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

Minerals Engineering 56 (2014) 10–28

Contents lists available at ScienceDirect

Minerals Engineering journal homepage: www.elsevier.com/locate/mineng

A critical review on solvent extraction of rare earths from aqueous solutions q 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, China Department of Materials Engineering, University of British Columbia, 309-6350 Stores Road, Vancouver, BC V6T 1Z4, Canada c Department of Materials Science and Engineering, University of California, Berkeley, 210 Hearst Mining Building, Berkeley, CA 94720, United States b

a r t i c l e

i n f o

Article history: Received 2 September 2013 Accepted 22 October 2013 Available online 15 November 2013 Keywords: Rare earths Solvent extraction Process configuration

a b s t r a c t Rare earth elements have unique physicochemical properties that make them essential elements in many high-tech components. Bastnesite (La, Ce)FCO3, monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4, are the main commercial sources of rare earths. Rare earth minerals are usually beneficiated by flotation or gravity or magnetic processes to produce concentrates that are subsequently leached with aqueous inorganic acids, such as HCl, H2SO4, or HNO3. After filtration or counter current decantation (CCD), solvent extraction is usually used to separate individual rare earths or produce mixed rare earth solutions or compounds. Rare earth producers follow similar principles and schemes when selecting specific solvent extraction routes. The use of cation exchangers, solvation extractants, and anion exchangers, for separating rare earths has been extensively studied. The choice of extractants and aqueous solutions is influenced 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 processes. Up to hundreds of stages of mixers and settlers may be assembled together to achieve the necessary separations. This paper reviews the chemistry of different solvent extractants and typical configurations for rare earth separations. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Contents 1.

2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Rare earths ores. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. Technological applications of rare earths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3. Primary rare earth extraction process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Solvent extraction separation of rare earths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Cation exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Carboxylic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Organophosphorous acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Chelating extractant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Solvation extractant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Anion exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Synergistic solvent extraction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process engineering and equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Molycorp-bastnesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Rhône-poulenc – monazite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. AS Megon–xenotime. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4. Mintek–apatite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.5. Industrial processes in China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 11 11 11 13 13 13 14 16 16 16 17 17 17 18 19 19 20 20

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits noncommercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. Tel.: +86 24 8368 7729. E-mail address: [email protected] (F. Xie).

0892-6875/$ - see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mineng.2013.10.021

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

4.

3.1.6. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Process simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

20 22 26 26 26

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).

1.1. Rare earths ores The term rare earths was originally used to designate the lanthanoids in row 3 of the periodic table which includes oxides of scandium (Sc, 21), yttrium (Y, 39), lanthanum (La, 57) and the 14 elements following lanthanum in the periodic table of elements, i.e. from cerium (Ce, 58) to lutetium (Lu, 71) inclusive. More recently, the term ‘‘rare earths’’ has been used to designate the elements themselves. Scandium and yttrium tend to occur in the same ore deposits as the lanthanoids and exhibit similar chemical properties (Jones et al., 1996; Wang et al., 2011). The term ‘‘rare’’ earth is a misnomer; they are relatively abundant in the Earth’s crust, however, they are typically dispersed and only rarely occur in concentrated and economically exploitable mineral deposits. Rare earth mines have operated in South Africa, India, and Brazil in the 1950s, but from the 1960s to the 1980s, the largest global producer was a mine in Mountain Pass, California. In the 1990s, China began large scale production and exported cheaper rare earths. Other producers were unable to compete economically, and began closing in the 1990s, with the Mountain Pass mine shutting down in 2002 (Tse, 2011). Consequently, China is currently the world’s largest producer of rare earth elements, providing more than 95% of the world’s total supply from its mines in Inner Mongolia (Chen, 2010). The world reserves and production of rare earths are summarized in Table 1 (U.S. Geological Survey 2011). There are a wide variety rare earth minerals known, but notably the 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 the largest percentage of the world’s rare earth resources. Notable occurrences include the carbonatite-hosted bastnesite deposit at Mountain Pass, California, several bastnesite deposits in Sichuan Province, China, and the massive deposit at Bayan Obo, Inner Mongolia, China. Monazite deposits in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, Thailand, and the United States constitute the second largest segment. Apatite, cheralite, eudialyte, loparite, phosphorites, secondary monazite, spent uranium solutions, and xenotime make up the remaining resources. There are also ion-adsorbed rare earth deposits, widely distributed in southern China formed through weathering were the rare earths have

Table 1 Global rare earth mine production and reserves (after U.S. Geological Survey, 2011). Mine production, metric ton

United States Australia Brazil China Common wealth of Independent States India Malaysia Other countries World total

11

2009

2010

– –

– –

550 129,000 –

550 130,000 –

2700 350

2700 350

– 132,600

– 133,600

Reserves, metric ton

13,000,000 1,600,000 48,000 55,000,000 19,000,000 3,100,000 30,000 22,000,000 113,778,000

1.2. Technological applications of rare earths Rare earth metals and their compounds are in demand, and are often crucial for, a broad and rapidly expanding range of applications that rely upon their chemical, catalytic, electrical, magnetic, and optical properties. Rare earths are widely used for traditional sectors including metallurgy, petroleum, textiles, and agriculture. As indicated in Table 2 (Commercial Applications for Rare Earth Technology, Http://reitausa.org), they are also becoming uniquely indispensable and critical in many high-tech industry such as hybrid cars, wind turbines, and compact fluorescent lights, flat screen televisions, mobile phones, disc drives, and defense technologies (Song and Hong, 2010a,b). Different rare earths are needed to supply the required functionality in these applications. In some cases, a single rare earth element may be required, such as La for nickelmetal hydride batteries, but other applications require a mixture of rare 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 rare earth elements and transition metals such as iron, nickel, and cobalt. Samarium-cobalt magnets were first developed in the 1970s (Liu et al., 2006). However, due to their higher cost and weaker magnetic field strength, these magnets are now used less than neodymium magnets, unless their higher Curie point is needed. Neodymium permanent magnets, a tetragonal alloy of neodymium, iron, and boron (Nd2Fe14B), have been used in a wide range of applications requiring a high energy product and high coercive force (Brown et al., 2002). Neodymium can be replaced by praseodymium and up to 5% cerium in high energy product magnets (Doyle et al., 2000; Benz et al., 2000). The addition of terbium and dysprosium can enhance the coercivity of Nd–Fe–B sintered magnets (Hu et al., 2008; Xu et al., 2011). Rare earths phosphors are widely used in high efficiency lighting, flat display screens, plasma screens, and liquid crystal screens due 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 the emission lines of rare earths is almost independent of the host lattice. Rare earth ions such as Tb3+ and Eu3+ emit at frequencies that enable high lumen efficacies and a good quality of white light (Liu and Chen, 2007; Tu et al., 2011). Replacement of some of the rare earth cations of a crystalline rare earth phosphor by ions of another rare earth element activates the phosphor, achieving a high degree of fluorescence. For example, terbium-activated gadolinium oxysulphide (Gd2O2S:Tb) gives a maximum fluorescence when about 0.3% of the gadolinium atoms have been replaced by terbium. Commercial plasma screens have used yttrium tantalates activated by thulium (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 are the main rare earth minerals of commercial importance. Typical

12

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

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

Rare earth (RE) technology

Enabling functionality

RE elements required

Hybrids, plug-In, and electric Vehicles

RE permanent magnets

Nd, Pr, Dy, Tb

Wind and hydro power generation

RE permanent magnets

Computer Disc Drives; Cordless Power tools Medical imaging – MRI

RE permanent magnets

Electric traction drives replacing or supplementing internal combustion engines Gearless generators for better reliability and online performance Compact, light weight and powerful motors

RE permanent magnets

Produce magnetic field

X-ray imaging Handheld wire-less devices

RE phosphors RE permanent magnets;

High energy efficiency Compact, light weight and powerful motors;

Flat screen display Catalytic converters and other emission reduction technologies Fiber optics Ni Metal Hydride Batteries

RE phosphors Ability to oxidize CO and ozone to CO2 and O2

Capacitors with high energy density

Rare earth- doped ceramic, tantalum and other types of capacitors

Unique luminescent properties Significantly less expensive than Pt metal group alternatives Signal amplification Proven and cost effective compared to Li ion battery alternatives High energy density compared to conventional capacitors

RE doped optical fibers Energy storage

compositions for these minerals are shown in Table 3 (there may be significant compositional variations depending on sources) (U.S. Bureau of Mines, 1985). Various processing routes have been developed to recover rare earths. After mining and comminution, ore is beneficiated by flotation, magnetic or gravity methods to produce rare earth concentrates, which then undergo hydrometallurgical 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, bastnesite concentrates are typically roasted to decompose the carbonate minerals before leaching with either hydrochloric or sulfuric acid. Cerium comprises about half of the rare earth content within bastnasite, so removing it prior to solvent extraction dramatically reduces the solvent extraction capacity for selective separation of individual rare earth elements. To minimize the separation costs cerium could be oxidized to CeO2 during roasting which will not dissolve readily in acidic lixiviants so would report to the leach residue from which it could be recovered separately. Hydrochloric acid may promote reduction of Ce(IV) and hence, incomplete separation of Ce(III) from the other trivalent lanthanides, Ln(III). Alternatively after leaching all the rare earths, cerium could be oxidized in the aqueous phase to precipitate it and then be recovered by filtration. For example, Ce(OH)4 was precipitated at the Thorium Table 3 Rare earth contents of principal minerals as percentage of total rare earth oxide (Mineral Facts and Problems, 1985). Oxide Y2O3 La2O3 CeO2 Pr6O11 Nd2O3 Sm2O3 Eu2O3 Gd2O3 Tb2O3 Dy2O3 Ho2O3 Er2O3 Tm2O3 Yb2O3 Lu2O3 Total

Bastnesite (California)

Monazite (Australia)

0.1 32.0 49.0 4.4 13.5 0.5 0.1 0.3

2.1 23.0 45.5 5.0 18.0 3.5 0.1 1.8

0.1

1.0

100.0

100.0

Xenotime (Malaysia) 60.8 0.5 5.0 0.7 2.2 1.9 0.2 4.0 1.0 8.7 2.1 5.4 0.9 6.2 0.4 100.0

Nd, Pr Nd, Pr, Dy, Tb Nd, Pr, Dy, Tb, Y, Eu, Tb Nd, Pr, Dy, Tb, Y, Eu Y, Eu, Tb, Gd, Ce Ce, La Y, Eu, Tb, Er La Various rare earths

plant using ammonium carbonate and ammonium persulfate (Huang et al., 2005). In some cases, sodium hypochlorite is also used to oxidize dissolved Ce(III). At Baotou, the largest producer of rare earths in China, the bastnesite concentrates contain a small amount of monazite. Fig. 1 shows the leaching process used for Baotou rare earth concentrates, which has been designated to be flexible and to accommodate different concentrates. The process starts with roasting with concentrated sulfuric acid at >300 °C (Huang et al., 2006) to ‘‘crack’’ the monazite. The rare earth sulfates formed during this process are then leached with water, and excess acid is neutralized with magnesia and filtered. The leach solution then proceeds to solvent extraction, alternatively a mixed rare earth chloride (for electrolysis to misch metal) could be produced by precipitation with ammonium carbonate, followed by dissolution with HCl and crystallization. Unfortunately, the radioactive element, thorium, is precipitated and reports to the leach residue. It cannot be recovered economically, resulting in both loss of the valuable thorium and potential environment hazards. HF and sulfur dioxide report to the off-gas from roasting. Large amounts of water or alkaline solutions are needed to remove them, resulting in large volumes of acidic effluents. The roasting process has been modified, for example by adding MgO or CaO and NaCl to stabilize fluorine in the leaching residue instead of releasing it to the waste gas phase (Wang and Liu, 1996; Wu et al., 2002a,b, 2004b; Li et al., 2004). Bastnesite has also been roasted with ammonium chloride, which decomposes into gaseous HCl that forms rare earth chlorides, which are readily leached with hot water (Chi et al., 2004). Another variant involves heating concentrate with sulfuric acid at 40–180 °C for two to four hours before roasting at 150–330 °C (Hu, 1998). This suppresses decomposition of sulfuric acid, resulting in a relatively high fraction of HF in the gas phase; this can be recovered as solid NH4F by reacting with (NH4)2CO3 in the off-gas pipe. In some plants, the bastnesite concentrates are first digested with concentrated NaOH to decompose carbonates and then leached with hydrochloric acid to produce mixed rare earth chlorides (Gupta and Krishnamurthy, 2005; Xu et al., 2012). The disadvantages of this process include high alkaline consumption and the radioactive thorium reporting to both the leachate and the residue, which hampers subsequent recovery. Monazite and xenotime concentrates can be leached either by sulfuric acid or by sodium hydroxide at elevated temperature to decompose the orthophosphate lattice. The sodium hydroxide

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

treatment is preferred in most commercial extraction plants because it better separates phosphate from the rare earth (Alex et al., 1998). The ion-adsorption type rare earth ores are usually leached 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 precise leaching process, the leach solution will usually contain dissolved impurities such as iron, which are removed by precipitation before proceeding to solvent extraction to separate the rare earths.

