A Comparison of Production Methods of High-Purity Perrhenic Acid from Secondary Resources

: Methods for obtaining high-purity perrhenic acid (with metallic impurities content below 100 ppm) of a high concentration > 200 g/dm 3 and entirely from secondary raw materials were compared. Comparative analyses of three methods were performed: electrodialysis, solvent extraction (research carried out directly as part of the Small Grant project acronym RenMet), and ion-exchange (developed as part of previous projects implemented by Łukasiewicz-IMN). The basic process parameters were selected as comparative indicators: efficiency and selectivity of the process, purity of the obtained product, availability and consumption of raw materials and reagents, equipment necessary to carry out the process, the profitability of the technology, and the ecological aspects, i.e., the possibility of managing the generated solid waste and post-production solutions. Analysis of the verified indicators allowed us to select the most economically and ecologically advantageous method of obtaining high-purity perrhenic acid from secondary raw materials. Its preparation using the ion-exchange method emphasizes the product’s purity and the process’s simplicity, using readily available waste materials and renewable ion-exchange resin, and is based on a sustainable circular economy.


Introduction
Perrhenic acid is an inorganic chemical compound, that is a colorless or slightly yellow liquid, highly soluble in water and organic solvents [1,2].It has weak oxidizing properties and forms numerous salts, such as perrhenate salts with metals such as Cu [3], Ni [4], Co [5], Pb [6], Zn [4], and Li [7] or Na [8], K [9], and Ca [10], as well as with the ammonium cation NH 4 + [11].Perrhenic acid, ammonium perrhenate, and metallic rhenium are among the most frequently produced rhenium products and constitute the substrate for the production of other rhenium-containing components with specific desired properties often used in niche industries, e.g., catalysis, pharmacy, and medicine [12][13][14].The application of perrhenate acid requires its use as a reagent with high catalytic purity and a high concentration in the solution.
The production of perrhenic acid is a troublesome process, not only due to the possibility of significant losses of rhenium but primarily due to the possibility of its contamination, mainly with cations such as NH 4 + , Na, Mg, Al, K, Ca, Ni, Co, Fe, Zn, Cu, and Cr.The production of perrhenic acid with higher concentrations, most often carried out by concentration, also causes an increase in the number of impurities in the finished product.Such perrhenic acid usually does not meet the applicable quality standards.
Many methods of obtaining perrhenic acid differ in the initial substrate, purity and concentration of the product, and the techniques used.
One of the common methods for the production of perrhenic acid involves the direct dissolution of metallic rhenium or its oxides, most often in solutions of hydrochloric acid with the addition of a strong oxidant, e.g., nitric acid or hydrogen peroxide [15], directly in concentrated nitric acid [16] or in H 2 O 2 solutions [17].
As-obtained perrhenic acid solution has a low concentration; therefore, to obtain commercial products, it is required to concentrate it and remove the excess acids used.There is also a method consisting first of the oxidative roasting of rhenium-bearing materials, such as rhenium sulfide, and then the production of volatile rhenium(VII) oxide, which, after adsorption in water, forms perrhenate acid [18], or the direct dissolution of rhenium(VII) oxide in water [19].
These methods require the use of expensive and complex equipment that, in the case of metallic rhenium, are produced by reducing ammonium perrhenate.The purity of the perrhenic acid obtained in this way is determined by the purity of the Re 2 O 7 substrate, which is a very hygroscopic substance that is difficult to obtain and store [20].
Perrhenic acid can also be obtained by solvent extraction [21][22][23].Literature reports on the preparation of perrhenic acid by solvent extraction are scarce.Most of them are based on the extraction of rhenium and then the conversion of it into the form of ammonium perrhenate by re-extraction with ammonia solutions, not on the direct preparation of perrhenic acid.
Methods for obtaining perrhenic acid directly by solvent extraction often use acidic ammonium perrhenate solutions, from which perrhenate ions are extracted using selected, dedicated extractants.Most often, a 50% solution of tributyl phosphate (TBP) in toluene is used as an extractant [24,25], but tri-n-octylamine, bis-iso-dodecyl amine, pyridine, Aliquat 336, trioctylphosphine oxide, cyclohexanone, ethyl, xanthate, and mesityl oxide [26,27], as well as tetraphenylphosphonium chloride or triphenylbenzylphosphonium chloride, can also be used [28].To re-extract rhenium from organic-phase acid, basic solutions are used, i.e., HCl, H 2 SO 4 , and perrhenates are extracted with NH 4 OH when ammonium perrhenate is produced or NaOH and KOH when the products are NaReO 4 and KReO 4 , respectively [17].The production of high-purity perrhenic acid by solvent extraction needs to be further condensed.The process of perrhenic acid concentration causes an increase in the content of impurities in proportion to the degree of its concentration as well.A disadvantage of these methods is often the large number of operations, which generate losses of rhenium, increase the possibility of contamination of the final product, and extend the time needed to obtain it.
