Production of Perrhenic Acid by Solvent Extraction

Abstract

The aim of this work was to develop an effective method for obtaining perrhenic acid from available ammonia waste solutions using the solvent extraction method. An ammonia waste solution was used as the test material, with Re and NH₄⁺ concentrations of 13.5 and 43.7 g/dm³, respectively. The scope of this study includes the following: the selection of an appropriate extractant for testing, and the examination of the impact of individual parameters on the efficiency and selectivity of extraction and stripping. The obtained results made it possible to determine the conditions for the production of perrhenic acid via the extraction method using organic solutions of trihexyl(tetradecyl)phosphonium chloride (Cyphos IL 101). The outcome of this study was the development of a method for obtaining perrhenic acid and the production of the acid sample with an efficiency of >90% and a Re concentration of >100 g/dm³.

1. Introduction

Rhenium is an element from the group of transition metals with a low prevalence in the Earth’s crust. Its unique properties, such as its high melting point, high density, and resistance to corrosion and deformation, make it a valuable material in many industries [1].

In the aviation and space industries, rhenium is an important component of superalloys used to produce jet engine turbine blades and space vehicle heat shields. Its use allows engines to operate at higher temperatures, which translates into better performance and more efficient fuel consumption [2,3,4,5].

Rhenium also plays an important role in the petrochemical industry as a catalyst in the production of high-octane gasoline. In addition, it is used to produce heating elements, electrical contacts, electrodes, electromagnets, and vacuum and X-ray tubes [6,7].

In the field of chemistry, rhenium is valued for its catalytic properties in processes such as metathesis, epoxidation, and dihydroxylation [8]. The most valuable rhenium compounds on the market are high-purity perrhenic acid and ammonium perrhenate, which are used to produce metallic rhenium and its other compounds [9].

The recovery of rhenium is a complex process due to its low concentration in the Earth’s crust. Most often, it is obtained from manganese ore or as a by-product in the copper production process. The most popular techniques used to recover Re from primary and secondary sources are usually solvent extraction, ion exchange, precipitation, adsorption, and membrane techniques [9,10,11].

Among the above-mentioned techniques, solvent extraction is a particularly popular technique due to its high efficiency, the possibility of regenerating and recycling extractants, the relatively low price of the process, and the small amount of waste.

Many effective rhenium extractants are known, although there are only a few reports in the literature on the production of perrhenic acid by solvent extraction [12]. Typically, this technique is used to separate Re from other metallic components, including molybdenum [13,14,15,16], tungsten [17,18], vanadium [19,20,21], arsenic [22], or precious metals [23], where the end product of the process is most often ammonium perrhenate.

For example, Zhan-fang et al. investigated the extraction of rhenium from alkaline solutions containing molybdenum, using a mixture of triotylamine and tributyl phosphate in kerosene as a selective extractant. They showed that the optimal extraction conditions included extraction with 20% trioctylamine with the addition of 30% tributyl phosphate at pH 9.0, room temperature, and a phase ratio of 1:1, obtaining a rhenium extraction efficiency of 96.8% with a very low molybdenum co-extraction of 1.7%. Effective stripping of rhenium into the aqueous phase was achieved using an 18% ammonia solution, achieving a stripping efficiency of approximately 99.3% [16].

In a study conducted by Truong et al., a methodology for separating rhenium from molybdenum, vanadium, and tungsten from synthetic solutions was developed. The authors conducted research using two different solutions, obtained by dissolving appropriate salts in distilled water and adjusted to the desired pH by acidification or alkalinization with HCl and NaOH. As a result of the experiments carried out, the following separation scheme was proposed: First, molybdenum and vanadium were selectively extracted using the LIX 63 extractant, and tungsten was extracted from the remaining raffinate using Alamine 336. As a result, pure rhenium solutions were obtained for further processing [17].

In another publication, the same research team analyzed the possibility of separating Re from Mo and V using solutions of tributyl phosphate in kerosene, using different concentrations of hydrochloric acid in the starting solution. The tests results showed that tributyl phosphate selectively extracted Mo and Re, while V remained in the solution. However, attempts to separate Mo and Re at the stripping stage did not bring satisfactory results [21].

