**9. Separation using chelating ion exchangers**

Contrary to the cation and anion exchangers chelating ion exchangers have varying degrees of affinity and selectivity with respect to rare earth elements(III). Their properties depend mainly on the nature of the functional group and less on the beads size and other physicochemical properties. The sorption selectivity particularly affects the relative position of functional groups, their spatial configuration, etc. while the less important are the properties of the matrix. Ion exchange capacity of ion exchange resins depends on the content of these groups and the pH of the solution. A negative feature is their low rate of sorption (Kunin & Gustafson, 1969).

120 Ion Exchange Technologies

skeleton was determined. Among the tested anion exchangers Dowex 2x8 and Dowex 1x8 in the form of EDTA proved to be most advantageous. Good results were also obtained for the

Studies on the separation of anionic complexes of rare earth elements(III) with CDTA [Ln(cdta)]- type, which is an analogue of EDTA showed that in the macro-micro system Y(III) from Sm(III), Eu(III) and Nd(III) can be separated (Hubicka, 1989a). It was found that the process of separation of these complexes affects the degree of cross linking and the anion exchanger form. As follows the best results of separation of Y(III) from Nd(III) and Sm(III)

Application of anion exchangers for separation of rare earth elements(III) complexes with EDTA and CDTA by the frontal analysis technique allows, in comparison with cation exchangers and elution process, to reduce the consumption of these chelating agents, obtaining higher concentrations of rare earth elements(III) in the eluate and shortening the process time which is important from the economical point of view. Additionally, alkali and alkaline earth metal ions forming unstable complexes with EDTA, and sometimes

Kutun and Akseli (1999, 2000) for the separation of milligram quantities (5 mg) of rare earth elements(III) in the anion exchange process used the solution of sodium trimethaphosphate as the eluent. The elution was carried out with a gradient of 0.007-0.01 M concentration on

The advantage of anion exchangers over cation exchangers in the separation of rare earth elements(III) using aminopolycarboxylic acids as complexing agents is associated with their lower consumption in comparison to other eluents used, much faster process time, achieving higher concentrations in the eluate and the lack of negative impact of alkali(I),

A particular attention has been paid to separation and removal of rare earth(III) elements nitrate complexes by means of frontal analysis from the polar organic solvent-H2O-HNO3 on anion exchangers of various types. The addition of organic solvent to an aqueous solution of rare earth complexes generally improves their ability of separation. Selection of an organic component and its concentration in the mixture is to a large extent arbitrary. The separation process is frequently carried out with the HNO3, H2SO4, NH4SCN and CH3COOH solutions.

Contrary to the cation and anion exchangers chelating ion exchangers have varying degrees of affinity and selectivity with respect to rare earth elements(III). Their properties depend mainly on the nature of the functional group and less on the beads size and other physicochemical properties. The sorption selectivity particularly affects the relative position of functional groups, their spatial configuration, etc. while the less important are the properties of the matrix. Ion exchange capacity of ion exchange resins depends on the

macroporous, weakly basic anion exchanger Lewatit MP-7080 in the EDTA form.

accompanying rare earth elements(III) have no effect on the separation result.

the strongly basic polystyrene anion exchangers of types 1 and 2.

The examples of such systems are presented in (Marcus, 1983).

**9. Separation using chelating ion exchangers** 

were obtained on Dowex 1x4 in the acetate form.

Ca(II) and Mg(II) ions on separation.

For the separation of rare earth elements(III) and their purification from uranium(IV, VI), thorium(IV), iron(III), chromium(III), copper(II), nickel(II), cobalt(II), manganese(II) the chelating ion exchangers with the following functional groups: phosphinic -PO(OH), phosphonic-PO(OH)2, phosphate -OPO(OH)2, iminodiacetate -CH2N(CH2COOH)2, iminodiacetate and aminoacetate > N-CH2COOH, -aminophosphonic -CH2NHCH2PO(OH)2 carboxylic -COOH, etc are used.

It is worth mentioning that the ion exchangers with the phosphonic functional groups in the medium acidic system effectively absorb the rare earth elements(III), uranium(VI) and iron(III). According to Hubicki (1989), uranium(VI) and thorium(IV) can be selectively separated from rare earth elements(III) on the phosphonic ion exchanger Duolite ES-63 by both the frontal analysis technique and elution with mineral acids. It was found that on this ion exchanger in the macro-micro component system Y(III) from Lu(III), Yb(III), Tm(III), Er(III), Dy(III) and Ho (III); La(III) from Pr(III) and Nd(III); Ce(III) from Pr(III), Nd(III), Sm(III), Eu(III), Y(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Lu(III) can be separated (Hubicka & Hubicki, 1978a,b). For Duolite ES-63 the affinity series of rare earth elements(III) can be as follows: Sc(III) > Lu(III) > Yb(III) > Tm(III) > Er(III) > Ho (III) > Dy(III) > Tb(III) > Gd(III) > Eu(III) > Sm(III) > Nd(III) > Pr(III) > Ce(III) La (III).

