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Of chemical reactors for nanoparticle precipitation. Aiche J. 2006, 52, 1877–1887. [Google Scholar] [CrossRef]De Santana, A.O.; Dantas, C.C. Scale up of the mixer of a mixer-settler model used in a uranium solvent extraction process. J. Radioanal. Nucl. Chem. 1995, 189, 257–268. [Google Scholar] [CrossRef]Geeting, M.W.; Brass, E.A.; Brown, S.J.; Campbell, S.G. Scale-up of Caustic-Side Solvent Extraction Process for Removal of Cesium at Savannah River Site. Sep. Sci. Technol. 2008, 43, 2786–2796. [Google Scholar] [CrossRef] [Green Version]Kurniawansyah, F.; Mammucari, R.; Tandya, A.; Foster, N.R. Scale—Up and economic evaluation of the atomized rapid injection solvent extraction process. J. Supercrit. Fluids 2017, 127, 208–216. [Google Scholar] [CrossRef]Carta, G. Scale-Up and Optimization in Preparative Chromatography: Principles and Biopharmaceutical Applications. Chromatographic Science Series, Volume 88 Edited by Anurag, S. Rathore (Pharmacia Corporation, Chesterfield, MO) and Ajoy Velayudhan (Oregon State University). Marcel Dekker, Inc.: New York, Basel. 2002. xvi + 342 pp. $150.00. ISBN 0-8247-0826-1. J. Am. Chem. Soc. 2003, 125, 3398–3399. [Google Scholar] [CrossRef]Heuer, C.; Hugo, P.; Mann, G.; Seidel-Morgenstern, A. Scale up in preparative chromatography. J. Chromatogr. A 1996, 752, 19–29. [Google Scholar] [CrossRef]Rathore, A.S.; Velayudhan, A. Guidelines for optimization and scale up in preventive chromatography. Biopharm Int. 2003, 16, 1. [Google Scholar] Figure 1. The REE general processing plant. 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) Figure 3. Leaching-cracking approaches for separation of REE from radioactive elements. Figure 4. Process flowsheet for separation of Th and U from REE by precipitation. Figure 5. Process of separating Th from REE of Baotou bastnäsite leaching. Figure 6. Separation of thorium and uranium from REE by solvent extraction method with TEHP in n-paraffin. Figure 7. Cation exchange extraction with Dionex Ion Pac CS5 column. Figure 8. Flow diagram of uranium removal from REE. Figure 9. Separation factor of actinides/Nd by graphene oxide (GO) and functionalized graphene oxide with PDA (GO-PDA) membranes. (Reproduced with permission from ref. [109], copyright (2012), Elsevier). Table 1. REE-bearing minerals and gangue minerals from deposits. TypeMineralFormulaAverage Composition (wt.%)Ref.REE OxideThO2UO2CarbonateAncyliteSr(Ce,La)(CO3)2OH·H2O460–0.40.1[6]Bastnäsite(Ce,La)CO3F740–0.3[6]ParisiteCa(REE)2(CO3)3F2590–0.50–0.3[6]PhosphateApatiteCa5(PO4)3(F,Cl,OH)19--[4,6]Britholite(REE,Ca)5(SiO4,PO4)3(F,OH)561.5-[4,6]Monazite(REE,Th)PO435–710–200–16[6,23]XenotimeYPO461-0–5[4,6]OxideBrannerite(U,REE,Ca)(Ti,Fe)2O66--[6]Perovskite(Ca,REE)TiO30–2[6]SilicateAllanite(REE,Ca)2(Al,Fe)3(SiO4)3(OH)300.3-[4,6]Cheralite(REE,Th,Ca)(P,Si)O45-[4,6] Table 2. Uranium and thorium separation from REE by a two-step cracking-leaching process. REE-Bearing MineralReagentOperating ConditionsOverall Recovery in Leach Liquor (%)Ref.Pressure (atm)Time (h)T (°C)ThUREEXenotimeNitric acid1-60->98[35]Concentrated monazite (87 wt.% REE)Ammonium carbonate6.5–101–28099952.5[31]Monazite (18.5 wt.% REE)Ammonium oxalate140100>40[33] Table 3. The pH ranges for precipitation of thorium, uranium and the REE in different liquors. ElementsPrecipitation pH (Approx.)Chloride LiquorSulfate LiquorTh4.8–5.81–2U5.5–76REE6.8–83–5.5 Table 4. Uranium and thorium separation from REE by precipitation technique. Original OreUpstream Liquor FeedPrecipitation ReagentFinal pHPrecipitation Recovery (%)Ref.ThUREEMonaziteSulfate1st step: Oxalic acid0.798-99[39]2nd step: Alkali leaching->99-Synthesized solutionNitrateKOH and NH4OH4.5-90low[51]Monazite, ApatiteChlorideHydrated lime and NH4OH2.5>99-5[48]Monazite, REE carbonateChlorideLime and H2O2-99>80[49]Bastnäsite, MonaziteHydroxide cakeHCl5.8>99>992.3[56]XenotimeSulfate1st step: NH4OH1.5–1.9>99-[46]2nd step: Oxalic acid--->98 Table 5. Performance

