Synthesis of perrhenic acid using solvent extraction

Hydrometallurgy 95 (2009) 325–332

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Synthesis of perrhenic acid using solvent extraction Katarzyna Leszczyńska-Sejda a,⁎, Grzegorz Benke a, Stanisław Krompiec b, Andrzej Chmielarz a, Krystyna Anyszkiewicz a, Leszek Gotfryd a a b

Hydroelectrometallurgy Department, Institute of Non Ferrous Metals, Sowinskiego 5; 44-100 Gliwice, Poland University of Silesia, Faculty of Mathematics, Physics and Chemistry, Szkolna 9; 46-006 Katowice, Poland

a r t i c l e

i n f o

Article history: Received 21 April 2008 Received in revised form 21 July 2008 Accepted 22 July 2008 Available online 30 July 2008 Keywords: Rhenium Solvent extraction Perrhenic acid Ammonium perrhenate TBP

a b s t r a c t The paper presents results of investigations into production of perrhenic acid from aqueous solutions of ammonium perrhenate of concentration 25 g/L by solvent extraction method. It was demonstrated that perrhenic acid can be obtained by extraction of ReO−4 with 50% tributyl phosphate (TBP) solution in toluene from aqueous solution of ammonium perrhenate acidified with sulphuric(VI) acid to pH of 1.0 and further stripping of ReO−4 from organic phase by water of temperature 80 °C. Complete separation of rhenium from ammonium ions was possible after repeated extraction and repeated stripping. In the result perrhenic acid of concentration about 70 g/L Re was obtained, which then was concentrated to reach the level of 300 g/L Re. Thus produced perrhenic acid of concentration 300 g/L Re contained no more than 0.01 g/L of impurities composed of metal (sodium, potassium, magnesium) and ammonium ions. © 2008 Published by Elsevier B.V.

1. Introduction Rhenium is one of the rarest elements in nature (Roskill, 2004). Launched in Poland in 2005 production of ammonium perrhenate from acidic waters originating from gas washing system of copper flashsmelting furnace provided possibility to begin research into production of perrhenic acid directly from NH4ReO4 (Benke et al., 2006; Chamer et al., 2004). In 2007 authors described possibilities of obtaining perrhenic acid by ion-exchange from aqueous ammonium perrhenate solutions using sorption on strongly acidic cation-exchange resin (C 160 (H)) and recovery of rhenium from that ionite with 32% nitric (V) acid, and then concentration of the produced perrhenic acid on a rotary vacuum evaporator (Leszczyńska-Sejda et al., 2007). The one obtained by that method perrhenic acid directly after sorption had concentration of about 20 g/L, therefore the solution was then concentrated to reach a desired concentration level. That method produced highly concentrated perrhenic acid solutions (rhenium concentration above 300 g/L) (Leszczyńska-Sejda et al., 2008). In the literature there are many methods describing production of perrhenic acid by ion-exchange technique (Colton, 1965) or by solvent extraction (Tartak et al, 1996). Most frequently, however, it is obtained by dissolving of metallic

⁎ Corresponding author. Hydrometallurgy Department, Institute of Non Ferrous Metals, Sowinskiego 5; 44-100 Gliwice, Poland. Tel.: +48 32 238 06 77; fax: +48 32 231 69 33. E-mail address: [email protected] (K. Leszczyńska-Sejda). 0304-386X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.hydromet.2008.07.010