Very interesting are the results of the studies on selective separation of micro quantities of scandium(III) on Duolite ES-63 from other rare earth elements(III) at the maximum concentration of 500 g Ln2O3/dm3 (Hubicki, 1990). This ion exchanger is particularly useful for separation of micro quantity of scandium(III) and the preparation of scandium(III) with a high degree of purity. In the technology of processing of rare earth elements(III) concentrates also phosphonic ion exchanger KFP of Russia production was used for the selective removal of micro quantities of Th(IV) (Sozański, 1981).

The selectivity of phosphonic ion exchangers can be increased by connecting different substituents to the phosphorus atom, which affects its electro-donor strength. In the case of the alkylphosphonic ion exchangers affinity for rare earth elements(III) increases with the increasing atomic number of the element, which can be used for the separation of individual rare earth elements(III) by the elution using mineral acid solutions. Separation coefficients of the heavy lanthanides (Lu(III), Yb(III), Tm(III), Er(III)) for alkylphosphonic ion exchangers are from 1.3 to 1.4. The ion exchangers with the phosphine groups are characterized by stronger affinity for lanthanide(III) than the phosphonic ion exchangers. The percentage of the complexation of europium(III) and ytterbium(III) in 0.5 M HCl using the phosphonic ion exchanger is 70% and 82%, while for the ion exchanger with the phosphonic groups is equal to 25% and 50%, respectively.

Marhol (1982) which synthesized a number of ion exchange resins with the functional groups containing phosphorus found that the phosphinic ion exchangers are characterized by strong affinity for scandium(III) in agreement with the affinity series: Sc (III)> Fe (III)> In (III)> Ga (III)> Al(III) > La(III).

Szczepaniak and Siepak (1973) as a result of the cross linking reaction of (5% DVB) vinylbenzylamine with formaldehyde and phosphoric acid(III) in the hydrochloric acid medium obtained the ion exchanger containing aminomethylphosphonic groups. The obtained K-AMF ion exchanger can be defined as a phosphonic analogue of the iminodiacetic ion exchanger Dowex A-1 (Chelex 100). It can be recommended for separation of cations on the +2 and +3 oxidation states. Based on the volume distribution coefficients the affinity series of selected rare earth elements(III) for this aminophosphonic ion exchanger is as follows: La (III) > Gd (III) > Sm (III) > Nd (III) > Dy(III) > Pr(III) > Y(III). This series is not consistent with that obtained for the commercial ion exchange resins such as Duolite ES-467 and Lewatit OC-1060. These ion exchangers are used for the purification of micro quantities of lanthanum(III) from praseodymium(III) and neodymium(III) by means of both the frontal analysis technique and elution with mineral acids.

The aminophosphonic ion exchangers were used for selective purification of rare earth elements(III) from U(VI), Th(IV), Fe(III), Zn(II), Co(II) and Ni(II); purification of yttrium (III) from micro quantities of ytterbium(III), erbium(III), dysprosium(III) and holmium(III) as well as cerium(III) from rare earth elements(III). Of special interest are the values of separation coefficients determined by Van der Walt and Coetzee (1996) for the commercial ion exchanger Purolite S-950, which shows that rare earth elements(III) can be selectively separated from a number of other elements.

As for the phosphate ion exchangers cellulose phosphate is commercially produced. For cellulose phosphate, the mass distribution coefficients were determined for Cs(I), Li(I), Ba(II), Ca(II), Sr(II), Co(II), Ni(II), La(III), Pr(III), Yb(III), Cr(III) and Al(III) ions depending on the concentration of hydrochloric acid. There was made quantitative separation of rare earth elements(III) and earth metals(II); rare earth elements(III) and earth metals(II) and aluminum(III) as well as rare earth elements(III) and alkali and alkaline earth metals. High selectivity can be found in the case of cellulose phosphate ion exchangers. Passing the solution obtained from decomposition of monazite through such ion exchanger a relatively selective sorption of thorium(IV) is achieved with the capacity of the ion exchanger of about 300 g/kg with respect to Th(IV). In the four-column battery (bed height 340 cm, flow rate 15 cm3/min) of which two columns worked in the sorption cycle and two were eluted, the recovery of thorium(IV) exceeds 99% (Brown and Coleman, 1956). The recovery of pure mixed rare earth elements as well as Th(IV) and U(VI) from the monazite concentrate is based on the reaction with sulphuric acid. Then, after adding hydrazine sulphate, Ce(IV) is reduced to Ce(III). This treatment is also based on the fact that Th(IV) ion with sulphate(VI) ions forms anionic [(ThSO4)4]4-2n, that could pass freely through a cation exchange resin while the rare earth elements are completely adsorbed (Sherief & Almasy, 1968). Also the uranyl ionUO22+ forms anionic complexes of the type [UO2(SO4)n]2-2n where n=1, 2 or 3 (Preuss & Kunin, 1958).

