**3. Solvent extraction of Ce(IV)**

Using synchrotron X-ray and Raman spectroscopies and EXAFS, Ellis et al. also found that in strong acidic nitrate solution, ammonium ceric nitrate is a dinuclear Ce(IV) complex with a

the present quantum chemical calculations confirm that the Ce4+ coordination number is 9 and the relative free energies of Ce4+ is the 10- and 8-coordinate isomers in aqueous solutions.

Ce(IV) is unstable in perchloric acid aqueous solution because its standard electrode potential in perchloric acid aqueous solution is 1.61 V [19]. Therefore, when water decomposes and releases oxygen, Ce(IV) would be slowly reduced to Ce(III). In addition, Ce(IV) is very easy to hydrolyze and polymerize like other tetravalent cations. It is necessary to maintain high acidity in the medium to avoid it. Due to these difficulties, the data of the stability constants

Although the studies on nitrato-cerium complexes started earlier, there is contradictory information about these complexes in the relevant literature [63–66]. It was found by potentiometric

nitric acid aqueous solution [67]. The other two methods, the spectrophotometry and the extraction method, indicate that the ligand number of nitrato-cerium complexes may vary from 1 to 6. The distribution of Ce(IV) species in aqueous media was studied by measuring the total optical absorbance of Ce(IV) species in different nitric acid-perchloric acid mixture solutions. The stability constants of the nitrato-cerium complexes were determined spectrophotometrically [67]. There are considerable evidences of complex formation between ceric ions and sulfate ions in aqueous solution. Jones et al. have measured the migration in high sulfur concentration solutions and found that the color migrated to the anode. Some researchers [68–70] have measured the electromotive force of the cerium sulfuric acid solution; the results show that a complex is certainly formed, but its nature cannot be determined clearly. Besides, evidence for complexing has been found by Moore et al. [71] from kinetic studies on the reaction of Ce(IV) with arsenite ions. The first complex of Ce(IV) and sulfate in perchloric acid solution was studied by spectroscopic method [72]. The results show that the instability constant of the first complex varies with the concentration of total Ce(IV) ion plus sulfate and a higher complex was also found in this system. Hardwick et al. [73] made a spectral study on the association of Ce(IV) with sulfate. The results show that Ce(IV) interacts with one, two, and three sulfate ions in turn to form complexes. As is expected, as the number of sulfate ions in the complexes increases, the trend of association becomes smaller. Nevertheless, there were

no higher complexes of more than three sulfate complexes with Ce(IV).

The stability constants of the fluoride complexes of cerium(IV) in 1 M (HCIO<sup>4</sup>

, ß<sup>2</sup> , ß<sup>3</sup>

have been measured potentiometrically using a fluoride ion-selective electrode. This procedure ensures the stability of the oxidation state and prevents hydrolysis and polymerization of Ce(IV).

, and ß<sup>4</sup>

CeIV-O-CeIV(OH<sup>2</sup>

) x ]

] < 3.2 mol/L, there was only one complex in the form of Ce(NO<sup>3</sup>

6+ (x = 6 or 7) [62]. On the contrary,

) 3+ in

, NaClO<sup>4</sup>

were estimated to be 7.57 ± 0.04, 14.50 ± 0.03,

) medium

O)<sup>x</sup>

**Figure 2.** A proposed structure of the cationic dinuclear Ce(IV) species present in perchloric acid.

bridging oxo ligand, formulated as [(H2

8 Cerium Oxide - Applications and Attributes

of Ce(IV) complexes are very scarce.

Logarithms of the average values of ß<sup>1</sup>

−

method that when [NO<sup>3</sup>

**2.2. Ce(IV) complexes with anions in aqueous solution**

Many extractants have been reported and applied in nitric acid and sulfuric acid for Ce(IV) extraction, for example, acidic organophosphorus extractants [22–27], neutral organophosphorus extractants [28–36], amines [37], and bifunctional ionic liquid extractants (Bif-ILEs) [38–40]. Among them, tributyl phosphate (TBP), di-(2-ethylhexyl) 2-ethylhexyl phosphate (DEHEHP), di-(2-ethylhexyl) phosphate (P204), 2-ethylhexylphosphoric acid mono 2-ethylhexylester (P507), and Cyanex 923 are the most commonly used extractants for Ce(IV) extraction (listed in **Table 2**). Synergistic extraction [26, 35] is also an important method to enhance the extraction efficiency. It was reported that P204 + P507 and P204 + Cyanex 923 had synergistic extraction effects for Ce(IV) extraction from sulfuric acid medium.