rhenium or rhenium oxides in 30% H2O2 or in concentrated HNO3 (Leddicotte, 1981). The disadvantage of those methods is a necessity to use metallic rhenium, which is usually produced from ammonium perrhenate, with application of expensive, specialist equipment, including furnace for conducting the process in hydrogen atmosphere (Krzysztofowicz, 1994). Solvent extraction method is used especially for separation of rhenium from other components, such as arsenic, tungsten or molybdenum (Sawanta et al., 2001). Most frequently, for rhenium extraction tri-n-octylamine, bis-iso-dodecylamine, pyridine, Aliquat 336, tributyl phosphate, trioctylphosphine oxide, cyklohexanon, ethyl xanthate and mesityl oxide were used (Gerhardt et al., 2001, Kertes et al., 1961; Kinjakov and Korszunov, 1976). In the process of rhenium stripping from organic phase, solutions of both acids and alkalies were used, i.e. aqueous HCl, H2SO4 and ammonium solution (for ammonium perrhenate separation) or aqueous solutions of NaOH and KOH (to separate suitable rhenium salts, i.e. NaReO4 and KReO4) (Colton, 1965). The objective of this study was to obtain high purity perrhenic acid of rhenium concentration above 300 g/L by solvent extraction method. The idea was to check whether it is possible to obtain perrhenic acid by selective extraction of rhenium (in a form of ReO−4ion) to organic phase, and then its stripping to aqueous phase — in a form of HReO4. That methods provided possibility for production of perrhenic (VII) acid — high purity product of a wider range of applications than ammonium perrhenate. Such acid can be used as a source rhenium compound for production of, among others, high purity perrhenates of various metals, e.g. nickel (II), calcium or zinc.

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2. Experimental 2.1. Materials Within the scope of the investigations seven organic extractants were tested. Their selection was based on literature data (Colton, 1965). The following organic extractants were tested: Aliquat 336 (quaternary ammonium salt — a mixture of octyl and capryl chains with octyl predominating), trioctylamine, tributyl phosphate (TBP), Cyanex 923 (mixture of tertiary octyl and hexyl phosphine oxides) and Cyphosil 164 (tetrabutylphosphonium chloride) and methyl ethyl ketone. The extractants were used in pure form (tributyl phosphate, Cyanex 923, methyl ethyl ketone, trioctylamine) or in a form of solutions in toluene (Cyphosil 164, tributyl phosphate and Aliquat 336). Toluene solutions were used because of high viscosity or solid state (Cyphosil 164) of the extractants. Moreover, application of tributyl phosphate in a form of toluene solution brought possibility to increase efficiency of aqueous and organic phases separation. 50% (vol.) tributyl phosphate and 50% (vol.) Aliquat 336 in toluene as well as toluene solution of Cyphosil 164 of concentration 100 g/dm3 were used. The rhenium stripping was conducted with: hot water, 15–30% ultra pure hydrogen peroxide (manufactured by POCh, Gliwice), 10– 15% ultra pure hydrochloric acid (manufactured by POCh, Gliwice), ultra pure 10–15% sulphuric(VI) acid (manufactured by POCh, Gliwice) and ultra pure 10–65% nitric(V) acid (manufactured by POCh, Gliwice). All the tests were conducted with application of synthetic aqueous ammonium perrhenate solutions, which were obtained by dissolving 99.99% purity NH4ReO4 (manufactured by IMN) of the following composition: Re — 69.4%; Ca 0.002%; K b 0.001%; Mg b 0.0005%; Cu b0.0 0 05%; Na b0.0 0 05%; Mo b0.0 0 05%; Ni b0.0 0 05%; Pb b 0.0005%; Fe b 0.0005%. Double distilled water of electrolytic conductivity b2 µS/cm was used. 2.2. Extraction of rhenium from aqueous ammonium perrhenate solutions Investigations into selection of an extractant for ReO−4/ammonium separation were conducted using synthetic 25 g/L ammonium perrhenate solution. This corresponds to 17.35 g/L of Re and 1.68 g/L of ammonium ions. Within the scope of the research, tests into influence of the following parameters – pH, concentration of ammonium perrhenate solution, and aqueous to organic phase ratio – on efficiency of rhenium and ammonium ions extraction were performed. The method used in experiments was as follows: 50 mL of extractant was added to 50 mL of synthetic ammonium perrhenate solution, and the whole volume was intensely stirred for 30 min at room temperature. The mixture was then poured into separation funnel and after several minutes delaminated phases were separated and their volume was measured. Organic phases were collected (for stripping studies), and aqueous phases were analyzed for rhenium and ammonium ions content. The investigations into influence of individual parameters on efficiency of rhenium and ammonium ions extraction were conducted with selected in preliminary investigations extractants (50% solutions of TBP and Aliquat 336 in toluene) only. The range of examined pH was 1 to 5. Acidification of NH4ReO4 solutions was performed: sulphuric (VI), nitric(V) or hydrochloric acid. In the investigations into influence of ammonium perrhenate solutions concentration on efficiency of rhenium and ammonium ions extraction, the solutions preliminarily acidified with sulphuric(VI) acid to pH of 1.0 were used. Examinations of ammonium perrhenate concentration influence on extraction efficiency were performed at concentrations of ammonium perrhenate solutions from 5.0 to 50.0 g/L and temperature of 60 °C.

