**1. Introduction**

The REE are fifteen lanthanide elements in the periodic table with atomic numbers of 57 to 71, including lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu) as well as scandium (Sc) and yttrium (Y) with atomic numbers of 21, and 39, respectively. There is a fast growth in new applications and demand for the REE, especially in energy, environment, and high technology fields with durability, high efficiency and low carbon emissions [1–5]. These elements are called "rare" owing to their difficult extraction from deposits that is attributed to the similarity in the physical and chemical properties of REE and gangue minerals and to the difficulty to find concentrated deposits. Another challenge for REE production is the heterogeneity of these elements in the deposits [4,6–9], which plays a vital role in configuring the unit operations regarding the geology, versatility, and composition of the minerals [4,6]. The production of REE requires several steps of magnetic, gravity, and electrostatic separations in addition to flotation to efficiently separate the REE from associated gangues with similar physical properties. The individual production of the REE is very challenging owing to their similar chemical properties, and specific extraction techniques are required to recover REE; however, europium and cerium are exceptions, i.e., cerium can be formed as, either trivalent or tetravalent during hydrometallurgical process where the tetravalent cerium can be separated from the trivalent REE [10,11]. Figure 1 shows a typical REE production process, including geology, mining, physical beneficiation, hydrometallurgy, and separation of individual elements [6].

**Figure 1.** The REE general processing plant.

Since mining and refining of low-grade REE-bearing minerals are technically infeasible owing to a lithophilic nature of the REE [12,13], high-grade REE-bearing minerals, such as bastnäsite, monazite, and xenotime, are used for economic extraction of these elements [4,12,14–16]. Table 1 summarizes both the low-grade and high-grade REE-bearing minerals.

Among various gangues, radioactive elements are a serious challenge in the REE production process, regarding specific regulations for safety management in the processing units (for more details refer to [17–19]). Thorium (Th) and uranium (U) are naturally occurring radioactive materials (NORM), which can be found in the REE deposits (Table 1). Monazite and xenotime are the most known REE-bearing minerals that contain radioactive elements. For instance, the REE-bearing ore at Mountain Pass, i.e., a major bastnäsite resource in California with rare earth oxides (REO) of 8.5 wt.%, contains thorium (Th) and uranium (U) of 0.02, and 0.002 wt.%, respectively [19]. In addition, the Bayan Obo bastnäsite and monazite deposit in China contains minerals such as fluorite, magnetite, barite, calcite and quartz with magnetic susceptibility, specific gravity, electrical conductivity, or floatability similar to REE-bearing minerals [20,21].

The low concentration of radioactive elements in the upstream rare earth ore processing units, e.g., in physical beneficiation, results in quite low emission of radioactivity, whereas the higher concentration of the radioactive elements in downstream separation lines requires safety measurements to be carefully considered. For example, an exposure of a worker to an ore containing 500 ppm thorium and 50 ppm uranium, staying 1 m away from a large mass of the ore for an entire working year, leads to a total exposure of 2.4 mSv that is below the dose limit for a NORM worker, 20 mSv [17]. This exposure is mainly caused by ore dust inhalation (at 1 mg/m3) and incidentally ore ingestion (at 100 mg/day). Therefore, a step for separation of thorium and uranium is required to minimize risks associated with REE production in terms of safety, environmental hazards, and quality of the final product [22].

In the present paper, various hydrometallurgical techniques applied during REE production process for separating thorium (Th) and uranium (U) are reviewed, including leaching, precipitation, solvent extraction, ion chromatography, and membrane to understand the advantages and limitations of each technique. In addition, the process selection with regards to the feed properties, waste management of the separated radioactive elements, and how they can be treated to produce valuable co-products are also discussed.


**Table 1.** REE-bearing minerals and gangue minerals from deposits.

It is worthy of mentioning that recovery of REE from secondary sources such as electronic wastes [24,25], red mud (Bauxite) [26–29], and coal [30] has also been recently investigated. The separation of radioactive elements during these processes is out of the scope of the present review article and requires further study.

#### **2. Separation by Leaching**

Leaching is a process based on the different solubility of elements in a leach liquor. To separate the radioactive elements from REE, leaching process is typically applied on, either fresh or concentrated ore [4], in order to maximize the solubility of radioactive elements [31–34] or REE [35] in the liquor.