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

DM ¼

½M ¼K ½M

ð1Þ

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

bM1=M2 ¼ 2. Solvent extraction separation of rare earths Solvent extraction producer starts by separating different groups of rare earths from the leachate. Depending on the process, some primary rare earths producers may choose to sell intermediate, mixed products, or perform different downstream separations to produce individual rare earth salts or oxides. Individual rare earths are difficult to separate from each other, due to their similar physical and chemical properties. Separation processes based on ion-exchange and solvent extraction techniques have thus been developed to produce high purity single rare earth solutions or compounds. Before the advent of industrial scale solvent extraction in the 1960s, ion exchange technology was the only practical way to separate the rare earths in large quantities (Jamrack, 1963; Kumar, 1994; Reddy et al., 2009). Nowadays, ion exchange is only used to obtain small quantities of high purity rare earth product for electronics or analytical applications (Taniguchi and Doty, 1989). Solvent extraction is generally accepted as the most appropriate commercial technology for separating rare earths due to the need to be able to handle larger volumes of dilute pregnant liquors (Peppard et al., 1953, 1957a,b; Peppard and Wason, 1961; Brown and Sherrington, 1979; Sherrington, 1983). Solvent extraction processes for separation and purification of rare earths have been reviewed during the 1990s (Zhu, 1991; Reddy et al., 1995). Table 4 summarizes commercial extractants reported in the literature for rare earth solvent extraction (Ritcey and Ashbrook, 1984; Rydberg et al., 2004). All three major classes of extractant, namely, cation exchangers (or acidic extractants), solvation extractants (or neutral extractants), and anion exchangers (or basic extractants), have been utilized for separating rare earths. Some chelating extractants have also been suggested for rare earth separations. Before considering the different types of extractant, it is useful to define two terms, the distribution coefficient and the separation

DM1 DM2

ð2Þ

2.1. Cation exchangers The overall extraction of rare earth elements from aqueous media by cation exchange extractants in their acidic form can generally be expressed as (Peppard et al., 1958):

Ln3þ þ 3HA ¼ LnA3 þ 3Hþ

Ln3þ þ 3H2 A2 ¼ LnðHA2 Þ3 þ 3Hþ

ð4Þ

Here H2A2 refers to the dimeric form of the organic acid. From inspection of Eqs. (3), (4), it is evident that the extraction of rare earths with cation exchangers is promoted by increasing the aqueous phase pH, while the stripping process, which reverses the extraction reaction, is promoted by increasing the acidity of the aqueous stripping solution. Two different classes of cation exchangers are use for rare earth separations, namely carboxylic or fatty acids, and organic derivatives of phosphorous acids. 2.1.1. Carboxylic acids The use of different carboxylic acids, including naphthenic acids and Versatic acids, for extracting rare earth metal ions has been reported (Bauer and Lindstrom, 1964; Korpusov et al., 1974). The

Roasting with concentrated H2SO4 Off-gas

Calcined concentrate water leaching MgO Neutralization/precipitation/filtration

Solvent extraction for separation of individual rare earth

ð3Þ

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

Rare earth concentrate (50% wt REO)

leachate

13

residue Precipitation with ammonium carbonate and dissolution with HCl to produce mixed rare earth

Fig. 1. Schematic leaching process used for Baotou rare earth concentrates (after Huang et al., 2006).

14

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

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

Structure

1, Cation exchangers Carboxylic acids

R1

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

CH3 C

R2

COOH

R2

R1 (CH2)nCOOH

R3 Phosphorous acids

Naphthenic acids: R1-R4: varied alkyl groups

R4 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)

O

R1 P R2

OH

R1

S

Monothiophosphorous acids R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-monothiophosphinic acid (Cyanex 302)

P R2

OH

R1

S

Dithiophosphorous acids R1 = R2 = CH3(CH2)3CH2CH(CH3)CH2–, di-2,4,4-trimethylpentyl-dithiophosphinic acid (Cyanex 301)

P SH

R2 2, Chelating exchangers

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

O R1 C CH2 C R2 O

3, Solvating extractants

O

R1 P

R3

R2 4, Anion exchanger

RNH2

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) Primary amines R = (CH3)3C(CH)2C(CH3)2)4 (Primene JMT, N1923) Quaternary amines: R1 = R2 = R3 = C8–C10 mixture (Aliquat 336, Adogen 464)

RNH2

CH3 Cl

R1 N R2

R3

extraction behavior of yttrium differs for these reagents; yttrium is extracted by Versatic 10 with the middle rare earths (La < Ce < Nd < Gd < Y < Ho < Yb) whereas it is extracted by naphthenic acid with the light rare earths (La < Ce < Y < Nd < Gd < Ho < Yb) (Preston and du Preez, 1990). Zheng et al. (1991) noted that the behavior of Y is correlated with the acidity of the extractant, while du Preez and Preston (1992) attributed the changing order to steric hindrance caused by the structure of the carboxylic acids and the atomic number of rare earth metal ions. With straight chain and non-hindered acids the behavior of yttrium most closely resembles that of light lanthanides (e.g., Ce, or Pr), while for the sterically hindered acids the behavior of yttrium most closely resembles that of middle lanthanides (e.g., Gd or Tb). Naphthenic acid has been widely used for separating yttrium from lanthanides in China (Li et al., 1994). However, the extractant composition changes with use and its high solubility in water lead to significant reagent losses (Li, 1995). Alternatively, the novel carboxylic acids, such as sec-nonylphenoxy acetic acid (CA-100) and sec-octylphenoxy acetic acid (CA-12), developed in China, have

much lower aqueous solubilities than naphthenic acids. A study on the extraction of trivalent lanthanides (Sc, Y, Ln) and divalent transition metal ions (Cu, Zn, Ni, Mn, Cd, Co) from acidic chloride solutions with CA-100 in heptane indicated that CA-100 can extract rare earth ions at lower pH values than Versatic 10. The extraction behavior of yttrium with CA-100 most closely resembled that of the heavy lanthanides (Wang et al., 2002; Li et al., 2007a; Li et al., 2007b; Li et al., 2007c). The extraction behavior of trivalent rare earths using cekanoic, neo-heptanoic, and some 2-bromo alkanoic acids has also been investigated (Preston, 1994; Xu et al., 2003; Singh et al., 2006). It was suggested that the introduction of the 2-bromo substitute in an alkanoic acid structure lowers the pKa values, enabling the substitute acids to be effective extractants at lower pH than the parent compounds. 2.1.2. Organophosphorous acids Various kinds of acidic organophosphorous extractants have been used in rare-earth separation processes, with D2EHPA (or HDEHP, di(2-ethylhexyl) phosphoric acid) and HEHEHP (or EHEH-

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

PA, 2-ethylhexyl phosphonic acid mono-2-ethylhexyl) are the main solvents widely used. Peppard et al. (1957a,b) noted that the distribution coefficients of rare earth ions at tracer concentrations between D2EHPA in toluene and aqueous chloride solutions had an inverse third-power dependency on the HCl concentration in the aqueous phase and a third-power dependency on the D2EHPA concentration in the organic phase. This indicates that only one of the acid groups in a D2EHPA dimmer in the organic phase dissociates and participates in the extraction reaction (following Eq. (4)). Gels formed in the organic phase at high metal loadings and low acidities, which is undesirable because of the associated viscosity and phase separation problems (Ferraro and Peppard, 1963). The selectivity order for extracting rare earths from 0.5 M HCl solution with 0.75 M D2EHPA in toluene was Lu > Yb > Tm > Tb > Eu > Pm > Pr > Ce > La (Fig. 2) (Peppard and Wason, 1961), with the log of the distribution coefficient (called log K by Peppard et al. (1957a,b)) increasing linearly with the atomic number, Z, of the rare earth. The average separation factor of two adjacent rare earth elements was 2.5. Yttrium was extracted between Tb and Tm in this solvent extraction system, corresponding to an artificial atomic number 67.6. The extraction of the lanthanide elements with D2EHPA in toluene was similar for perchloric acid solutions (Piece and Peck, 1963), but poorer in nitrate media (Reddy et al., 1995). It should be pointed out that although the distribution coefficients of rare earths generally increase with increasing atomic number, the precise separation factors depend on the acidity of the aqueous phase and nature of the anion. In 1965, Molycorp demonstrated the large scale application of D2EHPA for pre-concentrating europium to around 15% from rare earth chloride feed derived from bastnesite, containing about 0.1% Eu2O3 (Kruesi and Schiff, 1968). Preston and du Preez (1996) pre-concentrated europium from chloride containing 0.22 ± 0.01 M total rare earths (%: Eu 93, Sm 3, Nd 2, Ce 1, Pr 0.5 and Gd 0.5) using 0.4 M D2EHPA in xylene; 99.98% Eu solutions were obtained in a single extraction stage at pH 2.7. 1 M D2EHPA

3

2

Log K

1

Y

0

-1

-2

-3

-4 57

59

61

63

65

67

69

71

Z Fig. 2. Plot of Log K (Log D) as a function of atomic number (Z) (0.75 M HDEHP in toluene and 0.5 M HCl; Tracer concentrations of rare earths) (after Peppard et al., 1957a,b).