The investigations into influence of aqueous/organic (A/O) volumetric ratio on extraction of rhenium and ammonium ions were conducted using ammonium perrhenate solution of concentration 25 g/L, acidified with sulphuric(VI) acid to pH = 1.0. Various A/O ratios, in the range from 1:2 to 4:2, were applied. For all obtained results (with respect to rhenium and ammonium ion concentrations in aqueous phase) rhenium and ammonium extraction coefficients were calculated, as well as their extraction efficiencies and rhenium versus ammonium selectivity coefficients. 2.3. Investigations into rhenium stripping from organic phase In the tests the first organic phase originated from ammonium perrhenate solution extraction by 50% TBP solution and the second one by 50% Aliquat 336 solution. Before extraction the ammonium perrhenate solution (25 g/L) was acidified with sulphuric(VI) acid to pH of 1.0. The extraction was conducted at temperature 60 °C, and volumetric ratio of aqueous to organic phase was 1:1. The samples of the subjected to stripping organic phases had the composition as presented in Table 1. Stripping of rhenium and potentially ammonium ions was conducted in the following way: 50 mL of organic phase and 25 mL of stripping agent was stirred for 30 min at room temperature or at temperature of 80 °C, when stripping was performed with water. The mixture was then poured into separation funnel and after several minutes delaminated phases were separated and their volume was measured. Organic phases were collected, and aqueous phases were analyzed for rhenium and ammonium ions content. Basing on the obtained results the efficiencies of rhenium and ammonium stripping were calculated. 2.4. Repeated rhenium extraction and hot water stripping using tributyl phosphate The investigation was carried out according to the following procedure: 100 mL of 50% TBP in toluene was added to 100 mL of 25 g/L ammonium perrhenate solution and acidified with sulphuric(VI) acid to pH = 1.0. Then the whole volume was intensely stirred for 30 min at temperature of 60 °C. The mixture was then poured into separation funnel and after several minutes delaminated phases were separated and their volume was measured. The organic phase was subjected to stripping with 50 mL of water, at 80 °C, and the inorganic phase was analyzed for rhenium and ammonium ions content. The second extraction was conducted from 50 mL of aqueous solution produced in the first stripping process, by addition of a new batch of TBP in toluene (50 mL), and following the same steps as in the first operation. Basing on the results all extraction and stripping coefficients, explained in the point 2.2. and 2.3, were calculated. The solution produced after second stripping was concentrated on a rotary vacuum evaporator, until rhenium concentration exceeding 300 g/L was obtained. 2.5. Analytical methods All analyses were carried out in Analytical Chemistry Department of Institute of Non-Ferrous Metals. Rhenium was determined

Table 1 The composition of organics phase subjected to stripping Sample Composition

Concentration of Concentration of rhenium ammonium ions g/L

I II

50% TBP in toluene after extraction 8.6 50% Aliquat in toluene after extraction 8.6

g/L 0.12 0.14

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Table 2 Efficiency of rhenium and ammonium ions extraction from ammonium perrhenate solution Extractant

Tributyl phosphate 50% solution of TBP in toluene 50% solution of Aliquat 336 in toluene Cyanex 302 Cyphosil 164 solution of 100 g/dm3 in toluene Ethylmethylketone Trioctylamine

Volume of aqueous phase

Volume of organic phase

Rhenium concentration in aqueous phase

Concentration of ammonium ions in aqueous phase

L

L

g/L

g/L

Ammonium ions extraction efficiency

Rhenium extraction efficiency %

%

(±0.1%)

