Uranium(VI), Thorium(IV), and Lanthanides(III) Extraction from the Eudialyte Concentrate Using the N,O-Hybrid Heterocyclic Reagents

Abstract

N,O-donor hybrid heterocyclic extractants have great potential for separation of actinides from lanthanides in spent nuclear fuel reprocessing processes. We demonstrate that this type of reagents can be used for primary concentration of actinides contained in eudialyte, a promising mineral containing a heavy group of lanthanides. With respect to lanthanide ions, the efficiency of their extraction decreases in the series L3 >> L1 > L2, and the extraction of actinides decreases in the series L1 ≈ L3 >> L2. For the extractant L2 based on 2,2′-bipyridine-6,6′-dicarboxylic acid diamide, the efficiency of lanthanide purification from U, Th exceeds 50. The structure and stereochemical features of the ligands do not have a significant effect on the composition of the formed complexes. The solvation numbers are close to 1 for all range f-elements studied, except for thorium, which indicates the predominant formation of complexes with the composition ratio of 1:1. The solvation numbers 1.4–1.5 are observed for thorium(IV), and the established values indicate the formation of a mixture of complexes with the composition ratios of 1:1 and 2:1.

1. Introduction

Interest in the elements of the lanthanide group and other rare metals is rapidly growing in global science and industry. As a result, new technologies are being developed and high-tech products, based on rare and rare-earth metals (RMs and REEs), are being created. The dynamics of RM consumption growth are driven by their application in both traditional sectors (metallurgy, composite materials, electrical engineering and electronics, and nuclear energy) and in new high-tech industries [1,2,3,4,5]. The REE market represents an economic and geopolitical system based on the interaction of various entities (companies, corporations, countries, and governments) at different stages of the technological chain: exploration and extraction—processing—high-tech production—management of the global supply chain [6,7,8].

The scarcity of rare metals, along with increasing prices, has made it essential to develop new methods for producing rare metal concentrates, including the processing of low-grade ores and man-made waste [9,10,11]. Traditionally, the processes for producing metal concentrates involve extraction stages that separate and concentrate REEs from aqueous solutions [12,13], typically using phosphorus-containing extractants [14,15,16].

The most productive natural source of strategic metals (REE, Zr, Hf, Nb, Ta) are the minerals of alkaline rocks and carbonatites [17,18,19]. Of particular interest are the large deposits of eudialyte-group minerals (EGMs) in Lovozero alkaline massif at the Kola Peninsula (Russia) or other countries (South Africa, Brazil, Canada, Denmark) [20,21]. Among them, the Lovozero alkaline massif contains eudialyte ores, which attract interest as a valuable source of medium-heavy and heavy rare earth metals, as well as zirconium and hafnium. Moreover, because of the complex chemical composition of EGMs, the eudialyte deposits could be considered as complex ores, also containing manganese, niobium, scandium, as well as radioactive metals.

The off-balance reserves of eudialyte ores at the Alluaiv site of the Lovozero deposit are estimated to be 7.3 million tons, which is approximately equivalent to 1 million tons of ZrO₂. The average eudialyte content in these ores is around 12%, with coarse and medium-grained varieties containing up to 20–25%. Despite the wide distribution of eudialyte ores, there are no industrial-scale operations exploiting deposits of this type. However, technologies for eudialyte mineral processing are actively developed. In Australia, for instance, the technological processing for eudialyte ores from the Toongi deposit (which contains tantalum, niobium, REE, and zirconium) has been developed over the past two decades. Moreover, the preparations are also underway for the development of eudialyte ore deposits of Norra Karr (Sweden) and Tanbreez (Greenland).