15

was used to separate a 99.8% La2O3 product from didymium chloride solution (feed containing 45% La2O3, 35% Nd2O3, 10% Pr6O11 and 5% Sm2O3) (Nair and Smutz, 1967). Two multistage countercurrent extraction circuits were needed; the first, 12 stage cascade concentrated La in the raffinate, and the second, 14 stage cascade provided further purification. The overall recovery of La was 60%. Preston et al. (1996a,b,c) described a continuous solvent extraction process for separating the middle (Sm, Eu, Gd, and Tb) and the light rare earth fractions (La, Ce, Pr, and Nd) from a nitrate feed. The middle rare earths were first extracted into a 15% v/v of D2EHPA in Shellsol AB in an 8 stage counter-current circuit, followed by scrubbing with 1 mol/L HNO3 in 2–4 stages, and stripping with 1.5 mol/L HCl in 6–8 stages. Residual rare earths in the organic phase (mainly Dy, with some Tb and Gd) were removed in a secondary stripping circuit using 2.5 mol/L HCl in four stages. Over 1000 L of feed liquor was processed in two continuous countercurrent 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 to 8 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 rare earths to the strip liquors were relatively high (95–100%), whereas losses of the light rare earths were low (< 4%). D2EHPA has also been used to separate Sm, Eu, and Gd from the other rare earths in a mixed nitrate-chloride leachate from monazite (Rabie, 2007). D2EHPA can extract rare earth at low pH values, but because the equilibrium of Eqs. (1) and (2) lies strongly to the right it is difficult to strip the loaded metals. Thus, other acidic organophosphorus extractants have been widely examined for rare earth solvent extraction. Benedetto et al. (1995) reported that DS5834 (Zeneca Specialties, with a formulation similar to M2EHPA, mono-2-ethylhexyl phosphoric acid) could effectively extract Ga, In, Sm, and Gd from acidic media but was neither selective for Sm and Gd, nor effective 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 rare earths can be stripped at lower acidities than from D2EHPA (Reddy et al., 1995). In addition, HEHEHP can be more heavily loaded with rare earth than D2EHPA before the onset of saturation effects, which increases the extraction efficiency. The extraction of rare earths with HEHEHP follows Eq. (1), with the extraction order from chloride and nitrate media of La < Ce < Pr < Nd < Sm < Eu < Gd < Tb < Dy (Y) < Ho < Er < Tm < Yb < Lu (Sato, 1989). A process developed by Daihachi for separating rare earths using HEHEHP has been applied in commercial separation plants in Baotou, China (Zhu, 1991). Fontana and Pietrilli (2009) also suggested the use of HEHEHP for recovering rare earths resulted from spent NiMnH batteries. Some di-alkyl phosphinic acids have also been investigated for rare earth separation, although only Cyanex 272 (bis(2,4,4-trimethylpentyl) phosphinic acid) has been used commercially (Kolarik and Pankova, 1966; Li and Frieser, 1986; Banda et al., 2012). Sabbot and 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 toluene from acidic nitrate-acetato media and suggested the formation of La(Ac)2A.3HA (Ac denotes acetate ion and HA denotes the acidic form of Cyanex 272) in the organic phase at low La loading and LaA3 at higher loading. Studies on the extraction of samarium from chloride solutions with Cyanex 272 indicated that the extracted species was Sm(OH)A2.2HA (El-Hefny et al., 2010). Studies on the extraction of La, Pr, Eu, Ho and Yb into chloroform solutions containing dicyclohexylphosphinic acid (DCHPA) showed that the extraction selectivity of DCHPA was inferior to that of other di-alkyl phosphinic acids, presumably because the cyclohexyl groups in

16

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

DCHPA sterically hinder chelate formation (Cecconie and Frieser, 1989). 2.2. Chelating extractant As hydrogen ion donors, chelating extractants extract metals by a cation ion exchange mechanism similar to Eq. (1), but but the resulting organic complexes are stabilized by the organic anion coordinating the central cation in at least two positions (Hudson, 1982): Chelating extractants have been examined for extracting europium from nitrate solutions and cerium(III) and lanthanum(III) from chloride solutions, but performed unfavorably compared with acidic extractants (Urbanski et al., 1996; Arichi et al., 2006). 2.3. Solvation extractant Several types of solvation extractants have been used for rare earth separations. Peppard et al. (1957a,b) investigated the extraction of trivalent rare earths from chloride and nitrate solutions with TBP (tributylphosphate). The extractability of the lanthanides with TBP increased with increasing atomic number, but the distribution coefficients were much lower in chloride solutions than in nitric media. Concentrated nitric systems were promising for separating rare earths lighter than samarium. Rare earths heavier than samarium could not be separated effectively in nitric systems. The rare earths in neutral nitrato complexes are coordinated by the phosphoryl group of TBP, yielding an extractable complex. The overall reaction can be expressed as:

Ln3þ þ 3NO3 þ 3TBP ¼ LnðNO3 Þ3 ðTBPÞ3

ð5Þ

although there would be few simple Ln3+ cations in solution at the ionic strengths needed for effective extraction. Later work of Peppard et al. (1966) examined the influence of extraction conditions on the equilibrium constant of reaction 5 to infer the composition of the 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, while the extraction of Th(IV) increased with the aqueous acidity. A third phase formed at H2SO4 > 5 mol/L. The extraction of Ce(IV) and Th(IV) from sulfate media with Cyanex 923 can be represented by the reaction: 2 M4þ þ SO2 4 þ 2HSO4 þ 2B ¼ MðSO4 ÞðHSO4 Þ2  2B

ð6Þ

where M represents Ce or Th and B denotes Cyanex 923. The extraction of trivalent lanthanides and yttrium from nitrate medium with Cyanes 925 in heptane was suggested to follow the reaction (Li et al., 2007a,b,c):

M4þ þ 3NO3 þ 2B ¼ MðNO3 Þ2  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; on attaining steady state, the process was stopped and products of different composition were withdrawn from different stages. This configuration is costly compared to continuous processing and not amenable to scale up. Preston et al. (1996a,b,c) described a pilot-scale process for recovering a mixed rare-earth oxide product from calcium sulfate hemihydrate sludge generated during the manufacture 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 adding 2.5 M ammonium nitrate and extracting into 33% v/v DBBP (dibutyl butylphosphonate) in Shellsol 2325. The organic phase was stripped with water to yield a solution of rare earth nitrates from which the mixed rare earth oxide was recovered by adding oxalic acid and calcining the precipitate. Later work examined using TBP (15% in Shellsol K diluent) to selectively extract cerium(IV) from the rare earth nitrate feed (Preston et al., 1996b). The organic phase was stripped by reducing the cerium(IV) with dilute hydrogen peroxide in two stages, giving solutions containing up to 90 g/L of cerium(III). 2.4. Anion exchangers Anion exchangers extract metal ions as anionic complexes, and hence are only effective in the presence of strong anionic ligands. Early work indicated that the separation factors for adjacent rare earths with primary or tertiary amines were poor in chloride media but 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 sulfate solutions with Primene JMT (tri-alkyl methylamine) was extracted with the following reactions:

2RNH2 þ H2 SO4 ¼ ðRNH3 Þ2 SO4

ð8Þ

2 2LaðSO4 Þ3 3 þ 3ðRNH3 Þ2 SO4 ¼ 2ðRNH3 Þ3 LaðSOÞ43 þ 3SO4

ð9Þ

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

Ln3þ þ 3NO3 þ xðR4 Nþ NO3 Þn ¼ LnNO3  xnR4 Nþ NO3

ð10Þ

where Ln denotes the rare earth ion and R4 Nþ NO 3 the quaternary ammonium nitrate salt (Cerna et al. (1992) suggested a more complicated reaction). These reagents are strong-base anion exchangers and require lower concentrations of salting out reagents than amines. Chelation with EDTA improved the extraction and separation of rare earth pairs. In nitrate media Aliquat 336 extracts light rare earths more readily than the heavier ones. This behavior contrasts that of most of cation exchange and solvating extractants, for which the extraction of the rare earth metals increases steadily with increasing atomic number. Hence, quaternary ammonium salts provide a means of removing light rare earths from process solutions. Yttrium is anomalous, behaving as a heavy rare earth in nitrate media and as a light in thiocyanate media. This has been utilized for treating xenotime, which contains 60% Y2O3 (Table 2). Xenotime concentrate was leached with HNO3 and lighter rare earths were extracted with Aliquat 336 in an aromatic diluent. Yttrium and the heavy rare earths remained in the aqueous phase. Yttrium was then extracted by Aliquat 336 from a thiocyanate solution, yielding 99.99% Y2O3, while other heavy rare earths remained in the 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 a 45-stage extraction. Preston (1996) described a solvent-extraction process for recovering neodymium oxide (95% Nd2O3) from light rare earth nitrate solution using 0.50 M solution of Aliquat 336 nitrate in Shellsol AB.

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

2.5. Synergistic solvent extraction Numerous types of synergistic solvent extraction systems for extracting and separating rare earths have been reported, including mixtures of acidic extractants (e.g., carboxylic or organophosphorus 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 and du Preez (1994) examined the effect of the addition of a series of tri-alkyl phosphates (RO)3PO, di-alkyl alkylphosphonates (RO)2RPO, alkyl di-alkylphosphinates (RO)R2PO, and tri-alkylphosphine oxides R3PO on the extraction of the trivalent lanthanides and yttrium from chloride media by DIPSA (3,5-di-isopropylsalicylic acid). Synergistic effects were observed with all mixtures, albeit to different extents. For the series of compounds with R = n-butyl, the synergistic effect increased in the order (RO)3PS < (RO)3PO < (RO)2RPO < (RO)R2PO < R3PO. The synergistic effects were greater for lutetium(III) than for lanthanum(III) (the separation across the lanthanide series increased). Mixtures of DIPSA and TIBPO (tri-isobutyl phosphine oxide) gave somewhat better separation factors between the light and the middle lanthanides (bSm/Nd = 3.0) than Versatic 10 acid alone (bSm/Nd = 2.6). Separation factors were comparable to those with the latter extractant between the heavy lanthanides (thulium to lutetium). The authors suggested that the extracted rare earth complexes had a composition of LnA3L2 (where HA represents carboxylic acid and L the neutral organophosphorous compound) and the synergism resulted from the replacement of some or all the undissociated carboxylic acid molecular (see Eq. (4)) (Preston and du Preez, 1995). Comparison of the effects of some bi-functional ligands containing C@O, P@O or S@O groups upon the extraction of trivalent rare earth metals from chloride media by DIPSA in xylene indicated that the shifts generally increased in the order S@O < C@O < P@O for comparable ligands. The synergistic effect produced by the addition of a given bi-functional compound generally decreased across the lanthanide series (La to Lu), attributed to steric hindrance effect (Preston and du Preez, 1998). Reddy et al. (1999) reported that La(III) and Nd(III) were extracted from nitrate solution by Cyanex 301 (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 both the extraction efficiencies and selectivities, especially between yttrium and heavier lanthanides. Zhang et al. (2008) reported studies of the solvent extraction of cerium(IV) and fluoride from sulfate solutions using a mixture of Cyanex 923 and D2EHPA in n-heptane and Ce(III) was not extracted by the mixture. For binary acid extractant systems, Ying et al. (2005) examined the extraction of Yb3+ from chloride solution with Cyanex 272, P507 (HEHEHP), and their mixtures in n-heptane. The extraction of Yb3+ was higher with the mixture than with Cyanex272 or P507 alone. A synergistic effect was observed on the separation of Yb/Tm and Lu/Yb, but not for Tm/Er, Er/Ho, and Ho/Dy. Zhang et 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 metals at pH = 2.0. Li et al. (2007) also reported that the separation factor for Sm and Nd was significantly increased using a mixture of D2EHPA (40% v/v) and HEHEHP (60% v/v). Sun et al. (2006) examined the synergistic extraction of trivalent Sc, Y, La, Gd, and Yb from chloride media using a mixture of Cyanex 272 and sec-nonylphenoxy acetic acid (CA-100) in n-heptane. The separation factor for Yb and Y was much higher than that with CA-100 alone. The extraction of rare earth elements from chloride media by mixtures of CA100 with Cyanex 301 or Cyanex 302 was studied by Tong et al. (2009). In the CA100 + Cyanex 301 system, the synergistic enhancement coefficients decreased