(±0.1%)

Selectivity coefficient of rhenium versus ammonium ions

0.050 0.052

0.053 0.050

3.42 3.45

0.89 0.94

79.1 80.1

43.8 44.0

0.100

0.053

≤ 0.01

1.56

≥99.9

1.6

0.057 0.055

0.053 0.053

13.56 0.10

1.54 0.10

17.2 99.4

2.8 93.7

7.1 11.0

0.053 0.051

0.050 0.050

10.98 12.34

1.48 1.56

36.7 28.9

11.9 7.1

4.3 5.3

gravimetrically (Williams, 1985), while sodium, potassium, calcium and magnesium — by atomic absorption spectrometry (AAS method) using Slaars instrument by Thermo Elemental. Ammonium ions were determined using Nestler's reagent. 3. Results and discussion 3.1. Extraction of rhenium from aqueous solutions of ammonium perrhenate 3.1.1. Preliminary tests of extraction The results obtained from preliminary investigations into rhenium and ammonium ions extraction from aqueous ammonium perrhenate solutions using various extractants are presented in Table 2. Comparative tests of various extractants has shown that the highest rhenium recovery (above 99.9%) was reached for 50% Aliquat 336 solution in toluene. Slightly lower results were reached with Cyphosil 164 (99.4%). This extractant, however, extracted also ammonium ions with high efficiency (93.7%). Significantly lower rhenium extraction efficiencies were obtained when trioctylamine, methyl ethyl ketone, and Cyanex 923 were used. As it turned out, the

4.9 5.1 ≥100 000

most selective extractant of rhenium with respect to ammonium ions is 50% Aliquat 336 solution in toluene. It extracted ammonium ions with extraction efficiency of 1.6%, while rhenium was extracted with efficiency of 99.9%. Promising results were also obtained with TBP — in a pure form and in a form of solutions in toluene. Rhenium extraction efficiencies with TBP were high (79.1 and 80.1% for pure form and for toluene solution, respectively), and Re/NH+4 selectivity coefficients relatively good (4.9 and 5.1). Therefore, for further investigations the following extractants were selected: 50% (vol.) Aliquat 336, because of the highest rhenium extraction efficiency and very high coefficient of extraction selectivity of rhenium versus ammonium ions; 50% (vol.) TBP, because of relatively high rhenium extraction efficiency and satisfying selectivity coefficients. 3.1.2. Influence of the pH In the next investigations the most advantageous pH for conducting efficient rhenium extraction was examined. Results of studies into influence of pH on efficiency and selectivity of rhenium extraction with 50% solutions of TBP and Aliquat 336 in toluene are presented in Tables 3 and 4.

Table 3 Mineral acids addition influence on efficiency of rhenium and ammonium ions extraction (from aqueous ammonium perrhenate solution) with 50% of TBP in toluene pH Volume of aqueous phase L

Volume of organic phase

Concentration of rhenium in aqueous phase

Concentration of ammonium ions in Rhenium extraction aqueous phase efficiency

Ammonium ions extraction efficiency

L

g/L

g/L

%

%

(±0.1%)

(±0.1%)