## **3.1. Acidic organophosphorus extractants**

Several acidic organophosphorus extractants [22–27] were used to extract cerium(IV); among these extractants, P204 or P507 is a great extractant for Ce(IV) with a high capacity, extraction efficiency, and selectivity. Peppard et al. studied the extraction of Ce(IV) from HNO<sup>3</sup> solution with P204 in 1957. Tedesco et al. studied the extraction of cerium in kerosene from sulfuric acid solution by di(2-ethylhexyl) phosphate (P204, HA). The effects of DEHPA concentration and pH on the extraction of cerium were determined. However, the mechanism of extraction of Ce(IV) by P204 is not clear. Tedesco et al. considered that the possible extraction mechanism is as follows [22]:

When pH < 1.0

$$\text{Ce}^{4+} + 4\left(\text{HA}\right)\_2 = \text{CeH}\_4\text{A}\_8 + 4\text{H}^+ \tag{1}$$

$$\text{nCe^{4+} + 2 (HA)\_2 = \text{(CeA)}\_4 + 2nH^+} \\ \tag{2}$$

when pH = 1.7–2.0

$$\text{CeO}^{2+} + 2\text{ (HA)}\_{2} = \text{CeOH}\_{2}\text{A}\_{4} + 2\text{H}^{+} \tag{3}$$

$$\text{nCeO}^{\text{e}\*} + \text{n (HA)}\_{\text{2}} = \left< \text{CeOA}\_{\text{2}} \right>\_{\text{n}} + 2\text{nH}^{\text{e}} \tag{4}$$

When the R-O group in dialkyl phosphoric acid molecule is replaced by R group, such as P507, its pKa value increases and its acidity increases. The distribution ratio of rare earth elements extracted by P507 is lower than that of P204. Li et al. [24] have studied the separation of Ce(IV) with P507 in nitric acid system and sulfuric acid system. The mechanism of extraction of Ce(IV) in sulfuric acid system by P507 is as follows:


**Table 2.** The commonly used extractants for Ce(IV) extraction.

Low acidity:

$$\text{Ce}^{4+} + 3 \text{ (HA)}\_2 = \text{CeL}\_2 \text{(HL}\_2\text{)}\_2 + 4\text{H}^+ \tag{5}$$

Cyanex 923 (a mixture of straight-chain alkylated phosphine oxides) is considered to be

In recent years, our group has used extractant Cyanex 923 to extract and separate Ce(IV) from nitric acid system and sulfuric acid system. Lu et al. [30, 31] studied the extraction and separation of Ce(IV), Th(IV), RE(III), and Fe(III) from sulfuric acid system by Cyanex 923. The

Liao et al. had studied the thermodynamics and kinetics of Cyanex 923 extraction of Ce(IV) from the liquor of bastnaesite in detail. The results showed that Cyanex 923 could effectively

<sup>−</sup> + 2Cyanex 923 = Ce(SO4) (HSO4)

Li et al. studied the extraction and recovery Ce(IV) from nitrate solutions by Cyanex 923. The

<sup>−</sup> + 2Cyanex 923 → Ce (NO3)

<sup>−</sup> + F− + H<sup>+</sup> + Cyanex 923 → Ce(HF) (NO3)

The mechanism of extracting Ce(IV) from sulfuric acid solution with primary amine extractant N1923 was studied by Li et al. It was found that the mechanism of extraction of Ce(IV) by

2

SO<sup>4</sup> = (RNH3)

4

4

Ce (SO4)

Ce (SO4)

4

4

<sup>2</sup><sup>−</sup> + 2 (RNH3)

<sup>2</sup><sup>−</sup> + 4(RNH3) HSO<sup>4</sup> = (RNH3)

<sup>−</sup> + 2Cyanex 923 = Ce(HF)(SO4) (HSO4)

4

4

extract Ce(IV) from sulfuric acid system. The extraction mechanism was as follows:

SO<sup>4</sup>

http://dx.doi.org/10.5772/intechopen.79225

2

Extraction and Recovery of Cerium from Rare Earth Ore by Solvent Extraction

2

• 2Cyanex 923 (9)

• Cyanex 923 (10)

+ 2H2 SO<sup>4</sup> (12)

(11)

media was Ce(IV)

11

• 2Cyanex 923 (7)

• 2Cyanex 923 (8)

the most used extractant in Ce(IV) extraction [76].

