a critical review on solvent extraction of rare earths from aqueous solutions

Minerals Engineering 56 (2014) 10–28

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Minerals Engineering

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A critical review on solvent extraction of rare earths from aqueoussolutions

0892-6875 � 2013 The Authors. Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.mineng.2013.10.021

⇑ Corresponding author. Tel.: +86 24 8368 7729.E-mail address: [email protected] (F. Xie).

Open access under CC BY-NC-ND license.

Feng Xie a,⇑, Ting An Zhang a, David Dreisinger b, Fiona Doyle c

a School of Materials and Metallurgy, Northeastern University, 3-11 Wenhua Road, Shenyang 110004, Chinab Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canadac Department of Materials Science and Engineering, University of California, Berkeley, 210 Hearst Mining Building, Berkeley, CA 94720, United States

a r t i c l e i n f o

Article history:Received 2 September 2013Accepted 22 October 2013Available online 15 November 2013

Keywords:Rare earthsSolvent extractionProcess configuration

a b s t r a c t

Rare earth elements have unique physicochemical properties that make them essential elements in manyhigh-tech components. Bastnesite (La, Ce)FCO3, monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4, are themain commercial sources of rare earths. Rare earth minerals are usually beneficiated by flotation or grav-ity or magnetic processes to produce concentrates that are subsequently leached with aqueous inorganicacids, such as HCl, H2SO4, or HNO3. After filtration or counter current decantation (CCD), solvent extrac-tion is usually used to separate individual rare earths or produce mixed rare earth solutions or com-pounds. Rare earth producers follow similar principles and schemes when selecting specific solventextraction routes. The use of cation exchangers, solvation extractants, and anion exchangers, for separat-ing rare earths has been extensively studied. The choice of extractants and aqueous solutions is influ-enced by both cost considerations and requirements of technical performance. Commercially, D2EHPA,HEHEHP, Versatic 10, TBP, and Aliquat 336 have been widely used in rare earth solvent extraction pro-cesses. Up to hundreds of stages of mixers and settlers may be assembled together to achieve the neces-sary separations. This paper reviews the chemistry of different solvent extractants and typicalconfigurations for rare earth separations.

� 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.1. Rare earths ores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.2. Technological applications of rare earths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.3. Primary rare earth extraction process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. Solvent extraction separation of rare earths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1. Cation exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1.1. Carboxylic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.1.2. Organophosphorous acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.2. Chelating extractant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.3. Solvation extractant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.4. Anion exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.5. Synergistic solvent extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3. Process engineering and equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1. Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.1. Molycorp-bastnesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.2. Rhône-poulenc – monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.3. AS Megon–xenotime. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.4. Mintek–apatite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.1.5. Industrial processes in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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F. Xie et al. / Minerals Engineering 56 (2014) 10–28 11

3.1.6. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 1Global

UnitAustBrazChinCom

SIndiaMalaOtheWor

3.2. Process simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1. Introduction

1.1. Rare earths ores

The term rare earths was originally used to designate the lan-thanoids in row 3 of the periodic table which includes oxides ofscandium (Sc, 21), yttrium (Y, 39), lanthanum (La, 57) and the 14elements following lanthanum in the periodic table of elements,i.e. from cerium (Ce, 58) to lutetium (Lu, 71) inclusive. More re-cently, the term ‘‘rare earths’’ has been used to designate the ele-ments themselves. Scandium and yttrium tend to occur in thesame ore deposits as the lanthanoids and exhibit similar chemicalproperties (Jones et al., 1996; Wang et al., 2011). The term ‘‘rare’’earth is a misnomer; they are relatively abundant in the Earth’scrust, however, they are typically dispersed and only rarely occurin concentrated and economically exploitable mineral deposits.

Rare earth mines have operated in South Africa, India, and Brazilin the 1950s, but from the 1960s to the 1980s, the largest globalproducer was a mine in Mountain Pass, California. In the 1990s,China began large scale production and exported cheaper rareearths. Other producers were unable to compete economically,and began closing in the 1990s, with the Mountain Pass mine shut-ting down in 2002 (Tse, 2011). Consequently, China is currently theworld’s largest producer of rare earth elements, providing morethan 95% of the world’s total supply from its mines in Inner Mon-golia (Chen, 2010). The world reserves and production of rareearths are summarized in Table 1 (U.S. Geological Survey 2011).

There are a wide variety rare earth minerals known, but notablythe mined minerals are bastnesite (La, Ce)FCO3, monazite, (Ce, La,Y, Th)PO4, and xenotime, YPO4 (Clark, 1984; Jordens et al., 2013).Bastnesite deposits in China and the United States constitute thelargest percentage of the world’s rare earth resources. Notableoccurrences include the carbonatite-hosted bastnesite deposit atMountain Pass, California, several bastnesite deposits in SichuanProvince, China, and the massive deposit at Bayan Obo, Inner Mon-golia, China. Monazite deposits in Australia, Brazil, China, India,Malaysia, South Africa, Sri Lanka, Thailand, and the United Statesconstitute the second largest segment. Apatite, cheralite, eudialyte,loparite, phosphorites, secondary monazite, spent uranium solu-tions, and xenotime make up the remaining resources. There arealso ion-adsorbed rare earth deposits, widely distributed in south-ern China formed through weathering were the rare earths have

rare earth mine production and reserves (after U.S. Geological Survey, 2011).

Mine production,metric ton

Reserves, metricton

2009 2010

ed States – – 13,000,000ralia – – 1,600,000il 550 550 48,000a 129,000 130,000 55,000,000mon wealth of Independenttates

– – 19,000,000

2700 2700 3,100,000ysia 350 350 30,000r countries – – 22,000,000ld total 132,600 133,600 113,778,000

been adsorbed onto the surfaces of clay minerals such as kaolin,feldspar and mica (Liu, 2008; Chi and Tian, 2007; Yang et al.,2008; Wang, 2009).

1.2. Technological applications of rare earths

Rare earth metals and their compounds are in demand, and areoften crucial for, a broad and rapidly expanding range of applica-tions that rely upon their chemical, catalytic, electrical, magnetic,and optical properties. Rare earths are widely used for traditionalsectors including metallurgy, petroleum, textiles, and agriculture.As indicated in Table 2 (Commercial Applications for Rare EarthTechnology, Http://reitausa.org), they are also becoming uniquelyindispensable and critical in many high-tech industry such as hy-brid cars, wind turbines, and compact fluorescent lights, flat screentelevisions, mobile phones, disc drives, and defense technologies(Song and Hong, 2010a,b). Different rare earths are needed to sup-ply the required functionality in these applications. In some cases,a single rare earth element may be required, such as La for nickel-metal hydride batteries, but other applications require a mixture ofrare earths, for example Nd and Pr for rare earth magnets and Eu(or Tb) and Y for rare earth phosphors.

Rare earth-containing permanent magnets are alloys of rareearth elements and transition metals such as iron, nickel, and co-balt. Samarium-cobalt magnets were first developed in the 1970s(Liu et al., 2006). However, due to their higher cost and weakermagnetic field strength, these magnets are now used less than neo-dymium magnets, unless their higher Curie point is needed. Neo-dymium permanent magnets, a tetragonal alloy of neodymium,iron, and boron (Nd2Fe14B), have been used in a wide range ofapplications requiring a high energy product and high coerciveforce (Brown et al., 2002). Neodymium can be replaced by praseo-dymium and up to 5% cerium in high energy product magnets(Doyle et al., 2000; Benz et al., 2000). The addition of terbiumand dysprosium can enhance the coercivity of Nd–Fe–B sinteredmagnets (Hu et al., 2008; Xu et al., 2011).

Rare earths phosphors are widely used in high efficiency light-ing, flat display screens, plasma screens, and liquid crystal screensdue to their unique luminescent properties (Ronda et al., 1988;Nazarov and Noh, 2010; Rapaport and Miliez, 2006; Ye et al.,2011). Unlike transition metal ions, the spectral position of theemission lines of rare earths is almost independent of the host lat-tice. Rare earth ions such as Tb3+ and Eu3+ emit at frequencies thatenable high lumen efficacies and a good quality of white light (Liuand Chen, 2007; Tu et al., 2011). Replacement of some of the rareearth cations of a crystalline rare earth phosphor by ions of anotherrare earth element activates the phosphor, achieving a high degreeof fluorescence. For example, terbium-activated gadolinium oxy-sulphide (Gd2O2S:Tb) gives a maximum fluorescence when about0.3% of the gadolinium atoms have been replaced by terbium. Com-mercial plasma screens have used yttrium tantalates activated bythulium (YTaO4:Tm) or niobium (YTaO4:Nb) (Fauchera et al.,2002; Karsu et al., 2011).

1.3. Primary rare earth extraction process

As highlighted above, bastnesite, monazite, and xenotime arethe main rare earth minerals of commercial importance. Typical

http://www.reitausa.org

Table 2Some commercial applications of rare earths (after http://www.reitausa.org/).