17

with increasing atomic number of lanthanoids, but the separation factors between Y and all the lanthanoids were enhanced. Jia et al. (2009) reported that the separation factors of all adjacent trivalent rare earths were better in a mixture of sec-octylphenoxy acetic acid (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 solvent extraction chemistry and equilibriums, as discussed above, far less is known on the engineering details of rare earth separations. From the limited open literature, rare earth producers appear to follow similar approaches (Gupta and Krishnamurthy, 2005). There is often a need for a primary separation of rare earths from impurities in the original leach solution, along with concentration. D2EHPA has been widely used for primary separation because the distribution coefficients of the rare earth elements as a group differ markedly from those of typical impurities in leach liquors. D2EHPA is also suitable for concentrating the rare earth elements from dilute, acidic solutions. In general, rare earths are separated in the trivalent state. They are usually separated into two, three or sometimes four groups, followed by precipitation or subsequent separation of individual rare earth. Preferential separation of yttrium is desirable, and cerium and europium are often separated initially on the basis of different valance states (Ce4+ and Eu2+). The extractants and aqueous anion are generally selected considering both cost and technical requirements, and the impact on the process configuration (McGill, 1997). For example, cationic exchangers usually offer higher selectivity on rare earth ions compared to neutral and anionic exchangers. However, the reactive chemical requirement is greater with cation exchangers, because base is required to drive extraction, and acid is required for selectively washing the organic phase. In contrast, with solvation extractants and anion exchangers the reactive chemical requirement is lower. Thus there is a trade-off between selectivity (which lowers the number of stages, and hence capital and chemical inventory costs) and the operating costs. 3.1. Configurations Rare earth solvent extraction processes are generally classified as primary separations, which aim to separate rare earth elements from other elements (which is relatively straightforward, and comparable with other solvent extraction processes in hydrometallurgical operations), and secondary separations, which produce single or mixed (typically 2 or 3) rare earth products from the mixed rare earth stream produced by primary separations. The latter is often much more challenging, particularly when producing a single, pure rare earth, because of the chemical similarity of the rare earths. As mentioned above, D2EHPA has been widely used in primary separation processes because the distribution coefficients for the rare earth elements with D2EHPA are markedly different from those of other elements in the aqueous solution (leach liquor). D2EHPA is also suitable for concentrating the rare earth elements from dilute solutions because of the high distribution coefficients. A plant producing multiple single rare earth products may contain hundreds of stages of mixers and settlers. A classic countercurrent flow scheme for a simple solvent extraction circuit is shown in Fig. 3. The aqueous stream leaving the nth mixer settler would be pumped to the (n  1)th mixer settler, while the organic phase (which is ideally in equilibrium with the aqueous phase leaving the nth stage) would be pumped to the (n + 1)th mixer settler. This arrangement may also be used for simple rare earth separations, e.g., the primary separation of rare earths from impurities. How-

18

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

organic feed

organic product

1

n-1

n

n+1

n+m

aqueous feed

aqueous raffinate Fig. 3. Simplified countercurrent solvent extraction circuit.

ever, there is fundamental shortcoming with this configuration if one wishes to separate rare earths from a mixed feed. Consider an operation using a cation exchange extractant or a solvating extractant, for which the heavier rare earths have a stronger affinity for the organic extractant phase than have the lighter ones. Iif mixed aqueous feed were introduced into one end of a bank of mixer settlers, the organic phase leaving the other end would be somewhat enriched in heavy rare earths, but there would still be an appreciable amount of light rare earths in this stream, because of the low separation factors. This shortcoming is addressed by introducing the mixed aqueous feed near the middle of the bank of mixer settlers, as shown in Fig. 4. A different aqueous stream is admitted into the end (the (n + m)th stage) to allow an appropriate number of stages for the light rare earth to be scrubbed from the organic phase back into the aqueous phase. This scrub solution may exit the process midway, or may continue on with the aqueous feed. To minimize the dilution of rare earth concentrations caused by the introduction of the barren aqueous scrub and the organic phase, reflux is sometimes used; some of the light rare earths from the aqueous raffinate are loaded back into the organic phase entering the first mixer settler, and some heavy rare earths are stripped from the organic product and added to the aqueous scrub (Fig. 5). When operating under reflux, the mass transfer occurring at each stage becomes an exchange of different rare earths, according to their affinity for an extractant, as exemplified below for a liquid cation exchange extractant.

LaA3 þ Ce3þ ¼ CeA3 þ La3þ

ð11Þ

Reflux minimizes dilution and also reduces the number of stages need to achieve an effective separation. Nevertheless, many separation stages are typically needed to obtain pure product. Most banks of mixer settlers are set up to make a single separation between two adjacent rare earths, but some configurations have three or more product streams. Various processes for rare earths separation from concentrates/ore and aqueous solutions have been summarized in detail by Gupta and Krishnamurthy (2005). Some typical applications and process flowsheets for solvent extraction separation of rare earths used in practice are listed below. 3.1.1. Molycorp-bastnesite Fig. 6 shows the schematic flowsheet for the Molycorp process, used to extract europium oxide from the leachate of Mountain Pass bastnesite (Gupta and Krishnamurthy, 2005). A chloride solution (100 g/L REO) containing all the rare earth except Ce is generated by calcination and leaching with HCl solution. Two steps of solvent extraction with D2EHPA were applied. The chloride solution was first contacted with 10% v/v D2EHPA in kerosene, and the extraction was performed in five stages of mixers and settlers under conditions that ‘‘split’’ the rare earths with Sm and all heavier rare earths reporting to the D2EHPA solution, and Nd and all lighter elements reporting to the raffinate (this is designated in Fig. 6 by ‘‘(Nd/Sm)’’ in the first solvent extraction stage). There are two

organic feed

organic product

1

aqueous raffinate

n-1

n

n+1

n+m

aqueous feed aqueous feed Optional scrub exit

Fig. 4. Simplified configuration of countercurrent solvent extraction circuit with optional scrub exit.

organic feed

Partial reflux

organic product

1

aqueous raffinate

n

aqueous feed

n+1

n+m

Partial reflux

aqueous scrub

Fig. 5. Simplified configuration of countercurrent solvent extraction circuit with reflux.

19

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

Bastnesite concentrate (60% REO) Calcination Conc. HCl

Leaching

Residue for Ce recovery

Other REs, Fe 10% D2EHPA

Solvent extraction (Nd/Sm)/stripping Sm, Eu, Fe

Soda ash

Iron Precipitation

Raffinate (for recovery of La, Ce, Pr, Nd)

Iron precipitate

Sm, Eu 10% D2EHPA

Solvent extraction(Sm/Eu)/stripping

Raffinate fed to the first SX circuit

Eu(3) Chloride solution Zinc amalgam

Reduction

H2SO4

Precipitation EuSO4 solution for producing Eu2O3

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 initially. The first is that it is relatively easy to separate Nd and Sm since they are consecutive elements in any natural rare earth minerals, but are not consecutive elements on the periodic table and the intermediate rare earth element, promethium, does not occur in nature. Thus their separation factor is typically double that of any other consecutive rare earth pair. The second reason is that, referring back to Table 3, it is apparent that Sm and the heavier rare earths account for only a very small proportion of the rare earths in bastnesite. Thus they can be removed using a small volume of D2EHPA solution, and the resulting rare earth mixture can be further processed using much smaller mixer settlers, leaving La, Pr, and Nd (the bulk of the rare earths in the concentrate) in the aqueous raffinate. These were precipitated with ammonium and sodium hydrogen sulfide and further processed in much larger-scale equipment. More than 98% of europium in the solution was extracted. The loaded organic (containing 98% of the Eu from the leach liquor) was stripped with 4 mol/L HCl. The iron in the strip solution was precipitated through neutralization to pH 3.5, and the clarified Eu-bearing solution proceeded to a second solvent extraction circuit, also using 10% D2EHPA in kerosene and five stages of mixers and settlers. Europium and other heavy rare earths were loaded in the organic phase, with the light rare earths remaining in the raffinate, which were returned to the first solvent extraction circuit. The europium (and other rare earths) were stripped from the loaded D2EHPA with 5 mol/L HCl solution and the strip liquor was 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%). After europium recovery, the strip solution still contained Sm, Y, and other heavy rare earths. Gadolinium was extracted with D2EHPA in a 10-stage extraction circuit followed by a 5-stage scrub. The raffinate was neutralized with soda ash to precipitate Sm and the heavy rare earths. 3.1.2. Rhône-poulenc – monazite Rhône-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 is shown in Fig. 7. Monazite concentrate was first digested with NaOH. The rare earths reported to the solid residue as hydroxides, which after filtration were dissolved in HCl or HNO3. After clarification, the resulting solution proceeded to a series of solvent extraction circuits to produce individual rare earth oxides. Chloride media were used to prepare a mixture of rare earth compounds, such as dehydrated rare earth chlorides, which were used to produce misch metal. Nitrate media were used to produce individual rare earth oxides, e.g., in the first separation circuit, lanthanum (99.9995% La2O3) remained in the aqueous phase while a mixture of Ce, Pr, Nd, Sm, etc. was loaded into the organic phase. Similarly, CeO2 (>99.5%) was separated from Pr, Nd, Sm, Eu, etc., after removing lanthanum. Various extractants, including carboxylic acids, organophosphorous acids, neutral organophosphorous compounds, and quaternary amines have been used in these separation processes. Rhône-Poulenc could also produce high-purity individual rare earth oxides from bastnesite or euxenite. The Rhône-Poulenc solvent extraction process has been regarded as the standard for all industrial producers (Bautista, 1995).

3.1.3. AS Megon–xenotime AS Megon developed a process for producing high-purity yttrium oxide starting from the xenotime concentrates (Gaudernack, 1973). The schematic flowsheet is shown in Fig. 8. The solvent extraction circuit consisted of a selective extraction by D2EHPA followed by three scrubbing and stripping units. The light rare earths (La, Ce, Pr and partial Nd) and impurities including Fe2+ ions remained in the raffinate. The extracted yttrium and other rare earths were separated into three groups by selective washing. Yttrium nitrate solution was fed to the second circuit using the nitrate of tri-capryl methylamine as the extractant. The lighter rare earths (La, Ce, Pr, Nd and Gd, Tb, Er) were extracted while Y, Tm, Yb, and Lu remained in the raffinate, which was fed to the third solvent extraction circuit, which used tri-capryl methylamine-NH4SCN to produce high-purity yttrium oxide.

20

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

Monazite NaOH Digestion/Filtration RE-Th hydroxide

HCl

Na3PO4 solution HNO3

Dissolution

Dissolution

Separation non-RE/RE/Th

Separation non-RE/RE/Th

Separation, La-Nd/Sm-Lu

RE chlorides RE fluorides

Conversion Cl-/NO3La2O3, 99.995%

Separation, La/Ce, Pr, Nd…

CeO2, 99.5%

Separation, Ce/Pr, Nd, Sm…

Polishing agent Separation, Pr, Nd/Sm, Eu, Gd… RE carbonates Anhydrous RE chlorides

Mischmetal

Pr6O11, 96%

Didymium Separation, Pr/Nd

Separation, Sm, Eu/Gd-Lu

Nd2O3, 96% Sm2O3, 96%

Separation, Sm/Eu

Gd2O3, 99.99% Tb4O7, 99.9%

Eu2O3, 99.99% Y2O3, 99.99% Fig. 7. Rhone-Poulenc liquid–liquid extraction process for separation of the rare earth elements (after McGill, 1997).