Selectivity coefficient of rhenium versus ammonium ions

Sulphuric(VI) acid 1.0 0.056 1.9 0.052 3.0 0.054 4.1 0.052 5.0 0.052

0.100 0.100 0.100 0.100 0.100

≤0.01 0.90 1.20 2.30 3.45

1.40 1.20 1.20 1.20 0.94

≥99.9 94.5 92.7 85.9 80.1

13.3 24.3 24.3 24.3 44.0

≥10 000 53.6 39.4 19.1 5.1

Hydrochloric acid 1.0 0.056 2.0 0.054 2.8 0.050 4.0 0.051 5.0 0.052

0.100 0.100 0.100 0.100 0.100

0.26 0.89 1.57 2.5 3.45

1.2 1.2 1.1 1.1 0.94

98.4 94.5 90.4 84.7 80.1

25.7 22.9 30.6 30.6 44.0

182.5 57.5 21.4 12.6 5.1

Nitric(V) acid 1.0 0.056 2.0 0.054 2.8 0.050 4.0 0.051 5.0 0.052

0.100 0.100 0.100 0.100 0.100

0.26 0.89 1.57 2.5 3.45

1.2 1.2 1.1 1.1 0.94

98.4 94.5 90.4 84.7 80.1

25.7 22.9 30.6 30.6 44.0

182.5 57.5 21.4 12.6 5.1

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Table 4 Mineral acids addition influence on efficiency of rhenium and ammonium ions extraction (from aqueous ammonium perrhenate solution) with 50% of Aliquat 336 in toluene pH Volume of aqueous phase L

Volume of organic phase

Concentration of rhenium in aqueous phase

Concentration of ammonium ions in Rhenium extraction aqueous phase efficiency

L

g/L

g/L

Ammonium ions extraction efficiency

%

%

(±0.1%)

(±0.1%)

Selectivity coefficient of rhenium versus ammonium ions

Sulphuric(VI) acid 1.0 0.053 2.0 0.053 2.9 0.053 4.0 0.053 5.0 0.053

0.100 0.100 0.100 0.100 0.100

≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01

1.56 1.56 1.56 1.56 1.56

≥99.9 ≥99.9 ≥99.9 ≥99.9 ≥99.9

1.6 1.6 1.6 1.6 1.6

≥100 000 ≥100 000 ≥100 000 ≥100 000 ≥100 000

Hydrochloric acid 1.1 0.053 2.1 0.053 3.0 0.053 4.0 0.053 5.0 0.053

0.100 0.100 0.100 0.100 0.100

≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01

1.20 1.30 1.56 1.56 1.56

≥99.9 ≥99.9 ≥99.9 ≥99.9 ≥99.9

24.3 18.0 1.6 1.6 1.6

≥5000 ≥7400 ≥100 000 ≥100 000 ≥100 000

Nitric(V) acid 1.1 0.053 1.9 0.053 3.0 0.053 4.1 0.053 5.0 0.053

0.100 0.100 0.100 0.100 0.100

≤0.01 ≤0.01 ≤0.01 ≤0.01 ≤0.01

1.20 1.56 1.56 1.56 1.56

≥99.9 ≥99.9 ≥99.9 ≥99.9 ≥99.9

24.3 1.6 1.6 1.6 1.6

5000 ≥100 000 ≥100 000 ≥100 000 ≥100 000

High rhenium extraction recovery was reached for toluene solution of Aliquat 336, which extracted rhenium at the level of 99.9%, from solutions acidified with sulphuric(VI) acid, hydrochloric acid and nitric(V) acid — in the whole analyzed pH range. That extractant presented also high selectivity coefficient of extraction of rhenium versus ammonium ions. However, ammonium ions extraction efficiency and, as a consequence, Re/NH+4 selectivity coefficient were to some degree dependent on pH of the solution, and on the nature of acid used for solution acidification. When sulphuric(VI) acid was used for acidification, Re/NH+4 selectivity coefficient was identical throughout the examined pH range. In the case of acidification with

hydrochloric acid the high selectivity coefficient was obtained in the pH range from 3.0 to 5.0. For nitric(V) acid the high Re/NH+4 selectivity coefficient was reached when pH was in the range from 1.9 to 5.0, while for the sulphuric(VI) acid in the whole examined pH range. For 50% TBP solution in toluene, high rhenium extraction recovery (99.9%) was reached and at the same time the highest selectivity coefficient of rhenium versus ammonium ions obtained (about 10,000), when the extraction was conducted from NH4ReO4 solutions acidified with sulphuric(VI) acid (to pH of 1.0). Acidification of ammonium perrhenate solution with hydrochloric or nitric(V) acid was definitely less advantageous. Therefore, for further investigations

Table 5 Concentration of ammonium perrhenate solution influence on efficiency of rhenium and ammonium ions extraction with 50% TBP and Aliquat 336 solutions in toluene Concentration of ammonium perrhenate