There are also known methods of obtaining perrhenic acid using the electrodialysis process [28][29][30].Most often, the process of concentrated perrhenic acid production by electrodialysis is carried out using solutions of rhenium salts: ammonium or potassium perrhenate, as well as sodium with a rhenium concentration of 35-40 g/dm 3 .The process is carried out in multi-chamber electrodialysis with alternating anion-exchange (MA-40) and cation-exchange (MK-40) membranes, a graphite or platinum anode, and a stainless steel or platinum cathode.The most commonly used electrode solutions in the anode chamber are hydrochloric acid at a concentration of 10 g/dm 3 [28,29] or perrhenic acid solutions [30,31] with a concentration of about 1 g/dm 3 [28,29,32], while in the cathode chamber, 0.1 M potassium hydroxide solution [30] and a sodium hydroxide solution with a concentration of 100 g/dm 3 are used [31].By using these methods, it is possible to obtain a concentration of up to 600 g/dm 3 of perrhenic acid; however, due to the reduction in the selectivity of the process and the mechanical stability of the membranes as well as the current efficiency of the process (increasing electricity consumption), the production of perrhenic acid with a concentration higher than 350 g/dm 3 is unprofitable.The disadvantage of this method is not only the use of many expensive ion-exchange membranes to construct one membrane cell but also their rapid wear and destruction of and reduction in selectivity.
There is a method of HReO 4 production by electrodialysis that limits the process inconveniences described in the abovementioned publications, which eliminates losses of rhenium [33].Using bipolar membranes (BMEDs) enables the production of acid and base from solutions of their salts.The process is carried out in an electrodialyser of two main production streams (diluate and concentrate) and two near-electrode streams (catholyte and anolyte).The stream solutions circulate in a closed circuit and are separated from each other by cation-exchange, anion-exchange, and bipolar membranes.Ammonium perrhenate solution with an initial rhenium ions concentration of 5-150 g/dm 3 circulates as a diluate, and the concentrate solution is perrhenic acid at a concentration of about >5 g/dm 3 .Under the influence of electric voltage, perrhenate ions migrate selectively through the membranes.This allows for obtaining a concentrate enriched in ReO 4 − ions and thus perrhenic acid, with a rhenium ion concentration of up to 130 g/dm 3 .In contrary to numerous advantages, the disadvantage of this process is the insufficient purity of the obtained product of perrhenic acid.It does not meet the quality standards for HReO 4 required by customers, i.e., the content of total impurities < 100 ppm, including the content of each impurity ≤ 10 ppm.
Another group of methods for obtaining perrhenic acid are techniques based on the ion-exchange process using ion-exchange resins.This has been extensively researched and described and is most often used in industry [34][35][36].The method involves passing an ammonium perrhenate solution through a strongly acidic cation resin bed, primarily prepared in hydrogen form.During sorption, ammonium, potassium, or sodium ions are sorbed.The process is carried out at a temperature of 30-50 °C.The effluent from the column is a HReO 4 solution containing up to 20 g/dm 3 Re, which is then directed to concentration by evaporation under reduced pressure to obtain acid of the desired concentration.For further purification of perrhenic acid, active carbon is used [34].
The disadvantage of all these methods of perrhenic acid production by ion-exchange is low solubility of substrates such as ammonium, potassium, or sodium perrhenate in water [37][38][39].The low concentration of rhenium in the starting solutions influences the low concentration of perrhenic acid obtained.Thus, further concentration under pressurized conditions by evaporation is necessary, and it requires the use of evaporators that contain large amounts of energy.
Since all known and used methods for obtaining perrhenic acid have both advantages and disadvantages, it cannot be indicated which is the best and which allows for the production of perrhenic acid at both a high concentration and high purity.Also, using rhenium-bearing waste as a source of rhenium requires verification of all three methods to select the most economically and ecologically advantageous one for the production of highpurity perrhenic acid from secondary raw materials.Thus, in this article, a comprehensive analysis of production methods of high-purity and high-concentration perrhenic acid only from secondary materials is attempted.

