**8.8. Extraction of rare-earth elements**

**Fig. 19.** Flotation system with interrelating subsystems (chemistry, equipment and operating components)(a) and so‐ dium oleate in an anionic collector that can be used to render apatite hydrophobic in alkaline environments (b) [4].

The selectivity of froth flotation processes is highly influenced by the specificity of integra‐ tions between minerals and reagents, which are used to control the hydrophobic/hydrophil‐

The use of additives is a tool for the control of surface tension of the flotation system. Additives (flotation reagents) used in phosphate flotation are synthetic organic species. They are produced via the ethoxylation of fatty alcohols. Alcohols are obtained from vegetable oils or animal fats. Ethylene oxide comes from the petroleum industry. These reagents may exhibit variable molecular composition and number of carbon atoms in the hydrocarbon chain, as well as the presence of double bonds, different stereochemistry (*cis*-*trans* isomerism) and also several levels of ethoxylation. The additives employed in phosphate ore flotation contain the carbon chains of different lengths, with a predominance of 18 carbon atoms. The ethoxyla‐ tion level is represented by the average number of ethylene oxide groups in the molecule. Best results were achieved with three or four groups. The dosage of additives is 5% with respect to

The organic reagents, such as guar gum, cashew gum, tannins, dextrin, ethyl cellulose and carboxymethylcellulose, are capable of acting as depressor in the flotation of igneous phos‐ phate ores. The performance of corn starches was consistently superior to that of those reagents [53],[54]. The depressing ability of starch and ethyl cellulose appears to be related to steric compatibility between the positions of cations present on the mineral surface and

The role of surface and porosity was investigated by ZHONG et al [55]. When the samples were not aged prior to the collector (potassium oleate) addition, the floatability was controlled by the dissolution (of calcium) and adsorption (of oleate) behaviors, which, in turn, were governed by the surface area. It appears that the surface constituted by pores had lower influence on the adsorption and dissolution characteristics than the external surface. This was suggested to be due to slow diffusion of calcium through the pores, which resulted in reduced dissolution rate, as well as the non-participation of a substantial portion of pores in the adsorption process. When the samples were aged prior to the oleate addition, the bulk

ic character of mineral/water interfaces [52].

the collector dosage, reaching 10% under special conditions [53].

406 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

hydroxyl groups within the molecular structure of reagents [52].

The group of rare earths consists of 14 lanthanides or 4f elements in the periodic table along with three more elements: lanthanum, scandium and yttrium. Lanthanides comprise 15

elements with atomic numbers 57 – 71, which include lanthanum (La), cerium (Ce), praseo‐ dymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadoli‐ nium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). All elements occur in nature, while promethium (Pm) originates as a part of radioactive decay. Elements La, Sc and Y have physiochemical proper‐ ties similar to rare earths and are associated with the same minerals. Since they have similar chemical properties, the elements in the lanthanide series, yttrium and scandium, are considered as rare-earth elements (REE). Another classification used is light rare-earth elements (LREEs, atomic numbers 57 – 63: La, Ce, Nd, Pr, Pm, Sm and Eu) and heavy rareearth elements (HREEs, atomic numbers 64 – 71: Gd, Tb, Dy, Ho, Er, Tm and Yb) [59],[60].

**Fig. 20.** Applications of REE (a) [62] and the coordination numbers and abundances of LREEs (white cycle)- and HREEs (black cycle)-bearing minerals. The size of circle indicates rough abundance of REEs for each mineral class (b) [64].

Most of the REE deposits exist in China, America, India, Middle Asian nations, South Africa, Australia and Canada. The demand for REEs has increased in recent years due to the uncer‐ tainty of the supply and high technological applications associated with their characteristic electronic, optical and magnetic properties (**Fig. 20**). RE phosphate minerals, such as mona‐ zite, florencite, xenotime, cheralite and britholite, are the most naturally abundant forms that are associated with fluorapatite [47],[61],[62],[63],[64].

The techniques described in **Chapter 8** are usually used for concentrating REE minerals prior to the extraction of REEs from phosphate rocks (PR).10 A pre-leaching stage with mineral acid (**Eq. 19** and **Eqs. 13** – **16**) can be useful in order to selectively leach the FAP fraction as well as other impurities such as sodium, potassium, magnesium, aluminum, iron, manga‐ nese, uranium and thorium associated with the FAP lattice, resulting in REE-enriched concentrate [47].

<sup>10</sup> The processing chain for PR results in the majority of trace elements being lost either to waste disposal or to the environment (mainly soil and water) through fertilizer consumption and the food chain [47].

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2 + 20\text{ HCl} \rightarrow 10\text{ CaCl}\_2 + 6\text{ H}\_3\text{PO}\_4 + 2\text{ HF} \tag{13}$$

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2 + 20\text{ HNO}\_3 \to 10\text{ Ca}\text{(NO}\_3\text{)}\_2 + 6\text{ H}\_3\text{PO}\_4 + 2\text{ HF}\tag{14}$$

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_6\text{F}\_2 + 20\text{ HClO}\_4 \rightarrow \text{l0 Ca}\text{(ClO}\_4\text{)}\_2 + 6\text{ H}\_3\text{PO}\_4 + 2\text{ HF} \tag{15}$$

$$\text{Ca}\_{10}\text{(PO}\_4\text{)}\_{6}\text{F}\_2 + 14\text{ H}\_3\text{PO}\_4 \rightarrow 10\text{ Ca}\text{(H}\_2\text{PO}\_4\text{)}\_2 + 2\text{ HF} \tag{16}$$

However, leaching efficiencies can vary significantly depending on the mineralogy of the ore and the type of acid used. H3PO4 and HF acids formed during the leaching process of FAP with acids interfere and change the leaching efficiency [61].

