**6. Separation of rare earth elements**

Lanthanides separation and preconcentration of high purity compounds is one of the most difficult problems in inorganic chemistry as it makes use of the subtle differences between the physicochemical properties of these elements and their compounds like solubility, basicity, volatility and possibility of occurrence with different oxidation numbers (Kowalczyk & Mazanek, 1990; Jimenez-Reyes, 1993; Uda et al. 2000).

Most frequently used methods in the separation of rare earth elements are - fractional crystallization, selective precipitation, oxidation-reduction methods, ion exchange and liquid-liquid extraction. As the rare earth ions can be substituted for readily in crystal lattices and the most precipitates consist of crystals of almost the same rare earth mixture, the fractional precipitation is used for the nitrate(V), suphate(VI) and bromate solutions. However, fractional separation for adjacent heavy rare earths is extremely slow and tedious. If the lanthanides are differentiated in terms of their atomic number, their separation is simplified. It takes only a few partial precipitations, for example, to obtain a lanthanum– cerium–praseodymium fraction completely free of erbium, thulium, ytterbium, and lutetium. The separation of La(III) and Ce(IV) is even easier. In this case only a few fractions should be enough for their separation. Consequently, fractional precipitation is used in operations of rare earth concentrate pre-treatment and pure lanthanum and cerium compounds have been commercially available for many years (Nash, 1994).

The method of crystallization of phosphorus containing lanthanides at a temperature of 150- 200 oC depends on the acid concentration. This type of technology is used in the processing of apatites. The phosphate type mineral, apatite used in the production of phosphoric acid has rare earth oxide content between 0.4 and 0.9%. There are two main types of processing of apatites to phosphoric acid and fertilizers based on the decomposition of raw material with nitric(V) or sulphuric(VI) acids. In terms of recovery of rare earth elements favourable conditions occur during apatite dissolution in nitric(V) acid because this method provides a quantitative transition of lanthanides to liquid phase containing phosphoric(V) and nitric(V) acids as well as calcium nitrate(V). The concentration of lanthanide in this method can be up to 0.5% Ln2O3 in solution. However, with the continuous precipitation method to separate Ln(III) ions high concentrations of Ca(II) ions will result in their co-precipitation.

It should be also mentioned that in the case of application of sulphuric(VI) acid, depending on temperature, acid concentration, the ratio of the liquid phase to the solid phase as well as the phase contact time CaSO42H2O, CaSO40,5H2O or CaSO4 are obtained as byproducts. The concentration of lanthanides in phosphoric(V) acid is equal to 0.1%, while in the phosphogypsum exceeds 0.3%. One method of further processing consists in combining the process of hydration of CaSO40,5H2O to CaSO42H2O with the simultaneous process of lanthanides extraction using e.g. D2EHPA. Also application of resin in the leaching (RIL) process instead of solvent extraction eliminates the need for a costly solid/liquid separation unit operation (Padayachee et al. 1996). The authors found that hydrocycloning gypsum to concentrate the leachable rare earth elements into a smaller and finer particle size mass fraction resulted in the increasing rare earth elements concentration from about 2500 g/kg to about 9000 g/kg. Additionally, the application of ion exchange resins such as aminophosphonic Duolite ES-467 and sulphonic Duolite C 20MB to extract rare earth elements from the cyclone gypsum shifted the equilibrium reaction and allowed to increase their leaching efficiency up to five times. The technology developed by the Institute of Chemistry and Inorganic Technology of the University of Technology in Cracow (Poland) eliminates the storage of phosphogypsum, by processing prostproduction wastes and stocks into commercial products: anhydrous calcium sulphate (anhydrite) and rare earth metal oxide concentrates of the content up to 99% recalculated into La2O3 (Kowalczyk & Mazanek, 1987; Kowalczyk & Mazanek, 1990). The resulting product, which is a building material, maintains parameters of the cement anhydrite produced from natural materials, and other parameters (such as strength or white colour). The resulting anhydrite, among others, can be applied for producing self-leveling floor screeds, fully meeting all requirements of this type of materials. This technology was tested on the experimental scale processing with 1ton/h phosphogypsum. The technology allows for the elimination of phosphogypsum storage and elimination of the existing waste dumps of phosphogypsum (Kowalski et al. 2006).