Application Rare earth (RE) technology Enabling functionality RE elementsrequired

Hybrids, plug-In, and electric Vehicles RE permanent magnets Electric traction drives replacing or supplementinginternal combustion engines

Nd, Pr, Dy, Tb

Wind and hydro power generation RE permanent magnets Gearless generators for better reliability and onlineperformance

Nd, Pr

Computer Disc Drives; Cordless Powertools

RE permanent magnets Compact, light weight and powerful motors Nd, Pr, Dy, Tb

Medical imaging – MRI RE permanent magnets Produce magnetic field Nd, Pr, Dy, Tb,Y, Eu, Tb

X-ray imaging RE phosphors High energy efficiencyHandheld wire-less devices RE permanent magnets; Compact, light weight and powerful motors; Nd, Pr, Dy, Tb,

Y, EuFlat screen display RE phosphors Unique luminescent properties Y, Eu, Tb, Gd, CeCatalytic converters and other emission

reduction technologiesAbility to oxidize CO and ozone to CO2 and O2 Significantly less expensive than Pt metal group

alternativesCe, La

Fiber optics RE doped optical fibers Signal amplification Y, Eu, Tb, ErNi Metal Hydride Batteries Energy storage Proven and cost effective compared to Li ion battery

alternativesLa

Capacitors with high energy density Rare earth- doped ceramic, tantalum andother types of capacitors

High energy density compared to conventionalcapacitors

Various rareearths

12 F. Xie et al. / Minerals Engineering 56 (2014) 10–28

compositions for these minerals are shown in Table 3 (there maybe significant compositional variations depending on sources)(U.S. Bureau of Mines, 1985). Various processing routes have beendeveloped to recover rare earths. After mining and comminution,ore is beneficiated by flotation, magnetic or gravity methods toproduce rare earth concentrates, which then undergo hydrometal-lurgical processing to recover rare earth metals or compounds(Gupta and Krishnamurthy, 2005).

Bastnesite concentrates are relatively straightforward to treat(Huang et al., 2005). In order to reduce the acid consumption, bast-nesite concentrates are typically roasted to decompose the carbon-ate minerals before leaching with either hydrochloric or sulfuricacid. Cerium comprises about half of the rare earth content withinbastnasite, so removing it prior to solvent extraction dramaticallyreduces the solvent extraction capacity for selective separation ofindividual rare earth elements. To minimize the separation costscerium could be oxidized to CeO2 during roasting which will notdissolve readily in acidic lixiviants so would report to the leach res-idue from which it could be recovered separately. Hydrochloricacid may promote reduction of Ce(IV) and hence, incomplete sep-aration of Ce(III) from the other trivalent lanthanides, Ln(III). Alter-natively after leaching all the rare earths, cerium could be oxidizedin the aqueous phase to precipitate it and then be recovered by fil-tration. For example, Ce(OH)4 was precipitated at the Thorium

Table 3Rare earth contents of principal minerals as percentage of total rare earth oxide(Mineral Facts and Problems, 1985).

Oxide Bastnesite(California)

Monazite(Australia)

Xenotime(Malaysia)

Y2O3 0.1 2.1 60.8La2O3 32.0 23.0 0.5CeO2 49.0 45.5 5.0Pr6O11 4.4 5.0 0.7Nd2O3 13.5 18.0 2.2Sm2O3 0.5 3.5 1.9Eu2O3 0.1 0.1 0.2Gd2O3 0.3 1.8 4.0Tb2O3 1.0Dy2O3 8.7Ho2O3 2.1Er2O3 0.1 1.0 5.4Tm2O3 0.9Yb2O3 6.2Lu2O3 0.4

Total 100.0 100.0 100.0

plant using ammonium carbonate and ammonium persulfate(Huang et al., 2005). In some cases, sodium hypochlorite is alsoused to oxidize dissolved Ce(III).

At Baotou, the largest producer of rare earths in China, the bast-nesite concentrates contain a small amount of monazite. Fig. 1shows the leaching process used for Baotou rare earth concen-trates, which has been designated to be flexible and to accommo-date different concentrates. The process starts with roasting withconcentrated sulfuric acid at >300 �C (Huang et al., 2006) to ‘‘crack’’the monazite. The rare earth sulfates formed during this processare then leached with water, and excess acid is neutralized withmagnesia and filtered. The leach solution then proceeds to solventextraction, alternatively a mixed rare earth chloride (for electroly-sis to misch metal) could be produced by precipitation with ammo-nium carbonate, followed by dissolution with HCl andcrystallization. Unfortunately, the radioactive element, thorium,is precipitated and reports to the leach residue. It cannot be recov-ered economically, resulting in both loss of the valuable thoriumand potential environment hazards. HF and sulfur dioxide reportto the off-gas from roasting. Large amounts of water or alkalinesolutions are needed to remove them, resulting in large volumesof acidic effluents.

The roasting process has been modified, for example by addingMgO or CaO and NaCl to stabilize fluorine in the leaching residueinstead of releasing it to the waste gas phase (Wang and Liu,1996; Wu et al., 2002a,b, 2004b; Li et al., 2004). Bastnesite has alsobeen roasted with ammonium chloride, which decomposes intogaseous HCl that forms rare earth chlorides, which are readily lea-ched with hot water (Chi et al., 2004). Another variant involvesheating concentrate with sulfuric acid at 40–180 �C for two to fourhours before roasting at 150–330 �C (Hu, 1998). This suppressesdecomposition of sulfuric acid, resulting in a relatively high frac-tion of HF in the gas phase; this can be recovered as solid NH4Fby reacting with (NH4)2CO3 in the off-gas pipe. In some plants,the bastnesite concentrates are first digested with concentratedNaOH to decompose carbonates and then leached with hydrochlo-ric acid to produce mixed rare earth chlorides (Gupta and Krishna-murthy, 2005; Xu et al., 2012). The disadvantages of this processinclude high alkaline consumption and the radioactive thoriumreporting to both the leachate and the residue, which hamperssubsequent recovery.

Monazite and xenotime concentrates can be leached either bysulfuric acid or by sodium hydroxide at elevated temperature todecompose the orthophosphate lattice. The sodium hydroxide

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treatment is preferred in most commercial extraction plants be-cause it better separates phosphate from the rare earth (Alexet al., 1998). The ion-adsorption type rare earth ores are usuallyleached directly with inorganic acid, either in dumps or in situ,resulting in dissolution of most of rare earths in acidic solutions(Zuo et al., 2007; Qiu et al., 2008).

Regardless of the original rare earth mineral or the preciseleaching process, the leach solution will usually contain dissolvedimpurities such as iron, which are removed by precipitation beforeproceeding to solvent extraction to separate the rare earths.

2. Solvent extraction separation of rare earths

Solvent extraction producer starts by separating differentgroups of rare earths from the leachate. Depending on the process,some primary rare earths producers may choose to sell intermedi-ate, mixed products, or perform different downstream separationsto produce individual rare earth salts or oxides. Individual rareearths are difficult to separate from each other, due to their similarphysical and chemical properties. Separation processes based onion-exchange and solvent extraction techniques have thus beendeveloped to produce high purity single rare earth solutions orcompounds. Before the advent of industrial scale solvent extractionin the 1960s, ion exchange technology was the only practical wayto separate the rare earths in large quantities (Jamrack, 1963; Ku-mar, 1994; Reddy et al., 2009). Nowadays, ion exchange is onlyused to obtain small quantities of high purity rare earth productfor electronics or analytical applications (Taniguchi and Doty,1989). Solvent extraction is generally accepted as the most appro-priate commercial technology for separating rare earths due to theneed to be able to handle larger volumes of dilute pregnant liquors(Peppard et al., 1953, 1957a,b; Peppard and Wason, 1961; Brownand Sherrington, 1979; Sherrington, 1983).

Solvent extraction processes for separation and purification ofrare earths have been reviewed during the 1990s (Zhu, 1991; Red-dy et al., 1995). Table 4 summarizes commercial extractants re-ported in the literature for rare earth solvent extraction (Ritceyand Ashbrook, 1984; Rydberg et al., 2004). All three major classesof extractant, namely, cation exchangers (or acidic extractants),solvation extractants (or neutral extractants), and anion exchang-ers (or basic extractants), have been utilized for separating rareearths. Some chelating extractants have also been suggested forrare earth separations.

Before considering the different types of extractant, it is usefulto define two terms, the distribution coefficient and the separation

Rare earth concent

Roasting with co

Calcined concentrate

leaching

water

Neutralization/precipitation/fi

MgO

leachate

Solvent extraction for separation of individual rare earth

Precipitdissolu

Fig. 1. Schematic leaching process used for Baotou ra

factor, commonly used to quantitatively describe solvent extrac-tion. The distribution coefficient of a metal ion, M, DM (sometimesdesignated as K), particularly in earlier literature, is given by:

DM ¼½M�½M� ¼ K ð1Þ

where ½M� is the molar concentration of M in the organic phase and[M] is the concentration in the aqueous phase. The separation factorof two different metal ions, M1 and M2, bM1/M2, is defined as:

bM1=M2 ¼DM1

DM2

ð2Þ

2.1. Cation exchangers

The overall extraction of rare earth elements from aqueousmedia by cation exchange extractants in their acidic form can gen-erally be expressed as (Peppard et al., 1958):

Ln3þ þ 3HA ¼ LnA3 þ 3Hþ ð3Þ

where Ln denotes any rare earth, A denotes the organic anion, andoverscoring denotes species present in the organic phase. Generallythe process is more complicated then expressed in Eq. (3) where theacidic extractants are usually aggregated as dimmers or larger olig-omers in non-polar organic solutions, which lowers their polarity,and the rare earth complexes formed upon extraction may containundissociated organic acid. Thus a more accurate depiction of theextraction reaction is (Mason et al., 1978):

Ln3þ þ 3H2A2 ¼ LnðHA2Þ3 þ 3Hþ ð4Þ

Here H2A2 refers to the dimeric form of the organic acid. Frominspection of Eqs. (3), (4), it is evident that the extraction of rareearths with cation exchangers is promoted by increasing the aque-ous phase pH, while the stripping process, which reverses theextraction reaction, is promoted by increasing the acidity of theaqueous stripping solution.