<sup>2</sup><sup>−</sup> + 2HSO<sup>4</sup>

<sup>2</sup><sup>−</sup> + 2HSO<sup>4</sup>

extraction mechanism was as follows:

N1923 had a great relationship with acidity.

Ce4<sup>+</sup> + 2SO<sup>4</sup>

Ce4<sup>+</sup> + 2SO<sup>4</sup>

Ce4<sup>+</sup> + 4NO<sup>3</sup>

Ce4<sup>+</sup> + 4NO<sup>3</sup>

**3.3. Amines extractants**

> Th(IV) > RE(III) > Fe(III).

Ce4<sup>+</sup> + SO<sup>4</sup>

Ce4<sup>+</sup> + HF + SO<sup>4</sup>

Without fluorine:

With fluorine:

Low acidity:

High acidity:

experimental results show that the order of extracting metal ions in H<sup>2</sup>

Cyanex 923 extracts Ce(IV) from a sulfuric acid solution without fluorine:

Cyanex 923 extracts Ce(IV) from a sulfuric acid solution with fluorine:

High acidity:

$$\text{Ce}^{4+} + 2\text{HSO}\_4^- + \text{SO}\_4^{2-} + \text{(HL)}\_2 = \text{H}\_2\text{Ce} \text{(SO}\_4\text{)}\_3 \bullet 2\text{HL} \tag{6}$$

However, it is difficult to extract F− from sulfuric acid solution by P204 and P507 [30]. The addition of boric acid and other masking agents eliminates the effect of F− [74], which would introduce impurities and increase cost.

### **3.2. Neutral organophosphorus extractants**

Neutral organophosphorus extractants were applied widely in recently years for Ce(IV) extraction. Tri-butyl-phosphate (TBP) was used to recover Ce(IV) from high acidic HNO<sup>3</sup> solutions by adding an appropriate quantity of H<sup>3</sup> BO<sup>3</sup> , and high-purity CeO<sup>2</sup> was recovered [75]. However, high extraction acidity and severe co-extraction of mineral acids make TBP extraction of Ce(IV) not superior. Besides, a low extraction efficiency was also a serious problem for Ce(IV) extraction compared with others. Therefore, a new extractant named di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP) was used to extract and recover Ce(IV) from HNO<sup>3</sup> solution by Li's group [28, 29]. However, a low extractability and a low loading capacity limit the application of this extractant. In addition, the problem of reduction of cerium(IV) in DEHEHP extraction was also a challenge. Because of its low solubility in aqueous phase, liable hydroxylation, high miscibility with ordinary organic diluents, and low extraction acidities compared to other neutral organophosphorus extractants, Cyanex 923 (a mixture of straight-chain alkylated phosphine oxides) is considered to be the most used extractant in Ce(IV) extraction [76].

In recent years, our group has used extractant Cyanex 923 to extract and separate Ce(IV) from nitric acid system and sulfuric acid system. Lu et al. [30, 31] studied the extraction and separation of Ce(IV), Th(IV), RE(III), and Fe(III) from sulfuric acid system by Cyanex 923. The experimental results show that the order of extracting metal ions in H<sup>2</sup> SO<sup>4</sup> media was Ce(IV) > Th(IV) > RE(III) > Fe(III).

