**1. Introduction**

Rare earth (RE) elements exist in a variety of minerals, some of which, such as bastnaesite, monazite, and Bayan Obo mixed rare earth ores, are of great significance and commercial value [1–3]. As for the rare earth resources in the world, bastnaesite deposits in China and the United States account for the largest proportion. The carbonatite-hosted bastnaesite deposits at Mountain Pass, the bastnaesite deposits in Sichuan Province, and the large deposits in Bayan Obo area are the most noteworthy deposits. Monazite deposits in Australia, Brazil, China, India, Malaysia, South Africa, Sri Lanka, Thailand, and the United States are the second largest

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

deposits [4–8]. The Bayan Obo mixed rare earth ores, which is mainly composed of bastnaesite and monazite, is the largest light rare earth resource in the world [3, 9]. The Bayan Obo deposit is actually the tailings of the iron ore processing scheme of Fe-RE-Nb deposit [10, 11].

Cerium (Ce) is the RE element present in the highest concentration in light RE ores, CeO<sup>2</sup> /∑REO reaches 50% in bastnaesite (REFCO<sup>3</sup> ), while CeO<sup>2</sup> /∑REO is approximately as high as 45–50% in monazite (REPO<sup>4</sup> ) [12]. **Table 1** gives the chemical compositions of different light RE concentrates [12]. Cerium atoms' electronic configuration is [Xe]4f<sup>1</sup> 5d1 6s2 . Losing two 6s electrons and one 5d electron, it forms the most common Ce(III). Ce(III) has the tendency to lose electron to become 4f0 and forms Ce(IV) [13]. Ce(IV) can exist stably in the aqueous solution because of the lowest standard electrode potential of Ce(IV)/Ce(III) [14]. Ce(IV) exhibits a markedly different chemical behavior compared to other RE(III). The use of the variable valence properties of cerium and its stable structure is of great significance in RE separation [15]. Solvent extraction is reported to be one of the most effective techniques to extract Ce(IV) [16]. When Ce(III) was oxidized into Ce(IV), it can be easily separated from RE(III) because of high separation factor values of Ce(IV) to Th(IV) (>100) and RE(III) (>600) [17].

Compared with other rare earth elements, Ce(IV) has strong complexation and coordination ability because of its small ionic radius [18]. Ce(IV) can form stable complexes with F− , NO<sup>3</sup> − , HSO<sup>4</sup> − , and H2 PO<sup>4</sup> − in solution, which are easy to be extracted by organic extractants [19–21]. Utilizing complex properties of Ce(IV) with these anions, the recovery of Ce and these associated resources from rare earth ore by solvent extraction can be realized.

ore by utilizing the property of valence change of cerium. The diagram of this process is shown in **Figure 1**. When Ce(III) was oxidized to Ce(IV), F(I) and P can be easily recovered from leaching liquor by solvent extraction in virtue of the complex properties of Ce(IV) with F(I) and P

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

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

7

Ce is the only lanthanide element that can form stable molecular complexes in the +4 oxidation

However, much less is known about the properties of Ce(IV) aqueous species than those of Ce(III). For example, the hydration structure of Ce(II) has been extensively studied by many methods [52–58]. In most cases, it is a part of the systematic study of the trivalent rare earth series. In contrast, even for the simplest aqua species of purely hydrated complex, there is little success in the identification of Ce(IV) aqua complexes in solution. In fact, in the scientific literature, Ce(IV) aqua species are often described only as "Ce(IV)" or "Ce(IV) complex," without specifying their chemical species or composition, simply because of a lack of information [59]. Moreover, because of their high electric charge, Ce(IV) has a strong tendency toward hydrolysis in aqueous solution and undergoes polynucleation or further, leading to colloid formation [60]. Several precedent studies have also implied the formation of soluble polymeric species with oxo- and/or OH-bridging [61]. Based on an extended X-ray absorption fine structure (EXAFS) study and density functional theory (DFT) calculations, Ikeda-Ohno et al. have demonstrated that the Ce(IV) ion in perchloric acid exists in the form of oxo-bridged dimer (**Figure 2**) [59].

electron configuration [51].

[18]. After extraction, Ce products can be obtained by reductive stripping.

state. The stability of the +4 state of cerium is attributed to the 4f<sup>0</sup>

**2. Aqueous chemistry of Ce(IV)**

**Figure 1.** Oxidation and separation of Ce(IV) [18].

**2.1. Ce(IV) in aqueous solution**

A number of extractants such as acidic organophosphorus extractants [22–27], neutral organophosphorus extractants [28–36], amines [37], bifunctional ionic liquid extractants (Bif-ILEs) [38–40], and others [41–48] have been used for the extraction and separation of Ce. When Ce was extracted into organic phase, pure cerium solution can be obtained by stripping, and then cerium products can be obtained by adding ammonium carbonate, oxalic acid, ammonia water, and so on. The high purity cerium oxide can be produced by calcination of these cerium products.