Two different classes of cation exchangers are use for rare earthseparations, namely carboxylic or fatty acids, and organic deriva-tives of phosphorous acids.

2.1.1. Carboxylic acidsThe use of different carboxylic acids, including naphthenic acids

and Versatic acids, for extracting rare earth metal ions has been re-ported (Bauer and Lindstrom, 1964; Korpusov et al., 1974). The

rate (50% wt REO)

ncentrated H2SO4

Off-gas

ltration

residue

ation with ammonium carbonate and tion with HCl to produce mixed rare earth

re earth concentrates (after Huang et al., 2006).

Table 4Some commercial extractants for rare earth solvent extraction (after Ritcey and Ashbrook, 1984; Rydberg et al., 2004).

Reagents class Structure Extractants

1, Cation exchangersCarboxylic acids R1

R2

CH3

COOH

C

Versatic acids:R1 + R2 = C7, Versatic 10;R1 + R2 = C6–C8, Versatic 911

R2

R3 R4

R1

(CH2)nCOOH

Naphthenic acids:R1-R4: varied alkyl groups

Phosphorous acids R1

R2

O

OH

P

Phosphoric acids:R1 = R2 = C4H9CH(C2H5)CH2O–, di-2-ethylhexylphosphoric acid (D2EHPA)Phosphonic acids:R1 = C4H9CH(C2H5)CH2O–, R2 = C4H9CH(C2H5)CH2–, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester(EHEHPA, HEHEHP, P507, PC88A)Phosphinic acids:R1 = R2 = C4H9CH(C2H5)CH2–, di-2-ethylhexylphosphinic acid (P229)R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentylphosphinic acid (Cyanex 272)

R1

R2

S

OH

P

Monothiophosphorous acidsR1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-monothiophosphinic acid (Cyanex 302)

R1

R2

S

SH

P

Dithiophosphorous acidsR1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-dithiophosphinic acid (Cyanex 301)

2, Chelatingexchangers

R1 C CH2 C R2

O

O b-diketones:R1 = R-C6H5, R2 = CH3(CH2)5–, R: unknown side alkyl, (LIX 54)

3, Solvatingextractants R1

R2

O

R3

P

Phosphorous ester:R1 = R2 = R3 = CH2(CH2)2CH2O–, tri-n-butyl-phosphate (TBP)R1 = R2 = CH2(CH2)2CH2O–, R3 = CH2(CH2)2CH2–, dibutylbutylphosphonate (DBBP)Phosphine oxides:R1 = R2 = R3 = CH2(CH2)6CH2–, tri-n-octylphosphine oxide (TOPO, Cyanex 921)

4, Anion exchanger RNH2 Primary aminesR = (CH3)3C(CH)2C(CH3)2)4 (Primene JMT, N1923)

RNH2

R1

R2

CH3 Cl

R3

N

Quaternary amines:R1 = R2 = R3 = C8–C10 mixture (Aliquat 336, Adogen 464)


extraction behavior of yttrium differs for these reagents; yttrium isextracted by Versatic 10 with the middle rare earths (La < -Ce < Nd < Gd < Y < Ho < Yb) whereas it is extracted by naphthenicacid with the light rare earths (La < Ce < Y < Nd < Gd < Ho < Yb)(Preston and du Preez, 1990). Zheng et al. (1991) noted that thebehavior of Y is correlated with the acidity of the extractant, whiledu Preez and Preston (1992) attributed the changing order to sterichindrance caused by the structure of the carboxylic acids and theatomic number of rare earth metal ions. With straight chain andnon-hindered acids the behavior of yttrium most closely resemblesthat of light lanthanides (e.g., Ce, or Pr), while for the sterically hin-dered acids the behavior of yttrium most closely resembles that ofmiddle lanthanides (e.g., Gd or Tb).

Naphthenic acid has been widely used for separating yttriumfrom lanthanides in China (Li et al., 1994). However, the extractantcomposition changes with use and its high solubility in water leadto significant reagent losses (Li, 1995). Alternatively, the novel car-boxylic acids, such as sec-nonylphenoxy acetic acid (CA-100) andsec-octylphenoxy acetic acid (CA-12), developed in China, have

much lower aqueous solubilities than naphthenic acids. A studyon the extraction of trivalent lanthanides (Sc, Y, Ln) and divalenttransition metal ions (Cu, Zn, Ni, Mn, Cd, Co) from acidic chloridesolutions with CA-100 in heptane indicated that CA-100 can ex-tract rare earth ions at lower pH values than Versatic 10. Theextraction behavior of yttrium with CA-100 most closely resem-bled that of the heavy lanthanides (Wang et al., 2002; Li et al.,2007a; Li et al., 2007b; Li et al., 2007c). The extraction behaviorof trivalent rare earths using cekanoic, neo-heptanoic, and some2-bromo alkanoic acids has also been investigated (Preston,1994; Xu et al., 2003; Singh et al., 2006). It was suggested thatthe introduction of the 2-bromo substitute in an alkanoic acidstructure lowers the pKa values, enabling the substitute acids tobe effective extractants at lower pH than the parent compounds.

2.1.2. Organophosphorous acidsVarious kinds of acidic organophosphorous extractants have

been used in rare-earth separation processes, with D2EHPA (orHDEHP, di(2-ethylhexyl) phosphoric acid) and HEHEHP (or EHEH-


PA, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl) are themain solvents widely used. Peppard et al. (1957a,b) noted thatthe distribution coefficients of rare earth ions at tracer concentra-tions between D2EHPA in toluene and aqueous chloride solutionshad an inverse third-power dependency on the HCl concentrationin the aqueous phase and a third-power dependency on theD2EHPA concentration in the organic phase. This indicates thatonly one of the acid groups in a D2EHPA dimmer in the organicphase dissociates and participates in the extraction reaction (fol-lowing Eq. (4)). Gels formed in the organic phase at high metalloadings and low acidities, which is undesirable because of theassociated viscosity and phase separation problems (Ferraro andPeppard, 1963).

The selectivity order for extracting rare earths from 0.5 M HClsolution with 0.75 M D2EHPA in toluene was Lu > Yb > Tm > T-b > Eu > Pm > Pr > Ce > La (Fig. 2) (Peppard and Wason, 1961), withthe log of the distribution coefficient (called logK by Peppard et al.(1957a,b)) increasing linearly with the atomic number, Z, of therare earth. The average separation factor of two adjacent rare earthelements was 2.5. Yttrium was extracted between Tb and Tm inthis solvent extraction system, corresponding to an artificial atom-ic number 67.6. The extraction of the lanthanide elements withD2EHPA in toluene was similar for perchloric acid solutions (Pieceand Peck, 1963), but poorer in nitrate media (Reddy et al., 1995). Itshould be pointed out that although the distribution coefficients ofrare earths generally increase with increasing atomic number, theprecise separation factors depend on the acidity of the aqueousphase and nature of the anion.

In 1965, Molycorp demonstrated the large scale application ofD2EHPA for pre-concentrating europium to around 15% from rareearth chloride feed derived from bastnesite, containing about0.1% Eu2O3 (Kruesi and Schiff, 1968). Preston and du Preez(1996) pre-concentrated europium from chloride containing0.22 ± 0.01 M total rare earths (%: Eu 93, Sm 3, Nd 2, Ce 1, Pr 0.5and Gd 0.5) using 0.4 M D2EHPA in xylene; 99.98% Eu solutionswere obtained in a single extraction stage at pH 2.7. 1 M D2EHPA

-4

-3

-2

-1

0

1

2

3

57 59 61 63 65 67 69 71Z

Log

K

Y

Fig. 2. Plot of LogK (LogD) as a function of atomic number (Z) (0.75 M HDEHP intoluene and 0.5 M HCl; Tracer concentrations of rare earths) (after Peppard et al.,1957a,b).

was used to separate a 99.8% La2O3 product from didymium chlo-ride solution (feed containing 45% La2O3, 35% Nd2O3, 10% Pr6O11

and 5% Sm2O3) (Nair and Smutz, 1967). Two multistage counter-current extraction circuits were needed; the first, 12 stage cascadeconcentrated La in the raffinate, and the second, 14 stage cascadeprovided further purification. The overall recovery of La was 60%.

Preston et al. (1996a,b,c) described a continuous solvent extrac-tion process for separating the middle (Sm, Eu, Gd, and Tb) and thelight rare earth fractions (La, Ce, Pr, and Nd) from a nitrate feed.The middle rare earths were first extracted into a 15% v/v ofD2EHPA in Shellsol AB in an 8 stage counter-current circuit, fol-lowed by scrubbing with 1 mol/L HNO3 in 2–4 stages, and strippingwith 1.5 mol/L HCl in 6–8 stages. Residual rare earths in the organ-ic phase (mainly Dy, with some Tb and Gd) were removed in a sec-ondary stripping circuit using 2.5 mol/L HCl in four stages. Over1000 L of feed liquor was processed in two continuous counter-current trials lasting a total of 630 h. From feed containing Sm:3.5 g/L, Gd: 2.4 g/L, Eu: 0.8 g/L, and Nd: 20 g/L (together with 4 to8 g/L each of the lighter rare earths), strip liquors containing Sm:35 g/L, Gd: 20 g/L, and Eu: 8 g/L were obtained with neodymium(5 g/L) as the main impurity. The recoveries of the middle rareearths to the strip liquors were relatively high (95–100%), whereaslosses of the light rare earths were low (< 4%). D2EHPA has alsobeen used to separate Sm, Eu, and Gd from the other rare earthsin a mixed nitrate-chloride leachate from monazite (Rabie, 2007).