According to the literature, a one-step leaching process faces technical issues during separation of thorium and uranium from REE owing to the occurrence of undesired reactions and leaching of un-wanted components [36]. For instance, Lapidus and Doyle [33] applied a one-step leaching process to separate radioactive elements from a monazite concentrate using oxalate reagent for leaching out thorium oxalate in the liquid form while rare earth oxalate remains in the solid form. They observed that either Th(HPO4)2 or Th3(PO4)4 is re-precipitated in the liquor at a pH < 3, and oxalate reagent forms stable complexes with other metal impurities, instead of reaction with radioactive elements. To overcome the drawbacks of this process, a two-step cracking-leaching process was proposed. In the first step (cracking), an alkaline reagent, e.g., NaOH, cracks the concentrate of the REE-bearing mineral, either monazite or xenotime, to produce a hydroxide cake containing the REE, thorium, uranium, and some other impurities, reactions 1 and 2 [31,33,35]. This step eliminates the re-precipitation of thorium phosphate resulting in the formation of hydroxide forms of the REE and thorium, which can be separated in the second leaching step [34,37,38],

$$2\text{REPO}\_4 + 6\text{NaOH} \rightarrow 2\text{RE}(\text{OH})\_3(\downarrow) + 2\text{Na}\_3\text{PO}\_4 \tag{1}$$

$$\text{Th}\_3(\text{PO}\_4)\_4 + 12\text{NaOH} \rightarrow 3\text{Th}(\text{OH})\_4(\downarrow) + 4\text{Na}\_3\text{PO}\_4 \tag{2}$$

After the cracking step, acid leaching of the produced hydroxide cake is performed wherein the acidic oxalate reagents are employed to leach the radioactive elements while the REE oxalate is insoluble [34]. If the REE is preferred to be in the liquor solution, other acids such as nitric acid (reaction 3) [35] or hydrochloric acid (reaction 4) [37] can be used instead of oxalate reagents; however, the uranium also tends to leach out with the REE if the pH is not properly controlled,

$$\rm{RE(OH)\_3 + 3HNO\_3 \to RE(NO\_3)\_{3(aq)} + 3H\_2O} \tag{3}$$

$$\text{RE}(\text{OH})\_3 + \text{3HCl} \rightarrow \text{RECl}\_{\text{3(aq)}} + \text{3H}\_2\text{O} \tag{4}$$

The two-step approach can be improved by employing high-pressure leaching to stabilize products that are unstable at atmospheric conditions. Figure 2 shows an increase in the recovery of thorium by ammonium carbonate (250%), from 13% at atmospheric pressure to 98% at 6.5–10 atm at 70 ◦C, while no significant change occurs in the recovery of REE [31].

**Figure 2.** Effect of the pressure on the recovery of thorium (Th) and REE at 70 ◦C, adapted from [31]. (Reproduced with permission from ref. [31], copyright (2002), Elsevier)

In such a high-pressure leaching process, the ammonium carbonate leaches both the thorium, reaction 5, and uranyl hydroxides, reaction 6, while it produces a solidus complex after reaction with REE, reaction 7 [31,32]. Typically, an excess amount of reagent is employed in either atmospheric or high-pressure leaching owing to the consuming part of the reagent in the secondary reactions,

$$5\text{ Th}(\text{OH})\_4 + 5(\text{NH}\_4)\_2\text{CO}\_3 \rightarrow (\text{NH}\_4)\_6[\text{Th}(\text{CO}\_3)\_5] + 4\text{NH}\_4\text{OH}\tag{5}$$

$$2\text{ UO}\_2(\text{OH})\_2 + 3(\text{NH}\_4)\_2\text{CO}\_3 \rightarrow (\text{NH}\_4)\_4[\text{UO}\_2(\text{CO}\_3)\_3] + 2\text{NH}\_4\text{OH}\tag{6}$$

$$2\text{RE}(\text{OH})\_3 + 4(\text{NH}\_4)\_2\text{CO}\_3 \rightarrow \text{RE}\_2(\text{CO}\_3)\_3(\text{NH}\_4)\_2\text{CO}\_3(\downarrow) + 6\text{NH}\_4\text{OH}\tag{7}$$

Figure 3 illustrates a flowsheet summarizing the cracking-leaching approaches (two-step leaching process) wherein, either REE or radioactive elements can be extracted in the leach liquor, depending on the leaching reagent. Table 2 summarizes the experimental conditions and the overall recovery of REE, thorium, and uranium in leach liquor for the two-step cracking-leaching process.