Materials and Methods
Ammonium perrhenate used in the research was obtained by leaching superalloy scraps (with the main compositions shown in Table 1) using a mixture of H 2 SO 4 and HCl acids with the addition of oxidants (H 2 O 2 and HNO 3 ) as the leaching agent [35,37,40].A solution containing 1.1 g/dm 3 Re, mainly of nickel, cobalt, chromium, and aluminum, was obtained.This solution was directed to the sorption of rhenium using a weakly basic ion-exchange resin A170 (Purolite, King of Prussia, PA, USA, hydroxide form).Next, it was directed to the recovery of valuable metal components (Ni, Co).Rhenium adsorbed by resin was eluted with an aqueous ammonia solution (25%, Chempur, Piekary Śl ąskie, Poland, p.a.).The ammonium perrhenate was crystallized from the obtained ammoniacal eluate.The ammonium perrhenate was obtained with the following composition: min.64.4% Re, Al, Cu, Fe, Mg, Mo, Na, Ni, Pb, Zn < 0.0005%, Ag, Ba, Ca, K, Cd, Co, Cr, Pd, Rh, Ru, Sb, Sn, Tl < 0.001%, As, Se, Si, W < 0.002%, V, P, Ir < 0.005%, Hg < 0.0001%, and Cl 2 < 0.05%.This compound was used in all the described and tested methods for obtaining perrhenic acid.In the process of the production of perrhenic acid by the electrodialysis method, the starting concentrate and near-electrode solutions of perrhenic acid concentrations of 40 g/dm 3 and 30 g/dm 3 HReO 4 , respectively, were obtained from a solution of perrhenic acid with a concentration of 120 g/dm 3 containing the following: <1 ppm Ca, <10 ppm K, <1 ppm Mg, <1 ppm Cu, <1 ppm Na, <1 ppm Mo, <1 ppm Ni, <1 ppm Pb, <1 ppm Fe, <5 ppm NH 4 + , <1 ppm Co, <1 ppm Sign.This acid was obtained from secondary raw materials-superalloy scrap waste during previous projects realized in Łukasiewicz-IMN (projects: InRen and NanoRen) [35,38,41].
In all cases, vacuum evaporators were used to concentrate the solutions.First was a glass vacuum evaporator from Kavalier with an efficiency of 5 dm 3 /h (Figure 1b) with a temperature of ~80 • C and operating pressure of 400 ÷ 500 mbar.Second was the Hei-VAP rotary evaporator from Heidolph with parameters of 130 • C and 400 mbar, respectively.The choice of the evaporator depended on the volume of the concentrated solution and the assumed final concentration of perrhenic acid.The most favorable process parameters were used in all the presented methods for obtaining high-purity perrhenic acid.These process parameters and the proposed process diagrams for obtaining rhenic acid by electrodialysis and solvent extraction were selected based on research results carried out as part of the Small Grant RenMet project.The process based on ion-exchange was based on the previous experience of scientists from Łukasiewicz-IMN.
Figure 1 shows the laboratory equipment used in the research to obtain perrhenic acid, while Figure 2 presents the methodology for the production of perrhenic acid by ion-exchange.Ammonium perrhenate was dissolved in water and directed to the sorption of ammonium ions using a strongly acidic C160 cation-exchange resin (Purolite, King of Prussia, PA, USA, hydrogen form) [35,37].The postsorption solution, containing perrhenic acid, was sent to the concentration stage.Perrhenic acid obtained by solvent extraction is presented in the diagram in Figure 3, and Figure 4 shows the laboratory equipment used in the research on its preparation.For further purification and concentration of perrhenic acid, the equipment consisted of an ion column, and evaporators were used (Figure 1).
Perrhenic acid obtained by electrodialysis using bipolar membranes is shown in the diagram in Figure 5, and Figure 6 shows the laboratory equipment used in the research on its preparation.Figures 7 and 8 show the electrodialyser structure and ion-exchange diagram in this process, respectively.Perrhenic acid obtained by electrodialysis using bipolar membranes is shown in the diagram in Figure 5, and Figure 6 shows the laboratory equipment used in the research on its preparation.Figures 7 and 8 show the electrodialyser structure and ion-exchange diagram in this process, respectively.Perrhenic acid obtained by electrodialysis using bipolar membranes is shown in the diagram in Figure 5, and Figure 6 shows the laboratory equipment used in the research on its preparation.Figures 7 and 8 show the electrodialyser structure and ion-exchange diagram in this process, respectively.The process of perrhenic acid production using electrodialysis is carried out in the electrodialyser shown in Figures 6 and 7, in which anion-exchange membranes (PC SA) are mounted alternately with bipolar membranes (PC bip), and both electrode chambers are separated by the cation-exchange membranes (PC SK).The cathode is made of acidresistant steel, and the DSA anode is made of titanium covered with iridium and ruthenium oxides.The active surface of ion-selective membranes was 64 cm 2 .The process should be carried out cyclically, with the first cycle consisting of the initial concentration of the perrhenic acid solution and the second cycle being the purification and concentration of the perrhenic acid solution.Perrhenic acid solutions with a concentration of 30 g/dm 3 HReO 4 are used as electrode solutions (anolyte and catholyte) in both cycles.A solution of ammonium perrhenate of a concentration equal to 40 g/dm 3 NH 4 ReO 4 is used as the starting diluate in the first cycle, and the concentrate is a solution of perrhenic acid of a concentration of 40 g/dm 3 HReO 4 .The starting diluate in II is a solution of purified perrhenic acid, with a rhenium concentration higher than about 15 g/dm 3 (concentration of acid 20 g/dm 3 ), and the concentrate is a solution of high-purity perrhenic acid, in which the content of each of the cationic impurities does not exceed 5 mg/dm 3 .If the concentration of any cationic impurity in this concentrate exceeds 10 mg/dm 3 , it is withdrawn from the process and replaced with a fresh starting concentrate.This withdrawn concentrate is returned to the second cycle of the process as a starting diluate.The process of obtaining perrhenic acid by electrodialysis is carried out cyclically until the required concentration of perrhenic acid is obtained in the solution using the concentrate obtained in the previous cycles of the process as the diluate starting solution.In both cycles, diluate is withdrawn from the process if the rhenium concentration drops below 1 g/dm 3 , and the withdrawn solution is replaced with fresh diluate.The withdrawn diluate, after supplementing with an appropriate amount of NH 4 ReO 4 , should be returned to the first cycle of the process as a starting diluate.The solutions of all the streams circulate through the electrodialyser in a closed circuit.After applying an electric voltage, the ions contained in the solutions migrate selectively through the membranes, the concentrate solution is enriched with ReO 4 − ions, and the diluate solution is depleted in perrhenates ions.The process efficiency was calculated using formula: where C 0 , C 1 -concentration of diluate stream at the beginning and finish process, respectively, mol/dm 3 .
FOR PEER REVIEW 8 of 17    The process of perrhenic acid production using electrodialysis is carried out in the electrodialyser shown in Figures 6 and 7, in which anion-exchange membranes (PC SA) are mounted alternately with bipolar membranes (PC bip), and both electrode chambers are separated by the cation-exchange membranes (PC SK).The cathode is made of acidresistant steel, and the DSA anode is made of titanium covered with iridium and ruthenium oxides.The active surface of ion-selective membranes was 64 cm 2 .The process should be carried out cyclically, with the first cycle consisting of the initial concentration of the perrhenic acid solution and the second cycle being the purification and concentration of the perrhenic acid solution.Perrhenic acid solutions with a concentration of 30

