or lanthanum/neodymium sulfate, dependingon the composition of the strip solution. Precipitation occurs only in the aqueous phase.

Additional results on the single-stage selective stripping of Nd-Eu, representing thelight rare earths, Nd-Dy, and Dy-Er representing the heavy rare earths, from HDEHPwith sulfuric acid solutions containing Na2S04 have been reported by Zielinski andSzczepanik (1993). Light lanthanides are readily stripped and precipitated compared tothe heavy lanthanides. As a result, the concentration of heavy lanthanides in the solutionafter stripping of the light lanthanides is higher than their concentration in the originalsolution.

5. Ion-exchange

Separation and purification of microquantities of scandium (III) from macroquantities ofrare earths on selective ion-exchangers has been described by Hubicki (1990). The chelat-ing and ion-exchangers examined include phosphonic gel and macroporous, aminophos-phonic, various carboxylic, amino-acids with different matrix compositions, polyphenol,amphoteric (snake in cage polymers), cellulose phosphate, zirconium phosphate, andcopolymer of polystyrene with divinylbenzene. The phosphonic, aminophosphonic andcellulose-phosphate were the most effective.

The great stability of the scandium complexes with phosphonic and aminophosphonicacids enhances the selective separation of microquantities of Sc(III) from Y(III), La(III)and Ln(III). The concentration of the rare earth solution was up to 500 gil and the acidityof the rare earth solutions purified from scandium reached 6 M HCI. Phosphonic ion-exchangers are not only useful in the selective separation but also in the purification ofSc(III) since 6 M HCI solution does not elute Sc(III) by gradient elution with 0.5-6 Macid.

With the exception of the isoporous ion-exchangers with the functional group EDTA,the other amino-acid types examined can be used in the purification of rare earth elementsalts from Sc(III). The most effective carboxylic ion-exchanger is the polymethacrylictype for purifying La(III) from Sc(III).

Preparation of 99.99% SC203 from feedstock of around 70% SC203 by extractionchromatography at room temperature with greater than 90% recovery has been reportedby Gongyi and Yuli (1989, 1990). Purification was carried out on two chromatographiccolumns containing tributyl phosphate as the stationary phase supported by hydrophobicsilica gel or polystyrene-divinylbenzene copolymer. The feed solution containing 2-4 gil SC203 and a small amount of citric acid was passed through column 1 to removeimpurities in the stock solution with very little loss of scandium. The effluent fromcolumn 1, adjusted to 3--4 M perchloric acid, passed through column 2 for furtherpurification of scandium. Scandium was eluted with 1M HCI from column 2 andprecipitated with oxalic acid followed by calcination to scandium oxide.

6. Supercritical extraction

SEPARATION CHEMISTRY 2J

Synthesis of the normal carbonates of La3+, Nd3+, Sm3+, Eu3+, Gd3+, Dy3+, and H03+by the reaction of an aqueous suspension of the lanthanide oxide with CO2 at or overthe critical temperature of 31C and critical pressure of 72.9 atm has been reported byYanagihara et al. (1991). The carbonates of Pr3+, Tb3+, Er3+, and Yb3+ either did notform or gave very low yields under the experimental conditions studied.

In a patent based on the above results, Fernando et al. (1991) separated the trivalentlanthanide oxides or hydroxides from the tetravalent lanthanides which do not react withcarbon dioxide under supercritical conditions. The rare earth carbonates from the trivalentoxides or hydroxides of La, Nd, Sm, Eu, Gd, Dy, Ho, Pm, Tm, and Lu can readily beformed after one hour at 40C and 100 atm with yields of up to 95% of the normalcarbonates, instead of the hydroxy carbonates. The oxides or hydroxides of Pr, Tb, Er,Yb, and Ce do not form carbonates under these conditions.

The solid precipitates formed after supercritical reaction consist ofrare earth carbonatesand unreacted rare earth oxides and/or hydroxides. Addition of 0.5 M HCI to the solidsat ambient temperature and pressure solubilizes the rare earth carbonates. The unreactedrare earth oxides and/or hydroxides are left in the solid phase. The solution containingthe rare earths can then be further separated into individual rare earths by conventionalsolvent extraction or ion exchange. Separation of rare earth carbonates is carried out indilute acid. ThOz, 2rOz and CeOz either did not react or gave very low yields under theexperimental conditions.