3.1.4. Mintek–apatite Rare earths have been recovered from the calcium sulfate sludge generated during the production of phosphoric from apatite at Phaleborwa (Preston et al., 1996b). Fig. 9 shows the schematic flowsheet for the pilot plant. The sludge was leached with dilute nitric acid solution containing calcium nitrate. Rare earths were extracted from the leachate with TBP (40% v/v in Shellsol 2325). The raffinate was recycled back to leaching after removing entrained organic solvent. The loaded organic solution was stripped with water to yield a mixed rare earth nitrate aqueous solution that was treated with ammonia and oxalic acid to precipitate a mixed rare earth oxalate. This was calcined to give a mixed rare earth oxide (89–94% purity). The rare earth oxide contained considerable amount of the middle rare earths, particularly Nd, Sm, Eu, and Ga. In subsequent pilot tests, TBP, HDEHP and Aliquat 336 were used to produce different rare earth products from the mixed oxides. 3.1.5. Industrial processes in China The Shanghai Yue Long Chemical Plant, was reported to treat monazite concentrates in a process similar to the Rhône-Poulenc process (Zhang et al., 1982). The simplified flowsheet of this plant is shown in Fig. 10. After digesting the monazite in NaOH, filtration and leaching of the residue with HCl, the resulting rare earth chloride solution was extracted with D2EHPA and the rare earths were split into three groups, from which mixed and pure oxides, carbonate, or chlorides were produced. The ion-adsorption type rare earth ores are first leached with HCl or H2SO4. Cation exchange extractants, such as HEHHP and naphthenic acid, are frequently used to extract rare earths elements from the leachate, since these ores have high levels of heavy rare earths, which have a strong affinity for acidic extractants. Individual rare earth compounds (oxides or

chlorides) can be produced through controlled stripping (Huang et al., 2005, 2006). For bastnesite ore, which is the main rare earth resource in China, the ore or concentrates are typically roasted with H2SO4, followed by leaching with water or dilute sulfuric acid. Rare earths are recovered from the leachate by solvent extraction with P204 (D2EHPA). Preferential stripping is used to divide the rare earths into two groups, La-Nd and Sm-Gd (the concentration of heavier rare earths is usually small); these can be further separated into individual rare earth elements if desired (Huang et al., 2006). To reduce reagent consumption, modified separation processes have been tested at pilot plant scale. One approach used P204 or P507 to extract Th and most of Ce first, then the raffinate containing the remaining rare earths underwent further solvent extraction steps to separate individual rare earth compounds (Fig. 11) (Huang et al., 2006). The F, Th, and Ce(IV) were selectively stripped from the organic phase. Another approach used Cyanex 923 to separate Ce(IV) from the leachate first. The raffinate containing other rare earths then underwent solvent extraction with N1923 (a primary amine) to separate Th (Fig. 12) (Lu et al., 1998). Individual rare earth compounds were produced from the Th-free raffinate in a third solvent extraction circuit. 3.1.6. Miscellaneous Doyle et al. (2000) developed a novel solvent extraction configuration capable of producing a mixed Ce–Pr–Nd product (for magnet production) and pure Nd oxide simultaneously, with flexibility to alter the relative proportions according to market conditions. The schematic flowsheet for this process is shown in Fig. 13. Rare earth chloride solution generated by leaching oxide with HCl underwent solvent extraction with P507 in kerosene. In the first solvent extraction circuit, Sm and all heavier rare earths along with

21

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

Xenotime

H2SO4; H2O Residue

Digestion/dissolution/filtration RE sulfate solution, 20 g/L REO

4-stage extraction

30% D2EHPA in Shellsol 4-stage selective scrubbing

8-stage selective scrubbing

1.5 M HNO3

6 M HNO3

NdSm(GdTb)

LaCePr(Nd), Fe2+, and other impurities

4-stage stripping

YbLu(Th)

0.5 M H2SO4

NH3

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

26-stage extraction

40% quaternary amine in Solvesso 50

6-stage scrubbing

3-stage stripping

1.75 M NH4NO3

NH4SCN

Gd-Er

Dilute HNO3

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

26-stage extraction

6-stage scrubbing

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

Solvent treatment

8-stage stripping

NH4SCN

TmYbLu

0.5 M NH4NO3; 0.1M NH4SCN

Dilute HNO3

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

Phalaborwa apatite ore Phosphoric acid manufacturing process Calcium sulfate sludge H2O washing

Washed sluge

Washing solution

Ca(NO3)2, HNO3 residue

Leaching/filtration

40% v/v TBP in Shellsol 2325

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

5-stage extraction

Raffinate Sheellsol 2325

washing

Washed raffinate

6-stage-stripping

Strip liquor (REO 45-60 g/L) Organic to Sheellsol 2325 tank

Precipitation

Water

NH3, oxalic acid

Mixed rare earth oxalate 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).

22

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

Monazite NaOH Na3PO4 solution

Digestion/Filtration

(RE, Th, U) hydroxide HCl

Dissolution/Filtration

RECl3 solution

(Th, U) sludge Dissolution

Solvent extraction

La, Ce, Pr, Nd

Sm, Eu, Gd Solvent extraction

REO

Solvent extraction

Y2O3

La2O3 Polishing agent Pr6O11 RE carbonates

Solvent extraction

Nd2O3

Gd2O3

Tb4O7

CeO2

Sm2O3

Dy2O2

RE Chorides

CeCl3

Th(NO3)4 ThO2

phosphors Solvent extraction

Solvent extraction

Solvent extraction

Eu2O3

RE fluorides

Malten salt electrolysis

Tb, …Lu

Lu2O3

U3O8 (NH4)2U2O7 Radioactive waste treatment

Mischmetal 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 Nd and lighter rare earths underwent a second solvent extraction with P507 in kerosene. Through controlling the number of stages and reflux ratios, Pr and Nd and part of thel Ce were extracted into the organic phase, with the balance of Ce, and all the La remaining in the aqueous phase from which a marketable lanthanum product were produced. The loaded organic phase underwent selective stripping to produce high-purity neodymium oxide and a mixture of Ce, Pr, and Nd oxides. Huang et al. (2008) used a synergistic extraction system to produce different rare earth products from rare earth sulfate solutions resulting from leaching of bastnesite concentrates (Fig. 14). The non-saponified organic phase was used directly to extract rare earths from their sulfate or chloride solutions and by controlling operation conditions, as many as five (or more) commercial rare earth products could be produced simultaneously.

3.2. Process simulation Process development, analysis, control and optimization of rare earth solvent extraction are complex tasks. Computer simulation program for monitoring or optimizing the rare earth solvent extraction process requires a reliable model for the extraction equilibrium. However, very few models for describing the relevant equilibrium between rare earth elements and different extraction systems have appeared in the open literature, and these are usually only applicable to a limited and specific range of conditions. This probably reflects the similarities of the lanthanides, their propensity for interactions make it difficult to predict their behaviors in various extraction systems. Therefore, little progress has been made with regard to the development of a general approach for modeling rare earth solvent extraction systems.

The reported programs for simulating rare earth solvent extraction processes usually consider countercurrent circuits, due to their ubiquity. Stage-wise calculations offer efficiency and flexibility (Sharp and Smutz 1965; Sebenik et al., 1966). Most are based on the McCabe–Thiele method (Thiele and Geddes, 1933; McCabe et al., 2005). The technique was originally developed for graphical analysis of binary distillation, and later applied to liquid–liquid separation processes, especially for solvent extraction systems involving only one extractable species (Zhu, 1991; Rydberg et al., 2004). Considering an n-stage counter-current circuit separating the metal ion from an aqueous solution by solvent extraction, the aqueous stream leaving the ith mixer settler would be pumped to the (i1)th mixer settler while the organic phase would be pumped to the (i + 1)th mixer settler (i = 1, 2, . . ., n) (Fig. 15). The symbols in Fig. 15 are defined as follows: ½Mnþ1 : the molar concentration of metal ion in the aqueous feed; ½Mn : the molar concentration of metal ion in organic product; [M]i+1: the molar concentration of metal ion in the aqueous solution fed to the ith mixer settler; ½Mi : the concentration of metal ion in organic phase leaving the ith mixer settler; [M]i: the concentration of metal ion in the aqueous solution leaving the ith mixer settler; ½Mi1 : the concentration of metal ion in organic phase fed to the ith mixer settler; [M]1: the molar concentration of metal ion in the aqueous raffinate; ½M0 : the molar concentration of metal ion in the organic feed. Assuming that the two phases are totally immiscible, and defining:

23

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

Bastnesite concentrate (60% REO) Calcination H2SO4

Residue

Leaching/filtration

sulfate solution containing (RE, F, Th) P204 or P507

Solvent extraction

Raffinate (RE sulfate solution)

Loaded organic

Selective stripping F

Fluoride for recovery

Selective stripping Ce

CeO2, 99% -99.99%

Solvent extraction Individual RE compounds

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).

Bastnesite concentrate (60% REO) Calcination H2SO4

Leaching/filtration

Residue

sulfate solution containing (RE, F, Th) Cyanex 923

Solvent extraction

Raffinate (contaning RE, Th)

Loaded organic

Selective stripping F

Solvent extraction

Flouride for recovery

stripping Ce

Raffinate

CeO2, 99% -99.99%

Solvent extraction Individual RE compounds

N1923 (primary amine)

Loaded organic

stripping Th

ThO2, 99% -99.99%

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 expressed as:

V A ð½Miþ1  ½Mi Þ ¼ V O ð½Mi  ½Mi1 Þ

ð12Þ

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

24

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

“Ce-free” RE sulfate solution P507

Sm, Gd, Eu for market

Solvent extraction, La(Ce)PrNd/Sm

P507

Solvent extraction, La(Ce)Pr/Nd

Raffinate (La-Ce for market)

Loaded organic High-purity Nd2O3

Selective strip, (Ce)Pr/Nd

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

RE sulfate feed solution Mixed extractants

aqueous scrub (H2SO4 or HCl)

Multi-stage mixers settlers

La

LaCe

PrNd

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 extraction isotherm for the desired metal ion. The line BC is an ‘‘operating line’’ which is straight with a gradient equal to the ratio of the aqueous and organic flow rates (VA/VO). The points on this line reflect the composition of crossing streams in each stage, i. e., ([M]i+1, ½Mi ) for all values of i (i = 1, 2, 3, 4). The molar concentration of metal in the organic feed, ½M0 , is taken to be zero in this figure, although it need not be so in practice. When an aqueous feed (with a molar concentration of [M]4) enters the 3rd stage mixer-settler, the composition of the organic phase ‘‘crossing’’ the stream will be ½M3 . The composition of the aqueous phase leaving stage-3 ([M]3) is determined by the point on the equilibrium line with an organic concentration of ½M3 (specifically, the point ([M]3, ½M3 ). The of the organic stream that crosses the aqueous stream leaving stage 3 is then obtained from the operating line, as shown graphically on Fig. 16. This allows calculation of the composition of the raffinate, [M]1. Alternatively, the number of stages needed to attain specific stream compositions can be obtained graphically. A similar method can be used to calculate the number of stages needed for the stripping circuit and the corresponding metal concentration

in the different streams. An equilibrium line for stripping should be determined or defined first; this differs from that for extraction because the chemical composition of the aqueous phase will differ, for example, having a different pH in the case of cation or anion exchange extractants, or a different concentration of ligands in the case of a solvating extractant. A simple case occasionally encountered in rare earth solvent extraction processes is a linear equilibrium line through the ‘‘origin’’ (OA in Fig. 17), corresponding to a constant distribution coefficient (DM) for metal extraction in all stages. Thus:

½Mi ¼ DM ½Mi

ði ¼ 1; 2; . . . nÞ

ð13Þ

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

½M1 ¼ DM ½M1

ð14Þ

½M1  ½M0 VA ¼ ½M2  ½M 1 V O

ð15Þ

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

25

Defining the extraction factor, EM, as:

EM ¼ DM ðV O =V A Þ

ð20Þ

Eq. (19) can be expressed as:

  ½Mnþ1 ¼ ½M1 1 þ EM þ E2M þ . . . þ EnM

ð21Þ

or:

½Mnþ1 ¼ ½M1

Enþ1 M 1 EM  1

ð22Þ

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

½Mn  ¼ DM ½M1 Fig. 16. Schematic McCabe–Thiele diagrams for countercurrent solvent extraction process.