The polyfunctional ion exchanger Diphonix Resin is commonly used for separation of rare earth elements(III) (Fig. 9). It contains phosphonic, carboxylic and sulphonic groups. Another ion exchanger is Diphonix A with the phosphonic and ammonium or pyridine functional groups (Horwitz et al., 1993).

**Figure 9.** The structure of DiphonixResin, Dipxonix A (type 1) and Diphonix A (type 2).

Szczepaniak and Siepak (1973) as a result of the cross linking reaction of (5% DVB) vinylbenzylamine with formaldehyde and phosphoric acid(III) in the hydrochloric acid medium obtained the ion exchanger containing aminomethylphosphonic groups. The obtained K-AMF ion exchanger can be defined as a phosphonic analogue of the iminodiacetic ion exchanger Dowex A-1 (Chelex 100). It can be recommended for separation of cations on the +2 and +3 oxidation states. Based on the volume distribution coefficients the affinity series of selected rare earth elements(III) for this aminophosphonic ion exchanger is as follows: La (III) > Gd (III) > Sm (III) > Nd (III) > Dy(III) > Pr(III) > Y(III). This series is not consistent with that obtained for the commercial ion exchange resins such as Duolite ES-467 and Lewatit OC-1060. These ion exchangers are used for the purification of micro quantities of lanthanum(III) from praseodymium(III) and neodymium(III) by means

The aminophosphonic ion exchangers were used for selective purification of rare earth elements(III) from U(VI), Th(IV), Fe(III), Zn(II), Co(II) and Ni(II); purification of yttrium (III) from micro quantities of ytterbium(III), erbium(III), dysprosium(III) and holmium(III) as well as cerium(III) from rare earth elements(III). Of special interest are the values of separation coefficients determined by Van der Walt and Coetzee (1996) for the commercial ion exchanger Purolite S-950, which shows that rare earth elements(III) can be selectively

As for the phosphate ion exchangers cellulose phosphate is commercially produced. For cellulose phosphate, the mass distribution coefficients were determined for Cs(I), Li(I), Ba(II), Ca(II), Sr(II), Co(II), Ni(II), La(III), Pr(III), Yb(III), Cr(III) and Al(III) ions depending on the concentration of hydrochloric acid. There was made quantitative separation of rare earth elements(III) and earth metals(II); rare earth elements(III) and earth metals(II) and aluminum(III) as well as rare earth elements(III) and alkali and alkaline earth metals. High selectivity can be found in the case of cellulose phosphate ion exchangers. Passing the solution obtained from decomposition of monazite through such ion exchanger a relatively selective sorption of thorium(IV) is achieved with the capacity of the ion exchanger of about 300 g/kg with respect to Th(IV). In the four-column battery (bed height 340 cm, flow rate 15 cm3/min) of which two columns worked in the sorption cycle and two were eluted, the recovery of thorium(IV) exceeds 99% (Brown and Coleman, 1956). The recovery of pure mixed rare earth elements as well as Th(IV) and U(VI) from the monazite concentrate is based on the reaction with sulphuric acid. Then, after adding hydrazine sulphate, Ce(IV) is reduced to Ce(III). This treatment is also based on the fact that Th(IV) ion with sulphate(VI) ions forms anionic [(ThSO4)4]4-2n, that could pass freely through a cation exchange resin while the rare earth elements are completely adsorbed (Sherief & Almasy, 1968). Also the uranyl ionUO22+ forms anionic complexes of the type [UO2(SO4)n]2-2n where n=1, 2 or 3

The polyfunctional ion exchanger Diphonix Resin is commonly used for separation of rare earth elements(III) (Fig. 9). It contains phosphonic, carboxylic and sulphonic groups. Another ion exchanger is Diphonix A with the phosphonic and ammonium or pyridine

of both the frontal analysis technique and elution with mineral acids.

separated from a number of other elements.