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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): 3.2. Multi- or Single-Step PrecipitationThe 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]: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

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Of amine extractants for uranium and thorium separation from REE. AmineExperimental ConditionsExtraction (%)Ref.TypeNameLiquorOrepHTime (min)ThoriumUraniumREEPrimaryPrimene JM-TSulfateMonazite-595.48.80.32[58]N1923SulfateBastnäsite-->97-ECe: 3–8[60]OctylamineSulfateMonazite41570–8050–60-[59]SecondaryN-methylanilineSulfateMonazite41570–805–100[59]TertiaryAlamine 336SulfateMonazite-53.182.40.02[58]N,N-dimethylanilineSulfateMonazite4157015–20ECe, Eu, Y: 0[59]730055ECe, Eu, Y: 0[59]MixturePrimene JM-T and Alamine 336SulfateMonazite-545.558.80.04[58] Table 6. Phosphorous extractants for uranium and thorium separation from REE. Type of ExtractantFeed Liquor Phosphorous ExtractantExtraction (%)Ref.ThUREEAcidAcidicCyanex 27283-12[69]NitrateDEHEHP20 Ce: 95[65]NitrateMixture of HEH and EHP in kerosene95-Ce: 99[73]NeutralNitrateTiAP (2 solvent extraction steps)9995[74]Nitratep-phosphorylated calixarene99-5[75]NitrateTEHP in n-paraffin50–772.5Y: 0.17[68]OtherNitrateAliquat 3369754[32,76]Nitratepolyaramide 90->48[77] Table 7. Advantages and disadvantages of different methods for separating radioactive elements from REE. TableAdvantagesLimitationsRecovery (%)Scalability 4LeachingSimultaneous recovery of Th & U from REE;Use of cost-effective reagents;Must have a particle size of ore/cake similar or smaller than liberation-equivalent size to avoid higher loss of REE 1;Feed (REE-radioactive mixture) must be in solid phase;Th: >98 3aU: 65–95 3a+++PrecipitationSimultaneous recovery of Th & U from REE;Being pH dependent, possible individual precipitation of Th or U from REE;Use of cost-effective reagents;Highly dependent on reagent, pH and sometimes temperature;Possible co-precipitation of REE with Th & U if right pH is not maintained 2;Difficult to separate both Th & U simultaneously from REE with high recovery in one single step2 although it can be possible;Th ≥ 98U > 80 3aTh > 98U > 90 3b++Solvent ExtractionHigh selectivity towards Th & U;High recovery of Th & U in individual separation steps;Low recovery of Th & U in simultaneous separation;Multiple separation steps required for individual separation of Th & U;Use of chemicals: cost of reagents and environmental impact;Th ≥ 70U ≈ 55 3aTh > 95U > 95 3b+++Ion chromatographyExtraction of Th & U in one step;High recovery of both radioactive elements;Low flow rates;Selection of the scale-up technology and configuration (batch, column, expanded/fluidized-bed or suspended bed);Anion exchange only extracts U (with the presence of ppm);Th: 90–99U: 90–99++ Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( Share and Cite MDPI and ACS Style García, A.C.; Latifi, M.; Amini, A.; Chaouki, J. Separation of Radioactive Elements from Rare Earth Element-Bearing Minerals. Metals 2020, 10, 1524. AMA Style García AC, Latifi M, Amini A, Chaouki J. Separation of Radioactive Elements from Rare Earth Element-Bearing Minerals. Metals. 2020; 10(11):1524. Chicago/Turabian Style García, Adrián Carrillo, Mohammad Latifi, Ahmadreza Amini, and Jamal Chaouki. 2020. "Separation of Radioactive Elements from Rare Earth Element-Bearing Minerals" Metals 10, no. 11: 1524. APA Style García, A. C., Latifi, M., Amini, A., & Chaouki, J. (2020). Separation of Radioactive Elements from Rare Earth Element-Bearing Minerals. Metals, 10(11), 1524. Note that from the first issue of 2016, this journal uses article numbers instead of page numbers.. Ree - Ly is on Facebook. Join Facebook to connect with Ree - Ly and others you may know. Facebook gives people the power to share and makes the world