### **7. Cation exchange**

112 Ion Exchange Technologies

**6. Separation of rare earth elements** 

Lanthanides separation and preconcentration of high purity compounds is one of the most difficult problems in inorganic chemistry as it makes use of the subtle differences between the physicochemical properties of these elements and their compounds like solubility, basicity, volatility and possibility of occurrence with different oxidation numbers

Most frequently used methods in the separation of rare earth elements are - fractional crystallization, selective precipitation, oxidation-reduction methods, ion exchange and liquid-liquid extraction. As the rare earth ions can be substituted for readily in crystal lattices and the most precipitates consist of crystals of almost the same rare earth mixture, the fractional precipitation is used for the nitrate(V), suphate(VI) and bromate solutions. However, fractional separation for adjacent heavy rare earths is extremely slow and tedious. If the lanthanides are differentiated in terms of their atomic number, their separation is simplified. It takes only a few partial precipitations, for example, to obtain a lanthanum– cerium–praseodymium fraction completely free of erbium, thulium, ytterbium, and lutetium. The separation of La(III) and Ce(IV) is even easier. In this case only a few fractions should be enough for their separation. Consequently, fractional precipitation is used in operations of rare earth concentrate pre-treatment and pure lanthanum and cerium

The method of crystallization of phosphorus containing lanthanides at a temperature of 150- 200 oC depends on the acid concentration. This type of technology is used in the processing of apatites. The phosphate type mineral, apatite used in the production of phosphoric acid has rare earth oxide content between 0.4 and 0.9%. There are two main types of processing of apatites to phosphoric acid and fertilizers based on the decomposition of raw material with nitric(V) or sulphuric(VI) acids. In terms of recovery of rare earth elements favourable conditions occur during apatite dissolution in nitric(V) acid because this method provides a quantitative transition of lanthanides to liquid phase containing phosphoric(V) and nitric(V) acids as well as calcium nitrate(V). The concentration of lanthanide in this method can be up to 0.5% Ln2O3 in solution. However, with the continuous precipitation method to separate

(Kowalczyk & Mazanek, 1990; Jimenez-Reyes, 1993; Uda et al. 2000).

compounds have been commercially available for many years (Nash, 1994).

Ln(III) ions high concentrations of Ca(II) ions will result in their co-precipitation.

It should be also mentioned that in the case of application of sulphuric(VI) acid, depending on temperature, acid concentration, the ratio of the liquid phase to the solid phase as well as the phase contact time CaSO42H2O, CaSO40,5H2O or CaSO4 are obtained as byproducts. The concentration of lanthanides in phosphoric(V) acid is equal to 0.1%, while in the phosphogypsum exceeds 0.3%. One method of further processing consists in combining the process of hydration of CaSO40,5H2O to CaSO42H2O with the simultaneous process of lanthanides extraction using e.g. D2EHPA. Also application of resin in the leaching (RIL) process instead of solvent extraction eliminates the need for a costly solid/liquid separation unit operation (Padayachee et al. 1996). The authors found that hydrocycloning gypsum to concentrate the leachable rare earth elements into a smaller and finer particle size mass fraction resulted in the increasing rare earth elements concentration from about 2500 g/kg to about 9000 g/kg. Additionally, the application of ion exchange resins such as Ion exchange separation of rare earth elements was initiated by Spedding and Powell to separate fission products obtained from nuclear reactors (Spedding et al. 1956; Powell, 1961, 1964). For several years the cation exchange was the primary method used to obtain individual lanthanides(III). In previous years and at present the development of extraction methods for separation of rare earth elements(III) proceeded simultaneously to the ion exchange method which is the most successful to obtain these elements with a high degree of purity, as the final product of the concentrates obtained in the extraction process [Preston 1996; Preston et all, 1996].

In the process of cation exchange separation of rare earth elements(III) the polystyrenesulphonic cation exchangers are most often used and rare earth cations are exchanged with H+, ammonium ion or other cations derived from the ion exchange phase. The charge, size and degree of hydration of the exchanged ions are the most important factors affecting their affinity for the cation exchanger. In the case of ions with the same charge, the affinity depends on their size and degree of the hydration.