As mentioned above, despite numerous studies on the use of solvent extraction for the isolation and separation of Re from other metals, only a few references to the use of this method for the production of rhenic acid can be found in the scientific literature. Previously, our team had already made attempts to obtain rhenic acid, which were published in 2009. In the mentioned studies, perrhenic acid was obtained from aqueous solutions of ammonium perrhenate, using a 50% solution of tributyl phosphate in toluene as an extractant. The process included the extraction of ReO₄⁻ from an acidified ammonium perrhenate solution, stripping ReO₄⁻ from the organic phase with water at 80 °C, and repeated extraction and stripping, which allowed us to obtain perrhenic acid with a concentration of approximately 300 g/L Re [12].

In recent years, ionic liquids have gained significant popularity in extraction research due to their unique physicochemical properties. Their low vapor pressure, their high thermal stability, and the ability to adjust their properties through the appropriate selection of cations and anions make them attractive alternatives to conventional organic solvents [23]. Taking into account the growing trend in the use of this type of substance, our team undertook research on the use of the ionic liquid Cyphos IL 101 (trihexyl(tetradecyl)phosphonium chloride) to obtain perrhenic acid. The experiments focused on the analysis of key parameters of the extraction and re-extraction process. The aim of this study was not only to determine the optimal conditions for the process, but also to attempt to obtain pure perrhenic acid.

2. Materials and Methods

2.1. Reagents

A solution created by combining ammonia eluates from the production of ammonium perrhenate and waste solutions from the production of perrhenates of various metals was used in this study. As a result, the starting solution had a pH of 11 and Re and NH₄⁺ concentrations of 13.5 g/dm³ and 43.7 g/dm³, respectively.

Organic solutions such as Cyphos IL 101 (abcr GmbH, Karlsruhe, Germany), tributyl phosphate (Avantor, Gliwice, Poland), toluene (Chempur, Piekary Śląskie, Poland), and Exxsol D80 (ExxsonMobil Chemical, Machelen, Belgium) were used for the extraction tests.

Nitric acid (Chempur, Piekary Śląskie, Poland) was used as a stripping solution. The characteristics of the reagents used in the tests are presented in

Table 1. The characteristics of the reagents used in the tests.

Substance	Molecular Formula	Purity	CAS Number	Producer
Cyphos IL 101	C38H68ClP	93%	258864-54-9	abcr GmbH
Tributyl phosphate	C12H27O4P	>98%	126-73-8	Avantor
Toluene	C7H8	>99%	108-88-3	Chempur
Exxsol D80	aliphatic hydrocarbons	>99%	64742-47-8	ExxsonMobil
Nitric acid (65%)	HNO3	>99%	7697-37-2	Chempur

.

2.2. Methodology

2.2.1. Extraction Procedures

• Research on the influence of pH on the extraction efficiency

The tests were carried out in the pH range of 1–11. The pH of the solution was controlled by gradually adding appropriate volumes of sulfuric acid. Then, a series of extraction tests were performed for a 50% (V/V) solution of Cyphos IL 101 in toluene. For this purpose, each time, 20 cm³ of the test solution with the selected pH values and 20 cm³ of freshly prepared extractant solution were taken. Both phases were intensively mixed using a magnetic stirrer for 30 min. Then, the phases were separated using a glass separator, the volumes of both phases were measured, and the separated aqueous phase was analyzed for the content of Re and NH₄⁺.

2.

Research on the influence of the extractant concentration in the organic phase on the extraction efficiency

Samples of organic phases with different volume concentrations of the extractant in Exxsol D80 were prepared. In order to avoid the formation of dispersion systems and two organic phases, a 20% (v/v) addition of tributyl phosphate was used each time. The extraction was carried out at the pH specific to the starting solution—11. The contact time was 30 min at a volume ratio of 1:1.

3.

Research on the influence of the Vaq:Vo ratio on the extraction efficiency

Cyphos IL 101 solutions with appropriate concentrations were prepared based on the results obtained in the previous part of the present study. Extractions were carried out for 30 min and the ratio of the aqueous phase (Vaq) to the organic phase (Vo) was 5:1–1:1.

4.

Research on the influence of contact time on the extraction efficiency

The tests were performed for a time interval of 1–30 min, assuming a Vo:Vaq volume ratio of 1:1 and the compositions of the extraction phases determined in the previous work points. Based on the data obtained, the dependence of the extraction efficiency on the contact time was plotted and the process time required to ensure the maximum extraction efficiency of the ReO₄⁻ ion was indicated.