EGMs are complex zirconium and calcium silicates, which are characterized by the following general formula: [N(1)₃N(2)₃N(3)₃N(4)₃N(5)₃]{M(1)₆M(2)₃M(3)M(4)Z₃(Si₉O_27-3x(OH)_3x)₂(Si₃O₉)₂Ø_0–6}X(1)X(2), where M(1) = ^VICa, ^VIMn²⁺, ^VIREE, ^VINa, ^VIFe²⁺; M(2) = ^IV,VFe²⁺, ^V,VIFe³⁺, ^V,VIMn²⁺, ^V,VINa, ^IV,VZr; M(3) and M(4) = ^IVSi, ^VINb, ^VITi, ^VIW⁶⁺; Z = ^VIZr, ^VITi; Ø = O, OH; N(1)–N(5) are extra-framework cations (Na, H₃O⁺, K, Sr, Ln, Y Ba, Mn²⁺, Ca) or H₂O; X(1) and X(2) are extra-framework water molecules, halide (Cl⁻, F⁻) and chalcogenide (S²⁻) anions, and anionic groups (CO₃²⁻, SO₄²⁻); x = 0–1. The coordination numbers of cations are indicated with Roman numerals [22,23,24]. The crystal structure of EGMs is represented by a dense microporous framework built up by zirconium(IV) in octahedral environment, calcium octahedral rings and two types of silicate tetrahedral rings ([Si₃O₉] and [Si₉O₂₇]) (Figure 1), while the pores of the framework are filled by alkaline, alkaline- and rare-earth elements, as well as additional halide and chalcogenide anions and water molecules. The high chemical rank of R = 11–13 of EGMs makes influences the searching of the efficient schemes of the decomposing of EGMs and the extraction of RM and REM.

Many technologies have now been developed for processing eudialyte ores, containing 25–27% of eudialyte, 52% of feldspar, loparite and nepheline, and 20% of aegirine, which can be processed using combined flotation–gravity–magnetoelectric enrichment schemes to produce eudialyte and loparite concentrates [25,26,27]. The resulting eudialyte concentrates, with minimal radioactivity, contain 54 wt.% SiO₂, 11–13 wt.% ZrO₂, 1.8–2.5 wt.% REE₂O₃, 0.21–0.27 wt.% HfO₂, 0.06–0.1 wt.% Ta₂O₅, 0.6–1% wt.% Nb₂O₅ and TiO₂, as well 0.95–1.48 wt.% SrO [25,26,27,28]. Existing technologies for extracting Zr and REE from eudialyte concentrates are based on their multi-stage decomposition using acids and alkalis, followed by extraction separation of valuable components from the digestion solutions using extractants of various classes [25,29,30,31,32,33].

During the processing of REE-containing raw materials of both natural and technogenic origin, low-level radioactive waste is typically generated, primarily due to the presence of uranium and thorium at concentrations ranging from 0.5 to 2 mg/L [17,20]. However, current environmental legislation in the Russian Federation mandates the transfer of these REEs for storage with subsequent burial, which significantly reduces the economic viability of producing concentrates and other products based on RM and REE. Consequently, it appears advisable to pre-treat these solutions to allow for the selective separation of radioactive uranium and thorium into a separate fraction, enabling the resulting REE concentrate to be sent for further processing. To address this issue, a technological method based on the extraction–sorption separation of radionuclides from solutions is often employed. This operation allows for the highly selective separation of uranium(IV) and thorium(IV), resulting in solutions with significantly lower concentrations down to 0.1 μg/L [32,33].

As a practical solution to the problem of selective extraction of radionuclides, it is necessary to use specialized extractants. In particular, the derivatives of bipyridine dicarboxylic acid amides (L1 and analogs) have previously been successfully used for joint extraction of both (IV) and (VI) valence actinide [34].

In this work, we conducted studies of three types of tetradentate N-heterocyclic extractants (Scheme 1). Ligands L2 and L3 are phenatroline derivatives that reproduce tetradentate coordination core of L1. The extraction properties of L2 have not been described previously. L3 is a 2,9-alkyl-substituted diphosphonate, the extraction properties of which were previously characterized with respect to some actinides and lanthanides in model experiments [35]. The extraction capacity of polydentate extractants, with respect to actinides and lanthanides, was investigated in relation to the development of a method for the separation of radioactive elements during the production of REEs from eudialyte ore concentrates.