(Preuss & Kunin, 1958).

functional groups (Horwitz et al., 1993).

These ion exchangers are characterized by high affinity for uranium(VI), plutonium(IV), neptunium(IV), thorium(IV), americium(III), europium(III) and a number of outside transient elements. The removal factor of europium(III) from 1 M HNO3 solution at the phase contact time 30 min. for DiphonixResin is 98.3%.

An important group of chelating ion exchangers used for lanthanide separations are those with the iminodiacetate functional groups. As follows from the literature data in the pH range 2.5-3.5 with the Ln(III) ions the cationic complexes are formed and in the pH range 4.0-4.5 the anionic ones.

Christell et al. (1961) showed that the standard iminodiacetic ion exchanger Chelex 100 has stronger affinity for La(III) than Lu(III), which is opposed to the values of stability constants of iminodiacetate complexes of these elements. Affinity for Chelex 100 in a lanthanide series increases with the increasing atomic number of La(III) to Eu(III), and then decreases to Lu(III). By contrast, based on the distribution coefficient determined by Schrobilgen and Lang (1968) for the iminodiacetic ion exchanger Dowex A-1 the affinity series is as follows: La(III) < Pr(III) < Nd(III) > Sm(III) > Gd(III) > Tb(III) Dy(III) < Er(III) > Yb(III).

The gel and macroporous ion exchangers, iminodiacetate ion exchangers Amberlite IRC 718, Diaion CR-10, Duolite ES 466, Lewatit TP 207 and Lewatit 208 as well as Wofatit MC 50 were tested for separation of rare earth elements(III) by both the frontal analysis technique and elution. The results of these studies revealed their great applicability in the process of rare earth elements separation as well as for obtaining individual lanthanides with a high degree of purity. Of the group of the above mentioned ion exchangers Wofatit MC-50 proved to be the most useful. Particularly favourable results were obtained for purification of the concentrate of yttrium(III) from ytterbium(III) (Hubicka & Hubicki, 1978; Hubicka, 1989b), which is one of the most common contaminants of yttrium(III). It was also very useful for the ion exchange purification of samarium(III) from ytterbium(III); samarium(III) from europium(III); yttrium(III) from neodymium(III); lanthanum(III) from praseodymium(III) and neodymium(III); cerium (III) from praseodymium(III), neodymium(III), samarium(III), europium(III), yttrium(III), dysprosium(III), holmium(III), erbium(III), thulium(III), ytterbium(III) and lutetium(III) as well as scandium(III) from other rare earth elements. The determined affinity series for heavy lanthanides(III), neodymium(III) and yttrium(III) can be as follows: Yb(III) > Er(III) > Dy(III) > Ho(III) > Nd(III) > Y(III) and this is consistent with the values of stability constants of their complexes with iminodiacetic acid (Inczedy, 1972).

Noteworthy is the unusual position of yttrium(III). It was also found that Yb(III) has higher affinity for this ion exchanger than Er(III), Dy(III), Ho(III) and Tb(III), which is not consistent with the data obtained for Dowex A-l (Schrobilgen & Lang, 1968).

The selectivity of the carboxylic acid ion exchange resins in relation to the rare earth elements (III) is highly variable. Arnold and Son Hing (1967) set the separation coefficients and investigated the mechanism of sorption of lanthanides on the carboxylic ion exchangers Amberlite IRC-50 and Amberlite XE-89. They showed that with the decreasing ionic radii of rare earth elements(III) their affinity for Amberlite IRC 50 increases reverse to that in the case of polystyrene-sulphonic cation exchangers. The determined separation coefficients for the selected pairs of elements are equal to: Ce(III)-La(III) 1.86, Pr(III)-La(III) 2.40; Nd(III)- La(III) 2.60; Nd(III)-Pr(III) 1.1; Pm(III)-La(III) 3.50; Tb(III)-La(III) 5.60; Tb(III)-Ce(III) 3.00, respectively.

Both small ion exchange rate (compared to the polystyrene-sulphonic cation exchangers) and high affinity for the ion H+ excludes practical application of the carboxylic ion exchange resins for the separation of rare earth elements(III) in the acidic media. However, carboxylic ion exchangers, especially phenol-carboxylic ion exchangers can be used for separation of uranium(VI) from rare earth elements(III).

A relatively small number of selective ion exchange resins is produced commercially. Therefore, the authors propose various modifications of ion exchangers by sulphonated aromatic chelating agents. The development of new functional resins which have chelating properties, prepared by simple immobilization of complexing organic reagents by ion exchange or adsorption onto conventional anion exchange resins or non ionic adsorbents has acquired great importance.

