Extraction of Rhenium with Trialkylbenzylammonium Chloride

Abstract

This study investigates the extraction of rhenium using trialkylbenzyl ammonium chloride (TABAC) as an alternative to trialkylamine (TAA) for recovering rhenium from highly diluted solutions. Rhenium, present as ReO₄⁻ over a wide acidity range, was extracted via an anion exchange mechanism in single-stage experiments monitored by inductively coupled plasma mass spectrometry (ICP-MS). Key variables, including pH, acid concentration, and the concentrations of extractant and metal, were examined. The results demonstrated a high extraction efficiency exceeding 99% within a pH range of 2 to 7 and from solutions containing sulfuric or hydrochloric acid at concentrations of 0.1 to 3.0 M (mol/L). However, extraction from nitric acid solutions was less efficient, with less than 30% recovery. Performance for both TAA-kerosene and TABAC-kerosene followed the order H₂SO₄ > HCl > HNO₃. The optimal TABAC concentration was 10⁻² M (mol/L) in kerosene. TABAC also showed higher selectivity than TAA, with separation coefficients Re/Mo = 66.8 and Re/W = 55.8 in 0.1–1.0 M (mol/L) sulfuric acid. Based on equilibrium studies, the complexes formed during extraction were identified as [R₃R′NH]ReO₄. This approach may offer environmentally friendly and cost-effective benefits for large-scale industrial applications, enabling efficient recovery of valuable rhenium while reducing waste and environmental impact.

1. Introduction

Rhenium is a rare and scattered metal; its content in the earth’s crust does not exceed 7 × 10–8%. Due to its refractoriness, heat resistance and wear resistance, it is widely used in many fields of science and technology. In particular, it is indispensable in the production of parts in the aerospace industry and is effective as a catalyst for organic synthesis [1]. According to the USGS Mineral Commodity Summaries 2024, global rhenium mine production in 2023 was approximately 56,000 kg, with Chile (30,000 kg), the United States (9100 kg), and Poland (6300 kg) as the leading producers. The largest reserves are concentrated in Chile (1,300,000 kg), the United States (400,000 kg), and Kazakhstan (190,000 kg). Rhenium is predominantly recovered as a byproduct of roasting molybdenum concentrates derived from porphyry copper deposits. This underscores the critical importance of secondary recovery and highlights the global dependence on a few key sources for this scarce and strategically significant metal [2]. Rhenium does not have its own deposits and is extracted during the processing of copper, copper-molybdenum, and uranium ores. The main source of rhenium is the sulfuric acid washing solutions generated during the high-temperature and high-pressure leaching of molybdenum concentrates, where rhenium is dissolved in solution along with other elements.

The content of rhenium in these solutions varies from 0.1 to 0.5 g/L [3]. Furthermore, some spent catalysts and superalloys containing rhenium represent potential secondary sources of this metal [4]. Of particular difficulty is the separation and concentration of rhenium from highly dilute solutions, which is carried out by sorption and solvent extraction methods. More than 50% of the world’s rhenium production is based on its extraction. Rhenium recovery from highly diluted solutions remains challenging, typically relying on sorption or solvent extraction. Sorption employs activated carbons or low-basic anion exchangers (bearing primary/secondary amines), which allow selective uptake, ammonia-based desorption, and minimal environmental impact. However, activated carbons lose efficiency after multiple cycles. In contrast, solvent extraction dominates owing to its continuous operation, high product purity, and robust capacity, even for very dilute solutions, and due to its high efficiency and selectivity, it will maintain a leading position in applied technological schemes for a long time [1]. In solutions of wet sulfuric acid dust collection, rhenium is present together with molybdenum. Mo and W are present in the spent “new generation” alloys together with rhenium [5]. Therefore, one of the most important problems is the separation of molybdenum, tungsten, and rhenium. A key challenge lies in the efficient separation of these elements due to their similar physicochemical properties, which significantly complicates extraction and subsequent utilization processes.

The choice of an effective extractant for the separation of molybdenum, tungsten, and rhenium is associated with the ionic forms of the metals present in the solution. Rhenium in solutions in a wide range of pH values from 0 to 12 exists in the form of perrhenate ion ReO₄⁻ [6,7]. The dominant ionic form of rhenium has been confirmed by Eh–pH dependencies [8], as shown in Figure 1.