2. Materials and Methods

2.1. General Remarks

Heterocyclic derivatives have been synthesized according to the previously published procedures (see Supplementary Materials). Meta-nitrobenzotrifluoride (F-3) (PIM-Invest) was used as the extractant, whereas HNO₃ (special purity) was used as the solvent. GSO 8363-2003 [36] uranium oxide, certified for uranium content of 84.784 ± 0.016%, as well as chemically pure grade nitrates Ln(NO₃)₃ × 6H₂O (Ln = La − Nd, Sm − Lu) and Th(NO₃)₄ × 5H₂O were used. The solutions were prepared by the volume–weight method: aqueous solutions were prepared in bidistilled water, while the solutions of nitrates of the studied elements were prepared by dissolving a weighed portion of the corresponding nitrate in a 0.01 mol/L HNO₃ solution. The concentration of the solution of lanthanide and actinide nitrates (0.1 mmol/L) was refined by ICP MS (PlasmaQuant MS Elite, Analytik Jena AG, Jena, Germany). The concentration of HNO₃ solutions was determined by potentiometric titration of 0.1 mol/L NaOH using an S470 SevenExcellence^TM pH meter/conductometer (MettlerToledo, Columbus, OH, USA) with an accuracy of ±0.01 units. pH, while the concentration of the NaOH solution, was specified with a certified solution (FIXANAL, Sigma-Aldrich, St. Louis, MO, USA).

The eudialyte ore concentrate (enriched up to 40% of EGMs) was used. Before the breakdown, the granulometric and elemental composition of the concentrate had been determined. The granulometric composition of the concentrate was studied by the method of low-angle laser light scattering (LALLS, laser diffraction) using an Analyzette 22 Compact diffractometer (Fritsch, Pittsboro, NC, USA). The calculation of the particle size distribution was calculated using the Fraunhofer theory. The obtained average particle size was 120 μm, the sizes of ca. 90% of the particles were in the range between 30 and 200 μm.

Elemental analysis was performed using X-ray fluorescence spectrometry with an ARL Advant’X wavelength dispersive X-ray fluorescence spectrometer. The UniQuant software (version AA 83706) was used without any corrections. The obtained results (wt.%), excluding oxygen, are presented in

Table 1. Elemental composition of the eudialyte concentrate (enriched up to 40% of EGMs).

Element	Si	Al	Fe	Zr	Mn	Ca	Na	K	Ti	U
wt.%	49.97	11.65	11.55	8.67	3.81	3.13	2.63	2.01	1.96	0.012
Element	Sr	Cl	Nb	Mg	P(V)	Hf	Ba	Mo	Ta	Th
wt.%	1.25	0.675	0.503	0.309	0.292	0.122	0.064	0.039	0.031	0.023
Element	S	Ni	La − Lu + Y		La − Nd		Sm − Lu +Y		Y
wt.%	0.023	0.017	1.22		0.77		0.45		0.403

. The weight percentages of the individual components relative to the sum of REEs are given in

Table 2. Weight fractions of REEs’ groups as well as some individual REEs in the eudialyte concentrate per total REEs weight.

Element	La − Nd	Ce	Sm − Lu + Y	Y
wt.%	63.2	0.40	36.8	0.40

.

Before conducting the extraction studies, the metals containing in the ore eudialyte concentrate were transferred into solution by leaching. Sulfuric acid leaching of eudialyte concentrate occurs with significant losses of zirconium and other rare elements due to the entry of these elements into silica gels [37,38]. The formation of poorly soluble silica gel also complicates the filtration process. The formation of poorly filterable silica gel sediment during the decomposition of EGMs is the main problem in the leaching of eudialyte concentrate. In order to intensify the extraction of REEs, as well as uranium(IV), thorium(IV), and other rare metals from the host EGMs into solution, it was decided to carry out leaching in nitric acid with the maximal possible concentration during heating. Leaching of the eudialyte ore concentrate (10 g) was carried out in 13.8 mol/L HNO₃ (S:L = 1:10), with stirring and a temperature of 80 °C for 72 h.