Volume of aqueous phase

Volume of organic phase

Concentration of rhenium in aqueous phase

Concentration of ammonium ions in aqueous phase

g/L

L

L

g/L

g/L

Rhenium extraction efficiency

Ammonium ions extraction efficiency

%

%

(±0.1%)

(± 0.1%)

Selectivity coefficient of rhenium versus ammonium ions

50% TBP solution in toluene 5.0 0.052 10.0 0.054 15.0 0.052 20.0 0.052 25.0 0.056 30.0 0.052 35.0 0.055 40.0 0.052 45.0 0.053 50.0 0.054

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100

≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 0.1 3.4 8.3

0.3 0.6 0.9 1.2 1.4 1.2 1.3 1.2 1.2 1.3

≥99.7 ≥99.8 ≥99.9 ≥99.9 ≥99.9 ≈100.0 ≈100.0 99.6 87.8 75.9

7.1 5.3 5.3 5.3 13.3 40.4 44.7 55.3 55.3 51.6

≥4350 ≥11000 ≥17 000 ≥23 000 ≥10 000 ≥3000 ≥3000 223.3 5.8 2.9

50% Aliquat 336 solution in toluene 5.0 0.052 10.0 0.053 15.0 0.053 20.0 0.056 25.0 0.053 30.0 0.052 35.0 0.051 40.0 0.052 45.0 0.05 50.0 0.05

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100

≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 ≤ 0.01 0.1 3.4 6.7

0.32 0.6 0.9 1.1 1.6 1.9 2.2 2.5 2.9 3.2

≥99.7 ≥99.8 ≥99.9 ≥99.9 ≥99.9 ≈100.0 ≈100.0 99.6 89.1 80.7

0.9 5.3 5.3 8.3 1.6 3.5 4.5 3.2 4.0 4.7

≥36 600 ≥11600 ≥17 500 ≥13700 ≥102 000 ≥55 500 ≥50 000 8000 194.0 84.5

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present isotherms of rhenium extraction for 50% TBP solution in toluene and for 50% Aliquat 336 solution in toluene, respectively. High rhenium extraction recoveries were reached for TBP and Aliquat 336 in the whole examined range of NH4ReO4 concentrations. For 50% TBP solution in toluene, increase of ammonium ions recovery with increase of ammonium perrhenate solution concentration was observed. When Aliquat 336 was used, no increase of ammonium ions extraction efficiency with increase of NH4ReO4 solution concentration was noticed. The results presented in Table 5 show that maximum rhenium concentration in 50% TBP solution in toluene can reach 270.7 g/dm3, while in Aliquat 336 it can be as high as 284.2 g/dm3. It is advantageous to conduct extraction in the ranges where for a specific extractant rhenium concentration reaches the highest value — because in the further stage, i.e. in stripping, it gives possibility to obtain perrhenic acid of the highest possible concentration. Fig. 1. Isotherm of rhenium extraction with 50% TBP solution in toluene, in temperature of 60 °C.

Fig. 2. Isotherm of rhenium extraction with 50% Aliquat 336 solution in toluene, in temperature of 60 °C.

3.1.4. Aqueous to organic phases ratio Extractions were conducted with 50% TBP and 50% Aliquat 336 in toluene. The results of the studies into influence of phases ratio on extraction efficiency for 50% TBP solution in toluene are presented in Table 6. High rhenium extraction efficiencies (99.9%) were obtained when the ratio of aqueous phase volume to the volume of organic phase was in the range from 1:2 to 1:1. When the ratio of those phases was 3:2 rhenium extraction degree went down to 84.1%. Similar tendency was observed for 50% Aliquat 336 solution in toluene — 99.9% rhenium extraction degree was reached at the ratio of aqueous phase volume to the volume of organic phase varying from 1:2 to 3:2. Decrease of rhenium recovery to 88.4% was observed when the ratio was 4:2. Therefore extraction of rhenium from NH4ReO4 with 50% solutions of TBP and Aliquat 336 should be conducted at the ratio of aqueous phase volume to the volume of organic phase equal 1:1. That guarantees maximum effectiveness of extraction process, i.e. maximum efficiency and selectivity. These limitations are directly related to very high but limited capacities of the extractants for rhenium extraction. 3.2. Results of stripping of rhenium

ammonium perrhenate solutions acidified with sulphuric(VI) acid to pH of 1.0 were used. 3.1.3. Ammonium perrhenate concentration The next tests were aimed for determination of influence of ammonium perrhenate solution concentration on extraction efficiency. The results of investigations into rhenium extraction when 50% TBP solution in toluene was used are presented in Table 5. Figs. 1 and 2