Molybdenum and tungsten, depending on their concentration in solution and the concentration of mineral acid, may be present in the form of various mono- and polynuclear ions. When the content of molybdenum is less than 10⁻⁴ M (mol/L), mononuclear forms of molybdenum predominate in the solution, and cationic complexes (MoO₂(H₂O)₄²⁺, MoO₂(OH)(H₂O)³⁺) dominate in acidic solutions. With increasing pH values, the proportion of anionic complexes of HMoO₄⁻ and MoO₄²⁻ increases. An increase in the concentration of molybdenum leads to the formation of polynuclear ions of Mo₇O_24−n(OH)_n⁽⁶⁻ⁿ⁾⁻ and MoO₂A_n [9]. For tungsten, a relatively low rate of ion formation was noted with varying pH. At low concentrations of the metal, mononuclear ions HWO₄⁻ are present in the solution; at pH < 2, tungsten precipitates in the form of tungstic acid H₂WO₄. An increase in the concentration of tungsten in solution also leads to an increase in the content of polynuclear forms W₁₂O_41−2n^(10−4n)− [10].

Organophosphorus compounds [11,12,13,14,15] and nitrogen-containing reagents are used as effective rhenium extractants. The latter shows higher distribution coefficients for rhenium [16]. On an industrial scale, it is effective to use a tertiary amine—tri-n-octylamine (TOA) or its technical analog trialkylamine (C₇–C₉). Tertiary amines are also produced under the name Alamin336. Trialkylamine (TAA) is used in the form of a 10–20% solution in kerosene with the addition of higher alcohols. Since TAA is not selective for rhenium, it is separated from molybdenum at the stage of re-extraction [17,18]. Fang et al. [19] studied the extraction of rhenium from sulfuric acid solutions and found out that in the absence of interfering impurities and at the acid concentration of 0.1 M (mol/L), the degree of rhenium extraction reaches 99.9%. Lou et al. [20] studied the possibility of using a TritonX-100/N235(TAA)/isoamyl alcohol/heptane/NaCl microemulsion as an effective extractant for rhenium. It is shown that the use of the microemulsion makes it possible to separate rhenium from molybdenum. In [21], conditions for the separation of rhenium and molybdenum from alkaline solutions with a mixture of TOA and tributyl phosphate in kerosene were proposed. Spent alloys of a new generation in their composition contain such valuable components as rhenium, molybdenum, tungsten and vanadium in [4] a comprehensive step-by-step scheme for their selective extraction and separation using LIX 63 (α-hydroxyoxime) and Alamine336 as extractant was proposed.

Despite the fact that tertiary amines quantitatively extract rhenium from solutions, they are not selective extractants to rhenium, as confirmed by various studies [19,21,22,23,24]. On the contrary, salts of quaternary ammonium bases (QAB) are selective to rhenium. However, the process of its extraction with such extractants has not been widely studied due to the fact that the re-extraction process is difficult. Therefore, the industrially used salt of QAB, Aliquat336 (trioctylmethylammonium chloride), is often combined with other extractants [1,4,25].

Identifying selective extractants and refining conditions for rhenium separation from associated metals remain significant challenges. These challenges have gained importance due to the increasing demand for rhenium in aerospace engineering and catalytic applications, where its unique properties are essential.

This study aims to determine the optimal conditions for extracting rhenium with trialkylbenzylammonium chloride (TABAC) and compare its efficiency to the widely used trialkylamine (TAA). Additionally, it explores the possibility of separating rhenium from molybdenum and tungsten, which often occur together, to improve separation processes and advance hydrometallurgical techniques. It is important to highlight the growing industrial significance of rhenium and the urgent need for improved separation technologies. Moreover, there is a gap in current knowledge regarding alternative extractants like TABAC, particularly for large-scale applications where operational robustness and selectivity are crucial.

2. Materials and Methods

2.1. Materials

The study used high-purity substances: ammonium perrhenate (NH₄ReO₄, 99.99%, Sigma-Aldrich, Burlington, MA, USA, CAS number 13598-65-7), ammonium molybdate tetrahydtrate ((NH₄)₆Mo₇O₂₄·4H₂O, 99.98%, Sigma-Aldrich, CAS number 12054-85-2), and ammonium tungstate ((NH⁴)₂WO₄, 99.99%, Sigma-Aldrich, CAS number 11140-77-5). As extractants, trialkylbenzylammonium chloride (45% active ingredient, NII PAV, Volgodonsk, Russia), and trialkylamine (95% tertiary amine content) were used. Standard solutions of rhenium, molybdenum, and tungsten were prepared by dissolving accurately weighed portions of these high-purity substances—ammonium perrhenate (NH₄ReO₄), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), and ammonium tungstate ((NH₄)₂WO₄)—in distilled water. Lower-concentration solutions were obtained by diluting the standard solutions with distilled water. The concentration of rhenium in the studied solutions was 4.0 × 10⁻⁶ mol/L (750 µg/L). Solutions of trialkylbenzylammonium chloride and trialkylamine in kerosene were used. Kerosene is most commonly used as a diluent in industrial extraction processes [1]. Trialkylbenzylammonium chloride is a selective extractant for rhenium, which was prepared by diluting the appropriate volume of pure reagent in a diluent.