During nitric acid decomposition of eudialyte under the described conditions, REEs, zirconium(IV), as well as emanating uranium(VI) and thorium(IV) passed into solution. A released amorphous well-coagulated silica ensures good filterability of the pulp. It should also be noted that during leaching, zirconium passes into solution as Zr⁴⁺ ions. The content of oxo-cations ZrO²⁺ and Zr₂O₃²⁺ in a nitric acid solution of high concentration of HNO₃ is minimal. Figure 2 shows the content of elements in the solution after nitric acid leaching of eudialyte ore concentrate; the content of nitric acid in the resulting solution is 12.2 mol/L.

2.2. Liquid Extraction Experiments

The metal cation extraction was carried out in glass tubes with ground stoppers in a phase volume ratio of 1:1 at a temperature of 25 ± 1 °C. The composition of organic phase was similar for all three studied extractants: 0.05 mol/L in meta-nitrobenzotrifluoride.

The phases were mixed for 20 min in a rotator. The time of the establishment of the extraction equilibrium was checked by increasing the phase contact time to 120 min (the distribution coefficients had not been changed). Phase separation was performed by centrifugation. After phase separation, the metal cation concentration in the aqueous phase was determined by ICP-MS (PlasmaQuant MS Elite, Analytik Jena AG, Jena, Germany). For all cases, at least five independent experiments were performed. The total error of the obtained results was ~20%, taking into account the non-excluded and random components. The confidence interval of the determined metal concentrations in the experiment was 0.002 mmol/L. The distribution coefficients during extraction (D = [M]_org/[M]_w) were determined at constant concentrations of the extractant (0.05 mol/L in meta-nitrobenzotrifluoride) and initial concentrations of the metal in the experiment (0.1 mmol/L in the aqueous phase).

3. Results and Discussion

3.1. Phase Stability of the Extraction Systems

Since the eudialyte solution contains 12.2 mol/L nitric acid, experiments were previously conducted to assess the phase stability of such systems (

Table 3. Data on the phase stability of the systems “0.05 mol/L ligand in meta-nitrobenzotrifluoride—nitric acid”.

Type of Ligand	HNO3 (mol/L)
	4	5	6	8	12
L1	Stable	Emulsion	Emulsion	Emulsion	Emulsion
L2	Stable	Stable	Emulsion	Emulsion	Emulsion
L3	Stable	Emulsion	Sediment and emulsion	Sediment and emulsion	Sediment and emulsion

“Stable” means that phase separation occurs in 15 s without the use of centrifugation; “emulsion” means that emulsion is stable for 3 min.

). The selected solvent has a high density, as does a sufficiently (12.2 mol/L) concentrated solution of nitric acid, which leads to an increase in the phase separation time, or even makes it impossible to separate them, even when using centrifugation. Moreover, in the case of ligand L3, a precipitate was formed. Based on these data, further experiments were conducted with concentrations of 4–5 mol/L of nitric acid.

3.2. Extraction of Actinides and Lanthanides from Model Solutions

To assess the extraction capacity of the ligands under study, a model 4 mol/L HNO₃ nitrate solution was used. The concentrations of emanating uranium(VI) and thorium(IV) were 0.1 mmol/L, while the concentration of lanthanides in total was 0.1 mmol/L. The values of the established distribution coefficients for the extraction systems based on L1, L2 and L3 are presented in Figure 3.

The nature of the extractants significantly affects the extraction behavior of U(VI), Th(IV), and REEs. The extractant L3 has the highest extraction efficiency both with respect to lanthanides and emanating U(VI) and Th(IV). L3 organic ligand is certainly effective for separating all f-elements into the organic phase to obtain a bulk concentrate or purify raffinates in the case of additional waste conditioning. However, in the case of extractive separation of lanthanides and emanating U(VI) and Th(IV), the use of the extractant L3 will lead to the accumulation of lanthanides in the organic phase. Nevertheless, the minimal separation factor SF (U, Th/REE) is not less than 8 (see

Table 4. Values of selectivity factors SF(U, Th/REE), calculated based on the values of the obtained distribution coefficients.