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

Assuming ½M0 ¼ 0 (no metal in the organic feed) gives:

½M1 VA ¼ ½M2  ½M1 V O

ð16Þ

Combining Eqs. (14) and (16):

  VO ½M2 ¼ ½M1 1 þ DM VA

ð17Þ

Similarly, one can obtain:

½M3 ¼ ½M1 1 þ DM

½Mnþ1 ¼ ½M1

 2 ! VO VO þ DM VA VA

 2  n ! VO VO VO 1 þ DM þ DM þ    þ DM VA VA VA

ð18Þ

ð19Þ

EnM  1 EM  1

ð23Þ

The theoretical number of stages required and the metal concentration in relevant streams can be determined from Eq. (22) if the metal concentration in the feed and raffinate are defined and the extraction factor is known. Similar equations can be developed for stripping if there are linear stripping equilibrium and operating lines (constant stripping factor and constant ratio of aqueous to organic flow rate) (Jamrack, 1963). However rare earths are seldom extracted in systems containing single rare earths. The solvent extraction system chemistry is usually controlled to make the distribution coefficient of the desired element(s) much higher than that of the unwanted elements. Even so, some extraction of these undesired elements is usually unavoidable. In these circumstances, as shown in Fig. 4, a scrubbing circuit is commonly employed to wash impurities from the organic before it leaves the extraction circuit. Graphical methods such as the McCabe–Thiele diagram become unmanageable for analyzing several extractable species, each of which has their own equilibrium line, and each of which may displace or be displaced by other species in the organic phase. In this case, a large number of extraction, scrubbing, and stripping stages may be required to obtain desired product. Algebraic equations have been developed from the Kremser and mass balance equations to calculate the theoretical number of stages for a countercurrent circuit for separating two rare earth elements by solvent extraction, assuming constant distribution coefficients for rare earths in the extraction and scrubbing circuits (Xu, 1978). Constant distribution coefficients would not be expected in typical solvent extraction circuits, because the exchange of species between the aqueous and organic phases changes the chemistry of each stream. However, in rare earth circuits where one rare earth is often being exchanged with another (see Eq. (11)), the chemistry may be much more stable so this assumption becomes more realistic. The separation factors for rare earths are frequently fairly constant throughout a given circuit (Voit, 1988). As a result, the distribution coefficients of each lanthanide can be deduced if the distribution coefficient of one has been determined. Typically, separation factors depend only on the type of selected extractant and to a lesser degree on the anionic species in the aqueous phase. Given that the assumption of constant stream volumes incorporated in the Kremser equation is usually valid, the equations derived above can be solved computationally to analyze and simulate rare earth solvent extraction circuits. Voit (1988) developed a simple simulation program for producing 99.5% Nd2O3 from a mixed rare earth chloride feed (La, Pr, and

26

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

Nd) using HEHEHPA for extraction, scrubbing and stripping sections. The Kremser equation was used to calculate the separation occurring in each section of the circuit. Using same equation, Reddy et al. (1992) developed a modified simulation program for an integrated rare earth solvent extraction circuit. Distribution coefficient data for key rare earth elements were tabulated for different initial acidity and metal ion concentrations, based on operating conditions. The use of average separation factors was suggested for non-constant distribution coefficients (Zhong, 2008). Further studies have considered the separation of multi-rare earth elements with two or more outlets/products in a single countercurrent circuit (Ding et al., 2003; Wu et al., 2004a,b). A simulation system has been combined with on-line EDXRF analysis to monitor the steady or dynamic performance of stage-wise processes (Wenli et al., 2000; Jia et al., 2001; Jia et al., 2004). The latter authors claimed that their model was especially useful when there was no more than one intermediate feed point and the distribution ratio of the component involved was constant. The detail of the calculation method followed was not reported.

4. Summary Rare earth elements have unique properties and are becoming essential in many high-technology applications. China is currently the world’s largest producer of rare earth elements providing more than 95% of the world’s total supply. Various rare earth minerals have been identified, of which bastnesite (La, Ce)FCO3, monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4, are the most main minerals commercially targeted. Rare earth minerals are usually concentrated by flotation, magnetic or gravity methods to produce concentrates that undergo hydrometallurgical processing to recover rare earth metals or compounds. Rare earth concentrates or calcine residue are typically leached with an inorganic acid, such as HCl, H2SO4, or HNO3. After solution purification, separation processes based on solvent extraction techniques are used to yield individual rare earths or mixed rare earth solutions or compounds. Rare earth producers usually follow almost identical principles or schemes when selecting solvent extraction circuits to separate rare earths from each other. Usually trivalent rare earths are separated into two or more groups, followed by subsequent separation of individual rare earths and preferential separation of yttrium if possible. The choice of extractants and aqueous solution conditions is influenced both by cost considerations and technical requirements, such as selectivity. The use of cation exchangers, solvation extractants, and anion exchangers, for separating rare earths has been extensively studied. Commercially, D2EHPA, HEHEHP, Versatic 10, TBP, and Aliquat 336 are widely used commercially to separate rare earth elements. Up to hundreds of stages of mixers and settlers may be assembled together to separate all the individual rare earths in a feedstock. Typical configurations for rare earth solvent extraction circuit have been reviewed. Traditional graphical methods for simulating solvent extraction circuits, such as McCabe–Thiele diagrams, have limited practical application for rare earth solvent extraction circuits. However, more promising computational approaches based on the Kremser and mass balance equations have been developed and are continually being developed which better predicts the complex chemical interactions.

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

(‘‘973’’ Program, 2012CBA01205 and Science and Technology Supporting Program, 2012BAE01B02).

References Alex, P., Suri, A.K., Gupta, C.K., 1998. Processing of xenotime concentrate. Hydrometallurgy 50, 331–338. Arichi, J., Gotz-Grandmont, G., Brunette, J.P., 2006. Solvent extraction of europium(III) from nitrate medium with 4-acyl-isoxazol-5-ones and 4-acyl-5hydroxy-pyrazoles. Effect of salts and diluents. Hydrometallurgy 82, 100–109. Banda, R., Jeon, H.S., Lee, M.S., 2012. Solvent extraction separation of La from chloride solution containing Pr and Nd with Cyanex 272. Hydrometallurgy 121– 124, 74–80. Bauer, D.J., 1966. Extraction and separation of selected lanthanides with a tertiary amine. US Bureau of Mines, R.I., 6809. Bauer, D.J., Lindstrom, R.E., 1964. Naphthenic acid solvent extraction of rare earth sulfates, Report RI 6396, Bureau of Mines, U.S. Department of the Interior, Washington. Bautista, R.G., 1995. Separation chemistry. In: Gschneidner, K.A., Jr., Eyring, L. (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 21. North Holland, Amsterdam, pp. 1–28. Benedetto, J.D.S., de l Soares, M.L., Greval, I., Dresinger, D., 1995. Recovery of rare metals with a new organophosphorus extractant. Separation Science and Technology 30 (17), 3339–3349. Benz, M.G., Shei, J.C., Chen, C.H., Walmer, M.S., Doyle, F.M., 2000. High-energyproduct (Pr, Nd, Ce)FeB magnets produced directly from mixed-rare-earthoxide feed for MRI medical imaging applications. In: Kaneko, H., Homma, M., Okada, M. (Eds.), Rare-Earth Magnets and Their Applications, Sendai, Japan, September 2000, vol. 14. The Japan Institute of Metals Proceedings, pp. 99–108 (REMXVI 2000). Brown, C.G., Sherrington, L.G., 1979. Solvent extraction used in industrial separation of rare earths. Journal of Chemical Technology and Biotechnology 29, 193–209. Brown, D., Ma, B.M., Chen, Z., 2002. Developments in the processing and properties of NdFeb-type permanent magnets. Journal of Magnetism and Magnetic Materials 248, 432–440. Cecconie, T., Frieser, H., 1989. Extraction of selected tervalent lanthanides using dicyclohexylphosphinic acid. Solvent Extraction and Ion Exchange 7 (1), 15–29. Cerna, M., Volaufova, E., Rod, V., 1992. Extraction of light rare earth elements by amines at high inorganic nitrate concentration. Hydrometallurgy 28, 339–352. Chen, Z., 2010. Global rare earth resources and scenarios of future rare earth industry. Journal of Rare Earth 29 (1), 1–4. Chi, L., Tian, J., 2007. Review of weathered rare earth ore. Journal of the Chinese Rare Earth Society 25 (6), 641–652 (in Chinese). Chi, R., Zhang, X., Zhu, G., Zhou, Z.A., Wu, Y., Wang, C., Yu, F., 2004. Recovery of rare earth from bastnesite by ammonium chloride roasting with fluorine deactivation. Minerals Engineering 17, 1037–1043. Clark, A.M., 1984. Mineralogy of the rare earth element. In: Henderson, P. (Ed.), Rare Earth Element Geochemistry. Elesevier, pp. 36–38. Commercial Applications for Rare Earth Technologies, Rare Earth Industry and Technology Association 2009, . Desouky, O.A., Daher, A.M., Abdel-Monem, Y.K., Galhoum, A.A., 2009. Liquid–liquid extraction of yttrium using primene-JMT from acidic sulfate solutions. Hydrometallurgy 96, 313–317. Ding, Y., Yu, M., Liu, Z., Chen, L., 2003. Formulae for calculating the number of stages in multi-components rare earth elements cascade extraction. Chinese Rare Earths 24 (3), 8–10 (in Chinese). Doyle, F.M., Benz, M.G., Shei, J.C., Bao, D.S., Ku, H.X., Zhen, N.D., 2000. Direct production of mixed, rare earth oxide feed for high energy-product magnets. In: Bautista, R.G., Mishra, B. (Eds.), Rare Earths and Actinides: Science, Technology and Applications IV. TMS, Warrendale, PA, pp. 31–44. Du Preez, A.C., Preston, J.S., 1992. The solvent extraction of rare earths metals by carboxylic acids. Solvent Extraction and Ion Exchange 10 (2), 207–230. El-Hefny, N.E., El-Nadi, Y.A., Daoud, J.A., 2010. Equilibrium and mechanism of samarium extraction from chloride medium using sodium salt of CYANEX 272. Separation and Purification technoLogy 75, 310–315. El-Yamani, I.S., Shabana, E.L., 1985. Solvent extraction of lanthanum(III) from sulfuric acid solutions by Primene JMT. Journal of the Less-Common Metals 105, 255–261. Fauchera, M.D., Morlottib, R., Moune, O.K., 2002. The effects of added foreign ions in Gd2O2S: Tb3+; crystal field calculations, lifetimes, photo-luminescence and absorption spectra. Journal of Luminescence 96, 37–49. Ferraro, J.R., Peppard, D.F., 1963. Nuclear Science and Engineering 16, 389. Fontana, D., Pietrilli, L., 2009. Separation of middle rare earth by solvent extraction using 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester as an extractant. Journal of Rare Earths 27 (5), 830–833. Gaudernack, B., 1973. Process for separating yttrium values from the lanthanides, US patent, 3751553. Gupta, C.K., Krishnamurthy, N., 2005. Extractive Metallurgy of Rare Earths. CRC Press. Hsu, K.H., Huang, C.H., King, T.C., Li, P.K., 1980. Separation of praseodymium and neodymium in high purity (99.9%) by counter-current exchange extraction and its mechanism. Proceedings of International Solvent Extraction Conference’80 (ISEC’80), Liege, Belgium, 6–12 September, vol. 2. Society of Chemical Industry, London, pp. 80–82.