**Table 2.** Uranium and thorium separation from REE by a two-step cracking-leaching process.


**Figure 3.** Leaching-cracking approaches for separation of REE from radioactive elements.

#### **3. Separation by Precipitation**

The REE can be separated from other elements in a liquor via precipitation method using an appropriate reagent such as oxalic acid, reaction 8 [39–42],

$$\text{R}2\text{R}^{3+} + 3\text{H}\_2\text{C}\_2\text{O}\_4 + 10\text{H}\_2\text{O} \to \text{RE}\_2(\text{C}\_2\text{O}\_4)\_3 \cdot 10\text{H}\_2\text{O} (\downarrow) + 6\text{H}^+\tag{8}$$

To separate the REE from the radioactive elements through precipitation, the most important parameters, include the type of the liquor feed, REE concentration in the feed, the concentration of the precipitation reagent, and the mass ratio of the reagent to REE. The pH is another parameter that determines the performance of selective precipitation of uranium, thorium, and REE. The ideal case is to find distinct pH values to separate radioactive elements from REE. Uranium and REE precipitate at close pH values with a risk of co-precipitation, whereas a high recovery of thorium is achievable since its precipitation requires a different pH [43–46]. Table 3 presents the pH ranges required for the precipitation of uranium, thorium, and REE in both chloride and sulfate liquors. In an acid liquor, some of alkali precipitation reagents are preferred owing to higher selectivity towards radioactive elements [46–49]. For instance, ammonium hydroxide (NH4OH) at a pH close to 5 [50], and sodium hydroxide [47] precipitate thorium with a small loss of REE. Whereas, potassium iodate (KIO3) is inefficient in the precipitation of thorium due to the co-precipitation of REE [50]. In addition, ammonium hydroxide (NH4OH) precipitates uranium at a pH close to 4.5, which is far enough from the REE precipitation pH of about 6 [51].

**Table 3.** The pH ranges for precipitation of thorium, uranium and the REE in different liquors.


#### *3.1. Types of the Liquor and Reagent*

Typically, the main liquors produced through the industrial hydrometallurgical processes are chloride and sulfate [4]. In the chloride liquor, precipitation of the radioactive elements is usually achieved by adding an alkali reagent. If the pH is kept close to or below 5.5, the total thorium and a part of uranium are likely precipitated and recovered while the loss of the REE is about 2% [44,45]. A lower pH for precipitation of radioactive elements has been reported in a sulfate liquor compared to the chloride liquor. Table 3 presents the pH range required for precipitating thorium and uranium from chloride and sulfate liquors. For example, 100% thorium is precipitated from sulfate liquor at a pH close to 1 using ammonium hydroxide. However, REE (44.7% La, 63.5% Ce, and 63.2% Nd) are also co-precipitated under such a highly acidic condition [43,46].

In some cases, nitrate liquor has also been studied for separation of uranium from REE [51,52] wherein the uranium was precipitated by 90% using hydroxide reagents at a pH of 4.5 [51]. Further, a higher efficiency has also been obtained for a precipitation pH between 6 to 8, at which almost total recovery of uranium is achieved in the form of β–UO2(OH)2 [52].

The type of reagent is another critical parameter that determines which elements, e.g., thorium, uranium, or REE, are precipitated, while the others remain in the aqueous phase. A double sulphate reagent produces the REE precipitate in the form of MRE(SO4)2 with M representing sodium, potassium or ammonium, leaving radioactive elements in the aqueous phase, reaction 9 [53]. Precipitation of the trivalent REE is attributed to the low solubility of the REE sulfate in water, resulting in the separation from the radioactive elements and tetravalent REE, i.e., cerium (IV):

$$\text{R} \to \text{RE}(\text{SO}\_4)\_3 + \text{Na}\_2\text{SO}\_4 + 2\text{H}\_2\text{O} \to \text{Na}\_2\text{SO}\_4\text{RE}\_2(\text{SO}\_4)\_3 \cdot 2\text{H}\_2\text{O} \downarrow \tag{9}$$