In the lanthanide(III) group with the increasing atomic number decrease of the ionic radius is observed. However, due to similar values of ionic radii of individual lanthanide(III) ions there are not significant differences in their affinity for the polystyrene cation exchangers. Therefore, attempts to obtain individual rare earth elements from the solutions of mineral acids (HCl, HBr, HNO3 and H2SO4) did not yield positive results (Nelson et al. 1964; Korkish, 1967; Korkish & Ahluwalia, 1967). HCl and HNO3 can be used for the separation of lanthanide(III) from other metal ions occurring in the lower oxidation states (Strelow & Bothma, 1964). This relationship is also used for the separation of cerium(IV) from other rare earth elements using nitric acid(V). Some improvement of the separation of rare earth elements(III) can be obtained by using mineral acid solutions containing organic solvents. In this case rare earth elements(III) are much harder sorbed on cation exchangers than using aqueous solutions of these acids (Starý, 1966).

Therefore, both on a laboratory scale and in industrial separations the elution technique is usually applied. The complexing agents used as eluents form complexes with rare earth elements with different values of stability constants. Separation of rare earth elements, without the introduction of a complexing agent is not possible due to small differences in the values of the separation coefficients. In the case of the complexation of cations by the anionic ligands cation exchanger prefers a cation which forms a complex anion with the lowest average number of ligands and in a series of analogous complexes the one which forms the weakest complex.

The effectiveness of rare earth elements(III) separation on cation exchangers using complexing agents as eluents depends on both the affinity of a given element for the cation exchanger as well as on the kind of complexing agent. As the affinity of rare earths(III) elements is similar to the cation exchanger, the order of elution depends on the stability constants of complexes of individual elements. Thus the separation rate depends on the ratio of stability constants of these complexes. In this group: aminopolycarboxylic acids (EDTA, NTA, HEDTA, DTPA, CDTA), carboxylic acids (acetic acid, malonic acid, maleic acid, phtalic acid), hyroxylicacids (-hydroxyisobutyric acid, citric acid, lactic acid), ketoacids (pyruvic acid), aldehydeacids (glyoxilic acid), tioacids (tiodiglicolic acid), phosphonic (1-hydroxyethane-1,1-diphosphonic, HEDP) and aminophosphonic acids should be mentioned. The eluent selection and elution conditions are largely dependent on the composition of the mixture of separated rare earth elements.

Among eluents used in the process of cation exchange separation of rare earth elements EDTA and NTA were of the greatest industrial application. Stability constants of the formed complexes generally increase from light to heavy lanthanides(III) because lanthanides(III) are eluted in the order of decreasing atomic numbers. Y(III) location in the elution sequence changes with the change of stability constants of its complexes, therefore, is dependent on the type of eluent, for example, yttrium(III) elutes between Dy(III)-Tb(III) with 1% EDTA solution at pH 3.5, near Nd(III) with DTPA, near Pr(III) with HEDTA, near Eu(III) with citrate 10-20o C and Dy(III)-Ho(III) with citrate at 87-100o C, near Ho(III)-Dy(III) with lactate, near Dy(III)-Ho(III) with thiocyanate and between Sm(III)-Nd(III) with acetate (Powell, 1964).

The advantages of EDTA, in comparison with other complexing agents, are its high efficiency separation of adjacent pairs of rare earth elements(III) with the exception of the pair Eu(III)-Gd(III) (KEu(III)=2.241017; KGd(III)=2.341017). The separation factors () of rare earth elements using EDTA are as follows: La(III)-Ce(III) 3.3; Ce(III)-Pr(III) 2.4; Pr(III)-Nd(III) 2.0; Nd(III)-Pm(III) 1.9; Pm(III)-Sm(III) 1.8; Sm(III)-Eu(III) 1.5; Eu(III)-Gd(III) 1.1; Gd(III)-Tb(III) 3.5; Tb(III)-Dy(III) 2.7; Dy(III)-Ho(III) 2.0; Ho(III)-Er(III) 2.0; Er(III)-Tm(III) 2.0; Tm(III)-Tb(III) 1.8 and Yb(III)-Lu(III) 1.6. EDTA is readily available, inexpensive and easy to regenerate. However, due to its low solubility elution can not be conducted in an acidic environment (pH <3) and on the cation exchanger in the hydrogen form. Using a solution of EDTA at higher pH values the separation process is carried out with a cupric-ion retaining bed. Of the ions proposed by Spedding, Krumholz and Powell the most relevant retaining ions are Cu(II) and Zn(II) (Spedding et al. 1956; Powell, 1961).

The elution can be carried out even at elevated temperatures, which creates the possibility of recovery of EDTA and increases the separation factor of the Gd(III)-Eu(III) and Eu(III)- Sm(III) pairs (Powell & Burkholder, 1967). The partial complexation method proved to be advantageous for obtaining concentrated heavy lanthanides(III) as well as for separation of lanthanum(III) from other rare earth elements(III). NTA as EDTA is available, although it is more expensive (Fitch et al. 1951, Courtney et al. 1958; Mąkowska, 1970).