In China, there are three main RE ores, about 80% mixed RE ores in Bayan Obo, 10% bastnaesite in Sichuan and Shandong, and 2.9% ion adsorption ores in South of China. Bayan Obo ores, the largest light rare earth sources in the world, are known as the most refractory rare earth minerals for processing due to the complicated mineral structures and compositions [49]. There are 7–8% fluorine element (F) and 4–6% phosphorus element (P) in Bayan Obo mixed RE ore [15]. The classical process of decomposing mixed RE concentrate by roasting with concentrated sulfuric acid at a high temperature cannot recover F and P, which could cause environmental pollution and waste of resources [50]. Our group proposed a process for Bayan Obo mixed RE


**Table 1.** Chemical compositions of different light RE concentrates.

Extraction and Recovery of Cerium from Rare Earth Ore by Solvent Extraction http://dx.doi.org/10.5772/intechopen.79225 7

**Figure 1.** Oxidation and separation of Ce(IV) [18].

deposits [4–8]. The Bayan Obo mixed rare earth ores, which is mainly composed of bastnaesite and monazite, is the largest light rare earth resource in the world [3, 9]. The Bayan Obo deposit

one 5d electron, it forms the most common Ce(III). Ce(III) has the tendency to lose electron to

the lowest standard electrode potential of Ce(IV)/Ce(III) [14]. Ce(IV) exhibits a markedly different chemical behavior compared to other RE(III). The use of the variable valence properties of cerium and its stable structure is of great significance in RE separation [15]. Solvent extraction is reported to be one of the most effective techniques to extract Ce(IV) [16]. When Ce(III) was oxidized into Ce(IV), it can be easily separated from RE(III) because of high separation

Compared with other rare earth elements, Ce(IV) has strong complexation and coordination

Utilizing complex properties of Ce(IV) with these anions, the recovery of Ce and these associ-

A number of extractants such as acidic organophosphorus extractants [22–27], neutral organophosphorus extractants [28–36], amines [37], bifunctional ionic liquid extractants (Bif-ILEs) [38–40], and others [41–48] have been used for the extraction and separation of Ce. When Ce was extracted into organic phase, pure cerium solution can be obtained by stripping, and then cerium products can be obtained by adding ammonium carbonate, oxalic acid, ammonia water, and so on. The high purity cerium oxide can be produced by calcination of these cerium products.

In China, there are three main RE ores, about 80% mixed RE ores in Bayan Obo, 10% bastnaesite in Sichuan and Shandong, and 2.9% ion adsorption ores in South of China. Bayan Obo ores, the largest light rare earth sources in the world, are known as the most refractory rare earth minerals for processing due to the complicated mineral structures and compositions [49]. There are 7–8% fluorine element (F) and 4–6% phosphorus element (P) in Bayan Obo mixed RE ore [15]. The classical process of decomposing mixed RE concentrate by roasting with concentrated sulfuric acid at a high temperature cannot recover F and P, which could cause environmental pollution and waste of resources [50]. Our group proposed a process for Bayan Obo mixed RE

**REO CeO2**

Bayan Obo mixed ore 60% 50% 7–8% 6–7%

Monazite (Guangdong) 55–65% 45–50% 25–30%

Bastnaesite (Sichuan) 60% 50% 8–10%

**Table 1.** Chemical compositions of different light RE concentrates.

ability because of its small ionic radius [18]. Ce(IV) can form stable complexes with F−

) [12]. **Table 1** gives the chemical compositions of different light RE concen-

in solution, which are easy to be extracted by organic extractants [19–21].

**/∑REO F P2**

**O5**

and forms Ce(IV) [13]. Ce(IV) can exist stably in the aqueous solution because of

5d1 6s2 /∑REO

, NO<sup>3</sup> − ,

/∑REO is approximately as high as 45–50% in

. Losing two 6s electrons and

is actually the tailings of the iron ore processing scheme of Fe-RE-Nb deposit [10, 11].

Cerium (Ce) is the RE element present in the highest concentration in light RE ores, CeO<sup>2</sup>

), while CeO<sup>2</sup>

reaches 50% in bastnaesite (REFCO<sup>3</sup>

6 Cerium Oxide - Applications and Attributes

PO<sup>4</sup> −

trates [12]. Cerium atoms' electronic configuration is [Xe]4f<sup>1</sup>

factor values of Ce(IV) to Th(IV) (>100) and RE(III) (>600) [17].

ated resources from rare earth ore by solvent extraction can be realized.

monazite (REPO<sup>4</sup>

become 4f0

HSO<sup>4</sup> − , and H2

> ore by utilizing the property of valence change of cerium. The diagram of this process is shown in **Figure 1**. When Ce(III) was oxidized to Ce(IV), F(I) and P can be easily recovered from leaching liquor by solvent extraction in virtue of the complex properties of Ce(IV) with F(I) and P [18]. After extraction, Ce products can be obtained by reductive stripping.