7. Industrial producers

Production of rare earth compounds from bastnasite, a fluo-carbonate mineral containingmostly cerium, lanthanum, neodymium and praseodymium and small quantities ofsamarium, gadolinium and europium at Molycorp was described by Harrah (1967).High purity yttrium and europium oxides are produced, together with a concentrate oflanthanum, praseodymium/neodymium and samarium/gadolinium. The europium solventextraction circuit organic phase consists of 10% di-2-ethyl hexyl phosphoric acid in akerosene diluent. The europium strip solution is 4N hydrochloric acid. Samarium andgadolinium is precipitated with Na2C03 from the solution after europium is removed.

An outline of the Rhone-Poulenc separation flowsheet to produce high purity rare earthoxides from various ores such as monazite, bastnasite and euxenite was published byAgpar and Poirier (1976). After the leaching step, non-rare earth elements and radioactiveproducts are removed. The first solvent extraction stream in a chloride medium producesnon-separated rare earth compounds, such as dehydrated rare earth chlorides used as thefeedstock for the electrolysis to produce mischmetal.

A second solvent extraction stream in a nitrate medium is used to produce separatedrare earth oxides. Individual rare earths in solution are separated from each other ina series of solvent extraction streams. Lanthanum (99.995% La203) which remains in

22 R.O. BAUTISTA

Radioactivewaste treatment

Solvent +-- TBPextraction

Solventextraction

EU10J~Y10,

Phosphor

Solventextraction

~ Gd10 JLsm 1 0 3

*R&D

1SmCo powder

Ce01

CeCI 3

IMoltensaltelectro-lysis

Ce metal

Solventextraction

REO

Polishingpowder

REF,

Monazite

NaOH-- Decomposition~Washing

t-- Na3 P04(U, Th, RE) (OH)x

+HCI -----... Dissolution

r-RECl,----+---+Solvent Sludge

D2 EHPA - extraction dissolution ---+I.. by HNOJ

+ + tLa, Ce, Pr, Nd Sm, Eu, Gd Tb ... Lu+ +Solvent Solvent

extraction extraction

Fig. 1. Simplified ftowsheet of the Yao Lung Chemical plant.

the aqueous phase is separated from the Ce, Pr, Nd, Sm, Eu, Gd, Tb, Y, etc. mixturesthat go into the organic phase. Ce02 (99.5%) is separated at the Nd-Sm stream.Didymium chloride (after primary Ce removal) is produced from which the followingrare earth oxides are recovered in sequence: Pr60U, 96%; Nd20 3, 95%; Sm203' 96%;EU203, 99.99%; Gd20 3, 99.99%; Tb40 7, 99.9%; and Y20 3, 99.99%.

Separation of each element by solvent extraction is carried out in multistage batteriesof mixer-settlers for each rare earth element. A minimum of 50 mixer-settler stages perstream is required to obtain a product with a purity of 4 or 5 nines. For economic andtechnical reasons the ion-exchange method is only used for laboratory and pilot scaleproduction to produce small tonnage of rare earths.

SEPARATION CHEMISTRY 23

The extractants used in the solvent extraction processing include organo----phosphoruscompounds, amines and carboxylic acids. The Rhone-Poulenc solvent extraction separa-tion flowsheet has become the standard for all industrial producers.

Zhang et al. (1982) summarized the rare earth industry in China. The Peoples Republicof China has the largest proven reserve of rare earth oxides in the world. Their productionis mainly from the bastnasite-monazite concentrate from the Baotu iron ore deposit inInner Mongolia (Northern China), monazite ore deposits in Southern China and theadsorbed rare earth oxides on clay found in Jiangxi province. The Yao Lung Chemicalplant in Shanghai was the first of their rare earth processing plants, which went on streamin 1964. The Yao Lung Chemical Co. flowsheet for the separation and extraction ofrare earth oxides from monazite is reproduced in fig. 1.