Table 1. Some properties of extractants and diluents.

Name	Formula	M, g/mol	Density, g/cm3
Trialkylamine	R3N(R–C7–C9)	354	0.81
Trialkylbenzyl ammonium chloride	R3R′NCl(R–C7–C9;R′–C6H5CH2)	552	0.91
Kerosene	RH (R–C9–C16)	151	0.80

Analytical grade H₂SO₄, HCl, HNO₃, and NaOH were used in the work.

shows the properties of the extractants.

2.2. Methods

Standard solutions of rhenium, molybdenum, and tungsten were prepared by dissolving precisely weighed portions of ammonium perrhenate (NH₄ReO₄), ammonium molybdate tetrahydtrate ((NH₄)₆Mo₇O₂₄·4H₂O and ammonium tungstate ((NH₄)₂WO₄ in distilled water. The organic extractants were prepared by dissolving aliquot portions of trialkylamine (TAA) and trialkylbenzylammonium chloride (TABAC) in kerosene. A 10% solution of TAA in kerosene and a 0.01 M (mol/L) solution of TABAC in kerosene were used.

Before extraction, the pH of the aqueous phase was precisely adjusted using dropwise addition of 0.05 M (mol/L) NaOH or 0.05 M (mol/L) H₂SO₄ under continuous stirring. The pH was monitored with an I-160MI ionometer equipped with an ESL-43-07 combined electrode, calibrated using standard buffer solutions (pH 4.01 and pH 6.86). This process ensured a stable and consistent pH throughout the experiments.

The extraction process involved placing 25 mL of an aqueous rhenium solution (initial concentration C_Re = 4.0 × 10⁻⁶ M) with the required pH value or acid concentration into a separatory funnel. To this solution, 2.5 mL of the extractant was added. The phases were mixed for 5 min using an overhead mechanical stirrer to ensure uniform contact. The volume ratio of aqueous to organic phases was fixed at 10:1. All experiments were conducted at room temperature.

After mixing, the organic and aqueous phases were separated, and the equilibrium concentration of rhenium in the aqueous phase was determined. The phase separation proceeded rapidly and distinctly, with no evidence of third-phase formation. The concentration of rhenium in the aqueous phase was quantified using inductively coupled plasma mass spectrometry (ICP-MS) on an Agilent 7500a spectrometer (Santa Clara, CA, USA). Calibration standards ranging from 5 to 1000 µg/L were prepared using the Multi-Elemental Calibration Standard-4 solution to ensure measurement accuracy.

The concentrations of molybdenum and tungsten in their respective aqueous solutions were determined using the same ICP-MS method. Experiments involving molybdenum and tungsten were conducted under identical conditions to maintain consistency and comparability of results.

All experiments were performed in triplicate, and the standard deviations of the distribution coefficients were consistently below 5%, underscoring the reliability and reproducibility of the experimental data.

The percentage extraction (E, %), distribution coefficient (D), and separation coefficient ($\mathsf{\beta }$) were calculated using the following formulas:

$$\left(\mathrm{E},\%\right)={\displaystyle \frac{\mathrm{D}·100}{\mathrm{D}+{\mathrm{V}}_{\mathrm{aq}}/{\mathrm{V}}_{\mathrm{org}}}}$$

(1)

$$\mathrm{D}={\displaystyle \frac{{\mathrm{C}}_{\mathrm{org}}}{{\mathrm{C}}_{\mathrm{aq}}}}$$

(2)

$${\mathsf{\beta }}_{\mathrm{Me}1/\mathrm{Me}2}={\displaystyle \frac{\left.\mathrm{D}({\mathrm{Me}}_1\right)}{\left.\mathrm{D}({\mathrm{Me}}_2\right)}}$$

(3)

3. Results and Discussion

Trialkylamine (TAA) effectively extracts rhenium from sulfuric, hydrochloric, and nitric acid solutions, as well as their mixtures. In comparison to tertiary amines, quaternary ammonium base salts function as stronger bases, enabling the extraction of metals from both alkaline and acidic solutions. Since metal extraction by basic extractants is significantly influenced by the pH of the solutions, as well as the nature and concentration of mineral acids, the extraction of rhenium by TAA and TABAC was systematically investigated by varying the pH and using solutions of HCl, H₂SO₄, and HNO₃ at different concentrations.