### *3.2. Multi- or Single-Step Precipitation*

The precipitation for separation of radioactive elements and REE can be conducted in either multi-steps or a single step, regarding the types of the liquor and reagents. A secondary purification step is necessary when the reagent is selective to one of the radioactive elements, while the other one remains with REE. For instance, a 30% concentrated oxalic acid precipitates 99% thorium along with 98% REE at 30 ◦C, leaving uranium in the solution according to reactions 10 to 13 [35,39,46,54]. Next, a mixed alkali solution of Na2CO3 and NaHCO3 selectively leaches and recovers 99% thorium from the oxalate cake, reaction 14, while the REE remains in the solid form as carbonates [39]:

$$\rm{R}\rm{RE}\_2(\rm{SO}\_4)\_3 + 3\rm{H}\_2\rm{C}\_2\rm{O}\_4 \rightarrow \rm{RE}\_2(\rm{C}\_2\rm{O}\_4)\_3 \downarrow + 3\rm{H}\_2\rm{SO}\_4 \tag{10}$$

$$\rm{UO\_2SO\_4 + H\_2C\_2O\_4 \to UO\_2(C\_2O\_4) + H\_2SO\_4} \tag{11}$$

$$\rm{UO\_2(C\_2O\_4) + 3H\_2SO\_4 \to UO\_2SO\_4 + 2CO\_2 + 3H\_2O} \tag{12}$$

$$\text{Th}(\text{SO}\_4)\_2 + 2\text{H}\_2\text{C}\_2\text{O}\_4 \rightarrow \text{Th}(\text{C}\_2\text{O}\_4)\_2 \downarrow + 2\text{H}\_2\text{SO}\_4 \tag{13}$$

$$\text{Th(C}\_2\text{O}\_4)\_2 + 4\text{Na}\_2\text{CO}\_3 + 2\text{NaHCO}\_3 \rightarrow \text{Na}\_6\text{Th(CO}\_3)\_5 + 2\text{Na}\_2\text{C}\_2\text{O}\_4 + \text{CO}\_2 + \text{H}\_2\text{O} \tag{14}$$

Vijayalakshmi et al. [46] also reported a multi-step separation of radioactive elements from REE wherein the thorium is initially precipitated from a sulfate liquor by adding ammonium hydroxide (NH4OH). Then, REE was separated from uranium in a secondary precipitation step in the form of REE oxalate. They suggested employing an excess amount of oxalic acid to lower the pH of the solution, and adjust it between 1 and 2 to avoid co-precipitation of impurities, e.g., aluminum and iron, with REE [40]. In such a multi-step separation of radioactive elements and REE, control of pH is more comfortable during the process, and the efficiency of the recovery is high. However, a large amount of reagent is required, which is economically infeasible.

To overcome the drawbacks of the multi-step precipitation, various studies were conducted to recover either the REE or both thorium and uranium in a single-step process. Kul et al. [41] applied a double-salt single-step approach and reported 98% recovery of REE in the precipitate while only 15% thorium is co-precipitated. The hydroxide reagents have the potential for a single-step recovery of the radioactive elements from a chloride liquor [48,49,55]. For instance, the hydrated lime, Ca(OH)2, precipitates thorium at a pH of 2.5, while the loss of REE through co-precipitation is minimized [48]. Yu et al. [49] employed the hydrated lime to extract the thorium and uranium from a chloride liquor which was produced from the processing of monazite and REE carbonatite minerals from the Montviel

deposit in the North West of Quebec in Canada. They demonstrated that the addition of hydrogen peroxide (H2O2) is necessary to oxidize the iron and facilitate its precipitation. They reported recovery of 99% Th, 80% U, and over 90% iron and phosphate impurities, while minimizing the co-precipitation of REE to less than 2%. The advantage of the single-step separation process is a small amount of reagent required for the precipitation, whereas adjusting a proper pH for selective precipitation is difficult.