Type of Ligand	L1	L2	L3
SF(U, Th/REE)	50–100	4	8

), which potentially allows using L3 as an extractant for separating actinides in several extraction stages.

Ligand L2 has a low extraction capacity with respect to both actinide and lanthanide cations, and a significantly lower selectivity factor value, which does not allow us to speak about the prospects of this system.

The system based on ligand L1 is characterized by the best characteristics. Under the experimental conditions, the condition D(Ln) < 1 and D(U, Th) > 1 is observed for the separated components. The selectivity factor reaches high values in the range of 50–110 (Table 4). The minimal factor is observed for the Th/Er pair which is equal to 50. These data point out the possibility of selective separation of actinide cations.

3.3. Determination of Solvation Numbers Using L

To assess the stoichiometry of the complex compounds Ln(III), U(VI) and Th(IV) passing into the organic phase, the solvation numbers were determined. They were obtained by analyzing the slope angle in the logD(Me)–logC(Me) dependencies. The obtained values are presented in

Table 5. Values of solvation numbers for the extraction of metal cations from 3 mol/L nitric acid. Organic phase: 0.01–0.1 mol/L in meta-nitrobenzotrifluoride. Temperature 25 ± 1 °C. Error of values ±0.1.

Extractant	Ln(III)	Th(IV)	U(VI)
L1	1.0–1.1	1.5	1.2
L2	1.1–1.2	1.5	1.1
L3	1.3–1.4	1.4	1.3

, corresponding to the n value in the following equation:

$${Me}_{(aq.)}^{x+}+{{x\left(N{O}_3\right)}^{-}}_{(aq.)}+{nL}_{(org.)}={\left[L_{n}Me{\left({NO}_3\right)}_x\right]}_{(org.)}$$

The structure and stereochemical features of the ligands do not have a significant effect on the composition of the formed complexes. For Ln(III), the range of values from the minimum to the maximum in the lanthanide series is represented. The highest solvation numbers are observed for thorium(IV), and the established values indicate the formation of a mixture of complexes with the composition ratios of 1:1 and 2:1. In other cases, the solvation numbers are close to 1, which indicates the predominant formation of complexes with a composition ratio of 1:1.

In order to find the best conditions for the efficient extraction of uranium(IV) and thorium(IV) into the organic phase, the effect of nitric acid on extraction using L1 type ligand was studied (Figure 4). It should be noted that the extraction capacity of L1 with respect to uranium(VI) and thorium(IV) is high throughout the entire studied range of nitric acid. The degree of the extraction at a concentration of 0.5 mol/L nitric acid is more than 90% for both uranium(VI) and thorium(IV). With an increase in the concentration of nitric acid in the solution, the distribution coefficients increase and the degree of extraction of emanating actinides increases accordingly. At a concentration of 4 mol/L HNO₃, uranium(VI) and thorium(IV) are extracted into the organic phase almost completely.

In order to find conditions for efficient re-extraction of emanating uranium(IV) and thorium(IV) from the organic phase, the effect of sodium carbonate concentration in the aqueous phase on the extraction capacity of ligand L1 with respect to uranium(VI) and thorium(IV) was studied (Figure 5). Carbonate anion traditionally forms stable complex compounds with actinides (IV, VI), and solutions containing carbonate anions can be effectively used to extract these cations from the organic phase [39]. In the case of loaded organic phase stripping with a sodium carbonate solution, almost all of the extracted metal cations are re-extracted into the aqueous phase. However, it is possible to achieve group separation of the elements by stripping the loaded organic phase with nitric acid solutions with concentrations varying from 0.1 to 0.5 mol/L [40]. In the case (Figure 6) of extraction from eudialyte ore concentrate leaching solutions, when Zr(IV), Hf(IV) and Ti(IV) are successfully extracted into the organic phase in addition to uranium(VI) and thorium(IV), at the reextraction stage, it is possible to separate these extracted elements by washing the loaded organic phase with a 0.1 mol/L nitric acid solution with a separation factor of SF[Zr(VI)/U(VI)]~100 for zirconium and uranium cations as an example. This effect can be explained by the better affinity of actinides to the ligand during complexation in comparison with Zr(IV), Hf(IV) and Ti(IV).