Production of High-Purity Perrhenic Acid by Ion-Exchange
In the ion-exchange column, 1.0 dm 3 of C160 ion exchanger previously conditioned in a 32% HNO 3 solution was introduced.Then, five trials to produce perrhenic acid were performed.In the first step, 3.0 dm 3 of demineralized water was introduced into the glass reactor, where 65.0 g of ammonium perrhenate was dissolved.After filtering on a bag filter, the obtained solution was passed through an ion-exchange column; after the solution was exhausted, water was added.The solution and water were passed by the column at a flow rate of no more than 7.5 BVs (Bed Volumes).The effluent from the column was collected in portions of 250 cm 3 .The pH of the effluent was also monitored.A sample was taken from each portion, and the rhenium concentration was determined.After obtaining the analysis results, the first four portions, with a total volume of 1 BVs, were directed to the waste.The next portions of the effluents, with a total volume of 3.5 BVs, were combined and sent for concentration.Subsequent portions of the effluent, with a total volume of 2 BVs, were retained and used to dissolve further portions of NH 4 ReO 4 .Then, the ion exchanger was regenerated by passing a 32% HNO 3 solution with a volume of 1.5 BVs through the bed.The excess acid was washed out by passing 25.0 dm 3 of water through the column.The regenerated bed was used in the next four sorption tests, carried out in accordance with the methodology used in the first test.Table 2 contains a summary of the volumes of solutions intended for sorption and the concentrations of rhenium in these solutions.Figure 9 shows how the concentration of rhenium in the column effluent changed during the sorption of ammonium ions.The first part of the effluent, with a volume equal to 1 BVs, did not contain rhenium, and only water remained in the bed from washing after ion exchanger regeneration.This part of the effluent was removed.In the following portions of the effluent with a volume of 3 BVs, the rhenium concentration was close to the concentration in the feed.This part of the leakage contained the main product of the installation and was directed to concentration.In subsequent portions of the effluent (0.5 BVs), the concentration of rhenium decreased, but these solutions did not contain ammonium, so they were also sent for concentration.The last portions of the leak, ~2 BVs, already contained residual amounts of rhenium, so they were used to dissolve fresh portions of ammonium perrhenate.Table 3 lists the solutions collected in the samples, which were combined and sent for concentration.The first part of the effluent, with a volume equal to 1 BVs, did not contain rhenium, and only water remained in the bed from washing after ion exchanger regeneration.This part of the effluent was removed.In the following portions of the effluent with a volume of 3 BVs, the rhenium concentration was close to the concentration in the feed.This part of the leakage contained the main product of the installation and was directed to concentration.In subsequent portions of the effluent (0.5 BVs), the concentration of rhenium decreased, but these solutions did not contain ammonium, so they were also sent for concentration.The last portions of the leak, ~2 BVs, already contained residual amounts of rhenium, so they were used to dissolve fresh portions of ammonium perrhenate.Table 3 lists the solutions collected in the samples, which were combined and sent for concentration.A total of 17.5 dm 3 of the column effluents were concentrated on a vacuum evaporator to a volume of 0.7 dm 3 .The concentration of rhenium in the concentrated solution was 300.1 g/dm 3 .Then, half of this solution was concentrated to a volume of 0.116 dm 3 ; the concentration of rhenium in this solution was 905.6 g/dm 3 .Consequently, two highconcentration solutions of perrhenic acid were obtained, containing the following: (1) A total of 300.1 g/dm 3 Re and <0.0001% Ca, <0.0001% K, <0.0001% Mg, <0.0001% Cu, <0.0001% Na, <0.0001% Mo, <0.0001% Ni, <0.0001% Pb, <0.0001% Fe, <0.0001% NH 4 + , <0.0001% Bi, <0.0001% Zn, <0.0001% W, <0.0001%As and <0.0001% Al.
Both of these solutions of perrhenic acid were high-purity solutions in which the sum of the metallic impurities was <100 ppm, and none of the impurities exceeded 10 ppm, the exception being the Fe of acid with a concentration above 900 g/dm 3 .