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Overall recovery of REE, thorium, and uranium in leach liquor for the two-step cracking-leaching process. 3. Separation by PrecipitationThe REE can be separated from other elements in a liquor via precipitation method using an appropriate reagent such as oxalic acid, reaction 8 [39,40,41,42], 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 [43,44,45,46]. 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 [46,47,48,49]. 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]. 3.1. Types of the Liquor and ReagentTypically, 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

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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].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 [4,12,14,15,16]. 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 [17,18,19]). 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.It is worthy of mentioning that recovery

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Author / Affiliation / Email Article Menu Font Type: Arial Georgia Verdana Open AccessReview by Adrián Carrillo García 1, Mohammad Latifi 1,2, Ahmadreza Amini 1 and Jamal Chaouki 1,* 1 Process Development Advanced Research Lab (PEARL), Chemical Engineering Department, Ecole Polytechnique de Montreal, C.P. 6079, Succ. Centre-ville, Montreal, QC H3C 3A7, Canada 2 NeoCtech Corp., Montreal, QC H3G 2N7, Canada * Author to whom correspondence should be addressed. Submission received: 8 October 2020 / Revised: 13 November 2020 / Accepted: 13 November 2020 / Published: 17 November 2020 Abstract: Rare earth elements (REE), originally found in various low-grade deposits in the form of different minerals, are associated with gangues that have similar physicochemical properties. However, the production of REE is attractive due to their numerous applications in advanced materials and new technologies. The presence of the radioactive elements, thorium and uranium, in the REE deposits, is a production challenge. Their separation is crucial to gaining a product with minimum radioactivity in the downstream processes, and to mitigate the environmental and safety issues. In the present study, different techniques for separation of the radioactive elements from REE are reviewed, including leaching, precipitation, solvent extraction, and ion chromatography. In addition, the waste management of the separated radioactive elements is discussed with a particular conclusion that such a waste stream can be employed as a valuable co-product. 1. IntroductionThe 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 [1,2,3,4,5]. 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 [4,6,7,8,9], 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. Ree - Ly is on Facebook. Join Facebook to connect with Ree - Ly and others you may know. Facebook gives people the power to share and makes the world View the profiles of people named Ree Ly. Join Facebook to connect with Ree Ly and others you may know. Facebook gives people the power to share and

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Of REE from secondary sources such as electronic wastes [24,25], red mud (Bauxite) [26,27,28,29], 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 LeachingLeaching 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 [31,32,33,34] 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 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], 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, 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].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, 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

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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. 4. Separation by Solvent ExtractionThe 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 ExtractantsAmine 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, Table 5 lists some amines employed for solvent extraction of radioactive elements from the REE [58,59,60].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., (CnH2n+1)2CHNH2 (n = 9–11), is an alternative primary amine for this process, which has been employed in Baotou and Sichuan in China [60,63,64,65]. 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. Ree - Ly is on Facebook. Join Facebook to connect with Ree - Ly and others you may know. Facebook gives people the power to share and makes the world

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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]. 4.2. Solvent Extraction with Phosphorous ExtractantsPhosphorous extractants are alternative for amines to extract the radioactive elements [4,68,69,70,71,72]. 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. 4.2.1. Acid Organophosphorus ExtractantsAcidic 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], 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 [69,81,82,83]. 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