8. Summary and conclusions

The major industrial producers ofrare earths recover individual rare earths from monazite,bastnasite, euxenite, and other rare earth containing raw material, through a series ofunit operations. These unit operations include leaching with an appropriate lixiviant,separation and concentration by solvent extraction and/or ion exchange, precipitation asan oxalate and calcination to produce the individual oxide. Solvent extraction technologyis the most efficient and economical separation method presently available.

The development of new extractants with enhanced selectivity for each individualrare earth will have a major impact on their separation, concentration, and purification.Completely new separation technologies that can compete with the efficient andeconomical multistage solvent extraction operation may yet be developed from any ofthe emerging separation processes discussed in this chapter. However, the new technologymust be a "quantum" development to be adopted by the industry.

References

Abbruzzese, c., P. Fornari, R. Massidda and T.S.Urbanski, 1992, Hydrometallurgy 28, 179-190.

Agpar, H., and P. Poirier, 1976, Preprint, 12th RareEarth Research Conference, Vail, CO, 1976, 12 pp.

Banks, C.V, and D.W Klingman, 1961, Analyticalchemistry of the rare earths, in: The Rare Earths,eds P.H. Spedding and A.H. Daane, reprinted 1971(R.E. Krieger Publishing Co., Huntington, NY)pp. 570-593.

Bauer, 0.1., 1966, Extraction and Separation ofSelected Lanthanides with a Tertiary Amine, RI6809 (US Bureau of Mines) 13 pp.

Bauer, 0.1., and R.E. Lindstrom, 1964, NaphthenicAcid Solvent Extraction of Rare Earth Sulfates, RI

6396 (US Bureau of Mines) 19 pp.Bauer, OJ., and R.E. Lindstrom, 1968, Differential

extraction of rare earth elements in quaternaryammonium salt-ehelating agent systems, in: Proc.7th Rare Earth Research Conf. (CONF-681 020) pp.413-423.

Bauer, 0.1., and R.E. Lindstrom, 1971, DifferentialExtraction of Rare Earth Elements in QuaternaryAmmonium Compound-Chelating Agent Systems,RI 7524 (US Bureau of Mines) 16 pp.

Bauer, 0.1., R.E. Lindstrom and K.B. Higbie, 1968,Extraction Behavior of Cerium Group Lanthanidesin a Primary Amine-Chelating Agent System, RI7100 (US Bureau of Mines) 12 pp.

24 R.G. BAUTISTA

Bautista, R.G., 1990, Industrial extraction andpurification techniques for rare earths, in: RareEarths in Alaska, Proc. Symp., Aug. 17-18,1988, Fairbanks (Alaska Science and EngineeringAdvisory Commission) pp. 47-58.

Bautista, R.G., 1992, Mineral Proc. Extractive Met.Rev. 8, 175-182.

Bautista, R.G., and N. Jackson, 1992, Rare Earths,Resources, Science, Technology and Applications(TMS, Warrendale, PA).

Bautista, R.G., and M.M. Wong, 1989, Rare Earths,Extraction, Preparation and Applications (TMS,Warrendale, PA).

Bostian, H., and M. Smutz, 1964, 1. Inorg. Nucl. Chern.26, 825-836.

Boyd, G.E., 1. Schubert and A. Adamson, 1947, 1.Am. Chern. Soc. 69, 2818~2829.

Boynton, W.V, 1979, Neutron activation analysis,in: Handbook on the Physics and Chemistry ofRare Earths, Vol. 4, eds K.A. Gschneidner Jr andL. Eyring (North-Holland, Amsterdam) pp. 457-470.

Bril, K.J., 1964, Mass extraction and separation, in:Progress in the Science and Technology of theRare Earths, Vol. 1, ed. L. Eyring (Pergamon Press,Oxford) pp. 30-61.

Callow, R.I., 1966, The Rare Earth Industry (PergamonPress, London).

Callow, R.I., 1967, The Industrial Chemistry of theLanthanons, Yttrium, Thorium, and Uranium, 1stEd. (Pergamon Press, London).