As can be seen from Figure 5, with increasing sodium carbonate concentration in the aqueous solution, the extraction capacity of ligand L1 (with respect to uranium and thorium) decreases. Thus, for efficient stripping of uranium and thorium together with Zr(IV), Hf(IV) and Ti(IV) from the organic phase, it is quite sufficient to use a solution containing 0.5 mol/L Na₂CO₃.

Thus, the following conditions were selected to conduct an experiment to study the extraction of f-elements from the solution of the eudialyte ore concentrate recovery:

-

Organic phase: 0.05 mol/L solution of ligand L1 in meta-nitrobenzotrifluoride;

-

Aqueous phase: nitric acid content of 4 mol/L; it is necessary to dilute the concentrated solution of the eudialyte ore concentrate recovery by three times, due to the phase instability of the extraction systems at high nitric acid contents;

-

Stripping of the organic phase with 0.1 mol/L nitric acid solution for selective re-extraction of Zr(IV), Hf(IV) and Ti(IV).

-

Re-extraction of uranium and thorium—0.5 mol/L sodium carbonate solution.

3.4. Extraction of Metals from Eudialyte Nitric Acid Solution

Under the above conditions (Section 3.3), experiments were conducted with the solution of eudialyte ore concentrate recovery. The established distribution coefficients of elements are presented in Figure 6. The general nature of the dependence reproduces the results of model experiments. The results also indicate the possibility of separating actinides from the nitric acid solution of ore eudialyte concentrate leaching.

When using amide bipyridine dicarboxylic acid derivatives in meta-nitrobenzotrifluoride, uranium(VI), thorium(IV), zirconium(IV), hafnium(IV), scandium(III), and titanium(III) are extracted from the solution of eudialyte concentrate recovery quantitatively, while rare earth elements are concentrated in the raffinate. The obtained results can become the basis for a new industrial technology for conditioning waste in the production of rare and rare earth metals from natural and man-made raw materials.

4. Conclusions

N,O-donor hybrid heterocyclic extractants have great potential for separation of actinides from lanthanides in spent nuclear fuel reprocessing processes; however, their ability has not been sufficiently studied in the field of processing natural raw materials containing emanating elements. In this work, it is shown that reagents capable of extracting a wide range of actinide ions in different valence states [35] can be used for primary concentration of actinides contained in eudialyte, a promising mineral containing a heavy group of lanthanides. Among heterocyclic N,O-donor hybrid extractants, the most effective are 2,2′-bipyridin-6,6′-dicarboxylic acid amide L1 and phenanthroline2,9-diphosphonic acid ester L3, while phenanthroline-2,9-bis(phosphine oxide) L2 exhibits moderate selectivity and efficiency of f-element extraction. With respect to lanthanide ions, the efficiency of their extraction decreases in the series L3 >> L1 > L2, and the extraction of actinides decreases in the series L1 ≈ L3 >> L2. For the extractant L2 based on 2,2′-bipyridine-6,6′-dicarboxylic acid diamide, the efficiency of lanthanide purification from U, Th exceeds 50. The structure and stereochemical features of the ligands do not have a significant effect on the composition of the formed complexes. For Ln(III), the range of values is from the minimum to the maximum in the lanthanide series is represented. The highest solvation numbers are observed for thorium(IV), and the established values indicate the formation of a mixture of complexes with the composition ratios of 1:1 and 2:1. In other cases, the solvation numbers are close to 1, which indicates the predominant formation of complexes with the composition ratio of 1:1. The most probable mechanism for the extraction of metal ions is the formation of mononuclear coordination compounds with actinide and lanthanide nitrates. When using amide bipyridine dicarboxylic acid derivatives in meta-nitrobenzotrifluoride, uranium(VI), thorium(IV), zirconium(IV), hafnium(IV), scandium(III), and titanium(III) are extracted from the solution of eudialyte concentrate recovery quantitatively.