Production of High-Purity Perrhenic Acid By Solvent Extraction
As part of the research carried out in the Small Grant RenMet project, the possibility of obtaining perrhenic acid extractive was checked using selected, commercially available organic solvents, i.e., tributyl phosphate, trioctylamine, and Cyphos IL 101 ionic liquid.An ammonia solution was used as the research material waste, with Re and NH 4 + concentrations of 13.5 and 43.7 g/dm 3 , respectively.This solution came from the process of rhenium recovery from superalloy scraps.
The results of the preliminary tests showed that all three selected extractants demonstrate the high extraction efficiency of ReO 4 − ions.For each of the extractants, a series of tests were carried out regarding the extraction and re-extraction conditions, the influence of pH, the composition of organic phases, phase ratios, contact time, and the type and temperature of the re-extractant used on the efficiency and selectivity of both processes.This research allowed us to determine the extraction and re-extraction conditions that led to the production of perrhenic acid.It has been established that there are at least two possible procedures for obtaining perrhenic acid by solvent extraction: (1) By extraction with a 50% (V/V) solution of tributyl phosphate in Exssol D80, re-ex traction with demineralized water at an elevated temperature of about 50 • C, and then the concentration of the obtained re-extract, preferably under reduced pressure; (2) By extraction using three-component organic mixtures containing 5% (V/V) of Cyphos IL 101 ionic liquid and 20% (V/V) tri-butyl phosphate in Exssol D80 or 10% trioctylamine and 20% tributyl phosphate in ExssoluD80, re-extraction with once-diluted nitric acid, and then stripping of nitrogen oxides.
Although the developed methods allow for obtaining perrhenic acid with the Re concentration required to be >100 g/dm 3 in the project, with an efficiency exceeding 90% and the content of the sum of metal impurities <100 ppm, this technology is quite time-consuming and requires the use of many consecutive stages.
In the first method, to obtain the assumed efficiency, it is necessary to use at least two extraction stages with a phase ratio 1:1, followed by repeated re-extraction with demineralized water at a temperature of 50 • C. Despite obtaining high extraction and re-extraction efficiencies, the second method requires stripping the nitrogen oxides, which involves a high-energy input and compliance with appropriate health and safety procedures, which is particularly important in using the described methods on a scale larger than the laboratory.Moreover, for all three methods, including those selected as the most preferred, the insufficient selectivity of the extraction and re-extraction of ReO 4 − ions in relation to ammonium ions was observed.It is therefore recommended to carry out an additional stage of purification of the obtained rhenic acid (VII) using other techniques, e.g., the sorption of NH 4 + ions using strongly acidic Purolite C160 cation-exchange resins.The necessity of using an additional purification step to eliminate NH 4 + ions, namely applications in the solvent extraction process connected with the ion-exchange method using selected ion exchangers, even while maintaining an amount below 10 ppm for selected single pollutants (Na, Al, K, Ca, Fe, and Zn <10 mg/dm 3 and Mg, Cu, Ni, and Co <5 mg/dm 3 ), disqualifies this method as recommended for use in practice.