114 Ion Exchange Technologies

forms the weakest complex.

Therefore, both on a laboratory scale and in industrial separations the elution technique is usually applied. The complexing agents used as eluents form complexes with rare earth elements with different values of stability constants. Separation of rare earth elements, without the introduction of a complexing agent is not possible due to small differences in the values of the separation coefficients. In the case of the complexation of cations by the anionic ligands cation exchanger prefers a cation which forms a complex anion with the lowest average number of ligands and in a series of analogous complexes the one which

The effectiveness of rare earth elements(III) separation on cation exchangers using complexing agents as eluents depends on both the affinity of a given element for the cation exchanger as well as on the kind of complexing agent. As the affinity of rare earths(III) elements is similar to the cation exchanger, the order of elution depends on the stability constants of complexes of individual elements. Thus the separation rate depends on the ratio of stability constants of these complexes. In this group: aminopolycarboxylic acids (EDTA, NTA, HEDTA, DTPA, CDTA), carboxylic acids (acetic acid, malonic acid, maleic acid, phtalic acid), hyroxylicacids (-hydroxyisobutyric acid, citric acid, lactic acid), ketoacids (pyruvic acid), aldehydeacids (glyoxilic acid), tioacids (tiodiglicolic acid), phosphonic (1-hydroxyethane-1,1-diphosphonic, HEDP) and aminophosphonic acids should be mentioned. The eluent selection and elution conditions are largely dependent on

Among eluents used in the process of cation exchange separation of rare earth elements EDTA and NTA were of the greatest industrial application. Stability constants of the formed complexes generally increase from light to heavy lanthanides(III) because lanthanides(III) are eluted in the order of decreasing atomic numbers. Y(III) location in the elution sequence changes with the change of stability constants of its complexes, therefore, is dependent on the type of eluent, for example, yttrium(III) elutes between Dy(III)-Tb(III) with 1% EDTA solution at pH 3.5, near Nd(III) with DTPA, near Pr(III) with HEDTA, near Eu(III) with citrate 10-20o C and Dy(III)-Ho(III) with citrate at 87-100o C, near Ho(III)-Dy(III) with lactate, near Dy(III)-Ho(III) with thiocyanate and between Sm(III)-Nd(III) with acetate (Powell, 1964).

The advantages of EDTA, in comparison with other complexing agents, are its high efficiency separation of adjacent pairs of rare earth elements(III) with the exception of the pair Eu(III)-Gd(III) (KEu(III)=2.241017; KGd(III)=2.341017). The separation factors () of rare earth elements using EDTA are as follows: La(III)-Ce(III) 3.3; Ce(III)-Pr(III) 2.4; Pr(III)-Nd(III) 2.0; Nd(III)-Pm(III) 1.9; Pm(III)-Sm(III) 1.8; Sm(III)-Eu(III) 1.5; Eu(III)-Gd(III) 1.1; Gd(III)-Tb(III) 3.5; Tb(III)-Dy(III) 2.7; Dy(III)-Ho(III) 2.0; Ho(III)-Er(III) 2.0; Er(III)-Tm(III) 2.0; Tm(III)-Tb(III) 1.8 and Yb(III)-Lu(III) 1.6. EDTA is readily available, inexpensive and easy to regenerate. However, due to its low solubility elution can not be conducted in an acidic environment (pH <3) and on the cation exchanger in the hydrogen form. Using a solution of EDTA at higher pH values the separation process is carried out with a cupric-ion retaining bed. Of the ions proposed by Spedding, Krumholz and Powell the most relevant retaining ions are

the composition of the mixture of separated rare earth elements.

Cu(II) and Zn(II) (Spedding et al. 1956; Powell, 1961).

Of the group of salts of aminopolycarboxylic acids used as eluents in the process of cation exchange separation of rare earth elements(III), HEDTA proved to be effective for the separation of light and heavy lanthanides(III) (exhibits high separation efficiency of a mixture containing Er(III), Tm(III ), Yb(III) and Lu(III)) and for the separation of La(III) from other rare earth elements(III). However, it is completely useless for the separation of medium lanthanides (from Sm(III) to Ho(III)) (Powell, 1961). In the case of separation of rare earth elements(III) with the buffered solution of HEDTA on the cation exchanger Dowex 50 there was reported over 7-fold reduction of the number of theoretical plates with the reduction in the degree of cross linking of the cation exchanger (from 12% to 2% DVB DVB). In this system, the decrease of the number of theoretical plates as a result of the addition of neutral salt solution such as LiCl, NaCl or KCl to the eluent and with the increasing concentration of these salts (Merciny & Duyckaerts, 1966). However, so far, there has not been theoretical explanation of this phenomenon.