The objective of this part of the study was to determine conditions for stripping of rhenium from the extractants, i.e. 50% toluene solutions of TBP and Aliquat 336, to aqueous phases. The stripping operation should produce aqueous perrhenic acid solution of the possibly highest rhenium concentration and the lowest ammonium ions content. Therefore the stripping should not only be effective (efficient, quantitative if possible), but also selective (the point is mainly to avoid transfer of ammonium ions do aqueous phase). The

Table 6 Influence of aqueous to organic phase ratio on efficiency of rhenium and ammonium ions extraction using 50% TBP and Aliquat 336 solutions in toluene Aqueous Volume of aqueous phase to organic L phase ratio 50% TBP solution in toluene 1:2 0.052 1:1 0.103 3:2 0.153 4:2 0.201

Volume of organic phase

Concentration of rhenium in aqueous phase

Concentration of ammonium ions in aqueous phase

Rhenium extraction efficiency

Ammonium ions extraction efficiency

L

g/L

g/L

%

%

(± 0.1%)

(±0.1%)

0.100 0.100 0.100 0.100

≤0.01 ≤0.01 0.9 1.2

1.4 0.7 0.5 0.4

≥ 99.9 ≥ 99.9 84.1 72.2

13.3 14.2 8.9 4.3

50% Aliquat 336 solution in toluene 1:2 0.053 0.100 1:1 0.104 0.100 3:2 0.152 0.100 4:2 0.202 0.100

≤0.01 ≤0.01 ≤0.01 0.5

1.56 0.80 0.55 0.41

≥ 99.9 ≥ 99.9 ≥ 99.8 88.4

1.6 1.0 0.5 1.4

Selectivity coefficient of rhenium versus ammonium ions ≥1100 ≥5000 54.1 58.0

≥100 000 ≥86 650 ≥119 000 532.7

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Table 7 Results of efficiency of rhenium and ammonium ions stripping from 50% TBP and Aliquat 336 solutions in toluene Reextractant Volume of aqueous phase L 50% TBP solution in toluene Hot water 0.025 15% H2O2 0.025 30% H2O2 0.025 10% HCl 0.025 15% HCl 0.025

Volume of organic phase

Concentration of rhenium in aqueous phase

Concentration of ammonium ions in aqueous phase

L

g/L

g/L

0.050 0.050 0.050 0.050 0.050

50% Aliquat 336 solution in toluene Hot water 0.025 0.050 15% H2O2 0.025 0.050 30% H2O2 0.025 0.050 10% HCl 0.025 0.050 15% HCl 0.025 0.050 10% H2SO4 0.025 0.050 15% H2SO4 0.025 0.050 10% HNO3 0.025 0.050 0.025 0.050 30% HNO3 65% HNO3 0.025 0.050

16.7 16.5 16.8 16.7 16.7

0.06 0.9 0.9 0.2 0.2 0.4 0.5 2.1 4.5 5.4

results of investigations into stripping of rhenium and ammonium ions with various agents to aqueous phase are presented in Table 7 and Figs. 3 and 4. From the obtained results it can be observed that stripping of rhenium (from 50% TBP solution in toluene) to aqueous phase was efficient when hot water was used and when hydrogen peroxide or hydrochloric acid were applied. The rhenium stripping efficiencies in the examined conditions were about 100%. Unfortunately, in those conditions also intensive ammonium ions stripping was observed. For practical reasons it was decided to conduct the stripping from toluene TBP solutions with hot water, since when using that agent the lowest degree of ammonium ions stripping was reached (8.3%). In fact, hydrogen peroxide solutions were slightly more efficient but brought slow degradation of the extractant. Application of HCl solutions was also not especially advantageous — it was not possible to obtain high purity perrhenic acid free from chloride ions. When 50% Aliquat 336 solution in toluene was used, stripping of rhenium from organic phase to aqueous phase was not possible at all. The best result was obtained when 65% nitric(V) acid was used.