3.1. Extraction of Rhenium Depending on the pH

The extraction of rhenium with solutions of trialkylamine and trialkylbenzylammonium chloride in kerosene was studied by varying the pH values from 2 to 12. The dependence of the degree of metal extraction on equilibrium pH values is shown in Figure 2. The maximum degree of rhenium extraction was 90–96% in the pH range of 2–6.

As the pH increases from 0 to 12, the extraction efficiency of rhenium by TAA and TABAC decreases. This decline is attributed to the transformation of TAA from its protonated form to a neutral form, reducing its capacity to facilitate extraction. However, QAB salts, being stronger bases than tertiary amines, have the ability to extract rhenium from alkaline solutions. Moreover, the extraction efficiency of rhenium with TABAC is superior to that of TAA. The decrease in extraction efficiency with increasing pH is further explained by the reduced protonation of amines at higher pH, which diminishes their ability to exchange anions effectively.

The high recovery of rhenium can be explained by the presence of rhenium in solutions in the form of perrhenate ions, which are extracted by the anion exchange mechanism.

According to existing studies [1], extraction of rhenium with tertiary amines from solutions of mineral acids proceeds in 2 stages:

The formation of an amine salt:

R₃N_org + H⁺_aq + A⁻_aq = [R₃NH⁺]A⁻_org (HA − HCl, HNO₃)

(4)

In the case of sulfuric acid:

R₃N_org + H₂SO_4aq = [R₃NH]HSO_4org

(5)

Replacement of the mineral acid anion with perrhenate ion:

[R₃NH]A_org + ReO₄⁻_aq = [R₃NH]ReO₄⁺_org A⁻_aq

(6)

[R₃NH]HSO_4org + ReO₄⁻_aq = [R₃NH]ReO₄⁻_org + HSO₄⁻_aq

(7)

Extraction by QAB salts proceeds in one stage:

R₄NA_org + ReO₄⁻_aq = R₄NReO₄⁻_org + A⁻_aq(A⁻—mineral acid anion)

(8)

3.2. Extraction of Rhenium Depending on the Nature and Concentration of Mineral Acid

Rhenium extraction was carried out using TAA-kerosene and TABAC-kerosene solutions depending on the concentrations of HCl, H₂SO₄, and HNO₃. Acid concentrations varied in the range of 0.1–3.0 M (mol/L). The results obtained are graphically presented in Figure 3.

Rhenium was efficiently extracted by TAA and TABAC from sulfuric and hydrochloric acid solutions, achieving high extraction levels. However, as the acid concentration increased, the extraction efficiency decreased from 99.9% to 83%. It is important to note that the highest extraction efficiencies were achieved when TABAC dissolved in kerosene was used as the extractant for sulfuric acid solutions.

Extraction of rhenium from aqueous solutions of mineral acids with TAA-kerosene and TABAC-kerosene solutions proceeds by an anion exchange mechanism, and for both extractants, decreases in the order H₂SO₄ > HCl > HNO₃. The extraction processes are described by Equations (4)–(7).

The low recovery of rhenium from nitric acid solutions is likely attributed to the occurrence of competing reactions, which interfere with the extraction process:

R₃N_org + HNO_3aq = [R₃NH⁺]NO₃⁻_org

(9)

[R₃NH]NO_3org+ HNO_3aq = [R₃NH]NO₃·HNO_3org

(10)

According to [26], the higher the nucleophilicity of acid anions, the stronger the competition of its extractable anions. The low nucleophilicity of Cl⁻ and SO₄²⁻ ions minimizes their interfering impact. Rhenium from nitric acid solutions is practically not extracted by both extractants due to the high affinity of NO₃⁻ ions for the amine.

3.3. Extraction of Rhenium with TABAC-Kerosene Solution Depending on the Concentration of the Extractant

The process of rhenium extraction was investigated by varying the concentration of TABAC in kerosene within the range of 10⁻⁵ to 10⁻² M (mol/L). Rhenium was effectively extracted by TABAC from solutions within the pH range of 2 to 6, and thus, a pH of 3.5 was maintained in the experiments. The results of the study are presented in Figure 4.

The optimal concentration of the extractant for quantitative rhenium extraction was found to be 10⁻² M (mol/L). At lower extractant concentrations, the distribution coefficients of rhenium decrease, indicating reduced extraction efficiency.