The flowsheet in Figure 4 [39,46,48], illustrates various potential precipitation pathways to separate radioactive elements from REE in a sulfate or chloride liquor that would be produced in the upstream hydrometallurgical processes. Table 4 summarizes the recovery yield of the radioactive elements obtained from different precipitation processes.

**Figure 4.** Process flowsheet for separation of Th and U from REE by precipitation.



#### **4. Separation by Solvent Extraction**

The solvent extraction technique has received significant attention for separation of thorium and uranium from REE by using appropriate extractants, which can be dissolved into an organic solvent to provide an immiscible phase and enough interface with the aqueous liquor of the REE.

This process is typically conducted by two main groups of extractants, including amine and organophosphorus extractants. The amine group can be divided into primary and tertiary types where the primary amine is highly selective towards the thorium in either sulfate or chloride liquors, and the tertiary amine is selective towards the uranium in sulfate liquors. The organophosphorus extractants are usually divided into acid and neutral types, which are applicable to nitrate, chloride and sulfate liquors.

#### *4.1. Solvent Extraction with Amine Extractants*

Amine extractants have been employed in the AMEX process, which was developed in the late 1950s, for extracting radioactive elements from REE-bearing minerals. In this process, the amine extractants are mixed with a sulfate leach liquor produced from monazite sands [57]. Thorium is first extracted with a primary amine (reaction 15) followed by a nitric acid stripping step. Then, uranium is extracted with a tertiary amine (reaction 16) and stripped with sodium carbonate,

$$\mathrm{^4RNH\_2H^+HSO\_4^-}\_{4\text{ (org)}} + [\mathrm{Th(SO\_4)\_4}]\_{4\text{(aq)}}^{4-} \rightarrow \left(\mathrm{RNH\_2H^+} \right)\_4 [\mathrm{Th(SO\_4)\_4}]\_{(\text{org)}}^{4-} + 4\mathrm{HSO\_4^-} \tag{15}$$

$$4\text{R}\_3\text{NH}^+\text{HSO}\_4^- + [\text{UO}\_2(\text{SO}\_4)\_3]^{4-}\_{\text{(aq)}} \rightarrow \text{(R}\_3\text{NH}^+\text{)}\_4[\text{UO}\_2(\text{SO}\_4)\_3]^{4-}\_{\text{(org)}} + 4\text{HSO}\_{4(aq)}^- \tag{16}$$

Table 5 lists some amines employed for solvent extraction of radioactive elements from the REE [58–60].


**Table 5.** Performance of amine extractants for uranium and thorium separation from REE.

The Primene JM-T is the most applied primary amine to extract thorium, and the Alamine 336 is a tertiary amine that has been employed for uranium extraction from the REE-bearing minerals [58,61,62]. The N1923, i.e., (C*n*H2*<sup>n</sup>*+1)2CHNH2 (*n* = 9–11), is an alternative primary amine for this process, which has been employed in Baotou and Sichuan in China [60,63–65]. The N1923 extractant is characterized by low solubility in water and a high separation factor between thorium and the REE, especially in sulfate liquors [66]. For instance, the selectivity of the N1923 for thorium is 600 times higher than for cerium in a sulfate liquor produced from the bastnäsite and monazite concentrates [60,66]. The reaction between the N1923 and thorium takes place in the interfacial zone, and it is controlled by the thorium mass transfer [67]. Figure 5 [60] shows a flowsheet for the recovery of thorium from REE-bearing minerals, where a thorium recovery of over 99% is achievable in an organic phase using a primary amine in a multi-step process, including solvent extraction, scrubbing, and precipitation [60].

Employing a mixture of primary and tertiary amines is a promising method for the simultaneous extraction of thorium and uranium from the REE, which is economically and technically favored due to a decrease in the number of steps in the solvent extraction process. For instance, simultaneous separation of Th and U has been reported from sulfate liquor during the processing of monazite using a mixture of Primene JM-T (i.e., a primary amine) and Alamine 336 (i.e., a tertiary amine), wherein the optimized process conditions (concentration of amines, contact time, and pH) resulted in the extraction of 99.9 and 99.5% thorium and uranium, respectively, while the extraction of REE was less than 0.1% [58].