The effect of aliphatic and aromatic low molecular weight organic acid on the release of REEs and yttrium from phosphate minerals was investigated by GOYNE et al [65]. The performance of acid increases in the following order:

No ligand ≈ salicylic acid < phthalic acid ≈ oxalic acid < citric acid.

elements with atomic numbers 57 – 71, which include lanthanum (La), cerium (Ce), praseo‐ dymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadoli‐ nium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). All elements occur in nature, while promethium (Pm) originates as a part of radioactive decay. Elements La, Sc and Y have physiochemical proper‐ ties similar to rare earths and are associated with the same minerals. Since they have similar chemical properties, the elements in the lanthanide series, yttrium and scandium, are considered as rare-earth elements (REE). Another classification used is light rare-earth elements (LREEs, atomic numbers 57 – 63: La, Ce, Nd, Pr, Pm, Sm and Eu) and heavy rareearth elements (HREEs, atomic numbers 64 – 71: Gd, Tb, Dy, Ho, Er, Tm and Yb) [59],[60].

408 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**Fig. 20.** Applications of REE (a) [62] and the coordination numbers and abundances of LREEs (white cycle)- and HREEs (black cycle)-bearing minerals. The size of circle indicates rough abundance of REEs for each mineral class (b)

Most of the REE deposits exist in China, America, India, Middle Asian nations, South Africa, Australia and Canada. The demand for REEs has increased in recent years due to the uncer‐ tainty of the supply and high technological applications associated with their characteristic electronic, optical and magnetic properties (**Fig. 20**). RE phosphate minerals, such as mona‐ zite, florencite, xenotime, cheralite and britholite, are the most naturally abundant forms that

The techniques described in **Chapter 8** are usually used for concentrating REE minerals prior

acid (**Eq. 19** and **Eqs. 13** – **16**) can be useful in order to selectively leach the FAP fraction as well as other impurities such as sodium, potassium, magnesium, aluminum, iron, manga‐ nese, uranium and thorium associated with the FAP lattice, resulting in REE-enriched

10 The processing chain for PR results in the majority of trace elements being lost either to waste disposal or to the

environment (mainly soil and water) through fertilizer consumption and the food chain [47].

A pre-leaching stage with mineral

are associated with fluorapatite [47],[61],[62],[63],[64].

to the extraction of REEs from phosphate rocks (PR).10

[64].

concentrate [47].

The utilization of organophosphorus reagents, such as Talcher organic phosphorus solvent (TOPS 99), an equivalent to di-2-ethylhexyl phosphoric acid, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC-88A) and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272), etc., for the extraction of REEs was also reported [66],[67],[68],[69].

Systematic study of the thermal decomposition of monazite to remove phosphate in order to achieve more complete conversion of rare-earth phosphate into its oxides was performed by KUMARI et al [62]. The method is based on roasting of monazite with CaO, Na2CO3 and NaOH (**Fig. 21**):

$$2\text{ REEPO}\_4 + 3\text{ CaO}^{610-700^\circ \text{C}} \rightarrow \text{REE}\_2\text{O}\_3 + \text{Ca}\_3\text{(PO}\_4\text{)}\_2\tag{17}$$

$$2\text{ REEPO}\_4 + 3\text{ Na}\_2\text{CO}\_3 \stackrel{600-900^\circ \text{C}}{\rightarrow} \text{REE}\_2\text{O}\_3 + 2\text{ Na}\_3\text{PO}\_4 + 3\text{ CO}\_2\tag{18}$$

$$\text{REEPO}\_4 + 3\text{ NaOH} \overset{29\text{--}49\text{°C}}{\rightarrow} \text{REE} \text{(OH)}\_3 + \text{Na}\_3\text{PO}\_4\tag{19}$$

Washed monazite concentrate achieved from roasting was dried and leached by diluted HCl:

$$2\text{ REE}\_2\text{O}\_3 + 6\text{ HCl} \rightarrow 2\text{ REECl}\_3 + 3\text{ H}\_2\text{O} \tag{20}$$

**Fig. 21.** Process flow sheet for the separation of phosphate and recovery of REMs from monazite [62].

Optimal condition includes 2 h of leaching by 6 M HCl at the temperature of 80°C. The pulp density should be of 30 g·dm−3 [62].

The optimization of leaching operation of rare-earth-bearing ores is a complex process since many attributes simultaneously affect the operation, with some of them being conflicting in nature. Therefore, a proper selection of leaching process with pertinent attributes is crucial for the user in order to maximize the percentage recovery at minimal operating costs. The methodology is proposed by BARAL et al [70]. The parameters affecting the performance of leaching operation are listed in **Table 3**.



**Table 3.** Operating conditions affecting the performance of leaching procedure [70].