The other complexing agent - DTPA proved to be particularly useful for the separation of Y(III) from heavy lanthanides(III), since in a elution series with DTPA Y(III)occupies a position near the Nd(III) (Hale & Hammer, 1972). Of the group of carboxylic acids (acetic, malonic, maleic, phthalic acid) ammonium acetate is the cheapest and easily regenerated complexing agent. Elution using ammonium acetate gives good results in the separation of light lanthanides(III), yttrium(III) from the light and medium lanthanides(III). The best effects of separation were achieved using a solution of ammonium acetate at pH 6.8-6.9 and the gradient concentration 0.45-1.0 M. Yttrium(III) in the elution series occupies a position between Nd(III) and Sm(III).

In the group of hydroxyacids (-hydroxyisobutiric (-HIBA), citric and lactic acids) used as eluents, of significant importance are -hydroxyisobutiric and -hydroxy-2-methtlbutiric acids (Faris, 1967). -HIBA is one of the most favourable eluents in this group. In comparison to citric or lactic acids, using -HIBA high rates of separation of neighbouring pairs of rare earth elements(III) were achieved (Smith & Hoffman, 1956; Choppin & Chopoorian, 1961). This also gives good results of separation of Gd(III)-Eu(III) pair (Hubicka & Hubicki, 1982). The eluent can be applied at room temperature using the ion exchangers of low cross linking, such as Dowex 50x4 and Dowex 50x8 (Smith & Hoffman, 1956). It should be mentioned that -hydroxyisobutiric acid is not used to on a commercial scale. The most favourable ion exchange separation of lanthanides(III), compared to the -HIBA can be achieved using 2-hydroxy-2-methylbutyric acid.

As eluents of rare earth elements also other complexing agents such as pyruvic, glyoxylic and thiodiglycolic and 1-hydroxyethane-1,1-diphosphonic as well as aminophosphonic acids were used (Jegorov & Makarova, 1971; Hubicka & Hubicki, 1983a; Hubicka & Hubicki, 1983b).

Of them, special attention should be paid to pyruvic acid (Hubicka & Hubicki, 1983a; Hubicka & Hubicki, 1983b). The pyruvic acid solutions at the concentration 0.15-0.4 M at pH 3.5 and 5.0 proved to be useful for separating such pairs of elements as Y(III)-Nd(III), Sm(III)-Nd(III) as well as for separation of lanthanum(III) from other light lanthanides(III), yttrium(III) from heavy lanthanides using the cation exchanger Wofatit KPS with 4 and 8% DVB. Using pyruvic acid the elution of rare earth elements proceeds in the order of decreasing atomic numbers. Yttrium becomes similar to the medium lanthanides(III).

In the case of thiodiglycolic acid application for the separation of rare earth elements, the unusual position of Y(III) in the elution series can be seen. It can be as follows: Sm(III), Eu(III), Gd(III), Nd(III), Pr(III), Dy(III), Ho(III), Er(III), Yb(III), Lu(III), Y(III), La(III). Yttrium(III) elutes after heavy lanthanides, which enables its separation from Dy(III). The thiodiglycolic acid solution at the concentration 0.15 M and pH 5.5 can be applied for the separation of Y(III) from Nd(III); Sm(III) from light lanthanides(III) and Y(III) as well as Y(III) from Sm(III), Eu(III) and Gd(III).

Using -hydroxoethylideno-1,1-diphosphonic acid proved that Y(III) behaves as a medium lanthanide(III) and can be separated from heavy lanthanides(III) (especially from Lu(III), Yb(III) and Tm(III)) as well as from Nd(III) (Hubicka & Hubicki, 1980). Availability and low price of this acid also provides an opportunity to use it as an eluent in the purification process of lanthanum(III).

The disadvantage of ion exchange separation of mixtures of rare earth elements on the polystyrene-sulphonic cation exchangers is the lack of universal eluent, which would allow for selective separation of light, medium and heavy lanthanides(III) as well as to achieve high concentrations in the eluate.