**Figure 5.** Process of separating Th from REE of Baotou bastnäsite leaching.

## *4.2. Solvent Extraction with Phosphorous Extractants*

Phosphorous extractants are alternative for amines to extract the radioactive elements [4,68–72]. This group of extractants includes phosphate esters and amines. Table 6 presents the performance of different types of organophosphorus extractants during the solvent extraction process for separating Th and U from different REE-bearing liquors.


**Table 6.** Phosphorous extractants for uranium and thorium separation from REE.

#### 4.2.1. Acid Organophosphorus Extractants

Acidic organophosphorus extractants such as di-(2-ethylhexyl)-phosphoric acid (DEHP), (2-ethylhexyl) 2-ethylhexyl-phosphonic acid (EHEHP), and di-(2-ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP), have been proposed as promising reagents to separate radioactive elements from REE [64,65,78,79]. Reaction 17 shows absorption of thorium during treatment with DEHEHP [64],

$$\text{Th(NO}\_3\text{)}^{3+} + 3\text{NO}\_3\text{-}^{-} + \text{L}\_{\text{(aq)}} \rightarrow \text{Th(NO}\_3\text{)}\_4\text{-}\text{L}\_{\text{2(org)}}\tag{17}$$

where L is the ligand or extractant. The acid organophosphorus extractants are efficient, especially for the extraction of radioactive elements from highly acidic sulfate solutions. In addition, DEHP, EHEHP, and DEHEHP can be employed for individual separation of the REE [80], since these extractants have a low affinity to extract trivalent REE while cerium(IV) is simultaneously extracted with thorium and

uranium [65]. Nonetheless, to avoid a high loss of the tetravalent cerium, it would be individually recovered in a downstream stripping. Moreover, cerium(IV) can be recovered by applying a second solvent extraction process to the organic phase, owing to a high separation factor of cerium (IV) over thorium, which is 36.

The Cyanex is another type of acid organophosphorus extractant that is widely employed for thorium and uranium separation from REE [69,81–83]. The mechanism of metals extraction by Cyanex is the cation exchange [69], where the strength of the acid determines the performance of the Cyanex extractants. The commonly used Cyanex extractants are the Cyanex 272 (di-2,4,4-trimethyl phosphonic acid), Cyanex 301 (bis-2,4,4-trimethylprntyl phosphonic acid) [84], and Cyanex 302 [69]. The Cyanex 272 shows higher efficiency compared to the Cyanex 302 for separating thorium from lanthanides due to the higher strength of the latter acid [83]. In addition, applying a mixture of extractants, e.g., a mixture of HEH and EHP in kerosene, is a promising approach to increase the efficiency of the solvent extraction for separating thorium from REE (Table 6).

#### 4.2.2. Neutral Organophosphorus Extractants

The tri-n-butyl phosphate (TBP) is the most common neutral organophosphorus extractant for separating thorium and uranium from REE that is especially efficient in a nitrate medium. Tris(2-ethylhexyl) phosphate (TEHP), and tri-iso-amyl phosphate (TiAP) are other neutral organophosphorus extractants, which are usually dissolved in n-paraffin or in xylene [68,74,85–87]. In terms of efficiency, the TEHP has a special affinity to uranium, which leads to the loss of some thorium in the aqueous phase. Therefore, the TEHP should be employed in a two-step solvent extraction (Figure 6) to recover uranium and then thorium [68].

**Figure 6.** Separation of thorium and uranium from REE by solvent extraction method with TEHP in n-paraffin.

Calixarene is another group of neutral organophosphorus extractants appropriate for recovering radioactive elements from REE [71,73,75,88]. For instance, 5,11,17,23-tetra(diethoxyphosphoryl)- 25,26,27,28-tetrapropyloxy calixarene and p-phosphorylated calixarene have been employed, either as a resin in chromatography to concentrate thorium and uranium [88] or as a solvent extractant to recover thorium and uranium from the REE nitrate liquors [71,75]. The efficiency of thorium extraction (reaction 18) would vary with the concentration of nitric acid used in the upstream process. It is

reported that around 85% of thorium is extracted using 2 mol/L nitric acid while the extraction of REE (Gd, La, and Yb) is less than 10% [75],

$$\text{Th}^{4+} + 4\text{NO}\_3^- + \text{\textasciicircum} \rightarrow \text{Th}(\text{NO}\_3)\_4 \cdot \text{\textasciicircum} \tag{18}$$

4.2.3. Improvement of Solvent Extraction Process

Although various types of extractants have been applied in the solvent extraction process to separate radioactive elements from REE [32,76,77,89], the selection of an appropriate extractant is a function of its cost, selectivity towards one of the radioactive elements, and the feed liquor (sulfate, nitrate or chloride). For instance, Cyanex extractant shows poor metal extraction, is costly and requires high acidic environments, which make it infeasible for industrial applications.