Liao et al. had studied the thermodynamics and kinetics of Cyanex 923 extraction of Ce(IV) from the liquor of bastnaesite in detail. The results showed that Cyanex 923 could effectively extract Ce(IV) from sulfuric acid system. The extraction mechanism was as follows:

Cyanex 923 extracts Ce(IV) from a sulfuric acid solution without fluorine:

$$\text{Ce}^{4+} + \text{SO}\_4^{2-} + 2\text{HSO}\_4^- + 2\text{Cyanex }923 = \text{Ce(SO}\_4\text{)(HSO}\_4\text{)}\_2 \bullet 2\text{Cyanex }923\tag{7}$$

Cyanex 923 extracts Ce(IV) from a sulfuric acid solution with fluorine:

$$\text{Ce}^{4+} + \text{HF} + \text{SO}\_4^{2-} + 2\text{HSO}\_4^- + 2\text{Cyane} \times 923 = \text{Ce(HF)(SO}\_4\text{)(HSO}\_4\text{)}\_2 \bullet 2\text{Cyane} \times 923 \tag{8}$$

Li et al. studied the extraction and recovery Ce(IV) from nitrate solutions by Cyanex 923. The extraction mechanism was as follows:

Without fluorine:

Low acidity:

High acidity:

Ce(IV) from HNO<sup>3</sup>

Ce4<sup>+</sup> + 3 (HA)

**Table 2.** The commonly used extractants for Ce(IV) extraction.

Ce4<sup>+</sup> + 2HSO<sup>4</sup>

10 Cerium Oxide - Applications and Attributes

However, it is difficult to extract F−

introduce impurities and increase cost.

**3.2. Neutral organophosphorus extractants**

solutions by adding an appropriate quantity of H<sup>3</sup>

<sup>2</sup> = CeL<sup>2</sup> (HL2)

<sup>2</sup><sup>−</sup> + (HL)

Neutral organophosphorus extractants were applied widely in recently years for Ce(IV) extraction. Tri-butyl-phosphate (TBP) was used to recover Ce(IV) from high acidic HNO<sup>3</sup>

[75]. However, high extraction acidity and severe co-extraction of mineral acids make TBP extraction of Ce(IV) not superior. Besides, a low extraction efficiency was also a serious problem for Ce(IV) extraction compared with others. Therefore, a new extractant named di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP) was used to extract and recover

loading capacity limit the application of this extractant. In addition, the problem of reduction of cerium(IV) in DEHEHP extraction was also a challenge. Because of its low solubility in aqueous phase, liable hydroxylation, high miscibility with ordinary organic diluents, and low extraction acidities compared to other neutral organophosphorus extractants,

BO<sup>3</sup>

solution by Li's group [28, 29]. However, a low extractability and a low

<sup>−</sup> + SO<sup>4</sup>

addition of boric acid and other masking agents eliminates the effect of F−

2

**Extractants Functional group Ref.**

DEHEHP [28, 29] Cyanex 923 [30–34]

P204 [23, 24]

[A336][P204] [38]

Neutral organophosphorus TBP P=O [15]

Acidic organophosphorus P507 [25, 26]

Amines N1923 RNH<sup>2</sup> [37] Bif-ILs [A336][P507] [39]

<sup>2</sup> = H2 Ce (SO4)

3

from sulfuric acid solution by P204 and P507 [30]. The

, and high-purity CeO<sup>2</sup>

+ 4H<sup>+</sup> (5)

• 2HL (6)

[74], which would

was recovered

$$\text{Ce^{4+} + 4NO\_3^- + 2Cyanex 923 \to Ce (NO\_3)\_4 \text{ + 2Cyanex 923}}\tag{9}$$

With fluorine:

$$\text{Ce}^{4+} + 4\text{NO}\_3^- + \text{F}^- + \text{H}^+ + \text{Cyanex 923} \rightarrow \text{Ce(HF)} \text{(NO}\_3\text{)}\_4 \bullet \text{Cyanex 923} \tag{10}$$

#### **3.3. Amines extractants**

The mechanism of extracting Ce(IV) from sulfuric acid solution with primary amine extractant N1923 was studied by Li et al. It was found that the mechanism of extraction of Ce(IV) by N1923 had a great relationship with acidity.