These modified resins can react with RRE ions by complex formation and can be used to preconcentrate their traces. For example, the research carried out by Hubicki (1989a) connected with the selective separation of micro quantities of scandium(III) from macro quantities of yttrium(III) and lanthanum(III) (50g/dm3) on strongly basic anion exchangers with gel and macroporous skeleton modified with sulphonated organic reagents as prototypes of the new chelating ion exchange resins should be mentioned. To this end the anion exchangers were modified by alizarin S, arsenazo I, arsenazo III, beryllonite II, thymol blue, phenol red, cresol red, pyrogallol red, chrome azurol S, 8-hydroxyquinoline-5 sulphonic acid, sulphosalicilic acid, nitroso-R-soli, R-salt, SPANDS, tirone, torone as well as orange xylene. The best results of separation of Sc(III) from Y(III) and La(III) were obtained on the anion exchanger Merck MP-5080 in the chrome azurol S form. In addition, modified different types of anion exchangers were used for the purification of macro quantities of lanthanum chloride from micro quantities of Eu(III), Tb(III), Yb(III) and Lu(III). Of the anion exchangers tested for this purpose the most preferred proved to be the anion exchanger modified by 8-hydroxyquinoline-5-sulphonic acid (Hubicki, 1989a; Hubicki, 1989b).

Amberlite XAD-4 functionalized with o-vanillinsemicarbazone has been applied for the preconcentration of lanthanum, cerium, thorium and uranium ions (Jain et al., 2001). Complexing properties of the XAD-4 resin functionalized with the bicine ligand (N,N-bis(2hydroxyethyl) glycine) were investigated for La(III), Nd(III), Tb(III), Th(IV) and uranium(VI). Polydithiocarbamate chelating resin has been applied for the preconcentration of rare earth elements with Fe(III), Fe(II), Cr(VI), Cr(III), V(V), V(IV), Ti(IV), Mo(VI), W(VI), Th(IV) (Miyazaki & Barnes 1981). In the paper by De Vito et al. (1999) it was found that Amberlite XAD-7 resin with (o-[3,6-disulpho-2-hihydroxy-1-naphthylazo]-benzenearsonic acid) was successfully used for the separation and preconcentration of Sm(III), Eu(III) and Gd(III). The resin containing the fluorinated--diketone chelating group immobilized on solid support styrene divinyl benzene was applied for the simultaneous preconcentration of La(III), Ce(IV), Nd(III), Sm(III), Gd(III), Eu(III), Dy(III), Er(III), Yb(III) and Lu(III) (Rao & Kala, R. 2004; Waqar, et al. 2009). Preconcentration of La(III), Eu(III) and Yb(III) was also achieved by sorption of their 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (PMBP) complexes on a silica gel column (Liang & Fa, 2005).

124 Ion Exchange Technologies

respectively.

uranium(VI) from rare earth elements(III).

has acquired great importance.

Noteworthy is the unusual position of yttrium(III). It was also found that Yb(III) has higher affinity for this ion exchanger than Er(III), Dy(III), Ho(III) and Tb(III), which is not

The selectivity of the carboxylic acid ion exchange resins in relation to the rare earth elements (III) is highly variable. Arnold and Son Hing (1967) set the separation coefficients and investigated the mechanism of sorption of lanthanides on the carboxylic ion exchangers Amberlite IRC-50 and Amberlite XE-89. They showed that with the decreasing ionic radii of rare earth elements(III) their affinity for Amberlite IRC 50 increases reverse to that in the case of polystyrene-sulphonic cation exchangers. The determined separation coefficients for the selected pairs of elements are equal to: Ce(III)-La(III) 1.86, Pr(III)-La(III) 2.40; Nd(III)- La(III) 2.60; Nd(III)-Pr(III) 1.1; Pm(III)-La(III) 3.50; Tb(III)-La(III) 5.60; Tb(III)-Ce(III) 3.00,

Both small ion exchange rate (compared to the polystyrene-sulphonic cation exchangers) and high affinity for the ion H+ excludes practical application of the carboxylic ion exchange resins for the separation of rare earth elements(III) in the acidic media. However, carboxylic ion exchangers, especially phenol-carboxylic ion exchangers can be used for separation of

A relatively small number of selective ion exchange resins is produced commercially. Therefore, the authors propose various modifications of ion exchangers by sulphonated aromatic chelating agents. The development of new functional resins which have chelating properties, prepared by simple immobilization of complexing organic reagents by ion exchange or adsorption onto conventional anion exchange resins or non ionic adsorbents