Employing a mixture of acid and neutral organophosphorus extractants potentially improves the solvent extraction process that would result in a higher separation efficiency than using each extractant separately. For instance, although the selectivity in uranium extraction by acid organophosphorus PC88A (mono (2-ethyl hexyl) ester) showed better performance than the neutral organophosphorus TBP, the best result was obtained with a mixture of both, where the distribution of uranium was 0.1, 1.1, and 2.3 after 20 min treatment with TBP, PC88A, and their mixture (0.15 M TBP and 0.15 M PC88A in xylene), respectively [86,90]. This enhancement in performance is attributed to the role of TBP, which dehydrates uranium, resulting in an enhancement in the hydrophobic nature of the species and improving the extraction by the mixture.

In addition, solvent extraction is typically followed by a stripping step to separate the recovered radioactive element(s) from the accompanying reagent. Recovery of the radioactive elements is usually over 90% after stripping [74,82]. For the stripping step, an appropriate acidic reagent is employed depending on the feed of the solvent extraction [69,74,85,86,90]. For instance, for a feed of sulfate solution, nitric or sulfuric acid is employed as the stripping reagent [58]. For a feed of nitrate liquor, the stripping reagent is usually hydrochloric acid or sulfuric acid [69,76,82].

#### **5. Separation by Ion Chromatography**

#### *5.1. Cation Exchange Resin*

Cation exchange column has been employed to investigate chromatography principles on the separation of radioactive elements from REE [91–94]. In ion chromatography, it is possible to employ an active stationary phase or an active mobile phase, circulating in a neutral chromatography column. A configuration of a silica gel resin impregnated with ammonium nitrate (NH4NO3) in mobile phases of HDEHP (di-(2-ethylhexyl)-dithiophosphoric acid) and HDiBDTP (di(iso-butyl)dithiophosphoric acid) resulted in the separation of metal ions and the individual REE [94,95]. This method is based on different retention times of thorium, uranium, and REE in the column, which is affected by the employed resin [91]. For this purpose, various resins have been employed such as transUranic-Element Specific (TRU Spec), i.e., a resin composed of TBP and octyl(phenyl)-N,Ndiisobutylcarbamoylmethylphosphine oxide (CMPO) and supported on an inert polymer substrate [93], and the Dionex, i.e., a column for the separation of transition and lanthanide metals from contaminated water [92].

These columns adsorb all the elements of interest and separate them using different eluent. Once adsorbed, the order of elements elution can vary in function of the column resin. By employing appropriate eluents, thorium and uranium can thus be recovered individually with an efficiency of over 90% [91,93]. For instance, Figure 7 [91] shows that the elements of interest, i.e., thorium, uranium, and the REE, are adsorbed in the Ion Pac CS 5 column, then recovered separately with a mixture of hydrochloric acid and ammonium sulfate as eluent. Accordingly, uranium is initially recovered in the form of UO2Cl<sup>−</sup>, then thorium is recovered as ThSO4 <sup>2</sup><sup>+</sup> [91]. Another chromatographic investigation showed over 95% recovery of the light REE, which was adsorbed in a transuranic-element specific resin (TRU Spec), using a LN Spec resin column by a nitric acid elution; later, thorium and uranium

were recovered, respectively, by hydrochloric acid and a mixture of hydrochloric and hydrofluoric acids [93,96].

**Figure 7.** Cation exchange extraction with Dionex Ion Pac CS5 column.

In addition, a combination of different chromatographic columns with different resins can achieve a higher recovery and purity of the products, in addition to the separation of REE from one to another [97,98]. However, the low flow rate of the eluent, e.g., 0.25 to 1.5 mL/min, in the cation exchange resin columns hinder their industrial application. This technique might be suitable in a downstream step where high purity is required.