Low acidity:

$$\text{Ce}^{\text{+}} + 2\text{SO}\_{4}^{\text{-}} + 2\text{\{RNH}\_{3}\}\_{2}\text{SO}\_{4} = \text{\{RNH}\_{3}\}\_{4}\text{Ce}\left(\text{SO}\_{4}\right)\_{4} \tag{11}$$

High acidity:

$$\text{Ce}^{4+} + 2\text{SO}\_4^{2-} + 4\text{(RNH}\_3\text{)}\text{HSO}\_4 = \text{(RNH}\_3\text{)}\_4\text{Ce}\text{(SO}\_4\text{)}\_4 + 2\text{H}\_2\text{SO}\_4\tag{12}$$

#### **3.4. Bifunctional ionic liquid extractants**

In recent years, quaternary ammonium salts have been widely used in ionic liquids, including surfactants, extractants, catalysis and biodegradation, and many other fields [77–79]. Common ionic liquids are divided into four categories, including imidazole, pyridines, quaternary ammonium salts, and quaternary phosphine salts [80]. Among them, ammonium and phosphorus ionic liquid extractants were investigated in REs separation because of this low price and little toxicity [81]. The neutral complexation mechanism of Bif-IlEs has higher extractability and selectivity, and lower acid and base consumption and no emission of ammonia, nitrogen, Na+ or Ca2+ wastewater.

A series of high purity quaternary ammonium salt ionic liquids ([A336][P507], [A336][P204], [A336][C272]) have been synthesized by Chen's team using the simple and efficient acid-base neutralization method. The synthetic route is given in **Figure 3** [82].

[A336][P507] as shown in **Figure 4** can be used for the separation of Ce(IV) from sulfuric acid solution [39]. Zhang has studied the extraction equilibrium of Ce(IV) from sulfuric acid by using [A336][P507], the extraction mechanism was as follows [39]:

$$\text{Ce^{4+} + SO\_4^{2-} + 2HSO\_4^- + 2[A336][P507] = \text{Ce(SO\_4)} \text{(HSO\_4)}\_2 \bullet 2[A336][P507] \tag{13}$$

have found the enhanced extraction and separation of yttrium from heavy rare earth using BTMPPA (Cyanex 272) and bis(2,4,4- three methyl amyl) phosphoric acid (CA-12) [85]. In addition, Reddy et al. have studied the synergistic extraction of rare earth elements by Cyanex 923 and Cyanex 301 [86]. These findings have contributed to the study of a new mixed system

Li et al. [35, 36] reported the extraction of Cyanex 923 + P204 for Ce(IV) in sulfuric acid system. The experimental results show that Cyanex 923 + P204 has a positive correlation with the extraction of Ce(IV) solution. The largest synergistic coefficient of Ce(IV) is obtained at the mole fraction *X*Cyanex 923 = 0.8. The synergistic enhancement coefficients (*R*max) obtained for

Ce(IV) are 23.12 in Ce(IV) solution. The synergistic extraction can be expressed as

<sup>2</sup> + 1.5B = Ce (HSO4)

The synergistic extraction of cerium(IV) from sulfuric acid medium using a mixture of 2-ethylhexyl phosphoric acid mono 2-ethylhexyl ester (HEH/EHP, HL) and Di-(2-ethyl hexyl) phosphoric acid (HDEHP, HA) as extractants was studied [26]. The results showed that HEH/EHP = 0.6 was

<sup>2</sup><sup>−</sup> + H2 L<sup>2</sup> + H2 A2 = Ce (SO4)

Cerium and its compounds are widely used in many fields. For example, monocrystalline

 due to its excellent redox ability has an important application value in the fields of CO catalytic oxidation, organic synthesis catalysis, photocatalysis, biological oxidation resistance, and so on. Generally speaking, the preparation of Ce products can be divided into two main methods: one is the sulfuric acid double salt precipitation method and the other is the method of solvent extraction. The process of sulfuric acid double salt precipitation is as follows:

2

)0.5HL<sup>2</sup>

A2

A2 · 1.5B + 2H<sup>+</sup> + 2HSO<sup>4</sup>

Extraction and Recovery of Cerium from Rare Earth Ore by Solvent Extraction

http://dx.doi.org/10.5772/intechopen.79225

13

<sup>−</sup> (15)

. The synergistic extraction reaction

0.5 HL<sup>2</sup> A2 <sup>+</sup> 3H<sup>+</sup> (16)

using Cyanex 923 as an extractant to recover Ce(IV).