These modified resins can react with RRE ions by complex formation and can be used to preconcentrate their traces. For example, the research carried out by Hubicki (1989a) connected with the selective separation of micro quantities of scandium(III) from macro quantities of yttrium(III) and lanthanum(III) (50g/dm3) on strongly basic anion exchangers with gel and macroporous skeleton modified with sulphonated organic reagents as prototypes of the new chelating ion exchange resins should be mentioned. To this end the anion exchangers were modified by alizarin S, arsenazo I, arsenazo III, beryllonite II, thymol blue, phenol red, cresol red, pyrogallol red, chrome azurol S, 8-hydroxyquinoline-5 sulphonic acid, sulphosalicilic acid, nitroso-R-soli, R-salt, SPANDS, tirone, torone as well as orange xylene. The best results of separation of Sc(III) from Y(III) and La(III) were obtained on the anion exchanger Merck MP-5080 in the chrome azurol S form. In addition, modified different types of anion exchangers were used for the purification of macro quantities of lanthanum chloride from micro quantities of Eu(III), Tb(III), Yb(III) and Lu(III). Of the anion exchangers tested for this purpose the most preferred proved to be the anion exchanger

modified by 8-hydroxyquinoline-5-sulphonic acid (Hubicki, 1989a; Hubicki, 1989b).

Amberlite XAD-4 functionalized with o-vanillinsemicarbazone has been applied for the preconcentration of lanthanum, cerium, thorium and uranium ions (Jain et al., 2001). Complexing properties of the XAD-4 resin functionalized with the bicine ligand (N,N-bis(2-

consistent with the data obtained for Dowex A-l (Schrobilgen & Lang, 1968).

In the paper by Vigneau et al. (2001) the molecular imprinting resins containing DTPA and EDTA were also studied in lanthanides(III) separation. It was found that DTPA derivative monomers exhibited much higher Ga(III)-La(III) selectivity than EDTA ones. Also ionic imprinting resins can be used to this end. In the molecular imprinting process, the selectivity of a polymeric material is based on the size and shape of the template as well as the affinity between the host matrix and the guest molecule. More particularly in the case of ionic imprinting, the affinity partly depends on the number and the orientation of interaction points (ligand denticity) as well as on the counter ion. As described in the paper by (Krishna et al. (2005) ion imprinting allows significant enhancement in selectivity coefficients of neodymium(III) with respect to La(III), Ce(III), Pr(III), Sm(III) and Eu(III). The obtained selectivity coefficients are several times higher than those for the best extractant such as D2EHPA, for example selectivity coefficient for Nd(III) over Ce(III) and Pr(III) increases twentyfold, threefold over La(III) and Sm(III).

## **10. Separation of rare earth(III) by means of the extraction method**

Ion exchange was of most significant importance in uranium production. However, success of ion exchange methods in uranium technology does not apply to that of other elements. Compared to the extraction method, rate of ion exchange and concentration of purified rare earth elements(III) salts are, as a rule, much smaller. Therefore large size of industry installations, generating large investment costs, is necessary. During extraction the elements are divided, according to the Nernst division law, into two immiscible with each other phases: organic and aqueous. Tri-n-butyl phosphate (TBP) and D2EHPA are commonly applied extracting agents in separation of rare earth elements(III) (Preston & Du Preez, 1990). Their application results mainly from their good extraction properties with a relatively law price. The advantages of this method are large concentration of elements in the organic and aqueous phases, thus large yield of multi-stage extraction. Others have discussed the use of bis(2-ethylhexyl)phosphonic acid (HDEHP) in rare earth elements separations (Jensen et al. 2001; Fontana & Pietrelli, 2009; Yin et al. 2010). HDEHP, a liquid cation exchanger that is also a chelating agent, most typically forms a tris complex with trivalent lanthanides in the organic phase, simultaneously releasing three H+ for each trivalent metal ion transferred into the organic phase. The distribution ratios for the extraction of the lanthanides from mineral acid solutions vary by nearly 105 from La(III) to Lu(III) (Nilsson & Nash, 2007, Mel'nik et al. 1999).