F. Xie et al. / Minerals Engineering 56 (2014) 10–28 Hu, K.Q., 1998. New process for decomposing Baotou rare earth concentrate with acid, China patent, ZL98118153 (in Chinese). Hu, Z.H., Lian, F.Z, Zhu, M.G., Li, W, 2008. Effect of Tb on the intrinsic coercivity and impact toughness of sintered Nd–Dy–Fe–B magnets. Journal of Magnetism and Magnetic Materials 320, 1735–1738. Huang, C.H., Jin, T.Z., Li, B.G., Li, J.R., Xu, G.X., 1986. Studies on extraction mechanism of the rare earths with quaternary ammonium salts. In: Proceedings of Int. Solvent Extr. Conf., ISEC ‘86. Dechema, Frankfurt, pp. 215–221. Huang, X., Long, Z., Li, H., Ying, W., Zhang, G., Xue, X., 2005. Development of rare earth hydrometallurgy in China. Journal of Rare Earth 23 (1), 1–4. Huang, X., Li, H., Xue, Xi., Zhang, G., 2006. Development status and research progress in rare earth hydrometallurgy in China. Journal of The Chinese Rare Earth Society 24 (2), 129–133 (in Chinese). Huang, X.W., Li, H.W., Long, Z.Q., Peng, X.L., Li, J.N., Zhang, Y.Q., Cui, D.L., Yang, G.L., 2008. Research and development on green rare earth metallurgy. SiChuan Rare Earth 4, 15–19. Hudson, M.J., 1982. An introduction to some aspects of solvent extraction chemistry in hydrometallurgy. Hydrometallurgy 9, 149–168. Jamrack, W.D., 1963. Rare Earth Extraction by Chemical Engineering Techniques. Pergamon press, London, UK. Jia, J., Yan, C., Liao, C., Wu, S., Wang, M., Li, B., 2001. Automation system in rare earth countercurrent process. Journal of Rare Earths 19 (1), 6–10. Jia, J., Wu, S., Liao, C., Yan, C., 2004. Research and application progress in countercurrent solvent extraction. Journal of Rare Earths 22 (5), 576–581. Jia, Q., Tong, S., Li, Z., Zhou, W., Li, H., Meng, S., 2009. Solvent extraction of rare earth elements with mixtures of sec-octylphenoxy acetic acid and bis(2,4,4trimethylpentyl) dithiophosphinic acid. Separation and Purification Technology 64, 345–350. Jones, A.P., Wall, F., Williams, C.T., 1996. Rare Earth Minerals: Chemistry, Origin and Ore Deposits. Chapman & Hall, London. Jordens, A., Cheng, Y.P., Waters, K.E., 2013. A review of the beneficiation of rare earth element bearing minerals. Minerals Engineering 41, 97–114. Karsu, E.C., Popovici, E.J., Ege, A., Morar, M., Indrea, E., Karali, T., Can, N., 2011. Luminescence study of some yttrium tantalate-based phosphors. Journal of Luminescence 131, 1052–1057. Klinkenberg, A., 1951. Calculation of the efficiency of counter-current stage-wise mass transfer processes with constant distribution factor, when in the stationary state: I. Distribution of one component only. Chemical Engineering Science 1 (2), 86–93. Kolarik, Z., Pankova, H., 1966. Acidic organophosphorous extractants 1 – extraction of lanthanides by means of dialkyl phosphoric acids-effect of structure and size of alkyl group. Journal of Inorganic and Nuclear Chemistry 28, 2325–2333. Korpusov, A.V., Danilov, N.A., Krylov, Yu. S., Korpusova, R.D., Drygin, A.I., Shvartsman, V.Y., 1974. Investigation of rare earth elements extraction with different carboxylic acids, In: Proceedings of International Solvent Extraction Conference (ISEC 74), vol. II, Lyon, pp. 1109–1120 . Kremser, A., 1930. Theoretical analysis of absorption process. National Petroleum News 22 (21), 48. Kruesi, P.R., Schiff, N.N., 1968. Molybdenum Corporation of America’s Mountain Pass, Europium Process, AIME Annul Meeting. Kumar, M., 1994. Recent trends in chromatographic procedures for separation and determination of rare earth elements: a review. Analyst 119, 2013–2024. Li, D.Q., 1995. Some chemical problems in the hydrometallurgical industries of Rare Earth. Progress in Chemistry 7 (3), 209–213 (in Chinese). Li, K., Frieser, H., 1986. Extraction of Lanthanide metals with bis(2,4,4trimethylpentyl) phosphinic acid. Solvent Extraction and Ion Exchange 4, 739–755. Li, D.Q., Wang, Z.H., Meng, S.L., 1994. Recommended separation processes for ionadsorbed rare earth minerals. Hydrometallurgy’94. Chapman Hall, London, UK, pp. 627–634. Li, G., Hu, K., Liu, J., Wang, G., Xu, Q., Wang, W., Guo, L., Yu, J., Hu, T., Chen, P., Zeng, T., Xiao, R., 2004. A novel roasting method for decomposing mixed rare earth concentrate, China patent, 200410006443.8 (in Chinese). Li, J., Huang, X., Zhu, Z., Long, Z., Peng, X., Cui, D., 2007a. Extracting rare earth from D2EHPA-HEHEHP-H2SO4 system. Journal of The Chinese Rare Earth Society 25 (1), 55–58. Li, W., Wang, X., Meng, S., Li, D., Xiong, Y., 2007b. Extraction and separation of yttrium from the rare earths with sec-octylphenoxy acetic acid in chloride media. Hydrometallurgy 54, 164–169. Li, W., Wang, X., Zhang, H., Meng, S., Li, D., 2007c. Solvent extraction of lanthanides and yttrium from nitrate medium with CYANEX 925 in heptane. Journal of Chemical Technology and Biotechnology 82, 376–381. Liu, Y.J., 2008. Present State and main task of development of rare earth industry in China. Journal of The Chinese Rare Earth Society 25 (3), 257–262 (in Chinese). Liu, G., Chen, X., 2007. Spectroscopic properties of lanthanides in nanomaterials. In: Gschneidner, K.A., Bünzli, Jr., J. C.G., Pecharsky, V.K. (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol 37, pp. 99–169. Liu, J., Vora, P., Walmer, M., 2006. Overview of Recent Progress in Sm-Co Based Magnets. In: Proceedings of 19th International Workshop on Rare Earth Permanent Magnets & Their Applications, Journal of Iron and Steel Research 13 (Supplement) 319–323 . Lu, D., Horing, J.S., HOH, Y.C., 1989. The separation of neodymium by quaternary amine form didymium nitrate solution. Journal of the Less-common Metals 149, 219–224. Lu, J., Wei, Z., Li, D., Ma, G., Jiang, Z., 1998. Recovery of Ce (IV) and Th (IV) from rare earths (III) with Cyanex 923. Hydrometallurgy 50, 77–87.

27

Mason, G.W., Bilobron, I., Peppard, D.F., 1978. Extraction of U(VI), Th, (IV), Am (III) and Eu(III) by bis para-octyl phosphoric acid in benzene diluent. Journal of Inorganic and Nuclear Chemistry 40, 1807–1810. McCabe, W.L., Smith, J.C., Harriot, P., 2005. Unit Operations of Chemical Engineering, seventh ed. McGraw-Hill. McGill, I., 1997. Rare Earth Metals. In: Habashi, F. (Ed.), Handbook of Extractive metallurgy, vol. 3. Wiley-VCH, Weinheim, Germany, pp. 1695–1741. Nair, S.G.K., Smutz, M., 1967. Recovery of lanthanum from didymium chloride with di(2-ethylhexyl) phosphoric acids solvent. Journal of Inorganic and Nuclear Chemistry 29, 1787–1797. Nazarov, M., Noh, D.Y., 2010. Rare earth double activated phosphors for different applications. Journal of Rare Earths 28, 1–11. Peppard, D.F., Wason, G.W., 1961. Liquid-liquid extraction of trivalent rare earths using acidic phosphonates as extractants. In: Kleber, E.V. (Ed.), Rare Earth Research. The Macmillan Company, New York, pp. 37–50. Peppard, D.F., Farris, J.P., Gray, P.R., Mason, G.W., 1953. Study of the solvent extraction of the transition elements. I., Order and degree of fractionation of the trivalent rare earth. Journal of Physical Chemistry 57, 294–301. Peppard, D.F., Driscoll, W.J., Siromen, S.J., Mason, G.W., 1957a. Fractional extraction of the lanthanides as their di-alkyl orthophosphates. Journal of Inorganic and Nuclear Chemistry 4, 334–343. Peppard, D.F., Driscoll, W.J., Sironen, R.J., McCarty, S., 1957b. Nonmonotonic ordering of lanthanides in tributyl phosphate-nitric acid extraction system. Journal of Inorganic & Nuclear Chemistry 4, 326–333. Peppard, D.F., Mason, G.W., Driscoll, W.J., Sironen, R.J., 1958. Acidic esters of organophosphorous acid as selective extractants for metallic cations-tracer studies. Journal of Inorganic and Nuclear Chemistry 7, 276–285. Peppard, D.F., Mason, G.W., Andejasich, C., 1966. Two mono-octyl phosphinic acids, (R)(H)PO(OH), as extractants for metallic cations selected M(II), M(III) and M(VI)-Tracer studies. Journal of Inorganic and Nuclear Chemistry 28, 2347– 2359. Piece, T.B., Peck, P.F., 1963. The extraction of the lanthanide elements from perchloric acid by di-(2ethylhexyl) hydrogen phosphate. Analyst, 217–221. Preston, J.S., 1994. Solvent extraction of the trivalent lanthanides and yttrium by some 2-bromoalkanoic acids. Solvent Extraction and Ion Exchange 12 (1), 29– 54. Preston, J.S., 1996. The recovery of rare earth oxides from a phosphoric acid byproduct, Part 4: The preparation of magnet-grade neodymium oxide from the light rare earth fraction. Hydrometallurgy 42, 151–167. Preston, J.S., Du Preez, A.C., 1990. Solvent extraction processes for the separation of rare earths metals, In: Proceedings of International Solvent Extraction Conference (ISEC 90), Japan, vol. 1, p. 883. Preston, J.S., du Preez, A.C., 1994. Solvent extraction of the trivalent lanthanides and yttrium bv mixtures of 3,5-diisopropylsalicylic acid and neutral organophosphorous compounds. Journal of Chemical Technology and Biotechnology 60, 317–325. Preston, J.S., du Preez, A.C., 1995. Solvent extraction of neodymium(III) and erbium (III) bv mixtures of 3,5-diisopropylsalicylic acid and neutral organophosphorous compounds. Solvent Extraction and Ion Exchange 13 (1), 27–41. Preston, J.S., du Preez, A.C., 1996. The separation of europium from a middle rare earth concentrate by combined chemical reduction, precipitation and solventextraction methods. Journal of Chemical Technology and Biotechnology 65, 93– 101. Preston, J.S., du Preez, A.C., 1998. Synergistic effects in solvent extraction systems based on alkylsalicylic acids. IV. Extraction of the trivalent rare-earth metals in the presence of bifunctional C@O, P@O, and S@O compounds. Solvent Extraction and Ion Exchange 16, 687–706. Preston, J.S., Cole, P.M., Craig, W.M., Feather, A.M., 1996a. The recovery of rare earth oxides from a phosphoric acid by-product. Part I: Leaching of rare earth values and recovery of a mixed rare earth oxide by solvent extraction. Hydrometallurgy 41, 1–19. Preston, J.S., Cole du Preez, P.M., Fox, A.C.M.H., Fleming, A.M., 1996b. The recovery of rare earth oxides from a phosphoric acid by-product. Part II: The preparation of high-purity cerium dioxide and recovery of a heavy rare earth oxide concentrate. Hydrometallurgy 41, 21–44. Preston, J.S., du Preez, A.C., Cole, P.M., Fox, M.H., 1996c. The recovery of rare earth oxides from a phosphoric acid by-product. Part 3. The separation of the middle and light rare earth fractions and the preparation of pure europium oxide, Hydrometallurgy, 42, 131–149. Qiu, T., Fang, X., Cui, L., Fang, Y., 2008. Behavior of leaching and precipitation of weathering crust ion-absorbed type by magnetic field. Journal of Rare Earth 26 (2), 274–278. Rabie, K.A., 2007. A group separation and purification of Sm, Eu and Gd from Egyptian beach monazite mineral using solvent extraction. Hydrometallurgy 85, 81–86. Rapaport, A., Miliez, J., 2006. Review of the properties of up-conversion phosphors for new emissive displays. Journal of Display Technology 2 (1), 68–78. Reddy, M.L.P., Ramamohan, T.R., Bhat, C.S., Santhi, P.B., Damodaran, A.D., 1992. Mathematical modeling of the liquid-liquid extraction separation of Rare Earths. Mineral Processing and Extractive Metallurgy Review 9, 273–282. Reddy, M.L.P., Rao, P., Damodarm, A.D., 1995. Liquid–liquid extraction processes for the separation and purification of rare earths. Mineral Processing and Extractive Metallurgy Review 12, 91–113. Reddy, M.L.P., Bharathi, J.R.B., Peter, S., Ramamohan, T.R., 1999. Synergistic extraction of rare earths with bis(2,4,4-trimethylpentyl) dithiophosphinic acid and trialkylphosphine oxide. Talanta 50, 79–85.