Carnall, W T., 1979, The absorption and fluorescencespectra of rare earth ions in solution, in: Handbookon the Physics and Chemistry of Rare Earths, Vol.3, eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 171-208.

Cassidy, P.W, and A.R. Burkin, 1971, Extraction ofeuropium from sulfate-perchlorate solutions by di-2-ethylhexyl phosphoric acid, in: Proc. Int. SolventExtraction Conf. 1971, Vol. I, eds 1.G. Gregory,B. Evans and P.C. Weston (Society of ChemicalIndustry, London) pp. 476--482.

Cerna, M., E. Volaufova and V Rod, 1992,Hydrometallurgy 28, 339-352.

Coleman, C.P., 1963, Nucl. Sci. & Eng. 17,274-286.Conzemius, R.I., 1979, Analysis ofrare earth matrices

by spark source massed spectrometry, in: Handbookon the Physics and Chemistry of Rare Earths, Vo!.4, eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 377--404.

Cox Jr, E.C., and M.W Davis Jr, 1973, Sep. Sci. 8(2),205-243.

Davis Jr, M.W, 1. Mrochek and C.1. Hardy, 1966,1.Inorg. Nucl. Chem. 28, 2001-2014.

DeKalb, E.L., and VA. Fassel, 1979, Optical atomicemission and absorption methods, in: Handbook onthe Physics and Chemistry of Rare Earths, Vol. 4,eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 405--440.

Donohue, T., 1977, 1. Chern. Phys. 67(11), 5402-5404.

D'Silva, A.P., and VA. Fassel, 1979, X-ray excitedoptical luminescence of the rare earths, in:Handbook on the Physics and Chemistry of RareEarths, Vol. 4, eds K.A. Gschneidner Jr andL. Eyring (North-Holland, Amsterdam) pp. 441-456.

duPreez, A.C., and 1.S. Preston, 1992, Solv. Extr. IonExch. 10(2),207-230.

Ensor, D.o., M. Nicks and D.I. Pruett, 1988, Sep. Sci.Techno!. 23(12-13), 1345-1353.

Fassel, VA., 1961, Analytical spectroscopy of therare earth elements, in: The Rare Earths, edsP.H. Spedding and A.H. Daane (R.E. KriegerPublishing Co., Huntington, NY) pp. 594-613.

Fernando, Q., N. Yanagihara, 1.T. Dyke and K.Vemulapalli, 1991, Formation of Rare EarthCarbonates Using Supercritical Carbon Dioxide, USPatent 5045289, Sept. 3, 1991.

Fidelis,1., 1970,1. Inorg. Nucl. Chern. 32, 997~1 003.Fidelis, 1., 1971, Influence of temperature on the

extraction of lanthanides in the HDEHP-HNOJsystem, in: Proc. Int. Solvent Extraction Conf. 1971,Vol. II, eds 1.G. Gregory, B. Evans and P.C. Weston(Society of Chemical Industry, London) pp. 1004-1007.

Freiser, H., 1988, Solv. Extr. Ion Exch. 6(6), 1093-1108.

Gongyi, G., and C. Yuli, 1989, Preparation of highpurity scandium oxide by extraction chromatogra-phy, in: Rare Earths, Extraction, Preparation andApplications, eds R.G. Bautista and M.M. Wong(TMS, Warrendale, PA) pp. 163-170.

Gongyi, G., and C. Yuli, 1990, Hydrometallurgy 23,333-340.

Gruzensky, WG., and G.T. Engel, 1959, Trans. Metall.Soc. AIME 215, 738-742.

Gschneidner Jr, K.A., 1993, Physical properties ofthe rare earths, in: Handbook on the Physicsand Chemistry of Rare Earths, Cumulative IndexVolumes 1-15, eds K.A. Gschneidner Jr and

SEPARATION CHEMISTRY 25

L. Eyring (North-Holland, Amsterdam) pp. 509~521.

Gupta, C.K., and N. Krisnamurthy, 1992, Int. Mater.Rev. 37(5), 197-248.