As follows from the literature data (Yan et al. 2006) the novel solvent extraction process and its application in industry for separating HREEs (thulium(III), ytterbium(III) and lutetium(III)), yttrium(III) and scandium(III) has been developed recently. The most popular solvent extraction method is using PC-88A (2-ethylhexyl phosphonic acid mono-2 ethylhexyl ester) or Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid) as the extractant with lower equilibrium acidity. The additives of modified PC-88A were also adopted to separate Lu(III) in practice. Naphthenic acid or related carboxylic acid derivative were also used to separate the heavy rare earth elements in a very long cascade. Although the efficiency was not satisfactory, the chemical consumption was economical enough. The electrochemical method is also piloted to obtain Yb(III). This makes the separation of Tm(III), Yb(III) and Lu(III) more effective by solvent extraction.

As for the purification of Y(III) its chloride salt is separated from heavy rare earth elements using PC-88A - HCl process. The contaminants of light rare earth elements and bivalent ions such as Ca(II) were removed in PC-88A-HCl cascade and then yttrium (≥99.999%) is coprecipitated with Eu(III), Tb(III) and Zr(IV) by purified oxalic acid (Yan, et al. 2006).

Solvent extraction is also the most commonly used and effective technique in Ce(IV) separation. Cerium(III) is most likely to be oxidized to a tetravalent state either by bubbling oxygen during rare earth hydroxide precipitation or by drying the rare earth hydroxide in the presence of air. In acidic solutions, the oxidation of Ce(III) to Ce(IV) may occur by chemical oxidation with strong oxidants, such as peroxosulphates(VI), permanganates, lead oxide or silver oxide or by electrochemical oxidation or photochemical oxidation. Separation of insoluble cerium(IV) from rare earth elements can be carried out by selective dissolution of the trivalent rare earth hydroxides or through its selective precipitation from acid solution (Ura et al. 2005; Zhang et al. 2008; Luna et al. 2011). Many studies have been conducted on Ce(IV) separation using various extractants such as Cyanex 923, TBP, primary amine N1923, Aliquat 336, synergistic extractant and ionic liquids. The recovery of Ce(IV) and Th(IV) from rare earths(III) with Cyanex 923 has been applied in a industrial process. However, there are some disadvantages of solvent extraction of Ce(IV), for example such as a third phase formation.

The extraction of rare earth elements by amine and quaternary ammonium salts has been investigated in detail by many authors. As follows from the paper by Kovalancik and Galova (1992) low values of separation factors in the rare earth elements extraction with amines may be enhanced by the addition of a complexing agent. The greatest differences in the stability constant values are found in the case of CDTA and HEDTA.

In the extraction process the separation of Eu(III) from Gd(III) is exception. Similar to Zr(IV)- Hf(IV) this pair is the most difficult one to separate. Among rare earth elements, which can be reduced to divalent ions, Eu(III) has the highest standard redox potential, which makes its selective reduction and recovery from a mixture containing the other trivalent rare earth ions possible (Morais & Ciminelli, 1998). It can be accomplished by several techniques, such as chemical reduction by Zn or Zn-Hg, photochemical reduction and electrochemical reduction (Atanasyants & Seryogin, 1995; Preston et al. 1996). The recovery is finally accomplished by precipitation with sulphate, based on the fact that the chemical properties of Eu(III) are similar to those of the alkaline earth ions. Among different sulphate sources (NH4)2SO4, K2SO4, Na2SO4, NaHSO4 and H2SO4 can be used as precipitating agents. As follows from the Morais and Ciminelli paper (2001) the precipitation with sulphuric(VI) acid led to higher-grade europium oxide by keeping pH in a range that does not favour gadolinium co-precipitation. The continuous addition of sulphuric(VI) acid is mainly responsible for the improvement of europium recovery. Maximum recovery was achieved within 2 h or more. Based on the experiments the product assaying 99.99% Eu2O3 can be obtained from the feed containing 5.0 Eu2O3 g/dm3 and 138.2 Gd2O3 g/dm3 in two stages of reduction–precipitation. The overall recovery is about 94%.

126 Ion Exchange Technologies

a third phase formation.

Lu(III) (Nilsson & Nash, 2007, Mel'nik et al. 1999).