Production of High-Purity Perrhenic Acid by Electrodialysis
In all experiments for the production of perrhenic acid by electrodialysis using bipolar membranes, the process was carried out at the intensity of the process streams flowing through the electrodialyser of 3 dm 3 /h, current density of 200 A/m 2 , and the temperature of all the process streams maintained in the range of 40 • C.

Preliminary Concentration of Perrhenic Acid
In the first cycle of the preliminary concentration of perrhenic acid in test I-1, four process solutions were fed to the electrodialyser at a constant flow rate of process streams.As a concentrate, 1 dm 3 of a solution containing 40 g/dm 3 HReO 4 and less than 1 mg/dm 3 NH 4 + , <1 mg/dm 3 Na, and <1 mg/dm 3 K was introduced.As a starting diluate, 2 dm 3 of a solution of ammonium perrhenate of a concentration of 40 g/dm 3 was used.Finally, as a near-electrode solution, a solution of perrhenic acid of a concentration of 30 g/dm 3 HReO 4 in a volume of 1 dm 3 of each solution was introduced.After 185 min, a sudden increase in voltage was noticed and the process was stopped.The concentration of rhenium in the diluate decreased to 1.55 g/dm 3 .The determined concentration of rhenium in the concentrate was 82.1 g/dm 3 (110 g/dm 3 HReO 4 ), and the impurity content was 89.2 mg/dm 3 NH 4 + , <1 mg/dm 3 Na, and <1 mg/dm 3 K.The process efficiency calculated for the diluate was 94.4%, and the concentration of perrhenic acid in the concentrate increased 2.75 times.The concentration of rhenium in the electrode solutions did not change.
The process was repeated three more times (trials from I-2 to I-4) using the concentrate obtained from the previous process as the starting concentrate.A fresh solution of ammonium perrhenate with a concentration of 40 g/dm 3 NH 4 ReO 4 was used as the starting dilute.The test conditions were the same, with the difference that in the last fourth test (I-4), 1 dm 3 of dilute was used.
Table 4 shows the results obtained in all four tests of the first cycle of preliminary concentration of the perrhenic acid solution.The process efficiency calculated for diluate from samples I-2, I-3, and I-4 and was 96.5%, 95.9%, and 90.4%, respectively.The concentration of perrhenic acid in the concentrate increased 1.68, 1.39, and 1.11 times, respectively.The concentration of rhenium in the solutions at the electrodes did not change or change very slightly, ranging from 2 to 5%.The results obtained in the first cycle of the preliminary concentration of perrhenic acid by the electrodialysis process using bipolar membranes in four consecutive tests showed the possibility of concentrating the concentrate from the acid solution of 40 g/dm 3  to 283.3 g/dm 3 HReO 4 , simultaneously contaminating it with ammonium ions up to 275.0 mg/dm 3 .In all tests performed in the first electrodialysis cycle, the efficiency of the process exceeded 90%.Despite the contamination with NH 4 + ions appearing in the concentrates, these were successfully concentrated and enriched in rhenium while simultaneously reducing the rhenium concentration in the diluates from 27.5 g/dm 3 to values in the range of 0.98 to 2.67 g/dm 3 Re.Diluates after the process can be enriched in ReO 4 − ions by adding an amount of NH 4 ReO 4 and returning it to the process as a starting diluate.The concentrate solution contaminated with NH 4 + , K, and Na cations was sent to the second electrodialysis cycle, which included the purification and concentration of perrhenic acid.
3.3.2.Purification and Concentration of Perrhenic Acid >100 g/dm 3   The concentrate obtained in the first cycle of the electrodialysis of the initial concentration (after trial I-4) with a concentration of 283.3 g/dm 3 HReO 4 and contaminated with NH 4 + and Na and K ions (Section 3.3.1)was sent for a purification test using electrodialysis in a two-stream system to reduce the concentration of NH 4 + ions and Na and K to <10 mg/dm 3 .All tests of the second electrodialysis cycle were carried out using the exact solutions at the electrodes and other process conditions, except the test duration, as in the first cycle of the electrodialysis process described in Section 3.3.1.
In the first trial of the second cycle (II-1), 1 dm 3 of perrhenic acid solution to a rhenium concentration of about 29.6 g/dm 3 (40.0g/dm 3 HReO 4 ) containing <1 mg/dm 3 NH 4 + , <1 mg/dm 3 Na, and <1 mg/dm 3 K was introduced into the electrodialyser as a concentrate.As a diluate, a solution volume of about 1 dm 3 of concentrate obtained in the first cycle of the process was introduced, and it was the solution of perrhenic acid with a rhenium concentration of 210 g/dm 3 (283.3g/dm 3 HReO 4 ) containing impurities in the amount of 275 mg/dm 3 NH 4 + , 15 mg/dm 3 Na, and 35 mg/dm 3 K.After 8 h, the process was completed.After the process, the concentration of rhenium in the diluate decreased to 51.2 g/dm 3 .The process efficiency calculated for the diluate was 75.6%.The concentration of rhenium in the electrode solutions did not change.The determined concentration of rhenium in the concentrate was 179.5 g/dm 3 (242.2g/dm 3 HReO 4 ), and the impurities were 52 mg/dm 3 NH 4 + , 3.1 mg/dm 3 Na, and 6.1 mg/dm 3 K.The concentration of perrhenic acid in the concentrate increased more than six times.Still, due to the excess of impurities, this solution was sent as a starting diluate for the following process of the second electrolysis cycle.
In the second test carried out as part of the second cycle of the electrodialysis process (II-2), the concentrate from the previous test was used as the dilute at a volume of about 1 dm 3 of perrhenic acid solution with a rhenium concentration of about 179.5 g/dm 3  (242.2g/dm 3 HReO 4 ) and contents of impurities of 52.0 mg/dm 3 NH 4 + , 3.1 mg/dm 3 Na, and 6.1 mg/dm 3 K.As a concentrate, there was 1 dm 3 of solution with a rhenium concentration of about 85.2 g/dm 3 (115 g/dm 3 HReO 4 ) containing <1 mg/dm 3 NH 4 + , <1 mg/dm 3 Na, and <1 mg/dm 3 Na.After completing the process, which lasted 8 h, the concentration of rhenium in the diluate decreased to 20.5 g/dm 3 .The process efficiency calculated for diluate was 88.6%.The determined concentration of rhenium in the concentrate was 239.8 g/dm 3 (329.5 g/dm 3 HReO 4 ), and the contents of impurities were 6.1 mg/dm 3 NH 4 + , 1.1 mg/dm 3 Na, and 2.2 mg/dm 3 K.The concentration of perrhenic acid in the concentrate increased over 2.8 times.The concentration of rhenium in the electrode solutions did not change.
Since the concentration of rhenium in the diluate after trial II-2 was high 20.5 g/dm 3 , this solution was used in the subsequent trial of the second cycle (II-3) as the starting diluate.In trial II-3, the same concentrate was used as in the previous trial II-2; it was solution with a volume of 1 dm 3 and rhenium concentration of 85.2 g/dm 3 (115 g/dm 3 HReO 4 ).The conditions of the trials were the same.After 60 min, a sudden increase in voltage was noticed and the process was stopped.After completing the process, the concentration of rhenium in the diluate decreased to 0.41 g/dm 3 .The process efficiency calculated for diluate was 98.0%.The determined concentration of rhenium in the concentrate was 102.4 g/dm 3 (142.1 g/dm 3 HReO 4 ), and the impurity content was <2.6 mg/dm 3 NH 4 + , <1 mg/dm 3 Na, and <1 mg mg/dm 3 K.The concentration of rhenium in the electrolyte solutions has not changed.
Tables 5 and 6 present the results obtained in all three tests of the second cycle of the purification and concentration of the perrhenic acid solution.The results obtained in the second cycle of the purification and concentration of perrhenic acid by the electrodialysis process using bipolar membranes showed the possibility of concentrating the concentrate from the acid solution from 40 g/dm 3 to 329.5 g/dm 3 HReO 4 , simultaneously reducing its contamination with NH 4 + , K, and Na to the required level below 10 mg/dm 3 of each pollutant.After the process, the diluates can be combined with purified perrhenic acid; the starting diluate is returned to the process or enriched in ReO 4 − ions by adding an amount of NH 4 ReO 4 and used as a diluate in the first cycle of the initial HReO 4 concentration.