Habashi, E, 1990, The rare metals and their positionin the periodic table, in: Rare Metals '90, Proc.Int. Symp. on Processing of Rare Metals (OsakaUniversity, Japan) pp. 47-52.

Harada, I., M. Smutz and R.G. Bautista, 1971,Polymers of iron and rare earths and di-2-ethylhexylphosphoric acid, in: Proc. Int. Solvent ExtractionConf. 1971, Vo!. II, eds J.G. Gregory, B. Evans andP.C. Weston (Society of Chemical Industry, London)pp. 950-956.

Hardy, C.J., 1970, l Inorg. Nuc!. Chern. 32, 619-625.Harrah, H.W., 1967, Rare earth concentration at

molybdenum corporation of America, Part 2,Solvent extraction plant, Deco Trefoil 31(5,November-December) 9-16. (Denver EquipmentCo., Denver, CO.).

Harris, D.H., and E.R. Tompkins, 1947, l Am. Chern.Soc. 69, 2792-2800.

Haskin, L.A., and I.P. Paster, 1979, Geochemistry andmineralogy of the rare earths, in: Handbook on thePhysics and Chemistry of Rare Earths, Vol. 3, edsK.A. Gschneidner Jr and L. Eyring (North-Holland,Amsterdam, 1979) pp. 1-80.

Hayden, J.G., LH. Gerow and M.W Davis Jr, 1974,Sep. Sci. 9(4), 337-350.

Healy, R.M., and H.E. Kremers, 1961, Separation ofrare earths by fractional crystallization, in: TheRare Earths, eds FH. Spedding and A.H. Daane,reprinted 1971 (R.E. Krieger Publishing Co.,Huntington, NY) pp. 29-37.

Hesford, E., and H.A.C. McKay, 1960, J. Inorg. Nuc1.Chern. 13, 156-164.

Hesford, E., E.E. Jackson and H.A.C. McKay, 1959,J. Inorg. Nuc!. Chern. 9, 279-289.

Hoh, YH., and WK. Wang, 1980, Ind. Eng. Chern.,Proc. Des. Dev. 19,64-67.

Hsu, K.H., C.H. Huang, I.C. King and P.K. Li,1980, in: Proc. Int. Solvent Extraction Conf., Vol.2 (Universite de Liege, Liege, Belgium) paper 80-82.

Huang, C.H., and R.G. Bautista, 1983, Sep. Sci. &Techno!. 18, 1667-1683.

Huang, C.H., R.E Hu and GX. Xu, 1989, Solv. Extr.Ion Exch. 7(3), 489-497.

Hubicki, Z., 1990, Hydrometallurgy 23, 319-331.Ketelle, B.H., and G.E. Boyd, 1947, l Am. Chern.

Soc. 69, 2800-2812.

Kondo, K., F Nakashio and H. Ohtsubo, 1992, Solv.Extr. Ion Exch. 10(4), 655-662.

Kosinski, FE., and H. Bostian, 1969, J. Inorg. Nuc!.Chern. 31, 3623-3631.

Krumholz, P., 1964, Solution chemistry, in: Progressin the Science and Technology of the Rare Earths,Vol. I, ed. L. Eyring (Pergamon Press, New York)pp. 110-138.

Krumholz, P., K. Bril, S. Bril, l Behmoiras, EGottdenker and EW Lima, 1958, in: Proc. 2nd U.N.Int. Conf. on Peaceful Uses of Atomic Energy, AIConf. 15/P/2491 (Brazil) pp. 184-195.

Kubota, F, M. Goto and F Nakashio, 1993, Solv. Extr.Ion Exch. 11(3),437-453.

Lenz, T.G., and M. Smutz, 1966, l Inorg. Nuc\. Chern.28, 1119-1120.

Loriers, l, 1964, La chimie analytique des terres rares,in: Progress in the Science and Technology of theRare Earths, Vol. 1, ed. L. Eyring (Pergamon Press,New York) pp. 351-398.

Ludwig, R., K. Inoue and T. Yamato, 1993, Solv. Extr.Ion Exch. 11(2), 311-330.