D2EHPA can extract rare earth at low pH values, but becausethe equilibrium of Eqs. (1) and (2) lies strongly to the right it is dif-ficult to strip the loaded metals. Thus, other acidic organophospho-rus extractants have been widely examined for rare earth solventextraction. Benedetto et al. (1995) reported that DS5834 (ZenecaSpecialties, with a formulation similar to M2EHPA, mono-2-ethyl-hexyl phosphoric acid) could effectively extract Ga, In, Sm, and Gdfrom acidic media but was neither selective for Sm and Gd, noreffective for the separating these metals. The reagent HEHEHP,marketed variously as PC-88A, SME 418, Ionquest 801 and P-507,has gained more popularity for rare earth separations because rareearths can be stripped at lower acidities than from D2EHPA (Reddyet al., 1995). In addition, HEHEHP can be more heavily loaded withrare earth than D2EHPA before the onset of saturation effects,which increases the extraction efficiency. The extraction of rareearths with HEHEHP follows Eq. (1), with the extraction orderfrom chloride and nitrate media of La < Ce < Pr < Nd < S-m < Eu < Gd < Tb < Dy (�Y) < Ho < Er < Tm < Yb < Lu (Sato, 1989).A process developed by Daihachi for separating rare earths usingHEHEHP has been applied in commercial separation plants in Bao-tou, China (Zhu, 1991). Fontana and Pietrilli (2009) also suggestedthe use of HEHEHP for recovering rare earths resulted from spentNiMnH batteries.

Some di-alkyl phosphinic acids have also been investigated forrare earth separation, although only Cyanex 272 (bis(2,4,4-trim-ethylpentyl) phosphinic acid) has been used commercially (Kolarikand Pankova, 1966; Li and Frieser, 1986; Banda et al., 2012). Sabbotand Rollat (1985) described the preparation of pure Yb2O3 (99.3%)from a mixture of ytterbium and lutetium oxides (Yb2O3 = 87.5%,Lu2O3 = 12.5%) using 1 mol/L Cyanex 272 in kerosene. Saleh et al.(2002) investigated the extraction of La(III) by Cyanex 272 in tolu-ene from acidic nitrate-acetato media and suggested the formationof La(Ac)2A.3HA (Ac denotes acetate ion and HA denotes the acidicform of Cyanex 272) in the organic phase at low La loading andLaA3 at higher loading. Studies on the extraction of samarium fromchloride solutions with Cyanex 272 indicated that the extractedspecies was Sm(OH)A2

.2HA (El-Hefny et al., 2010). Studies on theextraction of La, Pr, Eu, Ho and Yb into chloroform solutions con-taining dicyclohexylphosphinic acid (DCHPA) showed that theextraction selectivity of DCHPA was inferior to that of other di-al-kyl phosphinic acids, presumably because the cyclohexyl groups in


DCHPA sterically hinder chelate formation (Cecconie and Frieser,1989).

2.2. Chelating extractant

As hydrogen ion donors, chelating extractants extract metals bya cation ion exchange mechanism similar to Eq. (1), but but theresulting organic complexes are stabilized by the organic anioncoordinating the central cation in at least two positions (Hudson,1982): Chelating extractants have been examined for extractingeuropium from nitrate solutions and cerium(III) and lantha-num(III) from chloride solutions, but performed unfavorably com-pared with acidic extractants (Urbanski et al., 1996; Arichi et al.,2006).

2.3. Solvation extractant

Several types of solvation extractants have been used for rareearth separations. Peppard et al. (1957a,b) investigated the extrac-tion of trivalent rare earths from chloride and nitrate solutionswith TBP (tributylphosphate). The extractability of the lanthanideswith TBP increased with increasing atomic number, but the distri-bution coefficients were much lower in chloride solutions than innitric media. Concentrated nitric systems were promising for sep-arating rare earths lighter than samarium. Rare earths heavier thansamarium could not be separated effectively in nitric systems. Therare earths in neutral nitrato complexes are coordinated bythe phosphoryl group of TBP, yielding an extractable complex.The overall reaction can be expressed as:

Ln3þ þ 3NO�3 þ 3TBP ¼ LnðNO3Þ3ðTBPÞ3 ð5Þ

although there would be few simple Ln3+ cations in solution at theionic strengths needed for effective extraction. Later work of Pepp-ard et al. (1966) examined the influence of extraction conditions onthe equilibrium constant of reaction 5 to infer the composition ofthe complexes formed under different conditions (Peppard et al.,1957a,b). Lu et al. (1998) studying the solvent extraction of Ce(IV)and Th (IV) from sulfate solutions with Cyanex 923 in n-hexane,found the extraction of Ce(IV) to be insensitive to acidity, whilethe extraction of Th(IV) increased with the aqueous acidity. A thirdphase formed at H2SO4 > 5 mol/L. The extraction of Ce(IV) andTh(IV) from sulfate media with Cyanex 923 can be represented bythe reaction:

M4þ þ SO2�4 þ 2HSO2�

4 þ 2B ¼MðSO4ÞðHSO4Þ2 � 2B ð6Þ

where M represents Ce or Th and B denotes Cyanex 923. The extrac-tion of trivalent lanthanides and yttrium from nitrate medium withCyanes 925 in heptane was suggested to follow the reaction (Liet al., 2007a,b,c):

M4þ þ 3NO�3 þ 2B ¼MðNO3Þ2 � 2B ð7Þ

where M and B represent the metal and Cyanex 925, respectively.During the 1960s, Thorium Limited in the United Kingdom used

TBP to separate light rare earths in nitrate media (Sherrington,1966). This process was operated batchwise with total reflux; onattaining steady state, the process was stopped and products of dif-ferent composition were withdrawn from different stages. Thisconfiguration is costly compared to continuous processing andnot amenable to scale up. Preston et al. (1996a,b,c) described a pi-lot-scale process for recovering a mixed rare-earth oxide productfrom calcium sulfate hemihydrate sludge generated during themanufacture of phosphoric acid from apatite mined at Phalaborwa,South Africa. Rare earths were recovered from leach liquor

containing 1.0 M nitric acid and 0.5 M calcium nitrate by adding2.5 M ammonium nitrate and extracting into 33% v/v DBBP (dibu-tyl butylphosphonate) in Shellsol 2325. The organic phase wasstripped with water to yield a solution of rare earth nitrates fromwhich the mixed rare earth oxide was recovered by adding oxalicacid and calcining the precipitate. Later work examined usingTBP (15% in Shellsol K diluent) to selectively extract cerium(IV)from the rare earth nitrate feed (Preston et al., 1996b). The organicphase was stripped by reducing the cerium(IV) with dilute hydro-gen peroxide in two stages, giving solutions containing up to 90 g/Lof cerium(III).

2.4. Anion exchangers

Anion exchangers extract metal ions as anionic complexes, andhence are only effective in the presence of strong anionic ligands.Early work indicated that the separation factors for adjacent rareearths with primary or tertiary amines were poor in chloride mediabut were more promising in sulfate media. (Rice and Stone, 1962;Bauer, 1966). El-Yamani and Shabana (El-Yamani and Shabana,1985) suggested that the extraction of lanthanum(III) from sulfatesolutions with Primene JMT (tri-alkyl methylamine) was extractedwith the following reactions:

2RNH2 þH2SO4 ¼ ðRNH3Þ2SO4 ð8Þ

2LaðSO4Þ3�3 þ 3ðRNH3Þ2SO4 ¼ 2ðRNH3Þ3LaðSOÞ43 þ 3SO2�4 ð9Þ

where RNH2 denotes the Primene JMT in the organic phase. Y(III)was found to behave similarly (Desouky et al., 2009). Quaternaryammonium salts such as tri-octyl methylammonium nitrate (Ali-quat 336) have proved promising for separating rare earths. Theextraction reaction can be simply represented as (Hsu et al., 1980;Huang et al., 1986):

Ln3þ þ 3NO�3 þ xðR4NþNO�3 Þn ¼ LnNO3 � xnR4NþNO�3 ð10Þ

where Ln denotes the rare earth ion and R4NþNO�3 the quaternaryammonium nitrate salt (Cerna et al. (1992) suggested a more com-plicated reaction). These reagents are strong-base anion exchangersand require lower concentrations of salting out reagents thanamines. Chelation with EDTA improved the extraction and separa-tion of rare earth pairs. In nitrate media Aliquat 336 extracts lightrare earths more readily than the heavier ones. This behavior con-trasts that of most of cation exchange and solvating extractants,for which the extraction of the rare earth metals increases steadilywith increasing atomic number. Hence, quaternary ammoniumsalts provide a means of removing light rare earths from processsolutions.

Yttrium is anomalous, behaving as a heavy rare earth in nitratemedia and as a light in thiocyanate media. This has been utilizedfor treating xenotime, which contains 60% Y2O3 (Table 2). Xeno-time concentrate was leached with HNO3 and lighter rare earthswere extracted with Aliquat 336 in an aromatic diluent. Yttriumand the heavy rare earths remained in the aqueous phase. Yttriumwas then extracted by Aliquat 336 from a thiocyanate solution,yielding 99.99% Y2O3, while other heavy rare earths remained inthe raffinate (Gaudernack, 1973). Lu et al. (1989) isolated >99%Nd2O3 from didymium nitrate solution (83% Nd, 15% Pr and 2%other rare earths) with 95% Nd recovery using Aliquat 336 in a45-stage extraction. Preston (1996) described a solvent-extractionprocess for recovering neodymium oxide (95% Nd2O3) from lightrare earth nitrate solution using 0.50 M solution of Aliquat 336 ni-trate in Shellsol AB.