Conclusions
Methods for obtaining high-purity perrhenic acid (with metallic impurities concentration below 100 ppm) with a high concentration 100-200 g/dm 3 were briefly characterized entirely from secondary raw materials.The obtained perrhenic acid should have a specific composition to meet quality and customer requirements or other standards based on future applications, i.e., 100-900 g/dm 3 Re and <1 ppm Ca, <10 ppm K, <1 ppm Mg, <1 ppm Cu, <1 ppm Na, <1 ppm Mo, <1 ppm Ni, <1 ppm Pb, <1 ppm Fe, <1 ppm Co, <1 ppm Zn, and <5 ppm NH 4 + .Two of the three methods presented-electrodialysis and solvent extraction-were the subject of research carried out directly as part of the Small Grant project with the acronym RenMet: "Innovative hydrometallurgical technologies for the production of rhenium compounds from recycled waste materials for catalysis, electromobility, aviation and defense industry", and the ion-exchange method was developed as part of previous projects implemented by Łukasiewicz-IMN.Table 7 below shows the advantages and disadvantages of the methods for producing high-purity and high-concentration perrhenic acid.The basic process parameters criteria were selected: efficiency and selectivity of the process, purity of the obtained product, availability and consumption of raw materials and reagents, equipment necessary to carry out the process, and the profitability of the technology and ecological aspects, i.e., the possibility of managing the generated solid waste and post-production solutions.In order to fully determine which of the methods is the most sustainable in terms of costs and environmental impact, it would be worth focusing in future work on conducting a life cycle assessment of these processes.To support the conclusions, the results of a performed multi-criteria decision analysis (MCDA) are outlined in Table 7.To support the conclusions, the results of a performed multi-criteria decision analysis (MCDA) are outlined in Table 7. Factors for selecting the most economically and ecologically advantageous method for high-purity perrhenic acid production from secondary raw materials were analyzed.The main disadvantages of using the electrodialysis process to obtain perrhenic acid are as follows: -Necessity use of many expensive ion-exchange membranes and their rapid wear and destruction; -Carrying out the process in two cycles: the first cycle is the initial concentration of the perrhenic acid solution, and the second cycle is the process of the purification of the perrhenic acid.
Each of these cycles requires carrying out many stages until the required purity and concentration of the obtained product perrhenic acid are achieved.
The main disadvantage that eliminates the use of the solvent extraction method to obtain perrhenic acid is the low selectivity of the extraction and re-extraction process and the need to use an additional purification operation to remove NH4 + ions using, e.g., the ion-exchange method.
High-purity and high-concentration, above 100 g/dm 3 , perrhenic acid was obtained using the ion-exchange method.The key element of this method is the product's purity and the process's simplicity, easily available waste materials, and renewable ion-exchange resin.It is, therefore, based on a sustainable circular economy.