Majdan, M., and Z. Kolarik, 1993, Solv. Extr. IonExch. 11(2),331-348.

Manchanda, VK., CA Chang and l Peng, 1988, Solv.Extr. Ion Exch. 6(5), 835-857.

Marinsky, lA., L.E. Glendenin and C.D. Coryell,1947, l Am. Chern. Soc. 69,2781-2785.

Moeller, I., 1961, The chemistry of the rareearths, in: The Rare Earths, eds EH. Speddingand A.H. Daane, reprinted 1971 (KE. KriegerPublishing Co., Huntington, NY) pp. 9-28.

O'Laughlin, J.W, 1979, Chemical spectrophotometricand polarographic methods, in: Handbook on thePhysics and Chemistry of Rare Earths, Vol. 4, edsK.A. Gschneidner Jr and L. Eyring (North-Holland,Amsterdam) pp. 341-358.

Peppard, D.F, 1961, Separation of rare earths byliquid-liquid extraction, in: The Rare Earths, edsFH. Spedding and A.H. Daane, reprinted 1971(R.E. Krieger Publishing Co., Huntington, NY)pp.38-54.

Peppard, D.F, 1964, Fractionation of rare earthsby liquid-liquid extracion using phosphorus-basedextractants, in: Progress in the Science andTechnology of the Rare Earths, Vo!. I, ed. L. Eyring(Pergamon Press, New York, 1964) pp. 89-109.

Peppard, D.F, J.P. Faris, P.R. Gray and G.w. Mason,1953, J. Phys. Chern. 57, 294-301.

Peppard, D.F., WI. Driscoll, KI. Sironen and S.McCarthy, 1957, l Inorg. Nuc!. Chern. 4, 326.

26 R.G. BAUTISTA

Peppard, D.P., G.M. Mason, WJ. Driscoll and R.J.Sironen, 1958, J. Inorg. Nucl. Chern. 7,276-285.

Powell, J.E., 1979, Separation chemistry, in: Handbookon the Physics and Chemistry of Rare Earths, Vol.3, eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 81-109.

Powell, J.E., and EH. Spedding, 1959a, Trans. Metall.Soc. AIME 215, 457-463.

Powell, J.E., and EH. Spedding, 1959b, Basicprinciples involved in the macroseparation ofadjacent rare earths from each other by meansof ion exchange, in: Adsorption, Ion Exchange,and Dialysis, Chemical Engineering ProgressSymposium Series, Vol. 55, no. 24 (AmericanInstitute of Chemical Engineers, New York)pp. 101-113.

Preston, J.S., 1985, Hydrometallurgy 14, 171-188.Qiu, L.E, X.H. Kang and T.S. Wang, 1991, Sep. Sci.

& Technol. 26(2), 199-221.Reuben, J., and G.A. Elgavish, 1979, Shift reagents

and NMR of paramagnetic lanthanide complexes,in: Handbook on the Physics and Chemistry ofRare Earths, Vol. 4, eds K.A. Gschneidner Jr andL. Eyring (North-Holland, Amsterdam) pp. 483-514.

Ryabchikov, D.L, 1959, Engl. Trans., 1960, RareEarth Elements, Extraction, Analysis, Applications(Academy of Sciences, USSR, Moscow).

Ryabchikov, DJ., and V.A Ryabukhin, 1964, Sovietresearch on analytical chemistry of the rare earths,in: Progress in the Science and Technology of theRare Earths, Vol. 1, ed. L. Eyring (Pergamon Press,London) pp. 399-415.

Sato, T., 1989, Hydrometallurgy 22, 121-140.Schuhmann, S., and J.A Philpotts, 1979, Mass

spectrometric stable-isotope dilution analysis forlanthanides in geochemical materials, in: Handbookon the Physics and Chemistry of Rare Earths, Vol.4, eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 471-482.

Sheka, Z.A., and E.G. Kriss, 1959, Russ. J. Inorg.Chern. 4, 1153-1156.