28

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

Reddy, B.R., Kumar, B.N., Radhika, S., 2009. Solid–liquid extraction of terbium from phosphoric acid medium using bifunctional phosphinic acid resin, tulsion, CH96. Solvent Extraction and Ion Exchange 27 (5–6), 695–711. Rice, A.C., Stone, C.A., 1962. Amines in liquid–liquid extraction of rare earth elements. US Bureau of Mines R.I., 5923. Ritcey, G.M., Ashbrook, A.W., 1984. Solvent Extraction; Principles and Applications to Process Metallurgy, Part I. Elsevier, Amsterdam. Ronda, C.R., Jüstel, T., Nikol, H., 1988. Rare earth phosphors: fundamentals and applications. Journal of Alloys and Compounds 275–277, 669–676. Rydberg, J., Cox, M., Musikas, C., Choppin, G.R., 2004. Solvent Extraction Principles and Practice. Marcel Dekker Inc., New York. Sabbot, J. L., Rollat, A., 1985. Process for separating rare earths by liquid-liquid extraction, EU Patent, EP0156735. Saleh, M.I., Bari, M.F., Saad, B., 2002. Solvent extraction of lanthanum(III) from acidic nitrate-acetato medium by Cyanex 272 in toluene. Hydrometallurgy 63, 75–84. Santhi, P.B., Reddy, M.L.P., Ramamohan, T.R., Damodaran, A.D., 1991. Liquid–liquid extraction of yttrium(III) with mixtures of organophosphorous extractants: theoretical analysis of extraction behavior. Hydrometallurgy 27, 169–177. Sato, T., 1989. Liquid–liquid extraction of rare-earth elements from aqueous acid solutions by acid organophosphorous compounds. Hydrometallurgy 22, 121– 140. Sebenik, R.F., Sharp, B.M., Smutz, M., 1966. Computer solution of solvent-extractioncascade calculations for the Mmonazite rare-earth nitrates-nitric acid-tributyl phosphate water system. Separation Science and Technology 1, 375–386. Sharp, M., Smutz, M., 1965. Stagewise calculation for the solvent extraction system monazite rare earth nitrate nitric acid–tributyl phosphate–water. Industrial & Engineering Chemistry Process Design and Development 4, 49–54. Sherrington, L.G., 1966. Kemp, Liquid extraction process for separation yttrium values from rare earths vales, British patent 1026791. Sherrington, L., 1983. Commercial process for rare earths and thorium. In: Handbook of Solvent Extraction. Willey Interscience, New York, p. 17. Singh, D.K., Sing, H., Mathur, J.N., 2006. Extraction of rare earths and yttrium with high molecular weight carboxylic acids. Hydrometallurgy 81, 174–181. Song, H.F., Hong, M., 2010a. The analysis and prediction of rare earth industry in China (I), Rare Earth, Information, No.2, 6–8 (in Chinese). Song, H.F., Hong, M., 2010b. The analysis and prediction of rare earth industry in China (II), Rare Earth, Information, No.2, 8–12 (in Chinese). Sun, X., Wang, J., Li, D., Li, H., 2006. Synergistic extraction of rare earths by mixture of bis(2,4,4 trimethylpentyl)phosphinic acid and sec-nonylphenoxy acetic acid. Separation and Purification Technology 50, 30–34. Taniguchi, V.T., Doty, A.W., 1989. Large scale chromatographic separation using ‘continuous displacement chromatography’’ (CDC). In: Bautista, R.G., Wong, M.M. (Eds.), Proceedings of Rare earths: Extraction, Preparation and Application. TMS, pp. 147–161. Thiele, E.W., Geddes, R.G., 1933. Computation of distillation apparatus for hydrocarbon mixtures. Industry and Engineering Chemistry 25 (3), 289–295. Tian, M., Song, N., Wang, D., Quan, X., Jia, Q., Liao, W., Lin, L., 2012. Applications of the binary mixture of sec-octylphenoxyacetic acid and 8-hydroxyquinoline to the extraction of rare earth elements. Hydrometallurgy 111–112, 109–113. Tian, M., Jia, Q., Liao, W., 2013. Studies on synergistic solvent extraction of rare earth elements from nitrate medium by mixtures of 8-hydroxyquinoline with Cyanex 301 or Cyanex 302. Journal of Rare Earths 31 (6), 604–608. Tong, S., Zhao, X., Song, N., Jia, Q., Zhou, W., Liao, W., 2009. Solvent extraction study of rare earth elements from chloride medium by mixtures of sec-nonylphenoxy acetic acid with Cyanex301 or Cyanex302. Hydrometallurgy 100, 15–19. Tong, H., Wang, Y., Liao, W., Li, D., 2013. Synergistic extraction of Ce(IV) and Th(IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media. Separation and Purification Technology 118 (30), 487–491. Tse, P.K., 2011. China’s rare earth industry, Open-file report 2011-1042, U.S. Department of the Interior/U.S. Geological Survey. Tu, D., Liang, Y., Liu, R., Li, D., 2011. Eu/Tb ionsco-doped white light luminescence Y2O3 phosphors. Journal of Luminescence 131, 2569–2573. Mineral Facts and Problems, 1985, U. S., Bureau of Mines. U.S. Geological Survey, Mineral Commodity Summaries 2011, U.S. Department of Interior, January 2011. Urbanski, T.S., Fonari, P., Abbruzzes, C., 1996. The extraction of cerium(III) and lanthanum(III) from chloride solutions with LIX 54. Hydrometallurgy 40, 169– 179. Voit, D.O., 1988. Computer Simulation of Rare Earth Solvent Extraction Circuits, Rare Earth Extraction, Preparation and Application. TMS, Warrendale, US, 213.

Wang, G.Z., 2009. Current status of rare earths resources development and strategy in China. Si Chuan Rare Earth 3, 4–7 (in Chinese). Wang, X., Liu, J., 1996. Study on the decomposition mechanism of roasting of the Baotou mixed rare earth concentrates. Rare Earth 21 (6), 6–10 (in Chinese). Wang, Y.C., Yue, S.T., Li, D.Q., Jin, M.J., Li, C.Z., 2002. Solvent extraction of scandium (III), yttrium (III), lanthanides (III) and divalent metal ion with secnonylphenoxy acetic acid. Solvent Extraction and Ion Exchange 20 (6), 701–706. Wang, X., Li, W., Meng, S., Li, D., 2006. The extraction of rare earths using mixtures of acidic phosphorus based reagents or their thio-analogues. Journal of Chemical Technology and Biotechnology 81, 761–766. Wang, W., Pranolo, Y., Cheng, C.Y., 2011. Metallurgical processes for scandium recovery from various resources: a review. Hydrometallurgy 108, 100–108. Wenli, L., Ascenzo, G.D., Curini, R., Chunhua, Y., Jianfang, W., Tao, J., Mingwen, W., 2000. Simulation of the development automatization control system for rare earth extraction process, combination of ESRECE simulation software and EDXRF analysis technique. Analytica Chimica Acta 417, 111–118. Wu, W., Sun, S., Tu, G., 2002a. Reaction mechanism of roasting of monazite with CaO. The Academic Journal of Northeastern University (Natural Science Version) 12, 1158–1160 (in Chinese). Wu, W., Sun, S., Yu, Q., 2002b. Study on the decomposition mechanism of roasting of mixed rare earth concentrates. Rare Metals 21 (1), 76–80 (in Chinese). Wu, S., Liao, C., Jia, J., Yan, C., 2004a. Static design for multi components and multi outlets rare earth countercurrent extraction, (I), algorithm of static design. Journal of the Chinese Rare Earth Society 22 (1), 17–21. Wu, W., Hu, G., Sun, S., Chen, X., Tu, G., 2004b. Decomposition of mixed rare earth concentrates and roasted with CaO and NaOH. Journal of the Chinese Rare Earth Society 22 (2), 210–214 (in Chinese). Xu, G.X., 1978. Theory of countercurrent solvent extraction 1, optimization functions and their application. Acta Scientiarum Naturalium Universitatis Pekinensis 16 (1), 51–66 (in Chinese). Xu, Y.H., Ma. P.Q., Zhang, X., Wu, G.Q., 2003. The use of long chain fatty acids for solvent extraction of rare earths, China Patent, CN1455009 (in Chinese). Xu, F., Zhang, L., Dong, X., Qiongzhen Liu, Q., Komuro, M., 2011. Effect of DyF3 additions on the coercivity and grain boundary structure in sintered Nd–Fe–B magnets. Scripta Materialia 64, 1137–1140. Xu, Y., Liu, H., Meng, Z., Cui, J., Zhao, W., Li, L., 2012. Decomposition of bastnasite and monazite mixed rare earth minerals calcined by alkali liquid. Journal of Rare Metals 30 (2), 155–158. Yang, R., Wang, W., Zhang, X., Liu, L., Wei, H., Bao, M., Wang, J., 2008. A new type of rare earth elements deposit in weathering crust of Permian basalt in western Guizhou. NW China, Journal of Rare Earth 26 (5), 753–759. Ye, S., Xiao, F., Pan, Y.X., Ma, Y.Y., Zhang, Q.Y., 2011. Phosphors in phosphorconverted white light-emitting diodes: Recent advances in materials, techniques and properties. Materials Science and Engineering R 71, 1–34. Ying, X., Wang, X., Li, D., 2005. Synergistic extraction and separation of heavy lanthanide by mixtures of bis(2,4,4-trimethylpentyl)phosphinic acid and 2ethylhexyl phosphinic acid mono-2-ethylhexyl ester. Separation Science and Technology 40 (11), 2325–2336. Zhang, B.Z., Lu, K.Y., King, K.C., Wei, W.C., Wang, W.C., 1982. Rare earth industry in China. Hydrometallurgy 9, 205–210. Zhang, Y., Li, J., Huang, X., Wang, C., Long, Z., Zhu, Z., Zhang, G., 2007. Synergistic extraction of rare earths by mixtures of di(2-ethylhexyl)phosphoric acid and (HDEHP) and 2-ethylhexyl phosphoric acid mono-2-ethylhexyl ester (HEH/ EHP) in sulfuric acid media. Journal of The Chinese Rare Earth Society 26 (6), 671–676 (in Chinese). Zhang, Z., Li, H., Guo, F., Meng, S., Li, D., 2008. Synergistic extraction and recovery of Cerium(IV) and Fluorin from sulfuric solutions with Cyanex 923 and di-2ethylhexyl phosphoric acid. Separation and Purification Technology 63, 348– 352. Zheng, D., Gray, N.B., Stevens, D.W., 1991. Comparison of naphthenic acid, Versatic acid, and D2EHPA for the separation of rare earths. Solvent extraction and ion exchange 9 (1), 85–102. Zhong, X., 2008. Theoretical analysis of effective separation factor for multicomponent rare earths in countercurrent extraction. The Chinese Journal of Nonferrous Metals 18 (10), 1926–1929. Zhu, T., 1991. Solvent extraction in China. Hydrometallurgy 27 (2), 231–245. Zuo, H., Wang, Y., Jiang, H., Cheng, X., 2007. Seepage properties of leaching solution in ion-adsorbed rare earth deposit under effect of electric field. Journal of the Chinese Rare Earth Society 25 (1), 80–84 (in Chinese).