**Figure 4.** Chemical structures of [A336][P507].

4 + (HA)

extracted as an organic phase in the form of Ce(SO<sup>4</sup>

Ce (HSO4)

Ce4<sup>+</sup> + 0.5SO<sup>4</sup>

**4. Preparation of Ce product**

is as follows:

CeO<sup>2</sup>

Zhang [21] has investigated the extraction of Ce(IV) in H<sup>2</sup> SO<sup>4</sup> /H3 PO<sup>4</sup> system [42]. The possible extraction equilibrium is shown in Eq. (5):

$$\begin{aligned} \text{[eq:uillbrium] is shown in Eq. (5):}\\ \text{[eq:SSO}\_4^{2+} + 0.5\text{H}\_2\text{PO}\_4^- + 0.5\text{HSO}\_4^-] \\ + 0.5\text{SO}\_4^{2-} + [\text{A336}][\text{P507}] &= \text{Ce}(\text{SO}\_4)[\text{H}\_2\text{PO}\_4]0.5(\text{HSO}\_4) \\ 0.5(\text{SO}\_4) &0.5 \cdot [\text{A336}][\text{P507}] \end{aligned} \tag{14}$$

#### **3.5. Synergistic extraction system**

Synergistic extraction is an important method to enhance the extraction efficiency [26, 35] and has been applied for the extraction and separation of rare earths [83]. Recently, Li et al. have reported the synergistic extraction of lanthanum and Y using a mixture of Cyanex 272(BTMPPA) and 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (HPMBP) [84]. Sun et al.

**Figure 3.** Synthesis route of quaternary ammonium salt bifunctional ionic liquid extractant [82].

**Figure 4.** Chemical structures of [A336][P507].

**3.4. Bifunctional ionic liquid extractants**

12 Cerium Oxide - Applications and Attributes

ammonia, nitrogen, Na+

Ce4<sup>+</sup> + SO<sup>4</sup>

In recent years, quaternary ammonium salts have been widely used in ionic liquids, including surfactants, extractants, catalysis and biodegradation, and many other fields [77–79]. Common ionic liquids are divided into four categories, including imidazole, pyridines, quaternary ammonium salts, and quaternary phosphine salts [80]. Among them, ammonium and phosphorus ionic liquid extractants were investigated in REs separation because of this low price and little toxicity [81]. The neutral complexation mechanism of Bif-IlEs has higher extractability and selectivity, and lower acid and base consumption and no emission of

A series of high purity quaternary ammonium salt ionic liquids ([A336][P507], [A336][P204], [A336][C272]) have been synthesized by Chen's team using the simple and efficient acid-base

[A336][P507] as shown in **Figure 4** can be used for the separation of Ce(IV) from sulfuric acid solution [39]. Zhang has studied the extraction equilibrium of Ce(IV) from sulfuric acid by

<sup>−</sup> + 2[A336][P507] = Ce(SO4) (HSO4)

−

Synergistic extraction is an important method to enhance the extraction efficiency [26, 35] and has been applied for the extraction and separation of rare earths [83]. Recently, Li et al. have reported the synergistic extraction of lanthanum and Y using a mixture of Cyanex 272(BTMPPA) and 1-phenyl-3-methyl-4-benzoyl-pyrazolone-5 (HPMBP) [84]. Sun et al.

2

0.5(SO4)0.5 · [A336][P507]

SO<sup>4</sup> /H3 PO<sup>4</sup>

] = Ce(SO4)(H2 PO4)0.5(HSO4)

• 2[A336][P507] (13)

system [42]. The possible

(14)

or Ca2+ wastewater.

neutralization method. The synthetic route is given in **Figure 3** [82].

using [A336][P507], the extraction mechanism was as follows [39]:

<sup>−</sup> + 0. 5HSO<sup>4</sup>

**Figure 3.** Synthesis route of quaternary ammonium salt bifunctional ionic liquid extractant [82].

<sup>2</sup><sup>−</sup> <sup>+</sup> [A336][P507

<sup>2</sup><sup>−</sup> + 2HSO<sup>4</sup>

<sup>2</sup><sup>+</sup> + 0. 5H2 PO<sup>4</sup>

+ 0. 5SO<sup>4</sup>

extraction equilibrium is shown in Eq. (5):

CeSO<sup>4</sup>

**3.5. Synergistic extraction system**

Zhang [21] has investigated the extraction of Ce(IV) in H<sup>2</sup>

have found the enhanced extraction and separation of yttrium from heavy rare earth using BTMPPA (Cyanex 272) and bis(2,4,4- three methyl amyl) phosphoric acid (CA-12) [85]. In addition, Reddy et al. have studied the synergistic extraction of rare earth elements by Cyanex 923 and Cyanex 301 [86]. These findings have contributed to the study of a new mixed system using Cyanex 923 as an extractant to recover Ce(IV).