2.5. Synergistic solvent extraction

Numerous types of synergistic solvent extraction systems forextracting and separating rare earths have been reported, includingmixtures of acidic extractants (e.g., carboxylic or organophospho-rus acids), mixture of neutral extractants (e.g. TBP and TOPO),and combinations of these (Santhi et al., 1991; Wang et al., 2006;Tian et al., 2012; Tian et al., 2013; Tong et al., 2013). Preston anddu Preez (1994) examined the effect of the addition of a series oftri-alkyl phosphates (RO)3PO, di-alkyl alkylphosphonates (RO)2-

RPO, alkyl di-alkylphosphinates (RO)R2PO, and tri-alkylphosphineoxides R3PO on the extraction of the trivalent lanthanides and yt-trium from chloride media by DIPSA (3,5-di-isopropylsalicylicacid). Synergistic effects were observed with all mixtures, albeitto different extents. For the series of compounds with R = n-butyl,the synergistic effect increased in the order (RO)3PS < (RO)3-

PO < (RO)2RPO < (RO)R2PO < R3PO. The synergistic effects weregreater for lutetium(III) than for lanthanum(III) (the separationacross the lanthanide series increased). Mixtures of DIPSA andTIBPO (tri-isobutyl phosphine oxide) gave somewhat better sepa-ration factors between the light and the middle lanthanides(bSm/Nd = 3.0) than Versatic 10 acid alone (bSm/Nd = 2.6). Separationfactors were comparable to those with the latter extractantbetween the heavy lanthanides (thulium to lutetium). The authorssuggested that the extracted rare earth complexes had a composi-tion of LnA3L2 (where HA represents carboxylic acid and L the neu-tral organophosphorous compound) and the synergism resultedfrom the replacement of some or all the undissociated carboxylicacid molecular (see Eq. (4)) (Preston and du Preez, 1995).

Comparison of the effects of some bi-functional ligands contain-ing C@O, P@O or S@O groups upon the extraction of trivalent rareearth metals from chloride media by DIPSA in xylene indicated thatthe shifts generally increased in the order S@O < C@O < P@O forcomparable ligands. The synergistic effect produced by the addi-tion of a given bi-functional compound generally decreased acrossthe lanthanide series (La to Lu), attributed to steric hindrance effect(Preston and du Preez, 1998). Reddy et al. (1999) reported thatLa(III) and Nd(III) were extracted from nitrate solution by Cyanex301 (HA) and Cyanex 923 (L) as LnA2.NO3.L, while Eu(III), Y(III)and heavier rare earths were extracted as LnA3

.HA.2L (Reddy

et al., 1999). Tri-alkylphosphine oxide significantly enhanced boththe extraction efficiencies and selectivities, especially between yt-trium and heavier lanthanides. Zhang et al. (2008) reported studiesof the solvent extraction of cerium(IV) and fluoride from sulfatesolutions using a mixture of Cyanex 923 and D2EHPA in n-heptaneand Ce(III) was not extracted by the mixture.

For binary acid extractant systems, Ying et al. (2005) examinedthe extraction of Yb3+ from chloride solution with Cyanex 272,P507 (HEHEHP), and their mixtures in n-heptane. The extractionof Yb3+ was higher with the mixture than with Cyanex272 orP507 alone. A synergistic effect was observed on the separationof Yb/Tm and Lu/Yb, but not for Tm/Er, Er/Ho, and Ho/Dy. Zhanget al. (2007) investigated the extraction of trivalent La, Nd, Sm,and Gd from sulfate media by a mixture of D2EHPA and HEHEHP.A synergistic effect was observed for the extraction of all four met-als at pH = 2.0. Li et al. (2007) also reported that the separation fac-tor for Sm and Nd was significantly increased using a mixture ofD2EHPA (40% v/v) and HEHEHP (60% v/v).

Sun et al. (2006) examined the synergistic extraction of triva-lent Sc, Y, La, Gd, and Yb from chloride media using a mixture ofCyanex 272 and sec-nonylphenoxy acetic acid (CA-100) in n-hep-tane. The separation factor for Yb and Y was much higher than thatwith CA-100 alone. The extraction of rare earth elements fromchloride media by mixtures of CA100 with Cyanex 301 or Cyanex302 was studied by Tong et al. (2009). In the CA100 + Cyanex301 system, the synergistic enhancement coefficients decreased

with increasing atomic number of lanthanoids, but the separationfactors between Y and all the lanthanoids were enhanced. Jia et al.(2009) reported that the separation factors of all adjacent trivalentrare earths were better in a mixture of sec-octylphenoxy aceticacid (CA12) and Cyanex301 in n-heptane than in Cyanex 301 alone.

3. Process engineering and equipment

Although there is an extensive literature on rare earth solventextraction chemistry and equilibriums, as discussed above, far lessis known on the engineering details of rare earth separations. Fromthe limited open literature, rare earth producers appear to followsimilar approaches (Gupta and Krishnamurthy, 2005). There is of-ten a need for a primary separation of rare earths from impuritiesin the original leach solution, along with concentration. D2EHPAhas been widely used for primary separation because the distribu-tion coefficients of the rare earth elements as a group differ mark-edly from those of typical impurities in leach liquors. D2EHPA isalso suitable for concentrating the rare earth elements from dilute,acidic solutions.

In general, rare earths are separated in the trivalent state. Theyare usually separated into two, three or sometimes four groups,followed by precipitation or subsequent separation of individualrare earth. Preferential separation of yttrium is desirable, and cer-ium and europium are often separated initially on the basis of dif-ferent valance states (Ce4+ and Eu2+).

The extractants and aqueous anion are generally selected con-sidering both cost and technical requirements, and the impact onthe process configuration (McGill, 1997). For example, cationicexchangers usually offer higher selectivity on rare earth ions com-pared to neutral and anionic exchangers. However, the reactivechemical requirement is greater with cation exchangers, becausebase is required to drive extraction, and acid is required for selec-tively washing the organic phase. In contrast, with solvationextractants and anion exchangers the reactive chemical require-ment is lower. Thus there is a trade-off between selectivity (whichlowers the number of stages, and hence capital and chemicalinventory costs) and the operating costs.

3.1. Configurations

Rare earth solvent extraction processes are generally classifiedas primary separations, which aim to separate rare earth elementsfrom other elements (which is relatively straightforward, and com-parable with other solvent extraction processes in hydrometallur-gical operations), and secondary separations, which produce singleor mixed (typically 2 or 3) rare earth products from the mixed rareearth stream produced by primary separations. The latter is oftenmuch more challenging, particularly when producing a single, purerare earth, because of the chemical similarity of the rare earths. Asmentioned above, D2EHPA has been widely used in primary sepa-ration processes because the distribution coefficients for the rareearth elements with D2EHPA are markedly different from thoseof other elements in the aqueous solution (leach liquor). D2EHPAis also suitable for concentrating the rare earth elements from di-lute solutions because of the high distribution coefficients.

A plant producing multiple single rare earth products may con-tain hundreds of stages of mixers and settlers. A classic countercur-rent flow scheme for a simple solvent extraction circuit is shown inFig. 3. The aqueous stream leaving the nth mixer settler would bepumped to the (n � 1)th mixer settler, while the organic phase(which is ideally in equilibrium with the aqueous phase leavingthe nth stage) would be pumped to the (n + 1)th mixer settler. Thisarrangement may also be used for simple rare earth separations,e.g., the primary separation of rare earths from impurities. How-

1 n n+1 n+m

organic product

aqueous feed

organic feed

aqueous raffinate

n-1

Fig. 3. Simplified countercurrent solvent extraction circuit.


ever, there is fundamental shortcoming with this configuration ifone wishes to separate rare earths from a mixed feed. Consideran operation using a cation exchange extractant or a solvatingextractant, for which the heavier rare earths have a stronger affin-ity for the organic extractant phase than have the lighter ones. Iifmixed aqueous feed were introduced into one end of a bank ofmixer settlers, the organic phase leaving the other end would besomewhat enriched in heavy rare earths, but there would still bean appreciable amount of light rare earths in this stream, becauseof the low separation factors. This shortcoming is addressed byintroducing the mixed aqueous feed near the middle of the bankof mixer settlers, as shown in Fig. 4. A different aqueous streamis admitted into the end (the (n + m)th stage) to allow an appropri-ate number of stages for the light rare earth to be scrubbed fromthe organic phase back into the aqueous phase. This scrub solutionmay exit the process midway, or may continue on with the aque-ous feed. To minimize the dilution of rare earth concentrationscaused by the introduction of the barren aqueous scrub and the or-ganic phase, reflux is sometimes used; some of the light rare earthsfrom the aqueous raffinate are loaded back into the organic phaseentering the first mixer settler, and some heavy rare earths arestripped from the organic product and added to the aqueous scrub(Fig. 5). When operating under reflux, the mass transfer occurringat each stage becomes an exchange of different rare earths, accord-ing to their affinity for an extractant, as exemplified below for a li-quid cation exchange extractant.

1 n

organic feed

aqueous raffinate aqueous feed O

n-1

Fig. 4. Simplified configuration of countercurrent so

1 n

organic feed

aqueous raffinate aqueous feed

Partial reflux

Fig. 5. Simplified configuration of countercurr

LaA3 þ Ce3þ ¼ CeA3 þ La3þ ð11Þ

Reflux minimizes dilution and also reduces the number of stagesneed to achieve an effective separation. Nevertheless, many separa-tion stages are typically needed to obtain pure product. Most banksof mixer settlers are set up to make a single separation between twoadjacent rare earths, but some configurations have three or moreproduct streams. Various processes for rare earths separation fromconcentrates/ore and aqueous solutions have been summarized indetail by Gupta and Krishnamurthy (2005). Some typical applica-tions and process flowsheets for solvent extraction separation ofrare earths used in practice are listed below.