2.2.2. Stripping Procedures

The organic phases obtained in the previous stages of the present study were used to determine the possibility of recovering the ReO₄⁻ ion from the extracts. The possibility of obtaining perrhenic acid by stripping was checked by mixing the extract and re-extractant for 30 min using a 1:1 volume ratio (20 cm³:20 cm³). The following stripping agents were used:

• Demineralized water at room temperature and 50 °C;
• A 32.5% nitric acid solution at room temperature and 50 °C.

For the stripping tests, the influence of the phase ratio and contact time on the stripping efficiency was checked, similarly to the extraction tests.

2.2.3. Pure Perrhenic Acid Preparation

Rhenium extraction was carried out at the pH typical for the test solution, i.e., 11. A mixture containing 5% (v/v) Cyphos IL 101 and 20% TBP (v/v) in Exxsol D80 was used as the extractant. A total of 200 cm³ of the organic phase was contacted with 400 cm³ of the aqueous phase. Then, the collected organic phase was washed with water. Stripping with a 32.5% nitric acid solution was carried out using a volume phase ratio of 1:1. The concentration of nitric acid in the aqueous solution was lowered by evaporating the solution to remove nitrogen oxides. Evaporation was carried out by maintaining the temperature at 90 °C and checking the progress using wet test strips for the semi-quantitative determination of nitrates. Evaporation was completed when the concentration of nitrogen oxides in the vapors dropped below the detection limit of the test strips (10 mg/dm³).

2.3. Analytical Techniques

The concentrations of Re and NH₄⁺ in the tested solutions were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 5110 SVDV ICP-OES spectrometer, Agilent Technologies, Santa Clara, CA, USA) and atomic absorption spectrometry techniques (AAS, iCE 3300 AAS spectrometer, Thermo Scientific, Waltham, MA, USA). The analyses were performed by the Łukasiewicz Research Network—Institute of Non-Ferrous Metals (Gliwice, Poland).

3. Results and Discussion

3.1. Extraction Studies

Figure 1 shows the results of testing the influence of pH on the extraction efficiency of the ReO₄⁻ anion. The obtained results indicate that the extraction efficiency was very high in the entire tested range. The extraction efficiency of the ReO₄⁻ ion at the pH specific to the test solution was 99.98%. Moreover, the co-extraction of the ammonium ion at this pH was relatively low and did not exceed 10%.

The results of the tests on the influence of the composition of the organic phase on the extraction efficiency of the ReO₄⁻ anion, presented in

Table 2. Studies on the influence of the extractant concentration on the extraction efficiency.

Volume [%]		Concentration in Raffinates [g/dm3]		Extraction Efficiency [%]
Cyphos IL 101	TBP	Re	NH4+	Re	NH4+
1	20	8.78	31.3	18.70	10.47
3	20	5.09	35.1	52.87	<1
5	20	2.04 × 10−3	31.9	99.98	8.75
10	20	0.55 × 10−3	30.0	99.99	14.19

, indicate that already a 5% (v/v) solution of Cyphos IL 101 in Exxsol D80 allowed for the extraction of over 99.9% of Re. The 20% (v/v) addition of tributyl phosphate was necessary due to the formation of two organic phases and was used as a modifier. The obtained results indicate that changing the solvent from toluene to Exxsol D80 did not negatively affect the Re extraction efficiency. The co-extraction of the ammonium ion remained at an appropriately low level, i.e., <8%.

The obtained results were used in the subsequent stages of the research, where each time a 5% (v/v) solution of Cyphos IL 101 in Exxsol D80 with 20% (v/v) TBP was used as the organic phase.

For the tests on the influence of the phase ratio, extraction efficiencies above 99% were obtained using the volume ratios Vaq/Vo of 1:1 and 1:2 (Figure 2a). The highest Re concentration obtained in the organic phase was 27.36 g/dm³ at a Vaq:Vo ratio of 3:1.

The conducted research on the influence of contact time on the extraction efficiency showed that the extraction of the ReO₄⁻ anion occurred very quickly (Figure 2b). The equilibrium state, i.e., the minimum process time beyond which no further increase in the efficiency of ReO₄⁻ extraction is observed, was achieved after only 3 min of contact time. Moreover, the contact time did not significantly affect the increase or decrease in the co-extraction of NH₄⁺ ions.