3.4. Extraction of Rhenium by TABAC-Kerosene Solution Depending on the Metal Concentrations

The extraction of rhenium by a TABAC-kerosene solution was studied by varying the concentration of rhenium in the aqueous phase from 10⁻⁴ to 10⁻⁶ M (mol/L). The results are shown in Figure 5. The relationship between logD and -logC_Re was found to be linear, with the slope (tg α = 1.32) close to unity. This indicates the formation of a complex with the composition [R₃R′NReO₄] in the organic phase, where the metal-to-extractant ratio is 1:1.

3.5. Study of the Possibility of Separating Molybdenum, Tungsten, and Rhenium Using the TABAC-Kerosene Solution

The ability of TABAC to selectively extract rhenium in the presence of molybdenum and tungsten was evaluated by varying the acidity of the aqueous phase (pH 3–12, H₂SO₄ concentration 0.1–3 M). The results shown in Figure 6 demonstrate the effective separation of rhenium from both molybdenum and tungsten under acidic and alkaline conditions. The separation coefficients of metals during the extraction by TABAC are shown in

Table 2. Separation coefficients of Metals During Extraction by TABAC − kerosene Solution.

pH	C(H2SO4), M (mol/L)	${\mathbf{\beta }}_{\raisebox{1ex}{$\mathbf{Re}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{Mo}$}\right.}$	D(Re)·D(Mo)	${\mathbf{\beta }}_{\raisebox{1ex}{$\mathbf{Re}$}\!\left/ \!\raisebox{-1ex}{$\mathbf{W}$}\right.}$	D(Re)·D(W)
4.0		2.7	21,780	1.3	44,286
6.0		20.9	23,853	1.6	322,750
8.9		11.7	91	53.2	20
11.2		9.2	11.1	19.1	1956
	0.10	66.8	865
	0.50	58.4	445
	1.0	17.3	401	55.8	124
	2.0	10.9	743	21.6	1203
	3.0	6.8	579

.

A comprehensive comparison of the extraction characteristics of TAA and TABAC for rhenium is provided in

Table 3. Comparative Table of Rhenium Extraction Efficiency: Trialkylamine (TAA) and Trialkylbenzylammonium Chloride (TABAC).

Parameter	Trialkylamine (TAA)	Trialkylbenzylammonium Chloride (TABAC)
Extraction mechanism	Anion exchange, two-step process	Anion exchange, single-step process
Optimal pH range	2–6	2–7
Extraction efficiency	~90–96%	>99%
Acid types for extraction	H2SO4, HCl, HNO3	H2SO4, HCl, limited efficiency in HNO3
Separation from Molybdenum and Tungsten	Limited selectivity	High selectivity
Compatibility with solvent	Kerosene	Kerosene
Efficiency in nitric acid	<30%	<30%
Formation of complexes	[R3NH]ReO4, [R3NH]HSO4	[R4N]ReO4
Industrial Use	Commonly used in acidic solutions	High potential due to high selectivity
Separation coefficient (Re/Mo)	Moderate	High
Separation coefficient (Re/W)	Moderate	High

. This summary highlights TABAC’s superior selectivity and efficiency, especially in separation from molybdenum and tungsten.

4. Conclusions

The principles of rhenium extraction using trialkylbenzylammonium chloride (TABAC) in kerosene were thoroughly investigated. A comparison of the extraction abilities of TABAC and trialkylamine (TAA) for rhenium was conducted, and the potential for separating rhenium, molybdenum, and tungsten using the TABAC-kerosene solution was assessed. The key findings of this study are summarized as follows:

• Quantitative recovery (>99%) of rhenium is achieved within the pH range of 2–7 and in sulfuric and hydrochloric acid concentrations ranging from 0.1 to 3.0 M (mol/L).
• The extraction of rhenium from nitric acid solutions does not exceed 30%, which is attributed to the competing extraction of HNO₃.
• The extraction efficiency of TABAC for rhenium is superior to that of the industrial extractant trialkylamine.
• Rhenium extraction by TABAC follows an anion exchange mechanism, resulting in the formation of complexes of the composition [R₃R′NH]ReO₄, which are extracted into the organic phase.
• Trialkylbenzylammonium chloride is a selective extractant for rhenium. Rhenium can be effectively separated from molybdenum and tungsten from solutions with a pH greater than 7, as well as from hydrochloric and sulfuric acid solutions.
• The separation factors for rhenium, molybdenum, and tungsten in TABAC extraction are higher than those in extraction using trialkylamine.

While this study comprehensively investigated the extraction of rhenium using trialkylamine (TAA) and trialkylbenzylammonium chloride (TABAC), the desorption of rhenium from the organic phase was not examined. Future studies should focus on optimizing desorption processes to facilitate the complete recovery of rhenium and assess the reusability of the extractants, which are crucial for practical applications.