Figure 1 .
Figure 1.Equipment for production of perrhenic acid by ion-exchange: (a) ion column to sorption and elution of ammonium ions; (b) and (c) evaporators to concentration of postsorption solutions.

Figure 1 .
Figure 1.Equipment for production of perrhenic acid by ion-exchange: (a) ion column to sorption and elution of ammonium ions; (b) and (c) evaporators to concentration of postsorption solutions.

Figure 1 .Figure 2 .
Figure 1.Equipment for production of perrhenic acid by ion-exchange: (a) ion column to s and elution of ammonium ions; (b) and (c) evaporators to concentration of postsorption sol

Figure 2 .
Figure 2. Scheme for production of perrhenic acid by ion-exchange.

Figure 4 .
Figure 4. Equipment for obtaining perrhenic acid using the solvent extraction process.

Figure 4 .
Figure 4. Equipment for obtaining perrhenic acid using the solvent extraction process.Figure 4. Equipment for obtaining perrhenic acid using the solvent extraction process.

Figure 4 .
Figure 4. Equipment for obtaining perrhenic acid using the solvent extraction process.Figure 4. Equipment for obtaining perrhenic acid using the solvent extraction process.

Figure 8 .
Figure 8. Scheme of ion-exchange for high-purity perrhenic acid production using the electrodialysis process and bipolar membranes: A-anion-exchange membrane; K-cation-exchange membrane; B-bipolar membrane.

Figure 8 .
Figure 8. Scheme of ion-exchange for high-purity perrhenic acid production using the electrodialysis process and bipolar membranes: A-anion-exchange membrane; K-cation-exchange membrane; B-bipolar membrane.

Figure 9 .
Figure 9. Changes in rhenium concentration in the effluent.

Figure 9 .
Figure 9. Changes in rhenium concentration in the effluent.

Table 1 .
Concentration of main elements in superalloys used in research.

Table 2 .
Solutions of NH 4 ReO 4 used in experiments to produce perrhenic acid using the ionexchange method.

Table 2 .
Solutions of NH4ReO4 used in experiments to produce perrhenic acid using the ion-exchange method.

dm 3 Re Concentration of Re, g/dm 3 Mass of Re, g
Sum of mass of Re 218.39.
Sum of mass of Re 210.12.
Sum of mass of Re 210.12.

Table 4 .
Concentration of rhenium and impurities in diluate before and after I cycle purification and concentration of perrhenic acid in electrodialysis process.

Table 5 .
Concentration of rhenium and impurities in diluate before and after II cycle purification and concentration of perrhenic acid in electrodialysis process.

Table 6 .
Concentration of rhenium and impurities in concentrate before and after II cycle purification and concentration of perrhenic acid in electrodialysis process.

Table 7 .
Comparative parameters for obtaining perrhenic acid using various techniques.

Table 7 .
Comparative parameters for obtaining perrhenic acid using various techniques.