Fig. 3. Comparison of efficiencies of rhenium and ammonium ions stripping from 50% TBP with hot water, H2O2 and HCl.

Rhenium stripping efficiency

Ammonium ions stripping efficiency

%

%

(±0.1%)

(±0.1%)

0.02 0.24 0.24 0.24 0.24

97.1 95.9 97.7 97.1 97.1

8.3 ≈100.0 ≈100.0 ≈100.0 ≈100.0

≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001 ≤0.001

0.3 5.2 5.2 1.2 1.2 2.3 2.9 12.2 26.2 31.4

≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4 ≤0.4

However, even in those conditions only 30% of the rhenium present was recovered. Application of more concentrated HNO3 for stripping of rhenium from Aliquat 336 resulted in degradation of organic phase only — development of nitric oxides was observed. 3.3. Repeated rhenium extraction with tributyl phosphate Previous results showed that complete separation of rhenium from ammonium ions in the process of single extraction and stripping is not possible. Therefore investigations into possibility of obtaining pure perrhenic acid (containing less than 0.001 g/L of ammonium ions) by repeated extraction – hot water stripping process using tributyl phosphate – were conducted. Table 8 presents results of these investigations. The results of the performed tests show that repeated extraction with 50% solution of TBP in toluene and stripping with hot water makes complete separation of rhenium from ammonium ions possible. Application of the procedure presented in Fig. 5 creates possibility to obtain perrhenic acid of rhenium concentration of about 70 g/L and ammonium ions concentration ≤0.001 g/L. Moreover, it was possible to use the developed method on larger scale. The repeated extraction and stripping was performed with 1 L of ammonium

Fig. 4. Comparison of efficiencies of rhenium and ammonium ions stripping from 50% Aliquat 336 with hot water, H2O2, HCl, H2SO4 and HNO3.

K. Leszczyńska-Sejda et al. / Hydrometallurgy 95 (2009) 325–332

331

Table 8 Results of synthesis of perrhenic acid from ammonium perrhenate solution using repeated extraction and double stripping of rhenium Stage

I Extraction I Stripping II Extraction II Stripping

Volume of aqueous phase

Volume of organic phase

Rhenium concentration in aqueous phase

Concentration of ammonium ions in aqueous phase

Rhenium extraction efficiency

Ammonium ions extraction efficiency

Rhenium stripping efficiency

Ammonium ions stripping efficiency

L

L

g/L

g/dm3

%

%

%

%

(±0.1%)

(±0.1%)

(±0.1%)

(±0.1%)

0.100

0.100

≤0.01

1.45

≥99.9

13.7

–

–

0.050

0.100

34.5

0.04

–

–

99.5

8.7

0.050

0.050

≤0.01

0.002

≥99.9

0.1

–

–

0.025

0.050

68.9

≤ 0.001

–

–

≥99.9

≤0.1

perrhenate solution. The obtained results were the same as the ones reached on smaller scale. 4. Conclusion Within the scope of the study new extractive method for obtaining perrhenic acid from aqueous ammonium perrhenate solutions was developed. It was established that when applying repeated extraction of rhenium with 50% tributyl phosphate in toluene from aqueous ammonium perrhenate solution acidified with sulphuric acid to pH of 1.0, and stripping of rhenium from

organic phase at temperature of 80 °C it is possible to obtain perrhenic acid of rhenium concentration 70 g/L. Thus obtained acid was about fourfold concentrated to obtain perrhenic acid of concentration 300 g/L Re, which contained no more than 100 ppm of impurities composed of metal (sodium, potassium, magnesium) and ammonium ions. Acknowledgment This work was supported by The State Committee for Scientific Research, Project No. 3T 08 A 049 28.

Fig. 5. Perrhenic acid synthesis by repeated extraction method.

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