Li et al. [35, 36] reported the extraction of Cyanex 923 + P204 for Ce(IV) in sulfuric acid system. The experimental results show that Cyanex 923 + P204 has a positive correlation with the extraction of Ce(IV) solution. The largest synergistic coefficient of Ce(IV) is obtained at the mole fraction *X*Cyanex 923 = 0.8. The synergistic enhancement coefficients (*R*max) obtained for Ce(IV) are 23.12 in Ce(IV) solution. The synergistic extraction can be expressed as

$$\text{Ce}\,\text{(HSO}\_4\text{)}\_4 + \text{(HA)}\_2 + 1.5\text{B} = \text{Ce}\,\text{(HSO}\_4\text{)}\_2\text{ A}\_2 \cdot 1.5\text{B} + 2\text{H}^+ + 2\text{HSO}\_4^-\tag{15}$$

The synergistic extraction of cerium(IV) from sulfuric acid medium using a mixture of 2-ethylhexyl phosphoric acid mono 2-ethylhexyl ester (HEH/EHP, HL) and Di-(2-ethyl hexyl) phosphoric acid (HDEHP, HA) as extractants was studied [26]. The results showed that HEH/EHP = 0.6 was extracted as an organic phase in the form of Ce(SO<sup>4</sup> )0.5HL<sup>2</sup> A2 . The synergistic extraction reaction is as follows:

$$\text{Ce}^{\text{+}} + 0.5 \text{SO}\_{4}^{\text{-}} + \text{H}\_{2}\text{L}\_{2} + \text{H}\_{2}\text{A}\_{2} = \text{Ce} \left\{ \text{SO}\_{4} \right\}\_{05} \text{HL}\_{2}\text{A}\_{2} + 3\text{H}^{+} \tag{16}$$

## **4. Preparation of Ce product**

Cerium and its compounds are widely used in many fields. For example, monocrystalline CeO<sup>2</sup> due to its excellent redox ability has an important application value in the fields of CO catalytic oxidation, organic synthesis catalysis, photocatalysis, biological oxidation resistance, and so on. Generally speaking, the preparation of Ce products can be divided into two main methods: one is the sulfuric acid double salt precipitation method and the other is the method of solvent extraction. The process of sulfuric acid double salt precipitation is as follows:

Sulfuric acid rare earth solution containing Ce(IV) → reduces Ce4+ to form Ce3+ → NaOH base decomposition → HCl solubilization → Oxalic acid precipitation → Calcination.

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ja00471a056

Chuan Rare Earth. 2009;**3**:4-7. (in Chinese)

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However, the process of this method is long; there are many steps of solid-liquid separation, the yield of rare earth is low (65–67%) and the purity of CeO<sup>2</sup> product is low (95–99%). In addition, the associated resources such as F, P, and Th in sulfuric acid leach liquor were not recovered effectively, resulting in waste of resources and radioactive pollution. In contrast, however, solvent extraction has many advantages. The loaded organic phases could be used to prepare nano-size CeF<sup>3</sup> and CePO<sup>4</sup> by reductive stripping.

Our group developed a method to prepare high-purity CeF<sup>3</sup> nano-powder. This method was to extract from the sulfuric acid liquor of bastnaesite to obtain the loaded organic phase firstly, and then the CeF<sup>3</sup> can be obtained by reduction stripping used H<sup>2</sup> O2 . The purity of CeO<sup>2</sup> can be reached to 99.99% [87]. The preparation of CePO<sup>4</sup> nanoparticle was also invented by our group. This method was to extract Ce(IV) by [A336][P507] in H<sup>2</sup> SO<sup>4</sup> /H3 PO<sup>4</sup> system, and then the CePO<sup>4</sup> can be obtained by reduction stripping also using H<sup>2</sup> O2 [26].