Spedding, EH., AE Voigt, E.M. Gladrow and N.R.Sleight, 1947a, J. Am. Chern. Soc. 69,2777-2781.

Spedding, P.H., A.E Voigt, E.M. Gladrow, N.R.Sleight, J.E. Powell, J.M. Wright, T.A. Butler and P.Figard, 1947b, J. Am. Chern. Soc. 69,2786--2792.

Spedding, P.H., E.L Fulmer, T.A. Butler, E.M.Gladrow, M. Gobush, P.E. Porter, J.E. Powell andJ.M. Wright, 1947c, J. Am. Chern. Soc. 69,2812-2818.

Spedding, EH., J.E. Powell and EJ. Wheelright, 1954a,1. Amer. Chern. Soc. 76, 612-{i13.

Spedding, P.H., J.E. Powell and EJ. Whee1right,1954b, J. Amer. Chern. Soc. 76, 2557-2560.

Szabadvary, E, 1980, in: Handbook on the Physicsand Chemistry of Rare Earths, Vol. 2, edsKA Gschneidner Jr and and L. Eyring (North-Holland, Amsterdam) pp. 33-80.

Tang, 1., and C.M. Wai, 1986, Anal. Chern. 58, 3233-3235.

Taylor, S.R., 1979, Trace element analysis of rareearth elements by spark source mass spectrometry,in: Handbook on the Physics and Chemistry ofRare Earths, Vol. 4, eds K.A. Gschneidner Jr andL. Eyring (North-Holland, Amsterdam) pp. 359-376.

Tedesco, P.H., v.B. de Rumi and J.A GonzalezQuintana, 1967, 1. Inorg. Nucl. Chern. 29, 2813-2817.

Thompson, L.C., 1979, Complexes, in: Handbook onthe Physics and Chemistry of Rare Earths, Vol. 3,eds K.A. Gschneidner Jr and L. Eyring (North-Holland, Amsterdam) pp. 209-297.

Tompkins, E.R., and S.w. Mayer, 1947, 1. Am. Chern.Soc. 69, 2859-2865.

Tompkins, E.R., 1.X. Khym and WE. Cohn, 1947, 1.Am. Chern. Soc. 69, 2769-2777.

Topp, N.E., 1965, The Chemistry of the Rare-EarthElements (Elsevier, London).

Urbanski, T.S., A Abbruzzese, P. Fornari and R.Massidda, 1990, Hydrometallurgy 25, 185-195.

Urbanski, T.S., C. Abbruzzese, P. Fornari and R.Massidda, 1992, Hydrometallurgy 28, 1-12.

Warf, J.C., 1949, J. Am. Chern. Soc. 71,3257-3258.Weaver, B., 1964, Liquid-liquid extraction of the rare

earths, in: Progress in the Science and Technologyof the Rare Earths, Vol. 1, ed. L. Eyring (PergamonPress, New York) pp. 85-88.

Weaver, B., 1968, Liquid-liquid extraction of the rareearths, in: Progress in the Science and Technologyof the Rare Earths, Vol. 3, ed. L. Eyring (PergamonPress, New York) pp. 129-148.

Weaver, B., 1974, Solvent extraction in the separationof rare earths and trivalent actinides, in: IonExchange and Solvent Extraction, Vol. 6, edsJ.A. Marinsky and Y. Marcus (Marcel Dekker, NewYork) pp. 189-277.

Weeks, M.E., 1956, in: Discovery of the Elements,6th Ed. (American Chemical Society, Easton, PA)pp.695-727.

SEPARATION CHEMISTRY 27

Yanagihara, N., K. Vemulapalli, Q. Fernando and 1.T.Dyke, 1991, 1. Less-Common Met. 167, 223-232.

Yoshida, H., 1964, 1. Inorg. Nucl. Chern. 26, 619-625,

Zhang, B.Z., K.Y. Lu, K.C. King, W.C. Wei and W.C.

Wang, 1982, Hydrometallurgy 9,205-210,Zielinski, S" and A. Szczepanik, 1993, Hydrometal-

lurgy 33, 219-226.Zielinski, S., M. Buca and A. Szczepanik