3.1.1. Molycorp-bastnesiteFig. 6 shows the schematic flowsheet for the Molycorp process,

used to extract europium oxide from the leachate of Mountain Passbastnesite (Gupta and Krishnamurthy, 2005). A chloride solution(100 g/L REO) containing all the rare earth except Ce is generatedby calcination and leaching with HCl solution. Two steps of solventextraction with D2EHPA were applied. The chloride solution wasfirst contacted with 10% v/v D2EHPA in kerosene, and the extrac-tion was performed in five stages of mixers and settlers under con-ditions that ‘‘split’’ the rare earths with Sm and all heavier rareearths reporting to the D2EHPA solution, and Nd and all lighter ele-ments reporting to the raffinate (this is designated in Fig. 6 by‘‘(Nd/Sm)’’ in the first solvent extraction stage). There are two

n+1 n+m

organic product

aqueous feedptional scrub exit

lvent extraction circuit with optional scrub exit.

n+1 n+m

organic product

aqueous scrub

Partial reflux

ent solvent extraction circuit with reflux.

Bastnesite concentrate (60% REO)

Calcination

Leaching

Solvent extraction (Nd/Sm)/stripping

Iron Precipitation

Solvent extraction(Sm/Eu)/stripping

Eu(3) Chloride solution

Reduction

Precipitation

EuSO4 solution for producing Eu2O3

Conc. HCl Residue for Ce recovery

10% D2EHPA Raffinate (for recovery of La, Ce, Pr, Nd)

Soda ash Iron precipitate

10% D2EHPA Raffinate fed to the first SX circuit

H2SO4

Zinc amalgam

Other REs, Fe

Sm, Eu, Fe

Sm, Eu

Fig. 6. Molycorp process for producing europium oxide from the bastnesite concentrate (after Gupta and Krishnamurthy, 2005).


engineering reasons for splitting the rare earths in this way ini-tially. The first is that it is relatively easy to separate Nd and Smsince they are consecutive elements in any natural rare earth min-erals, but are not consecutive elements on the periodic table andthe intermediate rare earth element, promethium, does not occurin nature. Thus their separation factor is typically double that ofany other consecutive rare earth pair. The second reason is that,referring back to Table 3, it is apparent that Sm and the heavierrare earths account for only a very small proportion of the rareearths in bastnesite. Thus they can be removed using a small vol-ume of D2EHPA solution, and the resulting rare earth mixturecan be further processed using much smaller mixer settlers, leav-ing La, Pr, and Nd (the bulk of the rare earths in the concentrate)in the aqueous raffinate. These were precipitated with ammoniumand sodium hydrogen sulfide and further processed in much lar-ger-scale equipment. More than 98% of europium in the solutionwas extracted.

The loaded organic (containing 98% of the Eu from the leach li-quor) was stripped with 4 mol/L HCl. The iron in the strip solutionwas precipitated through neutralization to pH 3.5, and the clarifiedEu-bearing solution proceeded to a second solvent extraction cir-cuit, also using 10% D2EHPA in kerosene and five stages of mixersand settlers. Europium and other heavy rare earths were loaded inthe organic phase, with the light rare earths remaining in the raf-finate, which were returned to the first solvent extraction circuit.The europium (and other rare earths) were stripped from theloaded D2EHPA with 5 mol/L HCl solution and the strip liquorwas passed through a column of zinc amalgam to reduce Eu(III)to Eu(II). Sulfuric acid was added to precipitate europium sulfate,which was then calcined to produce pure Eu2O3 (99.99%). Aftereuropium recovery, the strip solution still contained Sm, Y, andother heavy rare earths. Gadolinium was extracted with D2EHPAin a 10-stage extraction circuit followed by a 5-stage scrub. Theraffinate was neutralized with soda ash to precipitate Sm and theheavy rare earths.

3.1.2. Rhône-poulenc – monaziteRhône-Poulenc had the capability of producing all the rare

earth elements at a purity of >99.999% almost entirely by solvent

extraction (McGill, 1997). A schematic flowsheet of the process isshown in Fig. 7. Monazite concentrate was first digested withNaOH. The rare earths reported to the solid residue as hydroxides,which after filtration were dissolved in HCl or HNO3. Afterclarification, the resulting solution proceeded to a series of sol-vent extraction circuits to produce individual rare earth oxides.Chloride media were used to prepare a mixture of rare earthcompounds, such as dehydrated rare earth chlorides, which wereused to produce misch metal. Nitrate media were used to produceindividual rare earth oxides, e.g., in the first separation circuit,lanthanum (99.9995% La2O3) remained in the aqueous phasewhile a mixture of Ce, Pr, Nd, Sm, etc. was loaded into the organicphase. Similarly, CeO2 (>99.5%) was separated from Pr, Nd, Sm, Eu,etc., after removing lanthanum. Various extractants, includingcarboxylic acids, organophosphorous acids, neutral organophos-phorous compounds, and quaternary amines have been used inthese separation processes. Rhône-Poulenc could also producehigh-purity individual rare earth oxides from bastnesite oreuxenite. The Rhône-Poulenc solvent extraction process has beenregarded as the standard for all industrial producers (Bautista,1995).

3.1.3. AS Megon–xenotimeAS Megon developed a process for producing high-purity yt-

trium oxide starting from the xenotime concentrates (Gauder-nack, 1973). The schematic flowsheet is shown in Fig. 8. Thesolvent extraction circuit consisted of a selective extraction byD2EHPA followed by three scrubbing and stripping units. Thelight rare earths (La, Ce, Pr and partial Nd) and impurities includ-ing Fe2+ ions remained in the raffinate. The extracted yttrium andother rare earths were separated into three groups by selectivewashing. Yttrium nitrate solution was fed to the second circuitusing the nitrate of tri-capryl methylamine as the extractant.The lighter rare earths (La, Ce, Pr, Nd and Gd, Tb, Er) wereextracted while Y, Tm, Yb, and Lu remained in the raffinate,which was fed to the third solvent extraction circuit, which usedtri-capryl methylamine-NH4SCN to produce high-purity yttriumoxide.

Dissolution

Digestion/Filtration Na3PO4 solution

NaOH

HCl RE-Th hydroxide

Monazite

Didymium

Dissolution

HNO3

Separation non-RE/RE/Th

Separation non-RE/RE/Th

Separation, La-Nd/Sm-Lu

Separation, La/Ce, Pr, Nd…

Conversion Cl-/NO3

-

Separation, Ce/Pr, Nd, Sm…

Separation, Pr, Nd/Sm, Eu, Gd…

Separation, Sm, Eu/Gd-LuSeparation, Pr/Nd

Separation, Sm/Eu

Pr6O11, 96%

Sm2O3, 96%

Eu2O3, 99.99%

Gd2O3, 99.99%Tb4O7, 99.9%

Y2O3, 99.99%

Nd2O3, 96%

CeO2, 99.5%

La2O3, 99.995%RE chlorides

RE fluorides

Polishing agent

RE carbonates

Anhydrous RE chlorides

Mischmetal

Fig. 7. Rhone-Poulenc liquid–liquid extraction process for separation of the rare earth elements (after McGill, 1997).


3.1.4. Mintek–apatiteRare earths have been recovered from the calcium sulfate

sludge generated during the production of phosphoric from apatiteat Phaleborwa (Preston et al., 1996b). Fig. 9 shows the schematicflowsheet for the pilot plant. The sludge was leached with dilutenitric acid solution containing calcium nitrate. Rare earths wereextracted from the leachate with TBP (40% v/v in Shellsol 2325).The raffinate was recycled back to leaching after removing en-trained organic solvent. The loaded organic solution was strippedwith water to yield a mixed rare earth nitrate aqueous solutionthat was treated with ammonia and oxalic acid to precipitate amixed rare earth oxalate. This was calcined to give a mixed rareearth oxide (89–94% purity). The rare earth oxide contained con-siderable amount of the middle rare earths, particularly Nd, Sm,Eu, and Ga. In subsequent pilot tests, TBP, HDEHP and Aliquat336 were used to produce different rare earth products from themixed oxides.

3.1.5. Industrial processes in ChinaThe Shanghai Yue Long Chemical Plant, was reported to treat

monazite concentrates in a process similar to the Rhône-Poulencprocess (Zhang et al., 1982). The simplified flowsheet of this plantis shown in Fig. 10. After digesting the monazite in NaOH, filtrationand leaching of the residue with HCl, the resulting rare earth chlo-ride solution was extracted with D2EHPA and the rare earths weresplit into three groups, from which mixed and pure oxides, carbon-ate, or chlorides were produced. The ion-adsorption type rare earthores are first leached with HCl or H2SO4. Cation exchange extract-ants, such as HEHHP and naphthenic acid, are frequently used toextract rare earths elements from the leachate, since these oreshave high levels of heavy rare earths, which have a strong affinityfor acidic extractants. Individual rare earth compounds (oxides or

chlorides) can be produced through controlled stripping (Huanget al., 2005, 2006).