The data obtained as a result of the preliminary stripping tests indicate that nitric acid solutions were an effective stripping agent for ReO₄⁻, as a >99.99% Re stripping efficiency from the organic phase was achieved each time. Moreover, in the case of the 32.5% nitric acid solution at a temperature of 50 °C, a decrease in the efficiency of ammonium ion co-stripping was observed compared to nitric acid at room temperature. In the case of tests to strip ReO₄⁻ using demineralized water, the efficiencies were very low, i.e., <1%, regardless of the used stripping solution temperature (

Table 3. Stripping tests results.

Stripping Solution		H2O RT	H2O 50 °C	HNO3 RT	HNO3 50 °C
Concentration in the stripping solution [g/dm3]	Re	0.37 × 10−3	0.37 × 10−3	0.37 × 10−3	0.37 × 10−3
	NH4+	8.0 × 10−3	8.0 × 10−3	8.0 × 10−3	8.0 × 10−3
Stripping efficiency [%]	Re	<0.01	0.01	99.99	99.99
	NH4+	0.27	0.27	44.86	8.66

).

The results of the research on the influence of the ratio of the organic phase to the aqueous phase and the contact time presented in Figure 3a,b indicate that the appropriate phase ratio for the stripping step should be 1:1 while maintaining a contact time of 5 min.

3.2. Perrhenic Acid Preparation

The carried out preliminary research was used to develop a methodology to produce perrhenic acid.

Table 4. Results of obtaining perrhenic acid.

Extraction						Stripping
Extract		Raffinate		Efficiency [%]		Concentration [g/dm3]		Efficiency [%]
Concentration [g/dm3]		Concentration [g/dm3]
Re	NH4+	Re	NH4+	Re	NH4+	Re	NH4+	Re	NH4+
26.99	7.17	0.005	40.12	99.96	8.20	13.49	0.62	99.93	17.20

presents a summary of the results of the conducted research. The obtained extraction and stripping efficiencies were, as expected, high. The obtained re-extract was concentrated, resulting in a solution with concentrations of Re, NH₄⁺, and NO₃⁻, respectively, of 107.9, 4.9, and 0.3 g/dm³.

The obtained results were used to prepare a technological scheme (Figure 4) for the production of perrhenic acid. Since the obtained perrhenic acid was characterized by a relatively high concentration of ammonium ions, an additional purification step using ion exchange resins is proposed.

4. Conclusions

The tests carried out confirmed the effectiveness of Cyphos IL 101 as an extractant for the extraction of ReO₄⁻ anions. In the tests, a more than 99.9% Re extraction efficiency was obtained. Testing a wide range of parameters allowed for the optimization of the extraction conditions. It has been shown that the organic phase containing 5% (v/v) Cyphos IL 101 and 20% (v/v) tributyl phosphate in the Exxsol D80 diluent is an efficient extraction phase, enabling almost quantitative extraction of Re from the initial solution. Moreover, Vo:Vaq ratios of 1:1 and 1:2 and a contact time of 3 min were considered the most favorable extraction conditions. The tested range of the pH influence confirmed that the Cyphos IL 101 extractant remains effective even in the acidic environment.

In the conducted stripping tests, it was established that the 32.5% solution of nitric acid is an effective stripping agent. In the study of the influence of parameters on the efficiency of Re recovery, it was indicated that stripping should be carried out with a phase ratio of 1:1 and a contact time exceeding 5 min, which allows for the recovery of extracted Re with an efficiency >99%. The obtained data were used for further research on the preparation of perrhenic acid using the solvent extraction method.

The developed method allowed us to obtain perrhenic acid with a Re concentration >100 g/dm³ and an efficiency exceeding 90%. Despite obtaining satisfactory extraction and stripping efficiencies, the entire method is quite time-consuming due to the necessary step of removing nitrogen oxides, which additionally involves a large energy input. This is particularly important in the context of using the described method on a scale larger than the laboratory one. Moreover, the entire method was characterized by insufficient selectivity of the extraction and stripping of ReO₄⁻ ions in relation to ammonium ions. Therefore, it is recommended to carry out an additional stage of purification of the obtained perrhenic acid using other techniques, e.g., sorption of NH₄⁺ ions on an ion exchange resin—Purolite C160 [24].

To sum up, the developed solvent extraction method is a promising way to obtain perrhenic acid. However, it requires further work on optimization and increasing selectivity to fully exploit its potential, especially in the context of its possible use on an industrial scale.