Tm(III), Yb(III) and Lu(III) more effective by solvent extraction.

trivalent metal ion transferred into the organic phase. The distribution ratios for the extraction of the lanthanides from mineral acid solutions vary by nearly 105 from La(III) to

As follows from the literature data (Yan et al. 2006) the novel solvent extraction process and its application in industry for separating HREEs (thulium(III), ytterbium(III) and lutetium(III)), yttrium(III) and scandium(III) has been developed recently. The most popular solvent extraction method is using PC-88A (2-ethylhexyl phosphonic acid mono-2 ethylhexyl ester) or Cyanex 272 (bis(2,4,4-trimethylpentyl)phosphinic acid) as the extractant with lower equilibrium acidity. The additives of modified PC-88A were also adopted to separate Lu(III) in practice. Naphthenic acid or related carboxylic acid derivative were also used to separate the heavy rare earth elements in a very long cascade. Although the efficiency was not satisfactory, the chemical consumption was economical enough. The electrochemical method is also piloted to obtain Yb(III). This makes the separation of

As for the purification of Y(III) its chloride salt is separated from heavy rare earth elements using PC-88A - HCl process. The contaminants of light rare earth elements and bivalent ions such as Ca(II) were removed in PC-88A-HCl cascade and then yttrium (≥99.999%) is co-

Solvent extraction is also the most commonly used and effective technique in Ce(IV) separation. Cerium(III) is most likely to be oxidized to a tetravalent state either by bubbling oxygen during rare earth hydroxide precipitation or by drying the rare earth hydroxide in the presence of air. In acidic solutions, the oxidation of Ce(III) to Ce(IV) may occur by chemical oxidation with strong oxidants, such as peroxosulphates(VI), permanganates, lead oxide or silver oxide or by electrochemical oxidation or photochemical oxidation. Separation of insoluble cerium(IV) from rare earth elements can be carried out by selective dissolution of the trivalent rare earth hydroxides or through its selective precipitation from acid solution (Ura et al. 2005; Zhang et al. 2008; Luna et al. 2011). Many studies have been conducted on Ce(IV) separation using various extractants such as Cyanex 923, TBP, primary amine N1923, Aliquat 336, synergistic extractant and ionic liquids. The recovery of Ce(IV) and Th(IV) from rare earths(III) with Cyanex 923 has been applied in a industrial process. However, there are some disadvantages of solvent extraction of Ce(IV), for example such as

The extraction of rare earth elements by amine and quaternary ammonium salts has been investigated in detail by many authors. As follows from the paper by Kovalancik and Galova (1992) low values of separation factors in the rare earth elements extraction with amines may be enhanced by the addition of a complexing agent. The greatest differences in

In the extraction process the separation of Eu(III) from Gd(III) is exception. Similar to Zr(IV)- Hf(IV) this pair is the most difficult one to separate. Among rare earth elements, which can be reduced to divalent ions, Eu(III) has the highest standard redox potential, which makes its selective reduction and recovery from a mixture containing the other trivalent rare earth

the stability constant values are found in the case of CDTA and HEDTA.

precipitated with Eu(III), Tb(III) and Zr(IV) by purified oxalic acid (Yan, et al. 2006).

To overcome inconveniences of the extraction of rare earth elements there should be used various impregnating substances. Solvent impregnated resins (SIRs) were developed by Warshawsky (1981). It is now well known that the extraction of metal ions with macroporous polymeric supports impregnated by extractants is an attractive method for the separation and preconcentration of metal ions (Schmidt, 1987; Cortina & Warshawsky, 1997; Horwitz and Schulz, 1999). Solvent impregnated resins for lanthanides separation are produced by adsorbing extractants such as TBP or D2EHPA on a macroporous resin without any functional groups. Different types of highly selective resins can be obtained by adsorbing various extractants. However, it should be mentioned that SIRs are characterized by smaller adsorption capacity and shorter life compared with the common ion exchange resins (Shibata & Matsumoto, 1998). The disadvantage of impregnating substances is slow kinetics due to the limited size of interfacial area and wet ability with the aqueous solution. An essential drawback is the irreversible loss of active substances. However, using Cyanex 302 in industrial processes of scandium(III) purification should be highlighted.

In the paper by Turanov (2010) bis(diphenylphosphoryl-methylcarbamoyl)alkanes were synthesized and studied as extractants for La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III), Lu(III) and Y(III) from perchloric acid solutions. The influence of both HClO4 concentration in the aqueous phase and that of the extractant in the organic phase on the extraction of metal ions was considered. The stoichiometry of the extracted complexes has also been determined. Bis(diphenylphosphoryl-methylcarbamoyl)alkanes possess a higher extraction efficiency towards Ln(III) than their monoanalogue Ph2P(O)CH2C(O)NHC9H19.

In the paper the research on the applicability of different types of anion exchangers for the separation of rare earth elements in the presence of the complexing agents IDA, HEDTA and CDTA is presented. The effect of the addition of a polar organic solvent (methanol, ethanol, acetone, 1-propanol, 2-propanol) on separation of rare earth(III) elements in such system is discussed. The examples of the removal of rare earth elements nitrate complexes from the polar organic solvent-H2O-HNO3 are discussed in detail.