For bastnesite ore, which is the main rare earth resource in Chi-na, the ore or concentrates are typically roasted with H2SO4, fol-lowed by leaching with water or dilute sulfuric acid. Rare earthsare recovered from the leachate by solvent extraction with P204(D2EHPA). Preferential stripping is used to divide the rare earthsinto two groups, La-Nd and Sm-Gd (the concentration of heavierrare earths is usually small); these can be further separated intoindividual rare earth elements if desired (Huang et al., 2006). To re-duce reagent consumption, modified separation processes havebeen tested at pilot plant scale. One approach used P204 or P507to extract Th and most of Ce first, then the raffinate containingthe remaining rare earths underwent further solvent extractionsteps to separate individual rare earth compounds (Fig. 11) (Huanget al., 2006). The F, Th, and Ce(IV) were selectively stripped fromthe organic phase. Another approach used Cyanex 923 to separateCe(IV) from the leachate first. The raffinate containing other rareearths then underwent solvent extraction with N1923 (a primaryamine) to separate Th (Fig. 12) (Lu et al., 1998). Individual rareearth compounds were produced from the Th-free raffinate in athird solvent extraction circuit.

3.1.6. MiscellaneousDoyle et al. (2000) developed a novel solvent extraction config-

uration capable of producing a mixed Ce–Pr–Nd product (for mag-net production) and pure Nd oxide simultaneously, with flexibilityto alter the relative proportions according to market conditions.The schematic flowsheet for this process is shown in Fig. 13. Rareearth chloride solution generated by leaching oxide with HClunderwent solvent extraction with P507 in kerosene. In the firstsolvent extraction circuit, Sm and all heavier rare earths along with

Xenotime

Digestion/dissolution/filtration

RE sulfate solution, 20 g/L REO

4-stage extraction

4-stage selective scrubbing

Solvent treatment

1.75 M NH4NO3

26-stage extraction 6-stage scrubbing

Gd-Er

H2SO4; H2O

Residue

LaCePr(Nd), Fe2+, and other impurities

30% D2EHPA in Shellsol

NdSm(GdTb)

Yttrium nitrate solution, 75% Y2O3/REO; 6 M NH4NO3

NH4SCN

1.5 M HNO3

8-stage selective scrubbing

4-stage stripping

6 M HNO3 YbLu(Th) 0.5 M H2SO4NH3

3-stage stripping

Dilute HNO3

Yttrium nitrate solution, 95% Y2O3/REO; 4,9 M NH4NO3; 0.1 M NH4SCN

26-stage extraction 6-stage scrubbing 8-stage stripping

Y2O3, 99.999%; Yb<2.5 ppm; Er< 3ppm; Others <1pm

0.5 M NH4NO3; 0.1M NH4SCN

TmYbLu Dilute HNO3

NH4SCN

40% quaternary amine in Solvesso 50

Fig. 8. AS Megon process for high-purity yttrium oxide (after Gaudernack, 1973).

5-stage extraction

Leaching/filtration

6-stage-stripping

Phosphoric acid manufacturing process

washing

H2O

Washing solution

40% v/v TBP in Shellsol 2325

residue

Phalaborwa apatite ore

Leahate (REO 8-24 g/L; 1M HNO3, 3 M Ca(NO3)2

Water

Calcium sulfate sludge

Washed raffinate

Ca(NO3)2, HNO3Washed sluge

Raffinate Strip liquor (REO 45-60 g/L)

washingSheellsol2325

Organic to Sheellsol2325 tank

Precipitation

Mixed rare earth oxalate

NH3, oxalic acid

Calcination in rotary kiln

Mixed rare earth oxalate (89-94% purity)

Fig. 9. Schematic flowsheet of the pilot plant for recovering rare earths from the phosphoric acid plant residue at Phalaborwa (after Preston et al., 1997).


Dissolution/Filtration

Digestion/Filtration Na3PO4 solution

NaOH

HCl

(RE, Th, U) hydroxide

Monazite

Solvent extraction

Solvent extractionSolvent extraction

Dissolution

Malten salt electrolysis

La2O3

Pr6O11

Nd2O3

CeO2

CeCl3

Gd2O3

Sm2O3

Tb, …LuSm, Eu, Gd

REO

RE fluorides

Polishing agent

RE carbonates

RE Chorides

Mischmetal

RECl3 solution (Th, U) sludge

La, Ce, Pr, Nd

Solvent extraction

Solvent extraction

Eu2O3 Y2O3

phosphors

Solvent extractionSolvent extraction

Th(NO3)4

ThO2

U3O8

(NH4)2U2O7

Radioactive waste treatment

Tb4O7

Dy2O2

Lu2O3

Fig. 10. Simplified flowsheet of the Shanghai Yue Long Chemical Plant (after Zhang et al., 1982).


Y, were loaded into the organic phase. The raffinate containing Ndand lighter rare earths underwent a second solvent extraction withP507 in kerosene. Through controlling the number of stages and re-flux ratios, Pr and Nd and part of thel Ce were extracted into theorganic phase, with the balance of Ce, and all the La remaining inthe aqueous phase from which a marketable lanthanum productwere produced. The loaded organic phase underwent selectivestripping to produce high-purity neodymium oxide and a mixtureof Ce, Pr, and Nd oxides.

Huang et al. (2008) used a synergistic extraction system to pro-duce different rare earth products from rare earth sulfate solutionsresulting from leaching of bastnesite concentrates (Fig. 14). Thenon-saponified organic phase was used directly to extract rareearths from their sulfate or chloride solutions and by controllingoperation conditions, as many as five (or more) commercial rareearth products could be produced simultaneously.

3.2. Process simulation

Process development, analysis, control and optimization of rareearth solvent extraction are complex tasks. Computer simulationprogram for monitoring or optimizing the rare earth solventextraction process requires a reliable model for the extractionequilibrium. However, very few models for describing the relevantequilibrium between rare earth elements and different extractionsystems have appeared in the open literature, and these are usuallyonly applicable to a limited and specific range of conditions. Thisprobably reflects the similarities of the lanthanides, their propen-sity for interactions make it difficult to predict their behaviors invarious extraction systems. Therefore, little progress has beenmade with regard to the development of a general approach formodeling rare earth solvent extraction systems.

The reported programs for simulating rare earth solvent extrac-tion processes usually consider countercurrent circuits, due totheir ubiquity. Stage-wise calculations offer efficiency and flexibil-ity (Sharp and Smutz 1965; Sebenik et al., 1966). Most are based onthe McCabe–Thiele method (Thiele and Geddes, 1933; McCabeet al., 2005). The technique was originally developed for graphicalanalysis of binary distillation, and later applied to liquid–liquidseparation processes, especially for solvent extraction systemsinvolving only one extractable species (Zhu, 1991; Rydberg et al.,2004). Considering an n-stage counter-current circuit separatingthe metal ion from an aqueous solution by solvent extraction,the aqueous stream leaving the ith mixer settler would be pumpedto the (i�1)th mixer settler while the organic phase would bepumped to the (i + 1)th mixer settler (i = 1,2, . . .,n) (Fig. 15).

The symbols in Fig. 15 are defined as follows:

½M�nþ1: the molar concentration of metal ion in the aqueousfeed;½M�n: the molar concentration of metal ion in organic product;[M]i+1: the molar concentration of metal ion in the aqueoussolution fed to the ith mixer settler;½M�i: the concentration of metal ion in organic phase leaving theith mixer settler;[M]i: the concentration of metal ion in the aqueous solutionleaving the ith mixer settler;½M�i�1: the concentration of metal ion in organic phase fed to theith mixer settler;[M]1: the molar concentration of metal ion in the aqueousraffinate;½M�0: the molar concentration of metal ion in the organic feed.

Assuming that the two phases are totally immiscible, anddefining:


Calcination

Leaching/filtration

Solvent extraction

Solvent extractionFluoride for recoverySelective stripping F

Selective stripping Ce

H2SO4 Residue

P204 or P507

sulfate solution containing (RE, F, Th)

Individual RE compounds

Loaded organic Raffinate (RE sulfate solution)

CeO2, 99% -99.99%

stripping Th

ThO2, 99% -99.99%

Fig. 11. Simplified flowsheet for separating RE, F, Th by solvent extraction with P204 (after Huang et al., 2006).


Calcination

Leaching/filtration

Solvent extraction

Solvent extractionFlouride for recoverySelective stripping F

stripping Ce

H2SO4 Residue

Cyanex 923

sulfate solution containing (RE, F, Th)

Loaded organic

Loaded organic Raffinate (contaning RE, Th)

CeO2, 99% -99.99% stripping Th

ThO2, 99% -99.99%

N1923 (primary amine)

Raffinate

Solvent extraction

Individual RE compounds

Fig. 12. Simplified flowsheet for separating RE, F, Th by solvent extraction with Cyanex 923 (after Lu et al., 1998).


VA: the flow rate of the aqueous phase;VO: the flow rate of the aqueous phase;

The mass balance for metal ion in the ith stage can be expressedas:

VAð½M�iþ1 � ½M�iÞ ¼ VOð½M�i � ½M�i�1Þ ð12Þ

If the extraction equilibria are known, either through theoreti-cal calculations or experimentally, the theoretical number of stagerequired can be calculated by solving the mass balance equationsfor all stages if the concentrations of metal ion in the aqueous feedand in the raffinate, and the flow rates of the organic and aqueousphases are known. The concentrations of metal ion in the twophases in different mixer settlers can thus be calculated stage bystage. Fig. 16 shows a McCabe–Thiele diagram for a 3-stage solvent

Sm, Gd, Eu for market

Raffinate (La-Ce for market)

“Ce-free” RE sulfate solution

Loaded organic

Solvent extraction, La(Ce)PrNd/Sm

Solvent extraction, La(Ce)Pr/Nd

P507

P507

Selective strip, (Ce)Pr/Nd High-purity Nd2O3

(Ce, Pr, Nd) oxides

Fig. 13. Simplified flowsheet for solvent extraction process for producing low-cost permanent magnet feed (after Doyle et al., 2000).

PrNd

Multi-stage mixers settlers aqueous scrub (H2SO4 or HCl)

Mixed extractants

La

RE sulfate feed solution

LaCe SmEuGd Heavy REs (heavier than Gd)

Fig. 14. Simplified configuration of countercurrent separation of rare earths to produce various rare earths products by solvent extraction.

Fig. 15. Schematic diagrams for simple n-stage countercurrent solvent extraction circuit.


extraction circuit. The equilibrium line OA shows the extractionisotherm for the desired metal ion. The line BC is an ‘‘operatingline’’ which is straight with a gradient equal to the ratio of theaqueous and organic flow rates (VA/VO). The points on this line re-flect the composition of crossing streams in each stage, i. e., ([M]i+1,½M�i) for all values of i (i = 1,2,3,4). The molar concentration of me-tal in the organic feed, ½M�0, is taken to be zero in this figure,although it need not be so in practice. When an aqueous feed (witha molar concentration of [M]4) enters the 3rd stage mixer-settler,the composition of the organic phase ‘‘crossing’’ the stream willbe ½M�3. The composition of the aqueous phase leaving stage-3([M]3) is determined by the point on the equilibrium line with anorganic concentration of ½M�3 (specifically, the point ([M]3, ½M�3).The of the organic stream that crosses the aqueous stream leavingstage 3 is then obtained from the operating line, as shown graph-ically on Fig. 16. This allows calculation of the composition of theraffinate, [M]1. Alternatively, the number of stages needed to attainspecific stream compositions can be obtained graphically. A similarmethod can be used to calculate the number of stages needed forthe stripping circuit and the corresponding metal concentration

in the different streams. An equilibrium line for stripping shouldbe determined or defined first; this differs from that for extractionbecause the chemical composition of the aqueous phase will differ,for example, having a different pH in the case of cation or anion ex-change extractants, or a different concentration of ligands in thecase of a solvating extractant.

A simple case occasionally encountered in rare earth solventextraction processes is a linear equilibrium line through the ‘‘ori-gin’’ (OA in Fig. 17), corresponding to a constant distribution coef-ficient (DM) for metal extraction in all stages. Thus:

½M�i ¼ DM½M�i ði ¼ 1;2; . . . nÞ ð13Þ

Since the operating line, BC, in Fig. 17 has a gradient of VA/VO as de-fined in Eq. (12), then:

½M�1 ¼ DM½M�1 ð14Þ

½M�1 � ½M�0½M�2 � ½M� 1

¼ VA

VOð15Þ

Fig. 16. Schematic McCabe–Thiele diagrams for countercurrent solvent extractionprocess.

Fig. 17. Schematic diagram for countercurrent stagewise solvent extraction with aconstant distribution coefficient.


Assuming ½M�0 ¼ 0 (no metal in the organic feed) gives:

½M�1½M�2 � ½M�1

¼ VA

VOð16Þ

Combining Eqs. (14) and (16):

½M�2 ¼ ½M�1 1þ DMVO

VA

� �ð17Þ

Similarly, one can obtain:

½M�3 ¼ ½M�1 1þ DMVO

VAþ DM

VO

VA

� �2 !

ð18Þ

½M�nþ1 ¼ ½M�1 1þ DMVO

VAþ DM

VO

VA

� �2

þ � � � þ DMVO

VA

� �n !

ð19Þ

Defining the extraction factor, EM, as:

EM ¼ DM VO=VAð Þ ð20Þ

Eq. (19) can be expressed as:

½M�nþ1 ¼ ½M�1 1þ EM þ E2M þ . . .þ En

M

� �ð21Þ

or:

½M�nþ1 ¼ ½M�1Enþ1

M � 1EM � 1

ð22Þ

Eq. (22) is known as the Kremser equation (Kremser, 1930). Its con-sequences have been discussed widely and it has been illustrated invarious forms (Klinkenberg, 1951). This equation can applied to anyaqueous streams situated an integral number of stages along themixer settler, not just to streams leaving the mixer-settler circuit.The composition of the organic phase leaving any stage can be ob-tained by combining Eqs. (14), (22), and (23):

½Mn� ¼ DM½M�1En

M � 1EM � 1

ð23Þ

The theoretical number of stages required and the metal con-centration in relevant streams can be determined from Eq. (22) ifthe metal concentration in the feed and raffinate are defined andthe extraction factor is known. Similar equations can be developedfor stripping if there are linear stripping equilibrium and operatinglines (constant stripping factor and constant ratio of aqueous to or-ganic flow rate) (Jamrack, 1963).

However rare earths are seldom extracted in systems contain-ing single rare earths. The solvent extraction system chemistry isusually controlled to make the distribution coefficient of the de-sired element(s) much higher than that of the unwanted elements.Even so, some extraction of these undesired elements is usuallyunavoidable. In these circumstances, as shown in Fig. 4, a scrub-bing circuit is commonly employed to wash impurities from theorganic before it leaves the extraction circuit. Graphical methodssuch as the McCabe–Thiele diagram become unmanageable foranalyzing several extractable species, each of which has theirown equilibrium line, and each of which may displace or be dis-placed by other species in the organic phase. In this case, a largenumber of extraction, scrubbing, and stripping stages may be re-quired to obtain desired product.

Algebraic equations have been developed from the Kremser andmass balance equations to calculate the theoretical number ofstages for a countercurrent circuit for separating two rare earthelements by solvent extraction, assuming constant distributioncoefficients for rare earths in the extraction and scrubbing circuits(Xu, 1978). Constant distribution coefficients would not be ex-pected in typical solvent extraction circuits, because the exchangeof species between the aqueous and organic phases changes thechemistry of each stream. However, in rare earth circuits whereone rare earth is often being exchanged with another (see Eq.(11)), the chemistry may be much more stable so this assumptionbecomes more realistic. The separation factors for rare earths arefrequently fairly constant throughout a given circuit (Voit, 1988).As a result, the distribution coefficients of each lanthanide can bededuced if the distribution coefficient of one has been determined.Typically, separation factors depend only on the type of selectedextractant and to a lesser degree on the anionic species in theaqueous phase. Given that the assumption of constant stream vol-umes incorporated in the Kremser equation is usually valid, theequations derived above can be solved computationally to analyzeand simulate rare earth solvent extraction circuits.

Voit (1988) developed a simple simulation program for produc-ing 99.5% Nd2O3 from a mixed rare earth chloride feed (La, Pr, and


Nd) using HEHEHPA for extraction, scrubbing and stripping sec-tions. The Kremser equation was used to calculate the separationoccurring in each section of the circuit. Using same equation, Red-dy et al. (1992) developed a modified simulation program for anintegrated rare earth solvent extraction circuit. Distribution coeffi-cient data for key rare earth elements were tabulated for differentinitial acidity and metal ion concentrations, based on operatingconditions. The use of average separation factors was suggestedfor non-constant distribution coefficients (Zhong, 2008). Furtherstudies have considered the separation of multi-rare earth ele-ments with two or more outlets/products in a single countercur-rent circuit (Ding et al., 2003; Wu et al., 2004a,b). A simulationsystem has been combined with on-line EDXRF analysis to monitorthe steady or dynamic performance of stage-wise processes (Wenliet al., 2000; Jia et al., 2001; Jia et al., 2004). The latter authorsclaimed that their model was especially useful when there wasno more than one intermediate feed point and the distribution ra-tio of the component involved was constant. The detail of the cal-culation method followed was not reported.

4. Summary

Rare earth elements have unique properties and are becomingessential in many high-technology applications. China is currentlythe world’s largest producer of rare earth elements providing morethan 95% of the world’s total supply. Various rare earth mineralshave been identified, of which bastnesite (La, Ce)FCO3, monazite,(Ce, La, Y, Th)PO4, and xenotime, YPO4, are the most main mineralscommercially targeted. Rare earth minerals are usually concen-trated by flotation, magnetic or gravity methods to produce con-centrates that undergo hydrometallurgical processing to recoverrare earth metals or compounds. Rare earth concentrates or calcineresidue are typically leached with an inorganic acid, such as HCl,H2SO4, or HNO3.

After solution purification, separation processes based on sol-vent extraction techniques are used to yield individual rare earthsor mixed rare earth solutions or compounds. Rare earth producersusually follow almost identical principles or schemes when select-ing solvent extraction circuits to separate rare earths from eachother. Usually trivalent rare earths are separated into two or moregroups, followed by subsequent separation of individual rareearths and preferential separation of yttrium if possible. The choiceof extractants and aqueous solution conditions is influenced bothby cost considerations and technical requirements, such as selec-tivity. The use of cation exchangers, solvation extractants, and an-ion exchangers, for separating rare earths has been extensivelystudied. Commercially, D2EHPA, HEHEHP, Versatic 10, TBP, andAliquat 336 are widely used commercially to separate rare earthelements.

Up to hundreds of stages of mixers and settlers may be assem-bled together to separate all the individual rare earths in a feed-stock. Typical configurations for rare earth solvent extractioncircuit have been reviewed. Traditional graphical methods forsimulating solvent extraction circuits, such as McCabe–Thiele dia-grams, have limited practical application for rare earth solventextraction circuits. However, more promising computational ap-proaches based on the Kremser and mass balance equations havebeen developed and are continually being developed which betterpredicts the complex chemical interactions.

Acknowledgement

One of the authors, Ting An Zhang, appreciates the financialsupport of Ministry of Science and Technology of P. R. China

(‘‘973’’ Program, 2012CBA01205 and Science and Technology Sup-porting Program, 2012BAE01B02).

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a critical review on solvent extraction of rare earths from aqueous solutions

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